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The Space Environment of Io and Europa 1

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Fran Bagenal & Vincent Dols 3

Laboratory for Atmospheric and Space Physics 4

University of Colorado, Boulder CO 5

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Abstract 7

The Galilean moons play major roles in the giant magnetosphere of Jupiter. At the same 8 time, the magnetospheric particles and fields affect the moons. The impact of magnetospheric 9 ions on the moons’ atmospheres supplies clouds of escaping neutral atoms that populate a 10 substantial fraction of their orbits. At the same time, ionization of atoms in the neutral cloud is 11 the primary source of magnetospheric plasma. The stability of this feedback loop depends on the 12 plasma / moon-atmosphere interaction. The purpose of this review is to describe the physical 13 processes that shape the space environment around the two innermost Galilean moons – Io and 14 Europa – and to show their impact from the planet Jupiter out into interplanetary space. 15 16

1. Introduction 17

In the 60s and 70s ground-based observations indicated Io was peculiar: the moon 18 triggered radio emissions, appeared to brighten on emerging from eclipse, and optical emissions 19 indicated clouds of sodium atoms and sulfur ions around Io. The Voyager 1 and 2 flybys of Jupiter 20 in 1979 revealed volcanism on Io, detected strong UV emissions from a torus of sulfur and oxygen 21 ions surrounding Jupiter at Io’s orbit, and measured the torus plasma in situ. When Voyager 1 22 passed close to Io, perturbations in the plasma and magnetic field showed Io generating Alfven 23 waves propagating away from the moon, carrying million-Ampere electrical currents along the 24 magnetic field towards Jupiter. The Voyagers characterized a plethora of radio and plasma waves, 25 several associated with Io or the Io plasma torus. Neither of the Voyager spacecraft passed close 26 to Europa but distant images hinted at an intriguingly smooth, icy surface. 27

When the Galileo spacecraft went into orbit around Jupiter in 1995, close-up pictures of 28 cracks, and a mysterious brown, suspiciously organic-looking material on the surface made 29 Europa a major focus of attention. Moreover, magnetic perturbations measured near Europa 30 indicated an electrically-conducting liquid ocean under the ice. On several passes, the Galileo 31 spacecraft also measured magnetic and particle perturbations near Io– as well as discovering that 32 Ganymede has an internal magnetic field. In late 2000, Cassini spacecraft got a gravity kick from 33 Jupiter to speed its journey to Saturn. The UVIS instrument provided months of high-quality 34 observations with of the torus UV emissions, telling us the plasma composition and how the torus 35 changed after a volcanic eruption on Io. 36

The aim of the 2004 monograph Jupiter: The Planet, Satellites and Magnetosphere 37 (Bagenal et al. 2004) was to summarize the post-Galileo view of the jovian system. Chapters by 38 Kivelson et al. (2004) and Saur et al. (2004) reviewed the current understanding of the 39

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electrodynamics of the plasma-moon interactions, and Thomas et al. (2004) reviewed Io’s neutral 40 clouds and plasma torus, explaining how about 1 ton/s of sulfur and oxygen escapes Io, becomes 41 ionized and trapped in the magnetic field, and moves out to fill Jupiter’s vast magnetosphere. 42

In the meantime, new observations and models have emerged. In spring 2007 the New 43 Horizons spacecraft got a gravity assist from Jupiter to speed its journey to Pluto, observing the 44 jovian system as it passed through. The Japanese space agency (JAXA) put the Hisaki satellite into 45 orbit around Earth in September 2013 (Yoshikawa et al. 2014, 2016). Hisaki’s UV spectrometer 46 has been observing emissions from the Io plasma torus, determining composition (Yoshioka et 47 al. 2014, 2017), mapping the oxygen neutral cloud (Koga et al. 2018a,b), and measuring temporal 48 variations (Tsuchiya et al. 2015, 2018; Yoshikawa et al. 2017; Kimura et al. 2018; Koga et al. 2019). 49 The Juno spacecraft went into orbit around Jupiter in July 2016 (Bolton et al. 2017). While the 50 prime Juno mission does not bring the spacecraft inside Europa’s orbit, the particles and fields 51 instruments are measuring the consequences of the Io-Europa space environment both out in 52 the middle magnetosphere and close over the poles (Bagenal et al. 2017). Future missions to 53 Europa further emphasize the need to consider the possible impacts that the changing 54 environment near Io might have on the Europa system. 55

In past reviews the separate components of the Io-Europa system are generally discussed 56 in isolation. To understand physical processes and the interconnections of the components, we 57 need to look at the whole system. The goal of this review is to summarize the current 58 understanding of this multi-component system and to present the outstanding questions. The 59 remainder of Section 1 provides an overview of the system and a brief summary of what we know 60 about the moons Io and Europa. In Section 2 we compare the atmospheres of these moons. In 61 Section 3 we look at the interaction between these atmospheres and the plasma in which they 62 are embedded. Section 4 describes the extended clouds of neutral atoms that come the plasma-63 atmosphere interactions. The space environment between Io and Europa, the plasma torus, is 64 reviewed in Section 5. Sections 6 presents the evidence that these space environments spread 65 their influence out into interplanetary space and into the atmosphere of Jupiter. Finally, we 66 present a summary in Section 7, listing outstanding questions for future research. We have 67 attempted to provide extensive references but also provide introductory and summarizing 68 paragraphs without references to present a readable review that is accessible to a broad 69 audience. 70

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1.1 – The Io-Europa Space Environment 72

Figure 1 shows main components of the system. Central are the moons Io and Europa. 73 The interactions of the magnetospheric plasma with these moons’ atmospheres produce clouds 74 of escaping neutrals that extend for a substantial fraction of Io’s and Europa’s orbits around 75 Jupiter. Electron impact ionization of these neutral clouds produces the plasma that is trapped in 76 Jupiter’s strong magnetic field. It is the intriguing feed-back systems between the moons and the 77 magnetospheric plasma that begs understanding. The magnetospheric plasma primarily 78 corotates with the planet’s 10-hour spin period, but also moves radially outwards over several 79 weeks. Only about 10% of the torus material moves inwards from Io’s orbit. The inward flows are 80 about a factor 50 slower than the outward flow. Charge exchange reactions between the 81

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corotating plasma and the neutral clouds produces energetic neutral atoms (ENAs) that escape 82 Jupiter to form a neutral disk that extends 100s of RJ around Jupiter. Recent observations by the 83 Juno spacecraft of heavy ions in Jupiter’s upper ionosphere suggest that some of the ENAs hit the 84 planet’s equatorial atmosphere and form a source of heavy ions close to the planet. As the 85 iogenic plasma moves out through the giant magnetosphere, it is heated to 10s-100s keV 86 energies. Thus, there is a source of energetic electrons and ions (protons, sulfur and oxygen ions) 87 that move inward while the lower-energy plasma moves outward. When the energetic ions move 88 inward through the neutral cloud around Europa’s orbit, they charge exchange, making very 89 energetic neutral atoms (vENAs). These vENAs have high speeds in all directions and likely spread 90 into a huge sphere around the Jupiter system. Juno also detected new equatorial belt of energetic 91 radiation close to the planet produced when the vENAs bombard Jupiter’s atmosphere. This 92 system of ten coupled components comprises a wide range of physics and extends from 10s of 93 kilometers to Astronomical Unit scales. 94

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1.2 –Io and Europa: Bulk Properties 96

Io and Europa are similar in size to Earth’s Moon (Figure 2). Io, Europa and Ganymede are 97 locked in a tight orbital resonance (with orbital periods increasing by a factor of two) which drives 98 tidal interactions between the moons (see review by Schubert et al. 2004). Tidal forces produces 99 orbit-phase lock with Jupiter, so that when viewed from Earth, surface longitude facing the 100 observer and local time along the orbit are related – and we do not see the night side of the 101 moon. The strong radial dependence (1/R6) of tidal heating means Io is the most volcanically 102 active object and the densest moon in the solar system. Io emits ~1014 W internal heat, about 103 1/3 the solar heating (McEwen et al. 2004). Having lost any water it may have had when formed, 104 Io is covered with over 425 volcanic features (Figure 3), of which 50-100 are actively erupting at 105 any one time, resurfacing the moon at a rate of ~1 cm/year, equivalent to the whole moon 106 turning over ~40 times in 4.5 Ga (McEwen et al. 2004; Davies 2007; Lopes & Spencer 2007; 107 Williams et al. 2011). A recent study of long term variations by de Kleer et al. (2019b) suggest Io’s 108 volcanism may be modulated by periodic (~1.25 year) variations in orbital parameters. 109

Europa’s bulk density of 2989 ± 46 kg m-3 is intermediate between that of Io (3528 ± 3 kg 110 m-3) and Ganymede (1942 ± 5 g cm-3), suggesting it has lost much but not all of its primordial 111 water, resulting in a mantle of ~80:20% rock:water composition (Anderson et al. 1998; Schubert 112 et al. 2004). The magnetic field perturbations measured by Galileo on multiple flybys of Europa 113 determined the presence of the water ocean (Khurana et al. 1998), supported by the presence 114 of tidally-driven cycloidal fractures (Hoppa et al. 1999). The net tidal heating is less well known 115 for Europa, ranging between 1012-1013 W (Greenberg et al. 2002). Europa’s outer layer of water 116 is equivalent to about twice the mass of Earth’s ocean, forming a 100-130 km layer capped by a 117 5-30 km ice layer (Cammarano et al. 2006; Vance et al. 2018). The depth:diameter ratio of the 118 largest multi-ringed craters suggest penetration of the impactor through 19-25 km of ice (Schenk 119 2002). Geological features such as ridged plains, chaotic terrain, spreading bands, pits and domes 120 suggest convection of ductile ice under a cold brittle surface (e.g., Schmidt et al. 2011). The major 121 question that is driving future exploration of Europa (Pappalardo et al. 2013) is whether there 122 are regions where the ice is thin or broken – spurred on by the detection of water plumes (Roth 123

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et al. 2014a,b; Sparks et al. 2016, 2017) – that might be places to search for life (Hand & Chyba, 124 2007). 125

Despite very different surface compositions (lava, sulfur, SO2 vs. ice and sulfates) , Io and 126 Europa have similar albedos (0.62 and 0.64 respectively) and similar average surface temperature 127 of ~110 Kelvin (Rathbun et al. 2004; Spencer et al. 1999). But the differences only increase as we 128 look above the surface to their atmospheres. 129

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2. Io and Europa Atmospheres 131

Io and Europa both have significant atmospheres but, like their surfaces, they are very 132 different in character (Figure 4, Table 1). Io’s atmosphere is primarily due to sublimation of SO2 133 frost. Europa’s atmosphere comes from charged particle sputtering of the icy surface producing 134 H2O, O2 and H2. The water quickly freezes back onto the surface and hydrogen escapes, leaving 135 Europa with an oxygen atmosphere. Io’s atmosphere is confined to low- to mid-latitudes with the 136 polar pressures about 2% of equatorial pressures. Europa’s atmosphere is more uniform, with 137 pressures comparable to the poles of Io. Remembering that Io and Europa are spin-phase locked 138 and that the atmospheres are strongly affected by the plasma flowing from the trailing 139 (upstream) side around to the leading (downstream) sides of the moons, one might expect 140 variations with longitude. The sub-/anti-jovian longitudes (System III West) are 0°/180° while the 141 leading/trailing longitudes are 90°/270°. Io has at least 16 active plumes. Europa probably 142 sometimes has an active plume. The roles of these eruptions on each atmosphere is poorly 143 known – but keenly pursued research topics. 144

The total mass of Io’s atmosphere is ~50 times that of Europa. This may not seem a huge 145 difference, but, as we shall see in subsequent sections, the material escaping from Io’s 146 atmosphere totally dominates the surrounding space environment. 147

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2.1 Io’s Atmosphere 149

Io’s atmosphere is still surprisingly poorly known. Some atmospheric properties (density, 150 composition, asymmetries, equatorial extension) have been observed via telescopes on the 151 ground, Earth orbit or on spacecraft at Jupiter. Some atmospheric properties are also deduced 152 from in-situ plasma observation and numerical modeling of the plasma-atmosphere interaction. 153 From these observations, it is concluded that Io has a collisionally-thick SO2 atmosphere with a 154 surface pressure of 1-10 nbar, concentrated ±40° of the equator (see reviews by McGrath et al. 155 2004; Lellouch et al. 2007). 156

Earth-based telescopes provide important information about the dayside atmosphere in 157 from UV to millimeter wavelengths. These remote observations consist of brightness integrated 158 along the line of sight on the sunlit hemisphere of Io. They provide an estimate of the column 159 density on the dayside atmosphere but they give limited information on its radial extension or 160 variations with time of day. Io’s surface pressure is primarily controlled by the sublimation of SO2 161 frost (Tsang et al. 2012, 2013a,b, 2016; Lellouch et al. 2015), which is supported by the fact that 162 the atmospheric column density drops by a factor of 5 ± 2 when Io goes into eclipse (Tsang et al. 163

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2016; Retherford et al. 2007, 2019). The SO2 atmosphere is confined to latitudes within ±40° of 164 the equator with the density above the poles just ~2% of the equatorial density (Feaga et al. 2009, 165 Strobel and Wolven 2001; Lellouch et al. 2015). This distribution is likely primarily caused by the 166 equatorial distribution of the frost on the surface but also by the location of some of the main 167 volcanoes at low latitude. The vertical column density at the equator ranges from 1.5 x 1016 cm-2 168 at sub-jovian longitudes to 15 x 1016 cm-2 at anti-jovian longitudes (Feaga et al. 2009; Jessup et 169 al. 2004; Spencer et al. 2005; Lellouch et al. 2015). The anti-jovian atmosphere is not just denser 170 but also latitudinally more extended than the sub-jovian side (Feaga et al. 2009). 171

The condensation response was modeled by Moore et al. (2009) and Walker et al. (2010, 172 2012) who showed that most of the atmospheric column is probably contained in the first 200 173 km with a steep density profile under 20-30 km. The night side atmosphere of Io is poorly 174 constrained. The low temperatures at night would suggest SO2 condenses onto the surface and 175 the composition of the atmosphere changes drastically. Modeling by Wong and Johnson (1996) 176 indicate that the night-side atmosphere may be dominated by non-condensable gases (O2 and 177 possibly SO) with SO2 condensing onto the surface. Moore et al. (2009) show that a buffer could 178 be created by even a small amount of non-condensable gases close to the surface, limiting the 179 condensation of SO2. On the other hand, Lellouch et al. (2015) report that the column density 180 changes by a factor of ~2 before and after noon corresponding to sublimation driven by ±2 Kelvin 181 change in temperature. Furthermore, the factor factor 5 ± 2 collapse during eclipse (Tsang et al. 182 2016) indicates that the effect of non-condensable gases is negligible. The data are currently very 183 limited but it is reasonable to assume that the atmosphere is significantly less dense on the night-184 side of Io but probably does not completely condense out. 185

SO2 comprises 90% of the surface atmospheric pressure. Atmospheric SO was detected in 186 mm-wave observations at the 3-10% level (Lellouch et al. 1996; Moullet et al. 2010, 2013; deKleer 187 et al. 2019a). But the global coverage and column density of SO are uncertain. Moullet et al. 188 (2013) report some minor species (e.g. NaCl, KCl) that are likely linked to volcanic outgassing and 189 Spencer et al. (2000) reported enhancement of S2 over the Pele volcanic plume. From modeling 190 the photochemistry of the atmosphere, Moses et al. (2002a) suggest the SO/SO2 and S/O ratios 191 may increase during more highly volcanic epochs. 192

Photodissociation of molecular species within ~5 hour time frame means that the dayside 193 atmosphere will have a higher atomic composition (O, S, Na) than the nightside (Moses et al. 194 2002a,b). Post-eclipse brightening of sodium emissions suggest that photodissociation of the 195 volcanically-produced NaCl accounts for most of the atomic sodium produced by Io (Grava et al. 196 2014). 197

The Io atmosphere also has a remarkable high-altitude component of atomic species: S 198 and O (Wolven et al. 2001). There is a corona within Io’s Hill sphere (~5.8 RIo) as well as neutrals 199 that escape Io’s gravity and extend partly around Io’s orbit. While O is observed in both visible 200 and UV, there has only been sparse detection of S in the UV (Durrance et al. 1983, 1995; Wolven 201 et al. 2001). Auroral emissions of O and S are produced by electron impact excitation of the 202 neutral atmosphere (Roesler et al. 1999; Retherford et al. 2000, 2003; Geissler et al. 2001, 2004). 203 It is commonly accepted that the emissions are caused by thermal (5 eV) electrons impacting O. 204 The emissions depend on the neutral density as well as the electron density and temperature, all 205

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of which are thought to vary drastically in the environment close to Io. The relative importance 206 of each electron property as well as the neutral density can only be estimated through numerical 207 simulations of the plasma/atmosphere interaction (see Section 3.1). The S and O auroral 208 emissions display several structures: Two large equatorial spots (Retherford et al. 2000; 2003), 209 further emission downstream along the flanks of Io at low altitude (Roth et al. 2011, 2014a,b), a 210 coronal emission that extends smoothly to ~ 10 RIo from the surface (Wolven et al. 2001; 211 Retherford et al. 2019), and polar limb brightening over the pole that faces the plasma sheet 212 during the diurnal rotation of Jupiter (Retherford et al. 2003). The density profiles of O and S in 213 the atmosphere are still not well known. From modeling of the interaction and its resulting O and 214 S emissions, the O/SO2 ratio is estimated to be between 10-20% in the upper atmosphere (the 215 models do not resolve the deep atmosphere) and S/SO2 ~ 1.5% with a scale height slightly larger 216 than SO2 (Feaga et al. 2009; Roth et al. 2011, 2014a,b). 217

In summary, the atmosphere of Io is mostly sublimation-controlled SO2, with a vertical 218 column of SO2 ranging from 1.5 to 15 x 1016 cm-2, concentrated around the equator. Extending 219 far from Io’s surface are minor species S and O which form a tenuous corona stretching to 220 distances of ~10 RIo. There are still major uncertainties about the radial profile, longitudinal 221 asymmetries, minor species composition and how much the atmosphere collapses at night or in 222 eclipse. 223

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2.2 Europa’s Atmosphere 225

The existence of the atmosphere at Europa is supported by the observation of UV aurora 226 in atomic oxygen lines as well as the detection of an ionosphere by radio occultation. As at Io, 227 properties of the global atmosphere are also inferred from numerical simulations of the plasma-228 atmosphere interaction, constrained by Galileo (GLL) plasma observations (see Section 3.2). 229 Europa’s atmosphere is tenuous (surface pressure ~3 pbar), barely collisional, and mainly 230 composed of O2 that is fairly uniformly distributed over the moon (see reviews by Johnson et al. 231 2009, 2019; McGrath et al. 2004, 2009; Burger et al. 2010; Coustenis et al. 2010; Plainaki et al. 232 2018). 233

UV auroral emission of Europa’s atmosphere in the 130.4 and 135.6 nm oxygen lines are 234 observed with the Hubble Space Telescope (HST). These UV emissions of oxygen atoms are 235 consistent with electron impact dissociation of an O2 atmosphere with vertical column density of 236 2-15 x 1014 cm-2 (Hall et al. 1995a, 1998; McGrath et al. 2009; Roth et al. 2014a,b). The UV 237 emissions depend on the neutral density, as well as on the electron density and temperature. 238 Thus, deriving neutral densities and their distribution is difficult and requires numerical 239 simulations. The extensive analysis of the O aurora by Roth et al. (2016) argues for a more limited 240 range of 2-10 x 1014 cm-2. The O brightness is similar in daylight or in eclipse showing that the 241 atmosphere does not collapse at night like at Io (because O2 is non-condensable). Unlike Io, the 242 main auroral emissions are not on the flanks but above the pole, and specifically brighter on the 243 polar hemisphere facing the plasma sheet. It is concluded that, in contrast to Io, Europa’s 244 atmosphere extends from pole to pole (see Figure 4). 245

Five GLL occultations of Europa’s ionosphere (Kliore et al. 1997), showing average 246

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electron densities of ~10,000 cm-3, compatible with an atmospheric neutral scale height of ~120 247 km, but with large spatial and/or temporal variations. This scale height is consistent with later 248 observations of the O auroral emissions (Roth et al. 2016) and numerical simulations (Saur et al. 249 1998). The presence of an extended atomic O corona is supported by Cassini observations during 250 its Jupiter flyby (Hansen et al. 2005) and HST observations in UV (Roth et al. 2016). The resulting 251 picture is an O2-dominanted atmosphere up to 900km (1.6 REu) where the O mixing ratio varies 252 from 6% to 30%. De Kleer & Brown (2018) measured optical emissions in Europa’s atmosphere 253 (dominated by electron impact dissociation/excitation of O2) and concluded that the global O/O2 254 ratio is less than 35%. 255

Erupting plumes may also contribute water vapor to the atmosphere but such sources 256 are, at best, sporadic. They are observed in O lines and H-Lyman alpha lines (Roth et al. 2014a,b; 257 Sparks et al. 2016, 2017). Longitudinal asymmetries in the aurora brightness are observed but 258 their interpretation in terms of inhomogeneity of the atmosphere is still debated. Roth et al. 259 (2016), in a compilation of many observations at different Europa orbital longitudes, shows that 260 the dusk hemisphere is always brighter than the opposite hemisphere. This asymmetry of the 261 auroral brightness could be attributed to a local denser atmosphere caused by not only the 262 potential presence of plumes, but also ice thermal inertia (Plainaki et al. 2013) or effect of 263 Europa’s varying illumination (Oza et al. 2018, 2019). Johnson et al. (2019) argues that the dawn-264 dusk asymmetry could not be reproduced with just a sputtered (trailing) source and that an 265 additional sub-solar source is needed. But Roth et al. (2016) claims that some of these brightness 266 asymmetries could also result from the variation of the plasma interaction. 267

An upstream/downstream asymmetry of the atmosphere is not clearly established. 268 Pospieszalska and Johnson (1989) modeled the sputtering of Europa’s surface and show that the 269 sputtering flux is not uniformly distributes and peaks at the trailing hemisphere. But there is little 270 observational support for this. The oxygen auroral brightness is slightly lower on the leading 271 hemisphere compared to the trailing, which is interpreted as a global atmosphere distributed 272 upstream and downstream (Saur et al. 2011; Roth et al. 2016). The Kliore et al. (1997) non-273 detection of an ionosphere on the downstream hemisphere was interpreted as a lack of 274 ionization, not solely as a lack of atmosphere. 275

The source of the atmosphere is the sputtering of ions (thermal and hot) and maybe high-276 energy electrons on the icy surface of Europa. Lab experiments of ion bombardment of water ice 277 (Fama et al. 2008) suggests that H2O molecules are the dominant ejecta when magnetospheric 278 ions hit Europa’s surface. Molecules of O2 and H2 are also sputtered off the icy surface. Molecular 279 oxygen, O2, probably dominates the atmosphere because it neither freezes to the surface like 280 H2O, nor quickly escapes Europa’s gravity like H2 (Johnson et al. 1982; 2009). 281

There is no consensus on the relative contributions of thermal ions versus energetic ions 282 to the production of the O2 atmosphere (Mauk et al. 2004; Paranicas et al. 2009). Cassidy et al. 283 (2013) show that thermal heavy ions (S, O at 1-2 keV ram energy) could be responsible for 284 supplying the atmosphere. They estimate the O2 and H2 production rates by multiplying the ion 285 flux with the sputtering yields for O2. The yields are functions of the ion kinetic energy and are 286 provided by Teolis et al. (2010, 2017). The ion flux was calculated very simply, ignoring the 287 electromagnetic interaction that diverts the plasma flow around the moon and shields its surface. 288

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Other processes could contribute to the atmosphere production. Ip (1996) proposed that 289 ionization creates O2

+ ions which, when accelerated by local electric fields, could cause significant 290 sputtering. But Saur et al. (1998), based on plasma-atmosphere simulations, suggested that these 291 picked-up ions are too slow for efficient sputtering. Both Dols et al. (2016) and Luchetta et al. 292 (2016) argue that charge exchange reactions between in-coming ions and the neutral 293 atmosphere are key. Dols et al. (2016) proposed that fast neutrals resulting from multiple charge 294 exchange reactions between pickup O2

+ ions and atmospheric O2 provide supplemental 295 sputtering. But the sputtering rate of these fast neutrals has yet to be well quantified. 296

In summary, Europa’s tenuous, global atmosphere is still poorly constrained by 297 observations. We await better measurements of the number and frequency of the sporadic 298 plumes but their contribution to the global atmosphere seems to be limited. The source of the 299 atmosphere is primarily particle sputtering of the icy surface but a comprehensive model of the 300 atmosphere is still lacking. 301

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3. Plasma-Satellite interactions 304

Figure 5 illustrates the main components of plasma-satellite interactions, Figure 5a 305 presenting the basic geometry. Figure 5b indicates some of the chemical reactions as the plasma 306 impacts the atmosphere. Figures 5c and 5d show the UV and radio emissions excited at Jupiter 307 by streams of electrons that are generated by the plasma-satellite interactions. 308

Before the 1958 Explorer 1 discovery of Earth’s Van Allen radiation belts, bursts of radio 309 emission revealed Jupiter has a magnetic field that traps electrons (Burke & Franklin 1955). These 310 high frequency (GHz or decimetric wavelength) radio emissions come from the MeV electrons 311 trapped close (<2.5 RJ) to Jupiter (Bolton et al. 2004). The bursts of emission at decametric 312 wavelengths (few-40 MHz) showed modulations with Jupiter’s spin period and, to great surprise, 313 the orbital phase of Io (Bigg 1964). The Io modulation of radio bursts triggered ideas of an 314 electrically-conducting Io acting like a generator as it moved relative to Jupiter’s magnetic field 315 (a “unipolar inductor”), driving electrical currents that complete a circuit between the moon and 316 the planet’s ionosphere (Marshall & Libby 1967; Piddington & Drake 1968; Goldreich & Lynden-317 Bell 1969). 318

The radio emissions indicate processes close to the planet, involving electrons radiating 319 as they move along the magnetic field away from Jupiter, and say little about the generation 320 mechanism at Io. On 6 March 1979 Voyager 1 flew past Io where the instruments detected 321 plasma and magnetic field perturbations consistent with a packet of Alfvén waves forming an 322 Alfvén wing propagating from Io (Belcher et al. 1981; Acuna et al. 1981). Gurnett & Goertz (1981) 323 suggested that multiple ionospheric reflections of such Alfvénic disturbances between north and 324 south hemispheres of Jupiter (Figure 5d) could produce the pattern of arcs in the frequency-time 325 spectrograms of the Voyager Planetary Radio Astronomy instrument (Warwick et al. 1979). 326

As Earth-based telescopes improved, auroral emissions in Jupiter’s atmosphere – IR, UV, 327 – revealed features at first associated with Io (Connerney et al. 1993; Clarke et al. 1996; Prange 328

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1996), and then not just Io but also Ganymede, Europa (Clarke et al. 2002; Grodent et al. 2006; 329 Bonfond et al. 2017a,b) and, recently, Callisto (Bhattacharyya et al. 2018). These auroral 330 emissions (Figure 5c) indicate that the plasma-satellite interactions all involve electrodynamic 331 perturbations which generate Alfven waves propagating from the moon, carrying electric 332 currents parallel to the magnetic field, accelerating electrons that bombard Jupiter’s atmosphere 333 and generate auroral emissions. The Juno mission is revealing new details about these auroral 334 emissions and flying through the polar end of the fluxtubes that couple to the moons (Mura et 335 al. 2018; Szalay et al. 2018). In this review we focus on what is happening at Io and Europa. 336

The interactions of the jovian plasma with moon atmospheres produce large 337 perturbations of the magnetic field and local plasma properties (flow velocity, density, 338 temperature and composition). Over 33 orbits of Jupiter, the Galileo spacecraft made several 339 close passes of Io and Europa where the particles and fields instruments measured the local 340 perturbations (summarized by Kivelson et al. 2004). Table 2 summarizes the range of plasma 341 conditions upstream of Io and of Europa. 342

The atmosphere/plasma interaction is intrinsically time-variable for two main reasons. 343 The first is the inclination of the torus centrifugal equator by 7° relative to Io’s orbital plane 344 (further discussed in section 5). During a jovian synodic period (13 hours at Io and 11 hours at 345 Europa), the moons experience variable upstream plasma densities and thus, a variable strength 346 of the interaction. Secondly, the moons’ atmospheres are also variable depending on the 347 atmospheric sources. At Io, the SO2 atmosphere is primarily sublimation-supported, varying with 348 illumination and when there are particularly strong volcanic eruptions. For Europa, the 349 atmosphere results from sputtering of the surface by thermal and energetic ions but the O2 350 desorption from the icy surface seems to depend on illumination as well (Johnson et al. 2018, 351 2019). Thus, the atmospheric column and the interaction’s strength depend on the illumination 352 of the hemisphere impacted by the torus plasma. 353

In Figure 5b, the impact of thermal electrons on the neutral atmosphere is the main local 354 source of new ions, which are mostly molecular (SO2

+ at Io and O2+ at Europa). Photoionization 355

represents only ~10-15% of the total ionization rate (Saur et al. 1998, 1999). A molecular 356 atmosphere is very efficient at cooling the impinging thermal electrons (Saur et al. 1998, 1999; 357 Dols et al. 2008, 2012, 2016) via ionization and dissociation of molecular neutrals and also by 358 exciting molecular electronic, vibrational and rotational levels. These cooling processes are so 359 efficient that the primary electrons would rapidly become too cold to provide any further 360 ionization if they were not replenished by the content of the flux tube above and below the moon 361 impacting the moon’s atmosphere. 362

Ion charge exchange processes (asymmetrical and resonant) do not provide new ions but 363 they can change the ion composition and constitute an important sink of momentum for the 364 upstream plasma. After an ionization or a charge exchange, the new ion is initially at rest in the 365 frame of the moon. It is then “picked up” by the background bulk plasma flow and also starts a 366 gyro-motion at the local flow velocity (further discussed in Section 5.1 below). The pickup process 367 also affects the average ion temperature of the plasma. Depending on the location of this pickup, 368 the local flow velocity is larger than the upstream velocity (on the flanks) or smaller (in the deep 369 atmosphere) and the pickup process is either a gain or a loss of energy that affects the average 370

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ion temperature. For instance, an SO2+ ion picked up by a 60 km/s flow at Io will gain 1080 eV, 371

which represent a net heating of the upstream plasma (with a typical temperature of ~100 eV). 372 In Figure 5b, the ion/neutral process called “atmospheric sputtering” refers to ion/neutral elastic 373 collisions without exchange of charge and after multiple collisions, a neutral is finally ejected 374 from the atmosphere (McGrath & Johnson 1987). The resulting neutral clouds are discussed in 375 section 4. 376

The pickup and collision processes slow the plasma flow close to the moon and divert 377 some of the plasma around the flanks as illustrated in Figure 5. To first approximation, the 378 magnetic field is frozen to the plasma flow. Thus, this flow perturbation also produces large 379 magnetic field perturbations that propagate away from the moon in the three classical MHD 380 wave modes (fast, slow and Alfven). In particular, the magnetic perturbation propagates along 381 field lines as Alfven waves towards the ionosphere of Jupiter (Acuna et al. 1981) and forms a 382 stationary structure in the reference frame of Io called an Alfven wing (Belcher 1987). A large 383 current is carried along this Alfven wing (~ 3 MAmp for Io, according to Acuna et al. 1981). The 384 obstacle to the flow is not only the solid body of the moon but the entire Alfven wing extending 385 from Io to the ionosphere of Jupiter. 386

Approaches to modeling these complex plasma-moon interactions range from focusing 387 on the electrodynamics with limited chemistry to assuming simple electrodynamics and focusing 388 on the chemistry. Figure 6 illustrates an example of the latter approach for the Io case. A similar 389 approach can be taken for Europa, with appropriate modification of the atmosphere and 390 upstream plasma. Numerical simulations of this interaction are constrained by in-situ 391 observations of the plasma properties close to the moons (Voyager and Galileo) and by remote 392 observations of the moons’ auroral emissions. Beyond matching the observations, the modeling 393 approach allows an exploration of the main features of the interaction. Numerical simulations by 394 Linker et al. (1998) and Combi et al. (1998) illustrate the generation and propagation of the three 395 MHD wave modes. Numerical simulations (Saur et al. 1998, 1999, 2002, 2003; Dols et al. 2008, 396 2012, 2016; Blocker et al. 2016, 2018), reveal the relative importance of each plasma process 397 (ionization, charge-exchange, collision), constrain the neutral atmosphere distribution radially 398 and longitudinally, estimate the local plasma production and neutral loss and explore the 399 presence of an induced magnetic field either in Io’s asthenosphere (Khurana et al. 2011) and/or 400 core (Roth et al. 2017b), or in a subsurface ocean for Europa ( Zimmer et al. 2000; Schilling et al. 401 2007), plus test evidence for plumes (Blocker et al. 2016; Zia et al. 2018; Arnold et al. 2019). 402

While the physical processes (Figures 5, 6) are similar at Io and Europa, there are major 403 differences. Below we describe each interaction separately and then make a comparison in 404 section 3.3. 405

406

3.1 – Plasma-Io interaction 407

The Galileo spacecraft made 5 flybys of Io between 1995 and 2001 revealing very strong 408 plasma and field perturbations. The relative velocity of the torus plasma in Io’s frame is ~ 60km/s. 409 The plasma is mainly composed of S++ and O+ ions at temperatures of ~ 100 eV. The gyroradius 410 of the thermal ions ~ 2 km and the gyrofrequency ~ 1.5 Hz (see Table 2). When Io is in the 411

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centrifugal plane of the torus, the flow is quasi-stagnated at the point where the plasma impinges 412 on the atmosphere, with 95% of the plasma diverted around the flanks. The electron flow close 413 to Io is strongly twisted towards Jupiter because of the Hall conductivity of the ionosphere (Saur 414 et al. 1999). The magnetic perturbation reaches ~ 700 nT in a background field ~ 1800 nT (Kivelson 415 et al. 1996a). Downstream of Io, GLL detected a wake of plasma that was very dense (~ 30,000 416 cm-3), very slow (flow speed <1 km s-1) and very cold (Ti ~ 10 eV) (Frank et al. 1996; Bagenal 1997). 417 Bi-directional parallel electron beams above the poles and in Io’s wake were also detected with 418 an energy ranging from ~140 eV to several 10s keV (Frank and Paterson 1999; Williams et al. 419 1996, 2003; Mauk et al. 2001). Finally, Electro-Magnetic Ion Cyclotron (EMIC) waves were 420 observed far downstream of Io at the SO2

+ and SO+ ion gyro-frequencies, suggesting a pickup 421 process far (>10 RIo) from Io (Warnecke et al. 1997; Russell & Kivelson 2000; 2001). 422

Electrons bombarding Io’s atmosphere generate auroral UV emissions that were 423 observed by HST and Cassini (Roesler et al. 1999; Geissler et al. 2001, 2004). The auroral 424 morphology presents three main structures: bright equatorial spots, limb glow and extended 425 emissions (see review by Roth et al. 2011 and references therein). The equatorial spots are close 426 to the surface (< 100km), slightly downstream and longitudinally extended into the wake. They 427 appear to “rock” with the local direction of the Jovian magnetic field during Io’s synodic period. 428 The limb glow is also brighter on the hemisphere facing the centrifugal equator (Rutherford et al. 429 2003). These emissions result from electron impact excitation of the atomic sulfur and oxygen 430 components of the atmosphere and are modeled by Saur et al. (2000) and Roth et al. (2011). 431 These local auroral emissions provide additional constraints on the atmospheric distribution, 432 composition and plasma interaction, most importantly during eclipse. Roth et al. (2014c)’s 433 phenomenological model of the auroral emissions rely on data gathered from 1997 to 2001. They 434 conclude that during this 4 year period, the auroral emissions did not vary beyond the changes 435 of the periodically varying local environment caused by the latitudinal motion of Io in the torus. 436 This stability is remarkable considering the potential variation of Io’s atmospheric content caused 437 by sporadic major volcanic eruptions during such a long period. Furthermore, Roth et al. (2017b) 438 argue that the rocking of Io’s auroral spots is consistent with induction in the deep iron core 439 rather than shallow aesthenosphere as proposed by Khurana et al. (2011). Blocker et al. (2018) 440 supported Roth’s idea by claiming that most of the magnetic field perturbation observed during 441 Galileo flybys were caused by an asymmetric atmosphere and not by a strong induction signal. 442

In Figures 7 and 8, we illustrate the plasma-neutral interaction at Io using the multi-443 species chemistry approach sketched in Figure 6. Although the model results vary with the 444 assumptions of the simulation, they provide reasonable estimates of the contribution of each 445 process and the resulting plasma properties. These simulations are presented in Dols et al. (2008) 446 and are summarized below. The plasma flow is prescribed as an incompressible flow around a 447 conducting obstacle. The simulation shown in Figure 7 does not include the parallel electron 448 beams detected by GLL which probably provide much of the ionization in the wake, particularly 449 when a dense atmosphere is present (Saur et al. 2002; Dols et al. 2008, 2012). Figure 7 shows the 450 plasma properties in the equatorial plane of Io. The SO2 distribution is assumed cylindrically 451 symmetrical and thus does not include any day-night asymmetry resulting from a collapse of the 452 SO2 atmosphere at night. The radial distribution is exponential with a scale height ~500 km and 453 a denser component with a scale height of ~30 km where most of the SO2 column resides. The 454

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resulting vertical column density of 5 x 1016 cm-2 is consistent with dayside observations. The SO2 455 atmosphere extends 1 RIo along the background magnetic field direction (Strobel & Wolven 456 2001). The S and O atmosphere is spherically symmetric based on the UV emission radial profiles 457 of Wolven et al. (2001). 458

The plasma flow is slowed upstream and downstream and accelerated on the flanks. 459 Following Dols et al. (2008), we prescribe a radial slowing of the flow consistent with the ion 460 temperature observed along the GLL J0 (first) flyby. The ion temperature increases on the flanks 461 because of the pickup of SO2

+ ions in the local fast flow. An increased electron and SO2+ density 462

is carried along the flow while the electron temperature decreases because of the efficient 463 cooling process provided by the molecular atmosphere. The SO2 electron-impact ionization and 464 dissociation are located mainly on the upstream hemisphere because of the efficient electron 465 cooling processes. The SO2 resonant charge-exchange rate (SO2 + SO2

+ => SO2+ + SO2) is larger on 466

the flanks. In the wake of Io, along the GLL J0 flyby, the flux tubes are emptied of the upstream S 467 and O ions because of charge-exchange reactions with SO2 and SO2

+ becomes the dominant ion. 468

In Figure 8 we show the volume-integrated rates of each plasma-SO2 process. These 469 results are computed using a 3D MHD simulation of the plasma flow around Io and are somewhat 470 different in detail to the 2D simulations shown in Figure 7. Although the reaction rates depend 471 on the flow and the atmosphere prescribed, Figure 8 provides a reasonable estimate of the 472 relative contribution of each process. The dominant SO2 loss process is the electron-impact 473 dissociation, which provides slow atomic neutrals that potentially feed on an extended corona. 474 Another significant process is a cascade of resonant charge exchange reactions, which provides 475 slow and fast SO2 molecules, depending on the local flow speed where the charge exchange 476 reaction occurs. This cascade of charge exchange and dissociation reactions potentially feeds 477 neutral clouds of S, O atoms and SO2 molecules similar to the observed Na structures illustrated 478 in Figure 11 which extend along Io’ s orbit or through the whole magnetosphere (discussed in 479 section 4). 480

Such modeling efforts require a consistent and robust set of observational constraints, 481 mainly provided by Galileo instruments (PLS, MAG, EPD, PWS). The plasma measurements are 482 essential to infer the plasma flow perturbation, the ion temperature, density and composition. 483 Unfortunately, the PLS sensitivity evolved during the Galileo mission and PLS plasma densities 484 are often not consistent with those inferred from the PWS measurements (Dols et al. 2012). This 485 lack of reliable plasma observations limits the reach of the numerical modeling of the local 486 interaction at Io. The GLL flybys were also relatively far from the denser part of the atmosphere 487 (~200–900 km) and thus cannot constrain accurately the deep atmosphere. Moreover, no GLL 488 flybys were achieved in eclipse and on the night side. Thus, the spatial and temporal variabilities 489 of the plasma-atmosphere interaction remain poorly constrained. A new mission at Io is needed 490 to provide further major breakthrough in our understanding of its local interaction. 491

3.2 – Plasma- Europa Interaction 492

Galileo made 9 flybys of Europa between 1996 and 2000 on the upstream hemisphere, 493 the flanks and > 3 REu downstream. These flybys were far (~200-3500 km) from the deep part of 494 the atmosphere and, unlike Io, no flybys were achieved above the poles. 495

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The relative velocity of the upstream plasma in Europa’s reference frame is ~ 100 km/s. 496 The electron thermal population (Tel~20 eV) is warmer than Io’s with a 5% non-thermal 497 population at ~250 eV (Bagenal et al. 2015). The gyroradius of the average thermal ion is ~6 km. 498 The Europa interaction is similar to Io’s, but weaker because of the smaller background magnetic 499 field (~450 nT) and a less dense upstream plasma (ne~160 cm-3). The magnetic perturbation along 500 the E4 flyby ~50 nT is described by Kivelson et al. (1999) and Saur et al. (1998) estimate that ~80% 501 of the upstream plasma flow is diverted around the moon. 502

The most important GLL discovery at Europa is the existence of a salty ocean under the 503 icy crust, based on the magnetic perturbations measured along several flybys. Consequently, a 504 major focus of analyzing and modeling the GLL observations is the derivation of the induction 505 signal to sound the conductivity and depth of the sub-ice ocean (Khurana et al. 1998, 2002; 506 Kivelson et al. 1999, 2000; Zimmer et al. 2000; Schilling et al. 2004). 507

More recently, putative water plumes were discovered with the UV cameras onboard HST 508 (Roth et al. 2014a,b; Sparks et al. 2016), in GLL magnetometer observations along the E26 flyby 509 at low (~400km) altitude upstream of Europa (Blocker et al. 2016; Arnold et al. 2019) and along 510 the E12 flyby (Jia et al. 2018), and from GLL observations of Europa’s ionosphere (McGrath & 511 Sparks 2017). These plumes are at best intermittent (Roth et al. 2014a,b, 2017a) but if they are 512 to be sampled by future missions at Europa, they might provide direct information about the 513 composition of the sub-surface ocean. 514

The morphology of the Europa oxygen aurora is remarkably different from Io’s. It is 515 comprehensively studied by Roth et al. (2016) using the STIS UV image-spectrometer onboard 516 HST. These emissions are not located along the equatorial flanks like Io, but mainly above the 517 poles and they are almost systematically brighter on the polar hemisphere that faces the 518 centrifugal equator. These polar emissions are reminiscent of similar observations of the auroral 519 polar limb at Io. Contrary to Io, Europa’s atmosphere is not localized around the equator and the 520 vertical column is much lower than Io’s. But Two-fluid simulations by Saur (Figure 19.10 in 521 McGrath et al. 2004) predict a uniform limb brightening at all latitudes, which is not observed. 522 Roth et al. 2016 review possible explanations for the absence of emission along the equator, but 523 these hypotheses have not yet been tested with numerical simulations, let alone definitive 524 observations. 525

The numerical simulations of the Europa plasma-atmosphere interaction are similar to 526 Io’s: Two-fluid (electron and single O2

+ ion in a constant uniform magnetic field, Saur et al. 1998), 527 MHD (Schilling et al. 2007, 2008 ), multi-fluid MHD (Rubin et al. 2015) , hybrid (Lipatov et al. 2010, 528 2013) and multi-chemistry (Dols et al. 2016) with the goal of matching the GLL plasma 529 observations, understanding the main processes driving the interaction, constraining the global 530 atmosphere, estimating the neutral loss rates and constraining the subsurface ocean. As the GLL 531 flybys did not probe the denser atmosphere, the simulations usually assume a large scale global 532 O2 atmosphere with a scale-height ranging from 100-200 km and some upstream-downstream 533 asymmetries due to the thermal sputtering source upstream. 534

Saur et al. (1998) focused on the plasma interaction and the formation of an O2 535 atmosphere. They inferred an equilibrium atmospheric column by balancing the sputtering 536 sources and the atmospheric losses, constrained by the oxygen auroral emissions observed by 537

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HST. Using a relatively low upstream plasma density ~ 40 cm-3, they compute an equilibrium 538 atmosphere with a vertical column of 5 x 1014 cm-2, a 145 km scale height and ~ 70 km exobase. 539 The resulting atmospheric loss by ionization is estimated at ~ 6 kg s-1 and the atmospheric 540 sputtering loss by charge exchange and elastic collisions above the exobase ~ 40 kg s-1. 541

Dols et al. (2016) focused on the multi-species chemistry, using a simplified plasma flow 542 completely diverted around Europa’s surface. Figures 9 and 10 show results of similar simulations 543 where we assume that the flow is diverted around an obstacle that extends to 1.26 REu to provide 544 results similar to Io’s shown on Figures 7 and 8. The assumed O2 atmosphere is spherically 545 symmetrical, with a scale-height of 150 km and a vertical column = 5 x 1014 cm-2, consistent with 546 observations. As Europa’s atmosphere is more tenuous than Io’s, the interaction is weaker and 547 the plasma variations close to the moon are smaller. Figure 10 summarizes the volume-548 integrated rates for each plasma-O2 process using a MHD flow. As for Io, a cascade of resonant 549 charge-exchanges is an important neutral loss process but, in general, all rates are much smaller 550 than at Io. The H2 chemistry is included in the simulations but the H2 atmospheric loss rates due 551 to plasma-neutral reactions in the atmosphere are insignificant. The escaping flux of H2 (~2 x 1027 552 s-1) is probably lost by thermal escape after surface sputtering of the icy surface. H2 is probably 553 the source of the extended neutral cloud detected around Europa (see Section 4) as the O2 rates 554 calculated here (~0.5 x 1027 s-1) are not sufficient to feed a dense cloud of neutrals. 555

Blocker et al. (2016) use MHD simulations to study the effect of localized plumes on the 556 global interaction. They show that each plume forms an Alfven winglet embedded in the main 557 Alfven wing caused by the interaction with the global atmosphere. The plasma production or 558 neutral loss resulting from the direct interaction at the plume is very small compared to the effect 559 of the global atmosphere. 560

561

3.3 Io-Europa Comparison 562

Properties of the plasma interactions with Io and with Europa are summarized in Table 3 563 while Figure 11 shows the plasma densities and ion composition downstream of the interactions 564 with Io and Europa. In the Io case we did not include electron beams in this simulation so the 565 cold, dense wake is missing. In each case the inflowing atomic ions are replaced downstream 566 with molecular ions. 567

While our main conclusion is that the neutral loss at Europa is much smaller than at Io, 568 for both Io and Europa, comprehensive self-consistent models of the full, coupled 569 plasma/atmosphere/neutral cloud system are still needed. The atmospheric part of such model 570 could be built on Saur et al. (1998)’s concept of source and sink balance. The atmospheric sources 571 would include the detailed sputtering rates by thermal torus ions, pickup ions, hot ions, and 572 maybe fast neutrals and hot electrons. The atmospheric losses processes would include the 573 ionization, elastic collisions, charge-exchange and molecular recombination. These loss rates 574 would be used as input to a neutral cloud model similar to Smith et al. (2019). Such model could 575 also assess the significance of Europa’s atmosphere as a neutral and plasma source for the Jovian 576 magnetosphere. 577

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A remarkable difference between the interactions at Io and Europa is the absence of 578 parallel electron beams in the wake of Europa. It is thought that at the foot of Io’s Alfven wing, 579 electrons are accelerated toward Jupiter to produce the footprint aurorae and also in the other 580 direction to produce the parallel electron beams detected at Io (Bonfond et al. 2008). As footprint 581 auroras are sometimes detected at the foot of the Europa’s Alfven wing, it was expected that 582 parallel electron beams would be present at Europa as well. Moreover, Io’s dense wake is thought 583 to be caused by electron beams ionization in the downstream hemisphere and GLL 584 measurements in Europa’s wake did not reveal a dense plasma comparable to Io’s (Kurth et al. 585 2000; Paterson et al. 1999). One possible explanation of the non-detection of parallel electron 586 beams along the GLL flybys is that, because of Europa’s weaker interaction, the beams are located 587 further downstream and so GLL missed them. An alternative scenario is that the local interaction 588 at Europa does not systematically produce footprint auroral emissions and concurrent electron 589 beams. 590

In summary, the strong plasma-atmosphere interaction produces locally at Io only ~200 591 kg of ions but up to 2000 kg of neutral dissociation products are distributed around Io’s orbit. 592 Meanwhile at Europa the interaction is much weaker. The interaction produces at best 20 kg/s 593 of O2

+ locally and is probably a minor source of plasma to the magnetosphere of Jupiter. 594 Nonetheless, extensive neutral clouds (probably hydrogen) have been detected along the orbit 595 of Europa, as we discuss in the next section. 596

597

4. Neutral Clouds 598

While radio astronomers were still scratching their heads about Io triggering bursts of 599 radio emission, in 1973 astronomers detected indications of atmospheric escape from Io. 600 Emission from sodium was observed to emanate from a cloud around Io. Sodium is relatively 601 efficient at scattering visible sunlight. Other neutral species are much harder to detect, 602 particularly molecular species, so that our picture of the iogenic neutral clouds are largely based 603 on the behavior of sodium and on models (Figures 12, 13). Table 4 lists the sources of neutrals 604 and plasma at Io and Europa. 605

Models of neutral clouds take a flux of particles from the moon’s exobase (where 606 atmospheric collisions become negligible) and follow the motions of the particles under the 607 gravity first of the moon and then, farther away, Jupiter. The neutral particles interact with the 608 surrounding plasma being eventually ionized or charge exchanged. Photoionization also plays a 609 role in lower density regions of the torus. Figure 13 shows a model recently published by Smith 610 et al. (2019) that takes typical escape fluxes of different species from Io and Europa (e.g., Smyth 611 & Marconi 2006) and tracks particles under Jupiter’s gravity using a background plasma model to 612 predict when along its trajectory a neutral particle of a particular species will be changed (via 613 dissociation, ionization or charge exchange). 614

Figure 13 shows that molecular species are more easily dissociated and ionized, confining 615 the molecular neutral clouds to the vicinity of the moons. Atoms with longer lifetimes (O, H) lead 616 to atomic clouds that spread around the moon’s orbit. The color scale is the same for all five of 617 the contour plots and shows that both Io and Europa neutral clouds are of remarkably 618

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comparable density. Io is a more prolific neutral source (~250-3000 kg/s) than Europa (~25-70 619 kg/s) but the plasma is denser and the neutrals are quickly removed, especially outside Io’s orbit. 620

621

4.1 – Io neutral cloud 622

The first observational evidence of neutral atoms escaping Io was the detection of optical 623 sodium D-line emission from a cloud in the vicinity of Io by Brown (1974). The efficient resonant 624 scattering of sunlight by sodium produced the bright emission (Trafton et al. 1974; Bergstrahl et 625 al. 1975; Brown and Yung 1976). Early studies suggested that the neutrals could be removed from 626 Io by Jeans escape (Kumar 1982), charged particle sputtering of its surface (Matson et al. 1974), 627 atmospheric sputtering (Haff et al. 1981; McGrath and Johnson 1987), charge-exchange (Johnson 628 and Strobel 1983), direct collisional ejection (Ip 1982; Sieveka and Johnson 1984 ). More recently 629 electron impact molecular dissociation is added to the list (Dols et al. 2008). Voyager 1 630 observations revealed Io’s S02 atmosphere (Pearl et al. 1979) and a sulfur-dominated surface 631 (Sagan 1979), indicating that Na was just a trace element in a neutral cloud dominated by sulfur 632 and oxygen atoms and their compounds. While sodium is a trace component of the atmosphere 633 (~few %), its large cross section for scattering visible sunlight (30 times brighter than any other 634 emissions) makes it the most easily observed species in the neutral clouds. Many images of the 635 sodium cloud have been acquired (see Figure 12 and reviews by Thomas et al. 2004; Schneider & 636 Bagenal 2007) and models of the neutral clouds showed that electron impact ionization and 637 charge exchange with torus plasma are significant loss processes on timescales of a few hours 638 (e.g., Smyth and Combi 1988; Smyth 1992). Hence, the observed distributions of the neutral 639 clouds are dependent upon both the neutral source and the spatial distribution of plasma 640 properties in the torus. 641

Sodium, potassium and sulfur have relatively short (2-5 hours) lifetimes against electron 642 impact ionization in the densest regions of the torus. The rate coefficient for ionization of oxygen, 643 however, is at least an order of magnitude lower than for sulfur so that charge-exchange loss of 644 O with torus ions dominates over ionization. The minimum lifetime for O is around 20 hours (e.g., 645 Thomas 1992) resulting in significant densities of neutrals farther from Io and an O neutral cloud 646 that extends all the way around Jupiter. 647

After the initial detection by Brown (1981) of atomic oxygen emissions from Io’s neutral 648 cloud at the wavelength of 630.0 nm, further observations of these optical emissions from the Io 649 cloud by Thomas (1992) and close to Io by Oliversen et al. (2001) showed substantial variations 650 with longitude, local time and Io phase. Meanwhile, atomic emissions were also detected in the 651 UV. Atomic sulfur emissions at 142.9 nm and atomic oxygen emissions at 130.4 nm were 652 discovered with a rocket-borne telescope by Durrance et al. (1983). The first detections of neutral 653 O and S were around 180° in Io phase away from Io itself and analyzed by Skinner and Durrance 654 (1986) to infer densities of neutral O and S of 29 ± 16 cm-3 and 6 ± 3 cm-3 respectively. The IUE 655 satellite detected UV emissions from O and S near Io (Ballester et al. 1987). Using 30 observations 656 by the HST-STIS, Wolven et al. (2001) mapped out O emission at 135.6 nm in the UV to 10 RIo. 657 They also demonstrated that these neutral emissions downstream of Io were brighter than those 658 upstream, that there seemed to be a weak, erratic System III longitude effect, and that the local 659 time (dawn versus dusk) asymmetry depends on the Io phase angle. 660

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The Hisaki UV observatory in Earth orbit has provided new oxygen observations, mapping 661 out Io’s neutral oxygen cloud to show it comprises a leading cloud inside Io’s orbit and an 662 azimuthally uniform region extending to 7.6 RJ (Koga et al. 2018a,b, 2019). The peak number 663 density of oxygen atoms is estimated at 80 cm-3, spreading ~1.2 RJ vertically. The Hiskaki team 664 estimate the source rate of oxygen ions at 410 kg/s (roughly consistent with previous studies 665 Smyth & Marconi 2003; Delamere and Bagenal 2003; Yoshioka et al. 2018). When Io exhibited a 666 volcanic eruption in 2015, lasting ~90 days, Koga et al. (2019) used Hisaki observations of the 667 time variations of O and O+ to show volcanism shortens the lifetime of O+ and that Io’s neutral 668 oxygen cloud spread outward from Jupiter during the 2015 volcanic event. The number density 669 of O in the neutral cloud at least doubles during the active period. 670

From 1977 to 2011 Bill Smyth and colleagues published ~25 papers applying a neutral 671 cloud model to study satellite atmospheres and neutral clouds (Smyth & MacElroy 1977; Smyth 672 et al. 2011). The Smyth neutral cloud model involved solving the kinetic equations using the 673 Direct Simulation Monte Carlo (DSMC) approach. This involved solving the non-linear Boltzmann 674 equation in which the spatial domain is divided into cells and the particles in each cell are stepped 675 forward in time. For each small time-step, the state (location, velocity, ionization state) of the 676 different particle species is updated following the consequences of the effects of gravity and 677 collisions (elastic, inelastic, and chemically reactive), which are calculated for the average 678 conditions in each model bin. They applied their model, including various refinements, to study 679 Io’s Na, K, O, S, SO2 and SO corona, plus the plasma torus properties (Smyth & Marconi 2003, 680 2005; Smyth et al. 2011). The recent Smith et al. (2019) model shown in Figure 13 is similar to 681 the Smyth models, with updated reaction cross-sections and incorporating current data on the 682 variability of conditions at orbit of Io. 683

Detailed measurements of Io’s sodium cloud requires models to explain the main 684 “banana-shaped” cloud, a “directional feature” or jet, as well as a “stream” of fast neutral sodium 685 atoms that spread out into a nebula extending for 100s of RJ (Figure 12). The main sodium cloud 686 is consistent with a roughly uniform corona of sodium atoms (a little above escape speed) around 687 Io (Schneider et al. 1991; Burger et al. 1999, 2001) producing a uniform source of an extended 688 cloud that is shaped by interaction with the surrounding plasma (Smyth & Combi 1997). Jets of 689 fast sodium neutrals require a process that includes acceleration, probably as an atomic or 690 molecular ion, followed by a neutralizing charge exchange (or dissociative recombination) 691 reaction that sends out a fast (20 km/s) neutral (Wilson & Schneider 1994; 1999). We return to 692 the extended sodium nebula in section 6. 693

The main goal of Smith et al. (2019) was to show that Io’s neutral material, whatever its 694 composition, would not significantly reach Europa’s orbit. The Smith et al. (2019) model 695 illustrated in Figure 13 assumes that only SO2 escapes directly from Io with a canonical rate of 1 696 ton/s and a prescribed low velocity distribution. They produce iogenic neutral clouds that are 697 consistent with observations of O and S emissions (Skinner & Durrance 1986; Koga et al. 2018a). 698 But there are no observations of the neutral SO2 cloud around Io. Models of the neutral clouds 699 have to assume a source of SO2 based on models of the plasma-atmosphere interaction (section 700 3.1) of, say, 1-3000 kg/s of SO2 molecules escaping from Io. 701

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Smyth and Marconi (2003] and Smith et al. (2019) study the formation of such extended 702 neutral structures but to this day, a self-consistent approach of their formation, from the 703 processes in the atmosphere of Io, to the neutral cloud and finally to the torus ion supply, has 704 not yet been carried out. Such extended neutral structures of S, O and SO2 are notably difficult 705 to observe (Brown & Ip 1981; Durrance et al. 1983; Koga et al. 2018a,b), beyond the minor Na 706 species and we hope that in a near future, new creative observational methods will help to 707 constrain quantitatively these extended neutral structures and improve our understanding of 708 their atmospheric sources. 709

710

4.2 – Europa Neutral Cloud 711

Strong indications of an extended source of neutrals escaping from Europa were implied 712 by measurements of (a) depletion of energetic protons at about the orbital distance of Europa 713 (Lagg et al. 1998; 2003), and (b) very energetic (10s keV per neucleon) neutral atoms (vENAs) 714 observed by the MIMI instrument on Cassini as it flew past Jupiter (Krimigis et al. 2002; Mauk et 715 al. 2003, 2004). A stringent limit of ~8 cm-3 on the density of atomic oxygen from Cassini UVIS 716 measurements (Hansen et al. 2005) meant that the neutral cloud, estimated to have a density of 717 20-50 cm-3 to produce the observed vENAs, must be mostly hydrogen rather than oxygen 718 (consistent with simulations of Shematovich et al. 2005; Smyth & Marconi 2006; Smith et al. 719 2019). The fate of the vENAs is discussed in section 6. 720

The low mass of hydrogen, allows it to easily escape Europa so that hydrogen has a much 721 higher (x10) escape rate than oxygen. Roth et al. (2017a) detected an extended corona of atomic 722 H around Europa, consistent with electron impact dissociation of molecular H2, as modeled by 723 Smyth & Marconi (2006), but the net contribution of escaping atomic H is relatively minor (~5%). 724 The Smith et al. (2019) model is basically the same as Smyth & Marconi (2006) but applies 725 updated reaction cross-sections. The Europa neutral clouds are presented on the right side of 726 Figure 13, showing that the dominant species is H2 that extends all the way around Europa’s orbit 727 with oxygen (particularly in molecular form) being more limited to the Europa environment. 728 Smith et al. (2019) also conclude “Source rate changes take longer to impact the Europa neutral 729 cloud than the Io neutral cloud. The dominant processes at Europa’s orbit have lifetimes from at 730 least 2–3 days up to longer than a week, while at Io, the neutral particles start interacting (with 731 the plasma) in 8–13 hr. Additionally, the range in observable ambient plasma environments can 732 vary particle interaction rates by more than an order of magnitude.” 733

Recently, Nenon & Andre (2019) note the removal of energetic (~MeV) sulfur ions with 734 ~90° pitchangle near Europa’s magnetic flux-shell and argue that inward-transported heavy ions 735 interact with the extended hydrogen cloud while protons (which have smaller cross-section for 736 charge-exchange reactions) interact with the tenuous, more equatorially-confined oxygen cloud. 737 Nenon & Andre (2019) point out that since the heavy ions tend to be higher ionization states 738 (Clark et al. 2016), their interaction with Europa’s neutral clouds reduces their charge state so 739 that when they move inward to Io’s neutral cloud they are singly charged and therefore leave the 740 system as vENAs on charge-exchanging with Io’s neutral clouds (Mauk et al. 2004). While protons 741 are less likely to charge-exchange, when they do react with Europa’s neutral O cloud, they are 742 lost as vENAs – as detected by the Cassini MIMI instrument (Krimigis et al. 2002). 743

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Neutral clouds of sodium (Brown & Hill 1996) and potassium (Brown 2001) have also been 744 detected around Europa. Burger & Johnson (2004) model the distribution of sodium and found a 745 cloud shape similar to the O cloud of Smith et al. (2019) in Figure 13, extending farther on the 746 trailing (upstream) side where particles are moving away from Jupiter into lower plasma densities 747 compared with the leading (downstream) side where particles are moving towards Jupiter into 748 higher plasma densities. The big question is whether the Na and K come via sputtering of iogenic 749 material onto Europa or from the interior of Europa (Brown 2001; Leblanc et al. 2002, 2005). 750

751

5. Plasma Torus 752

The Io plasma torus comprises three main regions: the outer region that has a roughly 753 circular cross-section – sometimes called the “doughnut” – that contains 90% of the mass; just 754 inside Io’s orbit there is narrow but vertically-extended region – sometimes called the “ribbon” – 755 that is bright in UV and, particularly, visible wavelengths; extending inwards from the ribbon is a 756 thin disk or “washer”. Figure 14 illustrates these three regions and their properties are listed in 757 Table 5. The warm torus extends past the orbit of Europa (section 5.4) and merges into the 758 plasma sheet. 759

Optical emissions from a toroidal cloud of S+ ions surrounding the orbit of Io were first 760 detected in ground-based observations by Kupo et al. (1976), which Brown (1976) recognized as 761 coming from a cold, dense plasma. The Voyager 1 flyby of Jupiter in 1979 revealed Io spewing 762 out SO2 from active volcanoes and provided detailed measurements of the lo plasma torus both 763 from the strong emissions in the EUV, observed remotely by the Voyager Ultraviolet 764 Spectrometer (UVS) (Broadfoot et al. 1979), as well as in situ measurements made by the Plasma 765 Science (PLS) instrument (Bridge et al. 1979) and the Planetary Radio Astronomy (PRA) 766 instrument (Warwick et al. 1979). 767

Since Voyager, the Io plasma torus has been observed remotely via visible and UV 768 emissions as (Thomas 1992; Hall et al. 1994; Gladstone et al. 1998; Feldman et al. 2001; 2004) 769 well as in situ by the Galileo spacecraft (Gurnett et al. 1996, 2001; Frank & Paterson 2000). Each 770 of these techniques for measuring the plasma properties in the torus has its pros and cons. The 771 remote sensing techniques provide good temporal and spatial coverage but suffer from being 772 integral measurements along the line of sight as well as being dependent on calibration of the 773 instrument and accurate atomic data for interpretation of the spectra. Moreover, a very broad 774 wavelength range is needed to cover emissions from all the main ion species. The in situ plasma 775 measurements provide detailed velocity distributions but suffer from limited spatial and 776 temporal coverage as well as poor determination of parameters for individual ionic species in the 777 warm region of the torus where the spectral peaks for different species overlap. Since these two 778 data sets are complementary, they can be combined to construct a description of the plasma 779 conditions in the torus, i.e. an empirical model, that can be compared to theoretical models 780 based on the physical chemistry of the torus plasma (e.g. Bagenal 1994; Herbert & Hall 1998; 781 Herbert et al. 2003, 2008; Smyth & Marconi 2005; Smyth et al. 2011; Nerney et al. 2019). A 782 summary of observations and theoretical understanding at end of Galileo mission is provided by 783 Thomas et al. (2004). 784

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The dominance of sulfur and oxygen composition of heavy ions throughout the 785 magnetosphere tell us that these products of volcanic gases fill the vast volume of Jupiter’s 786 magnetosphere. At the same time, energetic particles from the outer magnetosphere are 787 transported inwards, bringing in supra-thermal ions that charge-exchange with the neutral 788 clouds, plus electrons that ionize and excite UV emissions. In the next sections we first describe 789 the underlying physical processes in the Io-Europa space environment and then summarize 790 current understanding of the three torus regions. 791

792

5.1 – Physical Processes 793

The key physical processes controlling the plasma in the Io-Europa environment are ion 794 pick-up, centrifugal confinement to the equatorial region, radial transport and collisional 795 processes (dissociation, ionization, charge-exchange, radiation, etc; gathered under the term 796 “physical chemistry”) that drive the radial distribution of different ion species as well as the 797 energy of the plasma. 798

Ion Pick-Up: As discussed in Section 3 above, a relatively small amount of plasma is 799 directly produced in the interaction with either Io’s or Europa’s atmospheres. Most of the plasma 800 is produced via electron impact ionization of the extended neutral clouds (Section 4 above) where 801 the neutral atoms that follow Keplerian orbits (~17 and 14 km s-1 at Io and Europa respectively) 802 around Jupiter. The plasma is coupled via Jupiter’s magnetic field to planet’s electrically-803 conducting ionosphere which corotates with the planet’s ~10 hour spin period (~75 and 118 km 804 s-1 at Io and Europa respectively). Minor deviations from rigid corotation mean that typical 805 plasma flow speeds are ~5% less than corotation at Io (71 km s-1) and ~15% sub-corotational at 806 Europa (~100 km s-1). When a slowly moving atom is ionized (either via electron impact or charge 807 exchange), the fresh ion starts with an initially high velocity relative to the corotating plasma (55 808 and 85 km s-1 at Io and Europa respectively). Each pickup ion therefore experiences an electric 809 field due to its motion relative to the local magnetic field (and the plasma) and is accelerated – 810 i.e. "picked up” – gaining a gyro-velocity (in the plane perpendicular to the local magnetic field) 811 equal to its initial motion relative to the corotating plasma. Each fresh ion gains a gyro-energy 812 dependent on its mass: 270 eV for O+, 540 eV for S+ and 1.8 keV for SO2

+ at Io's orbital distance; 813 650 eV for O+ at Europa’s orbital distance. Fresh O+ pick-up ions gyrate around the magnetic field 814 at ~2 and 0.5 Hz with a 5- and 40-km gyro-radius at Io and Europa respectively. The energy of 815 these fresh ions ultimately comes from Jupiter' s rotation, coupled to the plasma via the magnetic 816 field. If the gyro-speed were distributed into an isotropic Maxwellian distribution (e.g., via 817 Coulomb collisions) the O+ and S+ ions would have temperatures of 2/3 their initial pick-up 818 energy. The corresponding pick-up electrons gain negligible gyro-energy but are accelerated to 819 corotate with the surrounding plasma. Electrons are heated quite quickly via Coulomb collisions 820 with the ions but only in the inner torus does the plasma hang around long enough to get close 821 to full thermal equilibrium. Evidence of local pick-up comes from the Electro-Magnetic Ion 822 Cyclotron (EMIC) waves were observed far downstream of Io at the SO2

+ and SO+ ion gyro-823 frequencies (Warnecke et al. 1997; Russell & Kivelson 2000; 2001). These waves are generated 824 by the unstable “ring-beam” velocity distributions of fresh pick-up ions. The presence of a 825 background population of thermalized ions supresses the generation of EMIC waves which 826

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explains why emissions are not detected at the gyro-frequencies of the dominant ion species (S+ 827 and O+). 828

When a neutral particle is ionized it not only picks up gyro-motion but it is also accelerated 829 up to the bulk motion of the flowing plasma. Material is added to the torus at a rate of about 1 830 ton/s which corresponds to ~10-3 ions cm-3 s-1 or just ~5 ions cm-3 per 10-hour Jupiter spin period. 831 When integrated around Io’s orbit, the additional radial current to pick up these fresh ions and 832 accelerate them to corotation is 0.6 megaAmp (for a torus height of 2 RJ). The electrical currents 833 close along the magnetic field, coupling the plasma to the planet’s ionosphere (that is collisionally 834 coupled to the neutral atmosphere). The corotating torus plasma continually experiences an 835 inward J x B force associated with an azimuthal current through the torus (see derivations in 836 Cravens (1997) chapter 8). This corotation current is about 10 megaAmp (for 4 RJ

2 torus cross-837 section). On the other hand, keeping the plasma corotating as it moves out to into the plasma 838 sheet (as well as balancing pressure gradient forces) drives radial currents in the plasma sheet of 839 several 10s megaAmps, closing via parallel current from/to the ionosphere (Cowley & Bunce 840 2001; Nichols et al. 2015). 841

Centrifugal confinement. Torus plasma trapped by Jupiter 's magnetic field is confined 842 toward the equator by centrifugal forces. A corotating ion gyrates around local field lines several 843 times per second while bouncing along field lines every few hours. On field lines at Io's orbit, each 844 ion experiences about 1 g of centrifugal acceleration outward from the rotation axis. Ions in a 845 Maxwellian velocity distribution are distributed along the magnetic field line, centered around 846 the point farthest from Jupiter's rotation axis. The locus of all such positions around Jupiter is 847 called the centrifugal equator. In an approximately dipolar magnetic field tipped like Jupiter's, 848 ~10° from the rotation axis, the centrifugal equator has 2/3 of the tilt, or ~7° from Jupiter's 849 rotational equator. As the tilted torus corotates with Jupiter, the torus viewed from Earth appears 850 to wobble ±7°. Non-dipolar components to the field can measurably warp the centrifugal 851 equator. 852

In equilibrium, one can consider the plasma distribution along the field as a fluid in 853 balance between the effect of gradients in the thermal pressure of the plasma (nkT) and the 854 centrifugal force, much as the vertical distribution of a planet’s atmosphere comes from a 855 balance between pressure gradient and gravitational forces. Thus, the torus vertical structure is 856 a rough indication of ion temperature. The fluid approach can incorporate further complexity 857 associated with the electric field (~15 volts across the torus) arising from small charge separation 858 between ions (more strongly affected by the centrifugal force) and the much lighter electrons, 859 plus the addition of multiple ion species and thermal anisotropy. 860

For the approximation of a single, isotropic ion species, cold electrons (with net charge 861 neutrality) and a roughly dipole magnetic field, the north-south or vertical dimension (z-axis) 862 distribution of plasma density n(z) about the centrifugal equator is a Gaussian function 863

n(z) = n(z=0) exp -(z/H)2 864

where the scale height H is determined by the spin rate of Jupiter, W, the ion temperature, T, and 865 the mass of the ions, A: 866

H = [2/3 kT / (mp A W 2 ]1/2 = Ho [T(eV) / A(amu)]1/2 867

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For H in units of Jovian radii, RJ, T is in eV and A is average ion mass in atomic mass units, we have 868 Ho = 0.64 RJ. For a more detailed description of the “diffusive equilibrium” distribution of plasma 869 along the magnetic field for a multi-species, anisotropic plasma see Bagenal (1994). 870

Radial Transport: Sulfur and oxygen ions, assumed to originate in the Io plasma torus, are 871 detected throughout the magnetosphere. The transport of plasma in a rotation-dominated 872 magnetosphere such as Jupiter’s, is usually described to be via centrifugally-driven flux tube 873 interchange (e.g., Hill et al. 1981). And because Jupiter's magnetosphere is a giant centrifuge, 874 outward transport is energetically strongly favored over inward transport. While many have 875 looked, direct evidence of the instability has been very rare (Thorne et al. 1997; Bolton et al. 876 1997; Kivelson et al. 1997; Frank and Paterson 2000; Russell et al. 2005). The theoretical problem 877 turns out to be, less an issue of explaining why outward transport occurs, but more a matter of 878 explaining why it occurs so slowly. Or, put another way, why is the torus so long-lived, and 879 therefore so massive? 880

The interchange instability is the magnetospheric analog of the Rayleigh-Taylor instability, 881 the same instability that drives convection in a planetary atmosphere or a pot of boiling water. 882 The effective gravity, dominated by the centrifugal force of corotation, is radially outward. The 883 essence of the centrifugally-driven interchange instability is that heavily-loaded magnetic flux 884 tubes from the source region move outward and are replaced by flux tubes containing less mass 885 (the outer magnetosphere is relatively empty) moving inward, thereby reducing the (centrifugal) 886 potential energy of the overall mass distribution without significantly altering the configuration 887 of the confining magnetic field. The interchange rate is regulated by the electrical conductivity in 888 Jupiter's ionosphere, at the “feet” of the magnetic flux tubes. Early studies used Voyager 889 measurements of the onset of deviation from corotation (McNutt et al. 1981) to constrain the 890 value of ionospheric conductance (Hill et al. 1981). But theoretical models using only ionospheric 891 conductance to limit fluxtube interchange have difficulty in matching the long life times (~20-80 892 days) for plasma in the Io torus derived from physical chemistry models (discussed in the next 893 section). A detailed discussion of possible additional mechanisms (e.g., ring current 894 impoundment, velocity shears, etc;) to slow down radial transport from the torus is given in 895 section 23.7 of Thomas et al. (2004). 896

An alternative, empirical approach describes fluxtube interchange as a diffusive process 897 that depends on the radial gradient of the content of a magnetic flux tube, NL2 where N is the 898 number density integrated over an L-shell of (dipolar) magnetic flux (Richardson et al. 1980). A 899 diffusion coefficient (usually simplified as a constant times a radial power-law) is derived to 900 match observed radial profiles of fluxtube content. The strong preference for outward transport 901 via centrifugally-driven fluxtube interchange means that the derived diffusion rates are about a 902 factor 50 faster for transport outwards from the source at Io, than inwards. This is a major factor 903 explaining why there is ~100 less mass in the inner torus (Richardson et al. 1980, 1981; Bagenal 904 1985; Herbert et al. 2008). 905

One topic that theorists struggle with understanding is the scale on which centrifugally 906 driven interchange is being driven. Richardson and McNutt (1987) looked at the currents into the 907 Voyager PLS sensors at the highest temporal resolution (L modes) and found that on 0.24 second 908 timescale (which translates to an effective spatial resolution of ~20 km), the plasma density does 909

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not fluctuate more than 20% (more typically <5%). These observations ruled out theories such as 910 proposed by Pontius et al. (1986) that required interchange of full and (essentially) empty flux 911 tubes. In a survey of Galileo data, Russell et al. (2005) showed that small-scale (~2 second 912 duration corresponding to scales of 100-1000 km) magnetic perturbations indicative of inward-913 moving empty fluxtubes occurred rarely (~0.32% of the time) which suggests these fluxtubes 914 need to move in quite rapidly (10s km/s) compared with the slow outward motion of full 915 fluxtubes. Hess et al. (2011a) argues that these fast, inward-moving fluxtubes are analagous to 916 the Io Alfven wing and that Alfven waves propagating along the fluxtube are responsible for the 917 source of suprathermal electrons in the torus as observed by Frank & Paterson (2000) and as 918 required by physical chemistry models (as described in the next section). Probably whole 919 fluxtubes interchange in the inner magnetosphere where the field is dipolar. But as plasma moves 920 beyond Europa’s orbit, and as the plasma is hotter at larger distances, the plasma pressure begins 921 to dominate over the magnetic field and one expects that other more local instabilities likely take 922 over (Vasyliunas 1983; Kivelson & Southward 2005). 923

Physical Chemistry. In the bulk of the Io-Europa space environment the transport times 924 are relatively long (a month to years) so that substantial plasma densities accumulate (1000-3000 925 cm-3) and collisions are quite frequent (hours to days). This means that collisional reactions 926 (excitation, ionization, dissociation, charge-exchange) are important sources and losses of both 927 particles and energy. For each species the source and loss terms are different and depend on 928 properties (neutral and plasma density, temperature, etc.;) that can vary in space and time, so 929 one needs to solve a self-consistent system including both physical chemistry and diffusive 930 transport. Early models took a homogeneous box with an input of neutrals with specified 931 composition (e.g. 1 ton/s of O and S neutral atoms in the ratio 2:1 consistent with dissociation of 932 SO2) and a specified timescale for transport out of the box (e.g. 40 days). Some initial plasma is 933 included, reaction rates specified, and the system evolved to equilibrium. It was quickly realized 934 that an additional source of relatively hot electrons needs to be added to the system in order to 935 sustain the system (Shemansky 1988; Barbosa 1994). 936

A major difficulty in building such models, particularly in early studies, is the fact that the 937 atomic data for many of the various reactions are not agreed upon or even known. McGrath & 938 Johnson (1989) calculated critical charge exchange reaction rates for typical torus conditions. 939 Taylor et al. (1995) developed the Colorado Io Torus Emission Package (CITEP) that took atomic 940 data provided by Don Shemansky and the Bagenal (1994) torus model to predict emissions for 941 various species at different wavelengths and viewing geometries. In 1997 a group of UV 942 astronomers put together a database of UV emissions in the CHIANTI database (Dere et al., 1997). 943 This public database has been updated periodically, the most recent being CHIANTI 8.0 (Del 944 Zanna et al., 2013; Del Zanna and Badnell, 2016). For example, Nerney et al. (2017) illustrates the 945 effects of using different data bases on the analysis of UV observations obtained by Voyager, 946 Galileo and Cassini to derive ion composition in the Io plasma torus, finding up to 40% changes 947 in the derived abundances of some of the species between CHIANTI versions 4 and 8. 948

Such physical chemistry models of the Io plasma torus have been extended from a “cubic 949 centimeter” in the center of the torus (Shemansky 1988; Lichtenburg et al. 2001; Delamere & 950 Bagenal 2003) to radial profiles (Barbosa 1994; Delamere et al. 2005; Nerney et al. 2017; Yoshioka 951 et al. 2014, 2017) and azimuthal variations (Steffl et al. 2008; Copper et al. 2016; Tsuchiya et al. 952

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2019) as well as temporal variations apparently driven by volcanic outbursts on Io (Delamere et 953 al. 2004; Kimura et al. 2018; Yoshioka et al. 2018; Tsuchiya et al. 2018). Delamere et al. (2005) 954 considered the effect of an additional neutral source at Europa on the torus chemistry and found 955 that the faster radial transport rate (Vr increasing from ~100 m/s at Io to few km/s at Europa) 956 meant that the contribution was minimal. We return to the impact of Europa on the torus in 957 Section 5.4. 958

Finally, it must be noted that the physical chemistry models of the torus assume sources 959 of atomic O and S and do not include molecular species (e.g. SO2, SO, O2) either as neutrals or 960 ions and assume such molecules are quickly dissociated. While this is probably a reasonable 961 assumption for most of the main, warm, outer torus, molecules probably play an important role 962 in the ribbon and cold, inner torus (see section 5.3). 963

964

5.2 – Main, Warm Io Plasma Torus 965 The evolution of thinking about the Io plasma torus through the Voyager era illustrates 966

the disconnect between the fields of astronomy and space physics. Initially, the two groups did 967 not understand the other’s nomenclature. Pre-Voyager, the planetary astronomers talked of 968 emissions from SII excited by electrons with temperatures of log10 Te = 4.4 ± 0.6 K and densities 969 of log10 [ne] = 3.5 ± 0.6 cm-3 (Brown 1976). Meanwhile, the Pioneer space physicists talked about 970 in situ measurements of 100 eV protons with densities of 50–100 cm−3 (Frank et al. 1976). As 971 Voyager approached Jupiter, the UVS team said they were seeing surprisingly intense emissions 972 from high densities of sulfur and oxygen ions. The Voyager PLS teamed scaled down their 973 instrument sensitivities accordingly. A couple days later, Voyager 1 flew through the middle of 974 the torus and measuring peak densities up to 3200 cm-3 (Bridge et al. 1979; Warwick et al. 1979) 975 with PLS only saturated in one high resolution spectrum. 976

The doughnut-shaped main warm torus outside Io’s orbit dominates the mass and energy 977 of the system (Table 5). The plasma is warm (ion temperature ~60-100 eV) with a vertical scale 978 height of ~1.0-1.5 RJ,. The plasma moves slowly outwards, the electron density dropping from 979 ~2000-3000 cm-3 at Io’s 6 RJ orbit to 100-200 cm-3 by Europa’s 9.4 RJ orbit (Figure 15), which it 980 reaches in ~30-80 days (Bagenal et al. 2016; 2017). While there are small blobs of cool (~20 eV) 981 in the plasma sheet (~10-30 RJ, Dougherty et al. 2017) the bulk of the plasma rises from ~60-100 982 eV at Io to ~few 100 eV by Europa (Bagenal et al. 2015). 983

As the plasma expands radially outwards, in fluxtubes of increasingly weaker magnetic 984 field, one would expect the plasma to cool. But instead of cooling on expansion, the iogenic 985 plasma is in fact clearly heated (by an as-yet-unknown process), as it moves outwards, reaching 986 keV temperatures in the plasma sheet on timescales of weeks, requiring something like ~0.3-1.5 987 TW of additional energy input (Bagenal & Delamere 2011). 988

Composition. Despite multiple ways of observing the torus, no observation uniquely 989 determines all 5 of the dominant ion species (O+, O++, S+, S++, S+++) in the main Io torus region. The 990 Voyager PLS instrument obtained excellent compositional measurements in the cold inner torus 991 (<5.6 RJ) and in several cold blobs in the plasma sheet (Dougherty et al. 2017). But the analysis 992 of both Voyager and Galileo data to obtain density, temperature and flow in the warm torus 993

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(Bagenal et al. 2015; 2016; 2017) required making assumptions about composition derived from 994 a combination of UV emissions and physical chemistry models. 995

Bodisch et al. (2017) reports on minor ions detected in the torus and plasma sheet from 996 the re-analysis of Voyager PLS data. They found protons comprise 1–20% of the plasma between 997 5 and 30 RJ (nominally ~10% in the torus) with variable temperatures ranging by a factor of 10 998 warmer or colder than the heavy ions. These protons, measured deep inside the magnetosphere, 999 are consistent with a source from the ionosphere (rather than the solar wind) of ~1.5–7.5 x 1027 1000 protons s-1 (2.5–13 kg/s). Sodium ions are detected between 5 and 40 RJ at an abundance of a 1001 few percent (occasionally as much as 10%) produced by the ionization of the extended iogenic 1002 neutral cloud discussed in Section 4 above. 1003

Early discussions about the torus ion composition derived from UV emissions were 1004 plagued with poor knowledge of excitation and reaction rates, as well as debates about the role 1005 of hot electrons. The ionization state of the ions seems to increase with distance and the total 1006 emitted power (~1.5 TW) required an additional source of hot (say ~50–100 eV) electrons 1007 (Moreno et al. 1985; Smith & Strobel 1985; Smith et al. 1988; Shemansky 1988; Barbosa 1994). 1008 The Bagenal et al. (1992) study of torus composition combined analysis of in situ PLS data with 1009 Voyager UVS data analyzed by Shemansky (1987, 1988). This combined composition analysis 1010 became the basis of an empirical description of the Io plasma torus by Bagenal (1994) which was 1011 used to constrain early physical chemistry models of the torus (Schreier et al. 1998; Lichtenberg 1012 et al. 2001; Delamere & Bagenal 2003). The torus composition derived from Voyager 1 UVS 1013 emissions by Shemansky (1987) was strongly dominated by O+ ions (~40% of ne) which required 1014 neutral input to the physical chemistry model of Delamere & Bagenal (2003) with O/S~4, about 1015 twice what one would expect from the dissociation of SO2. 1016

The Cassini spacecraft flyby of Jupiter (with a primary goal of gaining a gravity assist to 1017 Saturn) provided an excellent opportunity to study UV emissions from the Io plasma torus 1018 between October 2000 and March 2001 (Steffl et al. 2004a,b). Spectral analysis of the torus 1019 emissions suggested the ion composition was very different at the Cassini epoch than that 1020 derived by Shemansky (1987) at the time of Voyager (bottom of Figure 15). Specifically, the 1021 abundance of O+ was reduced to 26% of ne which Delamere & Bagenal (2003) could model with 1022 a neutral source of O/S~2, compatible with an SO2 origin. 1023

Meanwhile, Delamere et al. (2005) expanded their physical chemistry model to 2 1024 dimensions, assuming azimuthal symmetry. They averaged reactions over latitude and calculated 1025 radial variations in plasma properties for a fluxtube of plasma that was slowly moving outwards 1026 via centrifugally-driven fluxtube interchange. Note that the bounce periods for electrons and ions 1027 trapped in Jupiter’s magnetic field and the collisional reaction rates are relatively short compared 1028 with the transport timescale of 30-80 days. Beyond ~8 RJ the densities have dropped and the 1029 radial transport sped up so that the reactions and radiation have effectively ceased, “freezing in” 1030 the composition. The production of neutrals needed by Delamere et al. (2005) to match the 1031 Cassini UVIS data was comparable to the Smyth & Marconi (2003) neutral cloud model at Io (~7 1032 x 1026 atoms/s) but required a steeper radial gradient. 1033

Recent re-analysis by Nerney et al. (2017) of the Voyager 1 UVS data using the current 1034 atomic data for emission rates suggests an ion composition more consistent with the UV 1035

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emissions observed in the Cassini era and with models of the torus physical chemistry matched 1036 to the Cassini UV data by Delamere et al. (2005) as shown in Figure 15. Nerney et al. (2017) also 1037 looked at the Galileo UVS data (June 1996) which actually showed a depletion of O+ emission 1038 compared with Cassini. Intriguingly, Herbert et al. (2001) also reported a decrease in O+ emission 1039 observed by EUVE at about the same time. We return to the topic of temporal variations in the 1040 torus below. 1041

A measurement from the Juno mission that has important implications for the torus is the 1042 ability of the JADE instrument (McComas et al. 2017) to separate the O+ and S++ ion species that 1043 have the same mass/charge ratio (M/Q=16) with the Juno-JADE time-of-flight detector. Kim et al. 1044 (2019) report a ratio of O+/S++ ion abundances of ~0.7 (±10%) at 35 RJ in the plasma sheet. This is 1045 a bit lower than typical values of derived from UV spectroscopy and physical chemistry models 1046 (e.g. Delamere et al. 2005 found 0.95 to 1.4 at 9 RJ). Note that the Juno in situ measurements 1047 extend up to 10s keV energies and the composition may reflect species-dependent heating 1048 between the torus and the plasma sheet. 1049

JAXA’s Hisaki satellite has been in orbit around Earth since September 2013 (Yoshikawa 1050 et al. 2010, 2014). The UV spectrometer has been observing emissions (including new emission 1051 lines identified by Hikida et al. 2018) from the Io plasma torus, determining a background torus 1052 composition that is very similar to the post-Io-eruption Cassini epoch (Yoshioka et al. 2014, 2017). 1053 The greatest value of the Hisaki mission has been in monitoring spatial and temporal variations 1054 over several seasons, including a couple periods of enhanced volcanic activity that we discuss 1055 below, after summarizing the main torus physical chemistry. 1056

Flow of Mass & Energy. Summing up the mass in the Io plasma torus comes to total of 1057 ∼2 megaton. A source of ∼1 ton/s would replenish this mass in ∼40 days. To get the total thermal 1058 energy of the torus, we multiply this total mass by a typical energy (Ti ≈ 60 eV, Te ≈ 5 eV) to obtain 1059 ∼6 x 1017 J. The torus emits (via more than 50 ion spectral lines, mostly in the EUV) a net power 1060 of ~1.5 TW. This emission is excited by electrons which, at a rate of ~1.5 TW, would drain all their 1061 energy in ∼7 hours. While pickup supplies energy to the ions, which is fed to the electrons via 1062 Coulomb collisions, the pickup energy is not sufficient to maintain the observed emissions. An 1063 additional source of energy, perhaps mediated via plasma waves and/or Birkeland currents, is 1064 needed to heat the electrons. 1065

Figure 16 shows the flow of mass and energy through the torus system from Nerney et 1066 al. (2019) taking the “cubic centimeter” physical chemistry model at 6 RJ (that matches conditions 1067 derived from the Cassini UVIS spectrum) as typical of the main torus. The model is not very 1068 sensitive to the temperature of the hot electrons (40-400 eV). Model inputs are: O/S source ratio 1069 =1.9, source of neutrals Sn=7.8 x 10-4 cm-3 s-1, transport timescale=72 Days, fraction of hot 1070 electrons feh = 0.25%, temperature of hot electrons Teh=46 eV. This neutral production rate is 1071 equivalent to a net neutral source of 700 kg/s for a volume of 68 RJ

3 . We call this the “low & 1072 slow” case. Similar results are found for ~50% higher source rates and correspondingly lower 1073 transport times – the “high & fast” case. Note that there is an anticorrelation of transport time 1074 and source to match the observed density (~2000 cm-3) in the torus. 1075

The “low & slow” example is shown in Figure 16a.. Sulfur is more easily ionized than 1076 oxygen so that the dominant sulfur ion becomes S++ while oxygen is quickly charge-exchanged 1077

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and much (34%) of the oxygen is lost from the region as fast neutrals. Hence the abundance ratio 1078 of the dominant (M/Q=16) ions O+/S++ is about unity and we expect a denser, more extensive 1079 neutral oxygen cloud (density ~50 cm-3) than neutral sulfur cloud (density ~11 cm-3), consistent 1080 with recent modeling by Smith et al. (2019) discussed in section 4.1 above. The S+, S+++ and O++ 1081 ions, with abundances of a few percent each, are transported out of the torus with the dominant 1082 O+ and S++ ions. The supra-thermal electrons contribute to ionization, particularly for the higher 1083 ionization states, but the thermal electrons play the major role in the mass budget overall. The 1084 hot electrons become more important when considering the energy budget. 1085

Figure 16b shows the flow of energy through the system for the “low & slow” case. Here 1086 hot, supra-thermal electrons play a major role, supplying 54% of the energy. Current physical 1087 chemistry models fix the contribution of hot electrons to the total density. This is a simplistic 1088 approach and we await more sophisticated models. Nevertheless, such an approach provides 1089 useful indications of the extent of the role played by of supra-thermal electrons. The model is 1090 very sensitive to the faction of hot electrons and Nerney et al. (2019) could not find realistic 1091 physical chemistry solutions for feh > 0.5%. Additional energy sources are from ion pick up of S+ 1092 (26%) and O+ (20%) ions. Most of this energy input to the torus is coupled via Coulomb collisions 1093 to the core, thermal electrons and radiated out via mostly UV emissions (86%) excited by electron 1094 impact. 1095

The mass flow for “high & fast” case is remarkably similar with a lightly lower average 1096 ionization state since there is less time to ionize the higher source of neutrals beyond the first 1097 ionization state. The energy flow for “high & fast” case requires fewer hot electrons (~40% of the 1098 energy supply) with the higher neutral source producing more pick-up ions. The ions remain 30-1099 40eV hotter, carrying more of the energy out of the system with less (73%) being radiated 1100 through UV emissions. Note that both “low & slow” and “high & fast” cases are matching typical 1101 torus conditions and emissions. 1102

Estimates of the hot electron fraction based on UV emissions range from <1% to 15% 1103 (Steffl et al. 2004a,b; Yoshioka et al. 2014, 2017, 2018; Nerney et al. 2017; Tsuchiya et al. 2017, 1104 2019; Hikida et al. 2018) with the fraction increasing from 6 to 8 RJ. Physical chemistry models 1105 consistently suggest less than 1% (typically ~0.25%) for the hot electron fraction necessary to 1106 power the torus at 6 RJ (Delamere & Bagenal 2003; Yoshioka et al. 2011) increasing to 1.5% at 9 1107 RJ (Delamere et al. 2005). Nerney et al. (2019) shows that one can match the UV emission 1108 spectrum with a range of Feh from 0.1 to 5%, far higher values than suggested by the physical 1109 chemistry limits (<0.5% at 6 RJ ). In situ measurements from Voyager PLS electron data show 1110 electron distributions that reasonably approximate a double Maxwellian for most of the torus. 1111 Beyond about 8 RJ the supra-thermal component increases, forming a tail to the thermal core. 1112 Sittler & Strobel (1987) derive values for the hot electron fraction of (0.15, 0.9, 10%) at (6, 8, 9 1113 RJ) respectively. 1114

While the need for a supply of supra-thermal electrons has been known since the mid-1115 80s, the source mechanism remains unknown. It is even debated whether they are produced 1116 locally within the torus (Hess et al. 2011a; Copper et al. 2016) or injected from the outer 1117 magnetosphere (Yoshikawa et al. 2017; Tsuchiya et al. 2018, 2019; Kimura et al. 2018). The 1118 Galileo flybys of Io showed that beams of supra-thermal electrons are produced in the wake of 1119

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the Io interaction (discussed in section 3.1 above) but no significant Io-modulation was found in 1120 the Voyager or Cassini UV emissions from the torus (Sandel & Broadfoot 1982a,b; Steffl et al. 1121 2004a,b). In the more recent and extensive monitoring of UV emissions by Hisaki, Tsuchiya et al. 1122 (2015) reports a ~10% modulation of the UV power associated with the phase of Io’s orbit. 1123

Spatial Variations and Modulations. Over the past forty years spatial and temporal 1124 variations of the Io plasma torus have intrigued observers and puzzled theorists. For the warm 1125 torus, there are four main types of variations: (A) tied to the magnetic field structure System III 1126 longitude variations at Jupiter’s spin period; (B) a pattern/periodicity that drifts at a few percent 1127 slower than System III, originally called System IV; (C) temporal variations associated with Io’s 1128 volcanic eruptions; (D) modulations in brightness of torus emissions and a radial shift of the peak 1129 location on the dawn vs. dusk side of Jupiter. 1130

The idea of a systematic longitudinal modulation of the magnetosphere of Jupiter was 1131 initially provoked by Pioneer observations of energetic electrons escaping the system with a 10-1132 hour periodicity (Chenette et al. 1974) which Dessler (1978) interpreted as due to an anomalously 1133 weak field at a specific longitude driving preferential outflow in this “magnetic anomaly” region 1134 (Dessler & Vasyliunas 1979; Vasyliunas & Dessler 1981). From 1980 onwards ground-based 1135 observations of optical emissions from S+ ions showed systematic longitude variations (see 1136 thorough review in Steffl et al. 2006). Voyager UV emissions showed longitudinal variations in S+ 1137 and S++ emissions (Sandel & Broadfoot 1982b; Herbert & Sandel 2000) while Lichtenberg et al. 1138 (2001) found similar variations in IR emissions from S+++ ions. The 45 days of monitoring the UV 1139 emissions from multiple ions the torus by the Cassini UVIS instrument in late 2000 allowed Steffl 1140 et al. (2006) to analyze the temporal variability of the main Io torus. The primary effect they 1141 found was a System III longitude ~25% modulation of S+ and S+++ emissions (anti-correlated) that 1142 are consistent with a longitudinal modulation of electron density ~5% and temperature ~10%. 1143 The primary ion species, S++ and O+ showed only few percent modulation. Steffl et al. (2008) 1144 modeled these emission modulations with a “cubic centimeter” physical chemistry model, taking 1145 the fraction of hot electrons around a baseline value of 0.23% of the total electron density and 1146 superposing a 25% variation in the with longitude, peaking at 290° longitude. This small change 1147 in the hot electron fraction is enough to alter the ionization state of sulfur ions. Hess et al. (2011a) 1148 showed that such a modulation of hot electrons could be explained by Alfvenic heating of 1149 electrons and a longitudinal variation (due to the non-dipole components of Jupiter’s magnetic 1150 field) of the magnetic mirror ratio – that is, the ratio of the magnetic field in Jupiter’s ionosphere 1151 to the field strength at the magnetic equator. They showed that the VIPAL magnetic field model 1152 of Hess et al. (2011b) has a strong longitudinal dependence of the magnetic mirror ratio 1153 (averaged between north and south hemispheres) with a peak at 280° System III longitude. The 1154 Juno-based JRM09 magnetic field model of Connerney et al. (2018) shows an even larger peak at 1155 the same longitude. Such a peak in magnetic mirror ratio around 280° longitude means fewer hot 1156 electrons are scattered by waves into Jupiter’s ionosphere at such longitudes. Thus the geometry 1157 of Jupiter’s magnetic field is able to explain the small enhancement of hot electrons that is able 1158 to modulate the abundances – and hence emissions – of S+ and S+++ ions. 1159

The second longitudinal modulation of the torus emissions – the System IV variation – is 1160 more complicated. Hints of variations on a timescale a few present longer than System III came 1161 from radio emissions (Kaiser & Desch 1980, Kaiser et al. 1996) and ground-based emissions 1162

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(Roesler et al. 1984). Sandel & Dessler (1988) noted that the Voyager UV emissions had a 1163 periodicity at 10.22 hours, a few percent longer than Jupiter’s 9.925 hour System III spin rate, 1164 which they labelled System IV. Brown (1995) found a similar (10.214 hour) periodicity in ground-1165 based S+ emissions, but noted that the phase of this System IV sometimes shifted (also noted by 1166 Reiner et al. 1993; Woodward et al. 1994, 1997). Steffl et al. (2006) found a periodicity in the 1167 Cassini UVIS data of 10.07 hours, just 1.5% longer than the System III. Steffl et al. (2008) were 1168 able to model the 45 days of emission modulation as a compositional wave propagating 1169 azimuthally around the torus, driven by a combination of two sinusoidal variations in hot electron 1170 fraction about an average value (0.235%): a 30% modulation with a peak at 290° System III 1171 longitude, plus a 43% modulation drifting 12.5°/day (System IV). The beating of these two 1172 modulations produced peak amplitude with a 29-day beat frequency. The Steffl et al. (2006, 1173 2008) analysis of the UVIS data provides an empirical description of the System III/IV modulations 1174 but does not explain them. Copper et al. (2016) adapted the Delamere et al. (2005) physical 1175 chemistry model of the torus allowing variation in two-dimensions: radial distance and azimuth. 1176 They simulated the UVIS variations in S+/S+++ abundance ratio by applying a System III variation 1177 in hot electron fraction (consistent with the Hess et al. (2011a) Alfvenic heating plus magnetic 1178 field asymmetry enhancing trapping of hot electrons at 280°) for all radial distances and then 1179 imposed subcorotation of 1 km/s (~12°/day) at 6 RJ, increasing to 4 km/s at 7 RJ and then 1180 decreasing to full corotation at 10 RJ (approximating the radial profile of subcorotation observed 1181 by Brown 1994). The model simulated the System III/IV beat structure found by Steffl et al. (2006) 1182 for the densest (and brightest) part of the torus (6-7 RJ) with the System IV effect fading out 1183 beyond ~7 RJ. This behavior provides compelling evidence that the physics of magnetosphere-1184 ionosphere coupling is key to the System IV variations. The short term changes in the System IV 1185 period (on timescales of months to years) mean it is unlikely that this erratic modulation stems 1186 from changes in the intrinsic planetary magnetic field as originally suggested by Sandel & Dessler 1187 (1988), analogous to the changing solar magnetic field. Instead, variations in 1188 thermospheric/ionospheric properties and/or variations in plasma torus properties driven by 1189 iogenic input likely play an important role. 1190

Temporal variations in the torus emissions suggest Io’s volcanic eruptions can drive 1191 changes in the plasma torus. There are multiple observations of changes in the amount of neutral 1192 gas and plasma escaping from Io (Brown & Bouchez 1997; Bouchez et al. 2000; Mendillo et al. 1193 2004; Nozawa et al. 2004, 2005, 2006; Steffl et al. 2004b; Delamere et al. 2005; Yoneda et al. 1194 2010, 2015; de Kleer & de Pater 2016; Koga et al. 2018a,b,2019; Morgenthaler et al. 2019). But 1195 it is still difficult to predict the timing, duration, and spatial extent of the activity. The transport 1196 of plasma from the Io plasma torus has been estimated to take tens of days (Delamere & Bagenal 1197 2003; Bagenal & Delamere 2011). To investigate the magnetosphere response to changes in the 1198 plasma supply rate from Io, it is essential to monitor both changes in the neutral cloud around Io 1199 as well as the plasma conditions in the torus. As Cassini approached Jupiter, the Galileo dust 1200 instrument reported a 1000-fold increase in dust production, assumed to be caused by volcanic 1201 eruptions on Io (Kruger et al. 2003). The dust outburst began in July 2000, Peaked in September 1202 and settled back to normal by December 2000, overlapping with reports of activity from Io’s 1203 Tvashtar volcano. The Cassini UVIS instrument started observing UV emissions from the Io torus 1204 in October 2000 and Steffl et al. (2004b) reported high intensities that dropped to normal values 1205

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by the end of the year. Delamere et al. (2004) matched the changing UV emissions from the torus 1206 reported by Steffl et al. (2004b) with a factor ~3 increase in source of iogenic atomic neutrals 1207 lasting about 3 weeks, peaking at about August 2000. By January 2001, the torus seemed to have 1208 relaxed back to more typical conditions. Why the gas production in the torus was only a factor of 1209 ~3 while the dust production increased by three orders of magnitude remains a mystery. 1210

The JAXA Hisaki satellite has produced quasi-continuous EUV spectral images of planetary 1211 exospheres since its launch in 2013 (Yoshikawa et al. 2014). Emissions from the warm outer torus 1212 show similar features to those of Cassini UVIS: dawn-dusk asymmetry a System III longitude 1213 modulation by hot electrons, plus a strong modulation by the phase of Io (Yoshioka et al. 2014; 1214 Tsuchiya et al. 2015). In addition to continually monitoring the Io plasma torus emissions the 1215 Hisaki instrument has made a spectacular measurement of the neutral atomic oxygen cloud 1216 spreading all around Io’s orbit (Koga et al. 2018a,b) which is discussed further in section 4.1 1217 above. Conveniently, Io had a major volcanic event in mid-2015 (probably associated with the 1218 eruption of Kurdalagon Patera observed in the NIR by de Kleer & de Pater 2016, 2017) and Hisaki 1219 was able to measure the response in the emissions of different ion species in the Io plasma torus 1220 (Tsuchiya et al. 2015, 2019; Yoshikawa et al. 2017; Kimura et al. 2018; Yoshioka et al. 2018). In 1221 particular, Yoshikoka et al. (2018) compared the Hisaki torus emissions at the peak of the 1222 enhancement (DOY 50 2015) with quiet-time (late 2013) to which they applied a physical 1223 chemistry model to derive a factor 4.2 increase in neutral source, a ~2-4 times faster radial 1224 transport rate, and a minor increase in hot electron fraction (from 0.4% to 0.7%). Yoshioka et al. 1225 (2018) also found a reduction in the O/S ratio of the neutral source (from 2.4 to 1.7), consistent 1226 with the analyses of an active period in the Galileo-era by Nerney et al. (2017) and by Herbert et 1227 al. (2001). Copper et al. (2016) explored the effects of increased mass-loading on their 2-1228 dimensional model, comparing with the mid-2015 Hisaki event. They found they could model the 1229 observed general changes in plasma conditions (enhanced emissions, adjustments of 1230 composition) with an enhanced (x2.4) plasma source, increased hot electron fraction and 1231 decreased radial transport time (43 to 34 days). Tsuchiya et al. (2019) collected Hisaki 1232 measurements of S+++ and S+ emissions over three seasons (the first halves of 2014, 2015, 2016) 1233 to derive S+++/S+ emission ratio variations with System III and IV. They show a shortening of the 1234 System IV period after periods of volcanic activity, with an exception of an anomalous change in 1235 February 2014 that was associated with a change in Io’s volcanic activity. 1236

Suzuki et al. (2018) made a time-series analysis of the total EUV emission from the torus 1237 observed by Hisaki to explore the lifetime of brightenings that occur on top of System III and IV 1238 modulations. They applied the technique of Volwerk et al. (1997) who derived a radiative 1239 timescale for torus emissions of <5 hours from Voyager UVS data by comparing emissions 1240 between dawn and dusk as the plasma torus rotated around Jupiter every 10 hours. Suzuki et al. 1241 (2018) applied the Coulomb collision rate for hot electrons heating the thermal electron 1242 population to derive the temperature of a fixed 0.4% fraction of hot electrons. Out of 42 1243 brightening events in 240 days of observation over December 2013 to May 2015, they found that 1244 the majority (83%) did not persist for 5 hours suggesting that the hot electrons were colder than 1245 150eV. The remaining 7 events (17%) persisted for longer than 5 hours, consistent with supra-1246 thermal electron energies of 270-530 eV. 1247

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The fourth variation of the Io plasma torus is an apparent radial shift and intensity 1248 modulation with local time. Ground-based observers measuring optical torus emissions 1249 (primarily from S+ ions) noticed that the peak of the plasma torus emission appeared shifted by 1250 about 0.2 RJ towards the dawn side of Jupiter (i.e. to the east in the night sky as observed from 1251 Earth) compared to the dusk side (i.e. west in the sky) (Morgan 1985a,b; Oliversen et al. 1991; 1252 Schneider & Trauger 1995). Observers measuring UV emissions noticed that the plasma torus 1253 emission intensity was usually stronger on the dawn side of Jupiter than the dusk side by as much 1254 as a factor of 2 (Sandel & Broadfoot 1982b). This dawn-dusk asymmetry was explained by plasma 1255 flow down the magnetotail imposing a dawn-dusk electric field across the magnetosphere (Ip & 1256 Goertz 1983; Barbosa & Kivelson 1983). As the torus plasma moves around Jupiter, the electric 1257 field causes the plasma to move a few percent closer to (farther from) Jupiter on the dusk (dawn) 1258 side. Just a small shift produces sufficient compression (enhancing density) and heating on the 1259 dusk side, to double the UV emission. The Voyager UVS instrument resolved the spatial structure 1260 of the inner edge of the warm torus, indicating that 80-85% of the emission came from the 1261 narrow (0.2 RJ wide) ribbon region that showed this strong dawn-dusk asymmetry (Sandel & 1262 Broadfoot 1982b; Volwerk et al. 1997). The Cassini UVIS measurements showed a similar 1263 dusk/dawn emission ratio (1.3 ± 0.25) but did not resolve the narrow ribbon (Steffl et al. 2004a,b). 1264 The 3 years of Hisaki emissions show a similar dusk/dawn emission ratio but also reveal a 1265 modulation by the phase of Io in its orbit around Jupiter (Tsuchiya et al. 2015, 2019), suggesting 1266 that local heating of electrons near Io is involved in the local time modulation. 1267

Thus, the four types of variations in the warm plasma torus (6-8 RJ) are indications of the 1268 processes that modulate the flow of mass and energy through the system. The bulk (~90%) of 1269 the plasma that comes from Io fills the warm plasma torus, and spreads out into the vast 1270 magnetosphere of Jupiter. We consider the 10% diffusing inward and the narrow spatial region 1271 around Io’s orbit and in the next section. 1272

1273

5.3 – Inner Io Plasma Torus: Ribbon & Washer 1274

The inner boundary to the warm torus is a narrow (width~0.2 RJ), often bright region 1275 called the “ribbon”, the location of the peak emission varying between 5.6 and 5.9 RJ. This ribbon 1276 has a similar vertical extension as the warm torus, suggesting it may just be the inner boundary 1277 of the main torus (Figure 14, Table 5). Inward of the ribbon, the plasma is much colder (dropping 1278 to <1 eV), forming a thin (H~0.2 RJ), washer-shaped disk between 5.6 and 4.7 RJ that is relatively 1279 stable over time. 1280

The thin, bright ribbon is most distinct in optical emissions of S+ ions, as illustrated in 1281 Figure 17. The brightness reflects a combination of high density (~3000 cm-3) and high abundance 1282 of S+ ions. Farther out in the main torus the presence of warmer electrons raises the sulfur 1283 ionization state with S++ ions becoming dominant. Both in situ measurements and high-resolution 1284 optical emission spectra show the electron and ion temperatures plummet inside ~5.7 RJ and the 1285 dominant S+ and O+ ions become tightly confined to the centrifugal equator. The greatly reduced 1286 temperatures and densities inside ~6 RJ complicates observations of the inner torus: the UV 1287 emissions turn off while the optical emissions from the tenuous inner regions are observed along 1288 a line of sight through the denser, hotter ribbon. 1289

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Just as in the main, warm plasma torus, the ribbon is modulated by System III, System IV, 1290 local time and volcanic activity. But the tight radial confinement of the ribbon means that of 1291 particular importance is the dawn-dusk shift of the plasma by ~0.2-0.3 RJ due to the global electric 1292 field, moving the center of the ribbon from 5.84 ± 0.06 RJ on the dawn side to 5.56 ± 0.07 RJ on 1293 the dusk side (Morgan 1985a,b; Schneider & Trauger 1995; Herbert et al. 2008; Smyth et al. 2011; 1294 Schmidt et al. 2018). This shift is comparable to the width of the ribbon and moves this region of 1295 peak emission relative to Io’s orbit (5.90 ± 0.023 RJ). The ribbon occupies a volume that overlaps 1296 the cloud of neutrals from Io, the source of the plasma torus. Thus, these spatial and temporal 1297 variations of the ribbon may be influencing – or influenced by – the plasma source. Furthermore, 1298 the ribbon is associated with a 5% dip in the azimuthal flow of the plasma relative to corotation 1299 (Brown 1994; Thomas et al. 2001). The dip is lowest at ~6-7 RJ (indicating the peak plasma source 1300 region), rising back up to corotation by Europa. Inside 5.4 RJ the plasma was sufficiently cold for 1301 the Voyager PLS instrument to resolve separate ion peaks and determine that the plasma flow in 1302 the cold torus is within 1% of corotation. 1303

Post-Voyager era the plasma inside Io’s orbit could be characterized (Bagenal 1985) by 1304 three regimes: (1) the ribbon (6.0 – 5.6 RJ) of high density plasma population where the ion 1305 distributions have substantial non-Maxwellian tails implying the presence of pick-up ions, a bulk 1306 speed with ~5% lag behind corotation, electrons a factor ~5 times colder than the ions; (2) a “gap” 1307 or “dip”(5.6 -5.4 RJ) where there are sharp drops in both density and temperature (illustrated in 1308 panel (d) of Figure 17), shown by Schmidt et al. (2018) to vary with longitude; (3) the cold, inner 1309 torus (< 5.4 RJ) where the bulk flow is close to corotation, the ion and electron species are in close 1310 thermodynamic equilibrium with dwindling supra-thermal components (panel (b) of Figure 17). 1311 The difference in plasma conditions in these regions and the sharp boundaries suggest that the 1312 structure of the inner torus cannot be explained in terms of simple models of steady radial 1313 diffusion. 1314

Little progress was made in modeling the inner torus until Herbert et al. (2008) re-1315 analyzed Nick Schneider’s 1991 observations of S+ emission and derived a quantitative 1316 description of the ribbon plus cold inner torus, including variations with local time and System III 1317 (further quantified using Galileo data by Smyth et al. 2011). Herbert et al. (2008) reported the 1318 scale height of the ribbon (proportional to the S+ temperature parallel to the magnetic field, 1319 between 20-100 eV) varies over time. They also report a vertical offset from the centrifugal 1320 equator with longitude that is probably due to the fact that with the very low temperatures and 1321 small scale height (<0.2 RJ) the location of the centrifugal equator is sensitive to high-order 1322 structure of magnetic field (as noted by Bagenal 1994). The high-order structure of Jupiter’s 1323 magnetic field derived from Juno’s close flybys of the planet (Connerney et al. 2018) may allow 1324 accurate modeling of such fine-scale features. The red line in Figure 17 shows latest magnetic 1325 field model is close but does not yet match on the fine spatial scale of the cold inner torus. 1326

Richardson & Siscoe (1981, 1983) first tried to address the issue of the flow of mass and 1327 energy inside Io’s orbit with a diffusive transport model and concluded that (i) the steeper 1328 gradient in fluxtube content inside the ribbon meant that the plasma diffuses inwards ~50 times 1329 slower than outwards; (ii) additional sources of both plasma and heat were needed in the cold 1330 torus. These early studies were hampered by lack of information about the neutrals, limited 1331 physical chemistry data, as well as the fact that the first reports of ion temperature from Voyager 1332

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PLS were over estimated by a factor of 2 (Bagenal et al. 1985). Subsequent models (Moreno et 1333 al. 1986; Barbosa & Moreno 1988; Herbert et al. 2001) still needed an additional local source of 1334 energy. 1335

One source of mass and energy in the inner torus could be molecular SO2 coming from 1336 the Io interaction either as neutrals or ions (see section 3.1). The molecular ion SO2

+ has been 1337 observed directly (Figure 17b) by Voyager PLS in the cold torus (Bagenal et al. 1985; Dougherty 1338 et al. 2017). The molecular ion has not been detected in the warm torus, partly because it is likely 1339 quickly dissociated and/or recombined but also because the heavy, molecular ion species is likely 1340 swamped by the supra-thermal tail of the major atomic ion species in the PLS spectrum. Further 1341 evidence of molecular ion species came from magnetic signatures measured on multiple flybys 1342 of Io by Galileo. To quote Russell (2005): “The existence of ion cyclotron waves at the SO2

+ and 1343 SO+ gyrofrequencies (Kivelson et al. 1996b; Russell & Kivelson 2000) was not so much a surprise 1344 in the Galileo data as were their amplitudes and spatial extent”. Analysis of such waves by 1345 Huddleston et al. (1997, 1998) and Blanco-Cano et al. (2001) showed these molecular species 1346 extend for many Io radii and correspond to a source of 8 x 1026 SO2

+ ions/s. Models by Smyth & 1347 Marconi (1998) and by Cowee et al. (2003, 2005) showed that SO2

+ could contribute to the source 1348 of plasma needed in the inner torus. 1349

While the relatively small volume and mass of material residing inside Io’s orbit plays a 1350 limited role in the total magnetosphere, there are major unresolved issues about spatial and 1351 temporal variability of these fine-scaled structures that could tell us about the nature of the 1352 source of neutrals and plasma from Io. 1353

1354

5.4 Influence of Europa on Plasma 1355

Figure 15 shows the UV emissions from the plasma torus indicate rising electron 1356 temperatures and increasing amount of ions with higher ionization states as the plasma moves 1357 out from Io towards Europa’s orbit. But there does not seem to be an enhanced plasma density 1358 nor change in ion composition associated with Europa. This is perhaps not surprising given the 1359 relatively small (~25-70 kg/s) escape rate of neutrals (mostly H2) from Europa (see sections 3.2 1360 and 4.2). Delamere et al. (2005) applied such a source to their physical chemistry model to 1361 explore the impact of such a source around Europa’s orbit and found that very modest (few 1362 percent) changes in the ion abundances (increasing O+, decreasing Sn+). The most significant 1363 impact of a Europa source is the production of pick-up ions (650 eV for locally produced O+ ions) 1364 that enhances the ion temperature. Unfortunately, it is not easy to differentiate between heating 1365 due to ion pick-up around Europa’s orbit with the general heating (via as-yet known process) that 1366 seems to be operating on the plasma as it moves outwards through the magnetosphere. 1367

The most significant impact of Europa on the magnetosphere of Jupiter is likely the effect 1368 of the neutral clouds (Figure 13) on the inwardly diffusing hot plasma populations, as discussed 1369 in the next section. 1370 1371 1372 1373

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6. Jovian Neutral Nebulae 1374

Since the early (pre-Voyager) detection of the sodium cloud near Io, there was the 1375 realization that charge exchange of Io torus ions could generate a wind of neutrals extending out 1376 into the magnetosphere and might be a source of plasma (Eviatar et al. 1976). The Voyager 1377 measurements of energetic sulfur and oxygen ions in the middle magnetosphere fed the idea 1378 that re-ionization of such iogenic neutrals could produce pick-up ions at 10s of keV energies, 1379 gaining further energy as they diffused inwards, perhaps powering Jupiter’s intense aurora 1380 (Cheng 1980; Eviatar & Barbosa 1984; Barbosa & Eviatar 1986). It was also realized that such a 1381 wind of neutrals might supply heavy ions upstream of Jupiter (Krimigis et al. 1980; Kirsh et al. 1382 1981; Baker et al. 1984). As it skims over Jupiter’s ionosphere, the Juno spacecraft is now showing 1383 heavy ions at both low (Valek et al. 2019) and high (Kollman et al. 2017) energies. Perhaps these 1384 ions come from sprays of Io and/or Europa neutrals impacting the atmosphere. 1385

Below we briefly summarize the limited knowledge we have about these neutral nebulae 1386 and the roles they may play. 1387

1388

6.1 Mendillodisk – Io ENAs 1389

By the late 80s telescopic capabilities reached a level of sensitivity to detect sodium 1390 emission far from Io and Mendillo et al. (1990) reported emission extending >400 RJ from Jupiter 1391 in a disk (Figure 12). They proposed that sodium ions in the Io plasma torus charge exchange with 1392 neutrals around Io’s orbit. Having cooled (losing their pick-up energy) via UV emission and 1393 thermalized via Coulomb collisions, torus ions share relatively low temperatures (50-100 eV) so 1394 their gyro-speeds are less than their bulk corotation speed (Vg<<Vco). This means that when 1395 neutralized via charge-exchange the atoms spray mostly outwards in a disk of neutral atoms with 1396 energies of ~400 eV. To honor the discoverer, this is sometimes called the “Mendillosphere” or 1397 “Mendillodisk”. Flynn et al. (1994) modeled two-years of observations of the extended sodium 1398 nebula and found that the flaring angle of the disk (20-27°) anti-correlated with the source 1399 production rate (as indicated by Io’s surface IR output or brightness of near-Io emissions). Studies 1400 of Io’s extended neutral clouds showed variations with local time and Io’s orbital phase (Mendillo 1401 et al. 1992, 2004; Flynn et al. 1994; Yoneda et al. 2015) and a source of 1-4 x 1026 atoms per 1402 second. Mendillo et al. (2004; 2007) show a variation in brightness and shape with Io’s volcanic 1403 activity – the disk having angular edges (like an anulus) when Io is particularly active compared 1404 with an oblate spheroid during quiet times. The suggested explanation is that under quiet times 1405 the sodium ENAs are produced (from neutralized Na+ in the torus) as a stream (Figure 12). When 1406 Io is more active, production (e.g. via dissociative recombination of NaCl+) as jets local to Io 1407 increases (Mendillo et al. 2007). 1408

Similar extended disks of escaping S and O atoms, as well as SO2 and SO molecules also 1409 probably exist. Physical chemistry models of the torus (e.g. Delamere et al. 2005) predict charge 1410 exchange processes will produce a flux of escaping neutralized atoms of 1-5 x 1028 atoms per 1411 second, mostly oxygen (Figure 16a). While this is ~100 times greater than the sodium flux, the 1412 radiation efficiency of other species is so much weaker that any neutral disk would be very hard 1413 to detect. Direct in situ detection of these escaping neutrals would require an instrument that 1414

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measures Energetic Neutral Atoms (ENAs) with energies less than ~300 eV (e.g., <20 eV/nucleon 1415 for oxygen atoms moving at 70 km/s). 1416

While corotating torus ions that are neutralized via charge exchange with Io’s neutral 1417 clouds will come off as an outward stream, charge exchange close to Io could come off as a jet or 1418 spray (Figure 12). For example, a recently-picked-up ion produced in the plasma-atmosphere 1419 interaction that charge exchanges on the sub-jovian side of Io could be directed towards Jupiter 1420 where re-ionization on impacting the planet’s atmosphere would be a source of cold, ionospheric 1421 heavy ions near the planet’s equator (Valek et al. 2019). 1422 1423

6.2 Neutral Nebula Sphere – Europa vENAs 1424

On route to Saturn, Cassini flew past Jupiter for a gravity assist in late 2000. Cassini carried 1425 an instrument that detected ENAs, initially assumed to be hydrogen atoms, in the range of 50-80 1426 keV emanating from what appeared to be Europa’s orbit (Krimigis et al. 2002). To distinguish 1427 these (very energetic) 10s keV/nucleon atoms from the iogenic ENAs we call them vENAs. Mauk 1428 et al. (2003, 2004) modeled these vENAs as products of charge exchange reactions of inward-1429 moving energetic (10s keV) protons with neutrals in a cloud surrounding Europa’s orbit (Figure 1430 18), consistent with depletion of energetic H+ (Lagg et al. 1998, 2003; Kollmann et al. 2016) and 1431 S+ (Nenon et al. 2019) at these distances in Galileo EPD data . The observed flux of vENAs (1025 s-1432 1) requires a local density of 20 to 50 cm-3 hydrogen atoms in the Europa neutral cloud (see 1433 Section 4.2 above). 1434

Note that the ions charge exchanging with the Europa neutral cloud are sufficiently 1435 energetic that their bulk motion is much less than their thermal or gyro motion (Vg >> Vco). Thus, 1436 when they become neutralized, the Europa vENAs spray pretty much equally in all directions, 1437 forming a spherical cloud (Figure 19), unlike the iogenic ENAs which form a disk (“Mendillodisk”). 1438 1439

6.3 Re-ionization Upstream of Jupiter 1440

Krimigis et al. (2002) showed a plot (Figure 19) of ions measured by the Cassini CHEMS 1441 instrument in the solar wind upstream of Jupiter indicating the presence of energetic (55-220 1442 keV) oxygen and sulfur ions (as well as possible Na+ or SO2

+ ions) which they ascribe to photo-1443 ionization of the escaping vENAs. The Cassini measurements of heavy ions upstream of Jupiter 1444 add to measurements by Voyager (Krimigis et al. 1980; Kirsh et al. 1981; Baker et al. 1981) and 1445 Ulysses (Haggerty et al. 1999), but it is not clear at this point in time how much these ions leak 1446 out of the magnetosphere as energetic ions vs. produced locally via re-ionization of escaping 1447 neutral atoms, whether Io-ENAs or Europa-vENAs. 1448

1449

7. Oustanding Issues 1450

The peculiar role of Io in the magnetosphere of Jupiter was first noticed in 1964. A half 1451 century and a thousand or so papers later it is a good time to consider our understanding of the 1452 system and what are the key open questions. First, we consider the individual components of the 1453 system and then address coupling between them. 1454

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Atmospheres. The tenuous atmospheres of Io and Europa are hard to study remotely and 1455 close-up observations are often selective in viewpoint, coverage, and gas components. From a 1456 planetary atmospheres perspective the main focus is often properties near the surface. From a 1457 space physics perspective we are more interested in the outer regions. Specific open questions 1458 are: 1459

• What is the radial, longitudinal distribution and composition of the neutral atmospheres 1460 of Io and Europa from the surface to high altitude in daylight, in eclipse and at night? 1461

• How do the atmospheres, particularly the upper regions, vary with volcanic activity of the 1462 moon? How does the location of any active sites alter the plasma-atmosphere 1463 interaction? 1464

• Why is Io’s atmosphere sometimes very stable for many years then suddenly seems to 1465 supply large amount of neutrals, dust to the Io environment? 1466

• How much of the ionospheric density (e.g. as observed via radio occultations) is produced 1467 via photoionization vs. plasma impact on the atmosphere, and/or material convected 1468 through the interaction? Why do the ionospheres of Io and Europa appear so variable? 1469

• Where is the exobase, where do collisions quench reactions? A specific height may be 1470 hard to define for the complex interactions at Io and Europa. Moreover, however defined, 1471 the exobase will likely vary with longitude on the moon, plasma flow direction, perhaps 1472 with solar illumination, and probably with magnetic latitude of the moon. 1473 1474 Plasma-Atmosphere Interactions. The aurora produced via electron bombardment of the 1475

atmosphere can provide useful information about the interaction of the surrounding plasma with 1476 a moon’s atmosphere. But to explore the full physics we need a combination of in situ 1477 measurements along close flybys and detailed modelling. Specific open questions are: 1478

• How much of the plasma interaction goes into heating the atmosphere? Where? What is 1479 the impact on the atmospheric distribution? 1480

• What are the roles of electron impact ionization, charge exchange, collisions, and electron 1481 beams in the plasma-atmosphere interaction? What are the net ionized products of these 1482 reactions that escape into the space beyond? 1483

• What are the composition, velocity distribution, and fluxes of the neutrals that escape the 1484 moon’s gravity and into the space beyond? 1485

• How do the plasma interactions vary around the moon (e.g. upstream vs. downstream, 1486 sub/anti-Jupiter, day/night), with magnetic latitude and longitude and with local time / 1487 phase of the moon along its orbit? 1488

• Where do the electrical currents flow in the electrodynamical interaction between the 1489 magnetospheric plasma and the moons? 1490

• What is the strength of the induction signal at Io? How much from the aesthenosphere 1491 and/or the core? Is the induction signal at Europa purely from the conducting ocean? 1492

• How much of the iogenic material reaches the surface of Europa? 1493 • How do the plasma interactions vary (qualitatively and quantitatively) with volcanic 1494

activity of the moon? 1495 1496

28 May 2019

37

Neutral Clouds. While sodium is clearly observed around both Io and Europa, the major 1497 species are poorly measured, except Io’s oxygen cloud that is now being carefully mapped out by 1498 the Hisaki mission. Specific open questions are: 1499

• What are the amounts and trajectories of different neutral species that escape Io and 1500 Europa? In particular, are there molecular clouds of SO2, SO, O2, H2? What roles do they 1501 play? Is there a way to detect them? Are there any features in other atomic and molecular 1502 neutral clouds similar to the Na neutral structures such as jets and streamers? 1503

• How are these neutral clouds shaped by the plasma that flows through them? At Io, how 1504 much of the neutral clouds reach inwards of Io’s orbit (as molecules and/or atoms) to 1505 provide a source for the ribbon and inner torus? At Europa, how do the neutral clouds 1506 change under the variable conditions in the outer torus? 1507

• What role do the neutral clouds play in controling the influx of energetic particles 1508 (electrons, protons and heavy ions) from the middle magnetosphere through to the 1509 radiation belts within a few RJ of Jupiter? 1510 1511 Plasma Torus and Sheet. The general structure and systematic modulations (System III, 1512

IV, local time) of the three components of the torus – cold inner torus, ribbon, warm outer torus 1513 and plasma sheet – are fairly well described but underlying physical processes remain unclear. 1514 Specific open questions are: 1515

• What is (are?) the source(s?) of hot electrons? Do waves within torus accelerate local 1516 electrons? Are hot electrons injected from outside? 1517

• How is plasma heated as it moves out from the torus to the plasma sheet? How do the 1518 suprathermal populations (both ions and electrons) evolve? 1519

• What is the specific nature of Io eruptions that cause enhanced sources of plasma? What 1520 are the physical process whereby changes in sources affect the System III/IV modulations, 1521 radial transport rate, dawn/asymmetries, etc? 1522

• How much plasma does Europa contribute to the magnetosphere? 1523 • What is the spatial distribution of the production of plasma in the three regions of the 1524

torus? Where is the separation between outward and inward transport? What is the 1525 nature and role of the torus ribbon region? 1526

• What processes drive and control the radial tranport rate? How do these processes vary 1527 with radial distance and (if at all) with plasma production rate? 1528 1529 Extended Neutral Nebulae. The only nebula that has been detected directly is that of 1530

sodium from Io. But it is clear that charge-exchange reactions must be generating clouds of (very) 1531 Energetic Neutral Atoms – and perhaps molecules. Specific open questions are: 1532

• What are the fluxes of different neutral species (composition, energy, direction) out of 1533 the jovian system? Are these neutrals re-ionized? Where? And what is their fate? 1534

• What are the fluxes of different neutral species (composition, energy, direction) inwards 1535 towards Jupiter and impacting the planet’s atmosphere? Are these neutrals re-ionized? 1536 Where? And what is their fate? 1537 1538

28 May 2019

38

These components of the Io-Europa space environment are clearly highly coupled. While 1539 there are modulations and temporal variations, such changes seem to be limited to factors of a 1540 few (rather than wild swings of orders of magnitude). Thus, there must be both negative and 1541 positive feedback systems. Full understanding of this complex system will require both 1542 comprehensive observations as well as systematic modeling – of each component separately as 1543 well as carefully coupled – to explore what factors control (qualitatively and quantitatively) the 1544 different components. The beauty of the Io-Europa system, however, is that the timescales for 1545 the different processes make the components separable: plasma takes ~minute to pass each 1546 moon, neutrals survive ~hours orbiting Jupiter, plasma moves through the magnetosphere over 1547 many days. This separation of time scales allows us to study each component independently and 1548 then couple them together. 1549 1550 7.2 Future Observations 1551

The Juno mission is currently about 2/3 through the primary mission, mapping out the 1552 polar regions as well as outer to middle magnetosphere (for review of magnetospheric science 1553 see Bagenal et al. 2017; for updated orbits see Bolton et al. 2017). As the line of apsides of the 1554 orbit precesses southward, the spacecraft crosses the jovigraphic equator closer to the planet. 1555 By the end of the primary mission (34 orbits, spring 2021) Juno reaches between the orbits of 1556 Ganymede and Europa. Moreover, the tilted magnetic field allows the spacecraft to cross 1557 magnetic fluxshells intersecting Europa’s and Io’s orbits and sample the torus. Should the Juno 1558 mission be continued beyond orbit 34, the spacecraft will directly pass through the Io plasma 1559 torus as many as maybe 15 times (between November 2022 and November 2024), as well as 1560 likely intersect the Alfven wing that stretches downstream of Io (Figure 5). 1561

While Juno in situ measurements of particles and fields are very valuable, it is key that the 1562 torus emissions are also monitored from the ground (Schmidt et al. 2018; Morgenthaler et al. 1563 2019) and from the Hisaki satellite (Yoshikawa et al. 2014). Ground-based telescopes seem to be 1564 ever expanding in wavelength and aperture. Io’s volcanism is monitored in the IR but perhaps 1565 the extension into the millimeter range of telescope systems such as Atacama Large Millimeter 1566 Array may allow detection of the molecular species (SO2, SO, O2, H2). In the meantime, perhaps 1567 the community might support semi-continuous monitoring the more observable species (Na, S+, 1568 O, O+…?) with modest-sized telescopes. 1569

Looking farther to the future, the development of ESA’s JUICE and NASA’s Europa Clipper 1570 missions promise close exploration of the Europa system with perhaps some forays closer to Io. 1571

1572

Acknowledgements 1573

We are grateful to our colleagues who have assisted us in pulling this review together and gave 1574 us feedback on earlier drafts……….. We thank Crusher Bartlett for graphics. Funding. 1575 1576

Figure 1: The Io-Europa Space Environment. The system comprises at least 10 components that span from the moons in to Jupiter and out into the interplanetary medium.(Credit: top – John Spencer, SwRI; bottom – Steve Bartlett)

Figure 2: Io and Europa Bulk Properties.

Io & Europa: Bulk & surface properties

Io Europa

Orbit Period 42.5 hours 85.2 hours

Semi-major axis 5.90 ± 0.048 RJ 9.38 ± 0.17 RJ

Radius 1822 km 1561 km

Mass 8.93 x 1022 kg 4.80 x 1022 kg

Density 3528 kg m-3 3013 kg m-3

Escape speed 2.6 km s-1 2.0 km s-1

Core : Rock : Water (% volume) 6 : 94 : 0 3 : 74 : 23

Albedo 0.62 0.64

Surface Temperature 110 K 110 K

Silicate lava. SO2 frost, concentrated at equator & longitudes with plumes.

Water Ice. Sulphates, hydrates, preferentially trailing/upstream side.

>425 volcanic basins, ~250 active volcanoes, 50-100 erupting

Magnetic perturbations show liquid ocean under the ice

Global resurfacing ~1 cm / year. Craters removed in ~1 million yearsTurns inside out 40 times in 4.5 Ga

Geophysical evidence ice 5-30 km. Geological features suggests thinner brittle layer over ductile ice.

Where/how are tides dissipated to drive volcanism?

How much contact between ocean and surface?

Figure 3: Io and Europa Surface Properties

Europa

Figure 4: Io and Europa Atmospheric Properties. a: Eruption of Tvashtar Patera imaged by

New Horizons' LORRI imager 2007; b: HST image Feaga et al. (2009); c,d: daytime column

density of SO2 Feaga et al. (2009); e: Galileo visible image of aurora and volcanic emissions

Geissler et al. (2001); f: scale of atmosphere; g, h, i, j: HST images Roth et al. (2016); k:

Cartoon of plume (NASA/ESA/SwRI); l: scale of atmosphere.

a b

c d

e f

g

i

k

h

j

l

Io EuropaOxygen Hydrogen

Io EuropaSource (1) • SO2 sublimation in daylight

hemisphere.• Volcanoes at night?• Surface sputtering negligible

• Sputtering by thermal, high energy ions (2)

• Pick-up ions? (3)• Plumes? (4)• Sub-solar source? (5)

Asymmetry(1, 6)

• Day/night• Upstream/downstream• Pole/equator<35 deg• Jovian/anti-jovian

• Upstream/downstream• Jovian/anti-jovian?• Day/night?

Composition (7, 6) SO2, O, S, SONaCl, KCl O2, O, H, H2

Surface pressure (8, 6) ~ 2 nbar SO2 ~ 200 pbar O2

Column density (9, 10) [15-150] x 1015 cm-3 [0.2-2] x 1015 cm-2

Ionosphere peak density (11, 6) 28 x 104 cm-3 at 80 km 1.3 x 104 cm-3 at 50 km

Time variability • With distance from Sun (12)• With volcanic activity? (13)

• Plume activity? (4)

Table 1: Io and Europa atmospheric properties. (1) Lellouch et al. 2007; (2) Cassidy

et al. 2013; (3) Ip 1996; Saur et al. 1998; (4) Roth et al. 2014a,b; (5) Johnson et al.

2018; Plainaki et al. 2013; (6) McGrath et al. 2009; (7) Moullet et al. 2010, 2013; (8)

McGrath et al. 2004;(9) Spencer et al. 2005; (10) Hall et al. 1998; Hansen et al.

2005; Roth et al. 2016; (11) Hinson et al. 1998; (12)Tsang et al. 2012, 2013b; (13) de Kleer et al. 2019a,b.

Figure 5: Plasma-Io Electrodynamic Interaction. a, b: schematic of interaction between the surrounding plasma and Io (from Schneider & Bagenal 2007). c: Hubble Space Telescope image of Jupiter's UV aurora (Clarke et al. 2004) showing auroral foot prints of Io, Europa and Ganymede. d: Pattern of Alfven waves generated by Io, bounding between hemispheres of Jupiter where bursts of radio emission are generated above the ionosphere (Gurnett & Goertz 1981).

a b

c d

Io EuropaGanymede

Io EuropaRange from magnetic equatorRange from centrifugal equator

±1 RJ±0.65 RJ

±1.5 RJ±1 RJ

Local jovian magnetic field 1720-2080 nT 420-480 nTPlasma-moon relative velocity 53-57 km/s 65-110 km/sElectron density 1200-3800 cm-3 65-290 cm-3

Alfven Mach number plasma sheet center 0.3 0.4

Thermal electron temperature 5 eV 10-30 eVHot electrons 0.2% at 40 eV 2-10% at 250 eV

Major ions S++ (20%) O+ (25%) S++ (20%) O+ (20%)

Average Ion temperature 20-90 eV 50-500 eVAverage ion gyroradius ~3 km ~10 km

Table 2: Plasma conditions upstream of Io and Europa (Kivelson et al. 2004; Bagenal et al. 2015)

Figure 6: Sketch of the concept of the multi-species chemistry approach of the local interactionat Io. A parcel of plasma of prescribed composition and energy is carried by the prescribedplasma flow into the atmosphere of Io. The ion-electron-neutral processes change thecomposition and energy of the plasma in the parcel, which are then collected along a specificGLL flyby of Io (here the J0 flyby in red) and compared to the observations. Such simulations aimat constraining the atmospheric distribution and composition of Io’s atmosphere. Dols et al.(2008, 2012, 2016).

Figure 7 Plasma-atmosphere interaction at Io. Plasma properties at Io from a multi-species chemistry approach of the atmosphere-plasma interaction sketched in Figure 6. The plasma flow is prescribed as an incompressible flow around a conducting ionosphere, which is assumed to extend to 1.26 RIo (see text). From upper left in clockwise direction: the prescribed SO2 neutral density of the corona; the plasma flow slowed upstream and downstream and accelerated on the flanks; the resulting average ion temperature, which increases where SO2

+ ions are picked up by a fast flow; the electron density resulting from the torus thermal electron ionization; the electron temperature, which decreases in the molecular atmosphere because of the efficient molecular cooling processes; and the resulting SO2

+ density. These simulations do not include the ionization caused by the parallel electron beams detected in the wake of Io and nor do they include a day-night asymmetry of the SO2 atmosphere.

SO2 V SO2+

ne Te Ti

Io

Figure 8: Net Production at Io. Estimate of the contribution of each plasma-neutral process above an exobase (at 150km altitude) using the multi-species modeling approach described in Figure 6. These calculations use a prescribed MHD flow different from the flow shown in Figure 7. Although these rates depend on the flow and the atmosphere prescribed, they provide a reasonable estimate of the relative contribution of each process. The most important loss processes are the SO2 electron-impact dissociation, which provide slow atomic neutrals and a cascade of SO2 resonant charge-exchanges, which provide faster neutrals depending on the location of these charge exchanges.

Io

Figure 9 Plasma-atmosphere interaction at Europa. Plasma properties at Europa from a multi-species chemistry approach of the atmosphere-plasma interaction sketched in Figure 6. The plasma properties can be compared to Io’s in Figure 7. The flow is similar while O2 is the main atmospheric component and O2+ the main ion. As Europa’s atmosphere is more tenuous than Io’s, the interaction is weaker and the plasma properties do not vary as strongly as at Io.

O2 V O2+

ne Te Ti

Europa

Figure 10: Net Reactions at Europa. Estimate of the contribution of each plasma-neutral O2 process

above an exobase at 70 km altitude. The plasma-neutral processes on H2 are not a significant loss of H2

and are not shown here. Although the rates vary with the prescribed flow and neutral distribution, they

are much lower than the rates at Io shown in Figure 8. Europa does not provide much plasma to the

magnetosphere of Jupiter.

Europa

Figure 11: Densities of the plasma downstream of the interactions with (above) Io and (below) Europa (Dols et al. 2008, 2012, 2016).

Io EuropaGalileo flybys J0, I24, I27, I31, I32 E4, E6, E11, E12, E14, E15, E16,

E17, E19, E26Flow diversion 95 % (1) 60 - 80 % (23, 2) Magnetic perturbation 700 nT (3) 80 nT (4, 23)Main ion-neutral process Collision and chex (1, 5) Collisions and chex (2, 6)

Ionization • Thermal electron impact (3, 5)• Photoionization ~ 15% (5)• Electron beams (3, 7)

• Thermal and hot electron impact (2, 6)

• Photoionization ~ 10 %? (2)Main ion produced SO2

+ ~200 kg/s (1, 5, 8) O2+ ~20 kg/s (2, 6)

WakeNe~30,000 cm-3

slow flow (~ 1 km/s) cold ions (~ 1 eV) (8, 9)

No clear plasma wake at Galileo altitude? (10)

Auroral emissions Mainly O (11) also S (12), Na, K (13)

Mainly O also H Lyman-alpha (14)

Aurora location

• Equatorial flanks (15)• polar emission stronger on side of

plasmasheet (11)• Corona (11)

• Polar emissions stronger on side of plasma sheet (14)

• No flank emissions• Corona (16)

Electron beams location In wake J0, above poles (I31, I32) (17) No beams detected by Galileo

Electron beams energy • >150 eV • Powerlaw distribution (18)

Footprint emissions Multi-spot structure + long tail (19)

Double-spot structure + short tail (20)

Induced mag field in subsurface

Yes (21) No (22, 23) Yes (24)

Table 3 – Plasma-atmosphere interactions. (1) Saur et al. 1999; (2) Saur et al. 1998 calculated for a small upstream density ~ 40 cm-3; (3) Kivelson et al. 1996a; (4) Kivelson et al. 1997; (5) Dols et al. 2008; (6) Dols et al. 2016; (7) Saur et al. 2002; (8) Bagenal 1997; (9) Frank et al. 1996; (10) Kurth et al. 2001; (11) Retherford et al. 2003, 2007; (12) Wolven et al. 2001; Ballester et al. 1987; (13) Bouchez et al. 2000; Geissler et al. 2004; (14) Roth et al. 2016; (15) Roth et al. 2011; (16) Hansen et al. 2005; (17) Williams et al. 1996, 2003; Frank et al. 1999; (18) Mauk et al. 2001; (19) Gerard et al. 2006; Bonfond et al. 2008; (20) Bonfond et al. 2017; Grodent et al. 2006; (21) Khurana et al. 2011;(22) Roth et al. 2017b; (23) Blocker et al. 2018; (24) Zimmer et al. 2000; Schilling et al. 2004.

Figure 12: Neutral Clouds of Io (based on Wilson et al. 2002). Left - Io's sodium cloud on three spatial scales, as imaged by ground-based observations of sodium D-line emission (the bottom image is from Burger et al. 1999). The features observed on the left are explained by the three atmospheric escape processes shown schematically on the right.

Figure 13: Io (left) and Europa (right) neutral clouds as modeled by Smith et al. (2019). The color contours show local densities (cm−3) in the XY plane (−Y axis points toward the Sun). Bottom Left is a plotted the vertically-integrated column density along the Y=0 axis in the equatorial plane.

Io Europa

Local Loss processes(1, 2)

• Exospheric collisions, chex• Atmospheric sputtering• SO2 electron impact

dissociation• Direct volcanic injection

negligible• Surface sputtering negligible

• Jeans escape?• Exospheric collision• O2 electron impact

dissociation • Direct plume injection

negligible

Net Neutral loss (1, 2) 250 - 3000 kg/s 25 - 70 kg/sNeutral clouds (3, 4)DetectedExpected

S, O, Na, K, SO2 , SO ?

H, O, Na, K, H2 , O2 ?

Sodium sources (5) 10 - 30 kg/s ??Local plasma source (6, 7) ~200 kg/s ~20 kg/s

Extended plasma source (11) 700- 2000 kg/s ~60 kg/s

Table 4 – Neutral and plasma sources at Io and Europa. (1) section 3.1 Figures 7, 8; (2) section 3.2 Figures 9, 10; (3) Thomas et al. 2004; (4) Smyth & Marconi 2005; Smith et al. 2019; (5) Thomas et al. 2004; Wilson et al. 2002; (6) Bagenal 1997; Saur et al. 2003; Dols et al. 2008; (7) Saur et al. 1998; Dols et al. 2016; (11) Delamere et al. 2004; Nerney et al. 2019.

Warm Torus

RibbonS+ at 6731Å

S++ at 685Å

UV emission

Cold Disk Figure 14: Io Plasma Torus

a

b

c

d

Three regions of the Io plasma torus: Cold disk, ribbon, warm torus. (a) This computed image shows optical S+ emission (b) EUV S++ emission. Note that S+

dominates the cold torus and S++ dominates the warm torus. The ribbon is a tall, narrow ring which appears bright at the torus ansa because of projection effects. The ribbon is typically the most prominent of the three regions for S+, while in S++

emission the ribbon is a slight brightening at the inner edge of the warm torus (Schneider & Bagenal 2007). (c) net emission across the EUV spectrum as observed by Cassini UVIS (Steffl et al. 2004a). The structure of the torus can exhibit strong longitudinal variations, and the relative brightnesses of different regions can vary with time.

Cold Disk Ribbon Warm Torus Europa Orbit

Location (RJ) 4.7 - 5.7 Height 0.2

5.6 - 5.9 ?Width 0.2

6 - 8 9.4

Mass 35 kton1%

200 kton10%

2 Mton90%

~72 kton/RJ

Radial Transport (/RJ)

Inward:months

?? Outward: 20 - 60 days

~20 hrs

Ne (cm-3) ~1000 ~3000 ~2000 ~160

Te (eV) <2 ~5 ~5 ~10 - 30

Hot Electrons ~0 <0.1% 0.2% @ 40eV 2-10% @ 250eV

Ti (eV) <2 20 - 40 20 - 90 50 - 500

Ions S+, O+ O+, S+, S++ O+, S++, S+

S+++, O++O+, S++, O++

S+++ , S+

Table 5 – Plasma Torus Properties (see section 5).

Figure 15 – Models of the Io plasma torus. Top: two-dimensional model of the electron density based on Voyager in situ measurements and distribution along field lines under diffusive equilibrium. Bottom: Cassini UVIS observations and physical chemistry models constrain torus composition (Nerney et al. 2017).

S" S+" S++" S+++"

O" O+" O++"

3.8"0.26" 5.7"

22"1.0"

8.5"

6.8"

9.4"24"

5.3"

23"

7.6"1.9"

11"3.0" 3.5"

0.019"0.60"

22"34"0.35"

12"1.4"46"

1.4"7.7"0.10"6.6"

2.2"0.002"0.27"

Recombina9on"Ioniza9on"(thermal"eA)"Ioniza9on"(hot"eA)""

Charge"Exchange"Radial"Transport"

4.3"

100%=3.1"(10A4"cmA3"sA1)"

Par9cle"Flow"

The"electron"density"is"determined"at"each"itera9on"by"the"quasiAneutrality"condi9on"(assuming"nH+/ne"="0.1):"ne=("nS+"+"2nS++"+"3nS+++""+"nO+"+"2nO++")/0.9"

5.7"

1.4"

1.4"

Figure 16: Flow of (a) mass and (b) energy through the Io Plasma Torus,

applying the physical chemistry model of Delamere & Bagenal 2003 for typical

torus conditions. High charge-exchange rates (McGrath & Johnson 1989) cause

about half (45%) of the material (primarily O) to be lost from the system as fast

neutrals. while the remainder is transported out as plasma. Most of the energy

is radiated away via UV emissions. Input of hot electrons is key for maintaining

the torus.

S" S+" S++" S+++"

O" O+" O++"

Hot""e)"

Core""e)"

0.67"0.05" 1.0"

15"0.67"5.6"

4.5"

0.32"

0.82"

8"1.7"

4.3"

2.7"

0.8"0.20"

1.3"0.34"

0.63"

0.38"0.002"0.066"

2.7"4.2"0.043"

3.8"0.47"15"

0.45"

54"

0.16"0.17"0.95"0.011"0.71"

0.23"0.0002"0.03"

0.86"1.3"

8.5"

1.6"17" 5.2"

0.20"0.57" 0.12"

0.29"

0.35"

0.03"

2.8"

Recombina>on"Ioniza>on"(thermal"e))"Ioniza>on"(hot"e))"Coulomb"Collisions"

Charge"Exchange"Radial"Transport"Radia>on"Ioniza>on"Poten>al"

86"

0.49"

Energy"Flow"

100%=0.27"(eV"cm)3"s)1)"

0.66"

0.14"

Total radial transport = 7.9% Total CHEX = 5.7% Total radiated = 86%

Figure 17 – (a) Ground-based observations of S+ emissions from the torus (Schneider & Trauger 1995). While S+ is a minor constituent of the warm outer torus it dominates the ribbon and the cold inner torus. (b) Voyager PLS measurement of ion species in the cold inner torus. (c) Herbert et al. (2008) re-analyzed Nick Schneider's 1991 observations of the cold torus to derive the 3-D distribution of S+ density shown in (d). The red and green lines in (c) are the centrifugal and magnetic equators derived from the JRM09+CAN magnetic field model of Connerney et al. (2018).

a

d

b

c

Europa

Observed Energetic Neutral Atoms ~100 keV H, O

Figure 18: Europa Neutral Cloud as observed by Cassini's MIMI instrument (Krimigis et al. 2002; Mauk et al. 2004) via very Energetic Neutral Atoms (H, O) at energies of ~100 keV.

Figure 19: Jovian Nebulae of (very) Energetic Neutral Atoms (based on Krimigis et al. 2002). Inserted bottom right is a histogram of counts versus mass/charge summed over days 338-362, 2000, before the Cassini bow shock encounter. Major peaks are identified on the plot but counts have not been normalized, so no relative elemental abundances are implied.

1

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