Multispectral Observations of Jupiter’s Aurora

J. H. Waite\. Jr., D. Grodent2, B. M. Mauk3, T. Majeedl, G. R. GladstoneI, S. J. Boltond, J. T. Clarkes. J.-C.

GCrard2, W. S. Lewis], L. M. Trafton6, R. J. Walkeri, A. P. fngersoll8, and J. E. P. Connemey9

ISouthwest Research Institute, PO Drawer 28510. San Antonlo. TX 782-78-0510. USA

JLaboratoire de Physique. Atmospherique. et Planetaire, Liege, 4000. BELGIUM

3Applied Phvsics Laboratov, Johns Hopkins University. Johns Hopkins Rd.. Laurel, MD 1’0723. USA

4Jet Propulsion Laboratov, 4800 Oak Grove Drive, Pasadena. CA 91109-8099. USA

5Space Phvsics Research Laboratory. Universitv of Michigan. Ann Arbor. MI 48 IO9. USA

blmiveris; of Texas, Mail Code CI400. .4ustin. T,Y 78712. CXA

7fGPP. UAiversitv of California. Los Angeles, 405 Hiigard Ave.. Los Angeles. CA 900241567 USA

%alifornia Institute of Technology. l-700 E. Cal$rnia Blvd.. Pasadena, CA 91125 USA

9NASAlGoddard Space Flight Center. Code 695. Greenbelt MD ._‘077f USA

ABSTRACT

Remote sensing of Jupiter’s aurora from x-ray to radio wavelengths has revealed much about the nature of the jovian aurora and about the impact of ionosphere-magnetosphere coupling on the upper atmosphere of Jupiter. As indicated by the combination of x-ray and ultraviolet observations, both energetic heavy ions and electrons energized in the outer magnetosphere contribute to aurora1 excitation. Imaging with the Hubble Space Telescope in the ultraviolet and with the InfraRed Telescope Facility at infrared wavelengths shows several distinct regions of interaction: 1) a dusk sector where turbulent aurora1 patterns extend well into the polar cap; 2) a morning sector generally characterized by a single spatially confined aurora1 arc originating in the outer or middle magnetosphere of Jupiter; 3) diffuse emissions associated with the lo plasma - spectroscopy has provided important information about the thermal structure of Jupiter’s aurora1 atmosphere and the altitude distribution of aurora1 particle energy deposition, while Lyman alpha line profiles yield clues to the nature of thermospheric dynamical effects. Galileo observations at visible wavelengths on the nightside offer a new view of the jovian aurora with unprecedented spatial information. Infrared observations have added much to the understanding of thermal structure at all latitudes. the dynamics of the thermospheric wind system, and aurora1 morphology, and may hold the key to understanding the role of Joule heating in Jupiter’s thermosphere. ROSAT observations have revealed soft x-ray emissions from Jupiter’s lower latitudes as well as from the aurora1 zones, implying that energetic particle precipitation also occurs at low latitudes in regions magnetically linked to the inner radiation belts. In this review, multispectral observations of jovian aurora1 emissions are presented within a theoretical/modeling framework that is intended to provide some insight into magnetosphere-ionosphere coupling and its effects on the upper atmosphere. 0 2000 (‘OSPAR. Published by Elsevier Science i-k. All righIs reserved.

INTRODUCTION

Imaging and spectral data at wavelengths across the electromagnetic spectrum are the foundation for our understanding of Jupiter’s aurora and provide a global context for the interpretation of data acquired in situ. In the last ten years we have benefited from Hubble Space Telescope (HST) imaging at ultraviolet wavelengths and from infrared imaging with the NASA InfraRed Telescope Facility (IRTF) as we struggle to understand the clues to aurora1 generation mechanisms that are locked in the details of the morphology. With the deployment of the Space

Telescope Imaging Spectrograph (STIS), the combination of sensitivity, signal to noise, and spatial resolution in the ultraviolet regime have improved dramatically. greatly facilitating the use of global images in conjunction with in-situ tields and particles data for interpreting magnetospheric processes. Important complements to such morphological studies are empirical and global magnetohydrodynamic (MHD) models of the jovian magnetosphere. Correlating observed aurora1 morphology with models and in-situ observations appears to hold the key to understanding the magnetospheric processes that drive aurora1 phenomena at Jupiter. These processes are likely to involve the complex interplay of internal plasma sources from the Galilean satellites, free energy from internal rotation of the planet, and coupling with the solar wind and interplanetary magnetic field. X-ray images are thought to be a sensitive indicator of energetic ion (as opposed to electron) precipitation and thus, despite their limited spatial resolution. represent an additional valuable source of information about the magnetospheric processes involved in the generation of aurora1 emissions. Furthermore, the unprecedented spatial resolution of the Galileo images at visible wavelengths has provided a high-resolution indicator of the altitude distribution of the aurora1 emissions and also has confirmed the aurora1 dusk/dawn asymmetry from a night side viewing perspective.

Spectral analysis of aurora1 emissions provides a revealing diagnostic of the huge energy input into Jupiter’s atmosphere from aurora1 particle precipitation, the dominant driver of thermospheric processes on a global scale. Spectral analysis at ultraviolet wavelengths yields valuable information about the precipitation influx as a function of pressure, as well as about the altitude of particle energy deposition with respect to the methane layer in the mesosphere. By contrasting the longer wavelength portion of the H2 far-ultraviolet spectrum with the shorter wavelength portion of the ultraviolet spectrum where methane absorbs (“color ratio”, cf. Yung ef al. (1982)). we obtain a marker of the boundary between the well-mixed and diffusively separated atmosphere. Because of the self- absorption of Hz, comparing Hz emission features at far-ultraviolet wavelengths with those at extreme-ultraviolet wavelengths facilitates probing the atmosphere at different pressure levels. From Lyman alpha line profiles we can derive important information about the altitude at which the aurora1 Lyman alpha emissions are generated and also about thermospheric winds (Prange et al., 1997; Gladstone, et al., 1999; Rego et al., 1999a). Infrared emissions at wavelengths between 7 and I4 microns are diagnostic of CH4 and C2H2 cooling in the mesosphere, while those near 3.5 microns from H3+ are a useful diagnostic of the thermal structure and dynamics of the upper thermosphere and ionosphere (Rego et al., 1999b). These multispectral data can be correlated and analyzed within the context of one-dimensional aeronomical models (Grodent et al., 1999) and by three-dimensional global circulation modeiing (Achilleos et al., 1998) to investigate the effects of aurora1 particle precipitation on the jovian thermosphere.

IMAGING THE AURORA

Jupiter’s aurora emits at wavelengths from the radio through the x-ray. Figure I presents images (from top left to bottom right) acquired at visible wavelengths with the Galileo imager on the nightside; with the ground-based NASA IRTF at near-infrared wavelengths; with the Hubble Space Telescope in the ultraviolet regime; and at x-ray wavelengths with ROSAT. The visible emissions (captured in this image by the Galileo Solid State Imager during the El 1 orbit (Vasavada et al., 1999)) result from energetic particle impact on molecular hydrogen, which produces Balmer series emissions from dissociative excitation of H2 and additional triplet and continuum emissions from H2 (Pryor et al.. 1998). Because of scattering of sunlight in the atmosphere, these emissions are only visible on the nightside of the planet. The importance of this image lies in its unprecedented spatial resolution, which is on the order of 30 km per pixel when a suf‘ficient star field is simultaneously imaged. The fact that the aurora1 arc disappears over the limb allows us to infer an altitude of 245 +30 km (Vasavada et al., 1999) above the 1 bar pressure level for the emission peak, thus clearly improving the absolute altitude scale for the aurora1 emissions, which had been previously determined in the W by Clarke er al. ( 1996) (* 150 km) and Prange et al. (1998) (* 100 km).

The infrared image (Satoh et al., 1996) has a spatial resolution of about 1 .O arcsecond, which corresponds to about 3500 km on the surface. A narrow band filter was used in this observation to isolate a portion of the H3+ ro- vibrational band emission in a spectral region where methane absorption in the lower atmosphere provides a dark contrast. The emission intensity scales with increases in both the column density of H3f and in the temperature of the thermosphere. H3+ column density in the aurora1 zone is closely correlated with the total energy deposition of energetic particle influx as a result of impact ionization of H2. However, the temperature of the thermosphere is intluenced by both Joule heating and dynamics as well as by particle impact. It should also be noted that the H3+ temperature depends on the altitude of the emission peak, which differs inside and outside of the aurora1 region, and depends on the energy of the primary particles or solar EUV photons (e.g., Achilleos et al.. 1998).

Ultraviolet emissions (the HST STIS image in the lower left panel (Clarke et al.. 1998a)). on the other hand. are a direct indicator of particle impact excitation of Hz Lyman and Werner bands, triplet and continuum emissions from Hz, and Lyman alpha dissociative excitation of Hz. The spatial resolution of STIS is less than 0.1 arcseconds (350 km on the surface). As will be discussed below, complex patterns observed in the high-latitude emissions in Jupiter’s aurora are important clues to the nature of the magnetospheric processes that drive the aurora.

Finally, the x-ray emissions imaged with ROSAT (Gladstone ef al., 1998) are thought to be line emissions produced by the interaction of precipitating energetic sulfur and oxygen ions with Jupiter’s neutral atmosphere (Cravens. 1995). (Although electron bremsstrahlung remains a possibility, aurora] energy requirements (Metzger ef al.. 1983) and theoretical and modeling studies (Waite et al., 1994; Cravens, 1995) favor ion precipitation as the most plausible mechanism responsible for producing the jovian x-ray aurora. It is expected that AXAF observations will permit the conclusive identification ofthe particles responsible for the x-ray emissions.) Thus, in contrast to the UV emissions, which are excited by both energetic ions and electrons, the jovian x-rays-if not bremsstrahlung+an provide an unambiguous signature of ion precipitation. Spatial resolution at x-ray wavelengths is currently limited to 5 arcseconds (corresponding to a distance on the surface of 12,000 km), thus limiting our ability to relate regions of ion precipitation to specific magnetospheric source regions. However, identification of an ion-precipitation-induced aurora1 emission, particularly when correlated with observations of diffuse ultraviolet emissions, will provide information about the relative roles played by pitch-angle scattering of energetic ions and electrons in the generation of the jovian aurora.

Because of the strong influence that Jupiter’s intense magnetic field has on the motions of the particles within it. multispectral images of Jupiter at high latitudes offer a valuable means of relating emission regions and patterns to magnetospheric source regions and processes. Energetic ions and electrons are free to travel along field lines, but require electric fields (i.e.. from planetary rotation, pickup processes. or the solar wind interaction) and/or gradient and curvature drift in the magnetic field (which becomes more important for very energetic particles) to move in a direction perpendicular to the magnetic field. Thus, given accurate knowledge of the three-dimensional magnetic field topology, the surface of the planet can be used as a “television screen” from whose images processes taking place in the magnetosphere can be deduced. However, there are two limitations to this approach: 1) the electric and magnetic fields are not perfectly known owing to the lack of measurements near the planet and of temporally varying currents in the system, such as the IO plasma torus current sheet and the field-aligned currents presumably associated with some discrete aurora] features; and 2) the higher the latitude, the more tightly spaced the footpoints of the magnetic field lines become, and thus higher spatial resolution is needed to distinguish morphological features that map into the outer magnetosphere (>30 RJ). The present state of the art in magnetic field mapping is described by Connemey er al. (l998), who discusses the origins and limitations of the two field models (06 and VIP4) presently in use. The VIP4 mode1 has been improved over the 06 model by the use of HST and IRTF images of the latitude track of the IO flux tube (IFT) as a fiducial point (Comremey et al., 1998). This guarantees better agreement in the field line mapping to the region of the IO orbit, but may not improve the fits for more distant regions (i.e., near the 30 RJ limit of the field model) or at low magnetic latitudes. However, magnetic field models in the outer magnetosphere have been greatly improved using the multiple orbits of the Galileo data set (cf. Khurana, 1999).

Using the 06 magnetic field model, Satoh et al. (1996) found that the main H3+ aurora] oval generally coincides with the footprint of field lines mapping to 30 RJ. More recently, Satoh and Connemey (1999) used improved images to again study the aurora1 morphology. They find the most intense emissions between 12 and 30 RJ representing the main aurora1 oval emission. Similarly, recent WPPC2 images of jovian ultraviolet aurora] emissions suggest that the main oval generally conforms to a reference oval that maps to 30 RJ in agreement with the analysis of FOC FUV observations by Prange et al. (1998). Although, as Clarke et al. (1998b) point out, mapping to magnetospheric regions as close in as I5 RJ could account for some of the emissions. In contrast, the aurora1 oval observed at visible wavelengths with the Galileo SSI appears to map to 13 RJ (Ingersoll et al., 1998). However, a more complete analysis using the complete aurora1 imaging data set of Galileo SST shows a main oval that varies from 13 to 30 RJ depending on the magnetic longitude, but closely paralleling the day side reference oval derived from WFPCZ images (Clarke et al.. 1998b) between the image longitudes of 155 to 210 degrees west longitude, even where they both diverge from magnetic field model contours. Thus, the HST FOC, WFPC2, and ground-based H3+ images show similar primary arc locations (Prangt et al., 1998; Gerard et al., 1994; Grodent et al., 1997; Satoh et al., 1996; Satoh and Connemey, 1999). The poleward deflection of the primary arc at longitudes ~155 degrees http is seen in some HST images and may be the “transpolar emission” noted in some studies (Gerard et al., 1994; Prange et al.. 1998). The visible counterpart to this arc appears to be located at slightly lower latitudes between 150 and I90 degrees longitude. As noted above, the spatial resolution of the x-ray images is not adequate

to permit the high-latitude emissions to be unambiguously associated with a particular magnetospheric source region; however, recent analysis of Voyager LECP hot plasma data suggests that the generation region for the ion aurora lies at L < 12 RJ, with the corresponding latitudinal ordering of the x-ray emissions (Mauk er al., 1996). In addition to the high-latitude x-rays, there are distinct x-ray emissions near the equator. These are believed to result from inner radiation belt precipitation (Waite et al.. 1997); corresponding emissions in the ultraviolet are not detectable due to solar scattered light at these wavelengths. Infrared emissions also show some equatorial brightening (Miller et al.. 1997).

As the above summary indicates, because of uncertainties in both the observations and the magnetic field models, many questions remain to be answered about the mapping of Jupiter’s aurora1 emissions to particular magnetospheric source regions. There is, in addition, the interesting question of the apparent difference between the recent characterizations of the aurora based on HST and ground-based imaging and the characterization that was obtained from Voyager UVS measurements. That is, the Voyager investigators (Broadfoot et al., 1981; Herbert et al.. 1987) concluded that the intense aurora mapped along quasi-dipolar magnetic field lines to equatorial regions just beyond the IO torus (7-12 RJ), although they do not rule out emission polweard of 12 RJ due to the limited spatial resolution of the UVS spectrograph slit. The emissions were attributed to precipitation of inward diffusing hot ions (Thome, 1982; Gehrels and Stone, 1983). In contrast, as we have seen. recent images (with the exception of the Galileo SSI images) suggest that the aurora maps predominantly to magnetodisc regions out to 30 RJ, but certainly beyond 12 RJ. The differences between the Voyager and more recent observations of aurora have typically been accounted for in terms of the limitations of the Voyager measurements (Connemey et ~1.. 1993; Clarke et al.. 1996; Satoh et al., 1996).

There is another possibility, however. Recent comparison of Galileo EPD data with Voyager LECP data suggests that an epochal change in Jupiter’s magnetosphere may have occurred between the time of the Voyager observations and the times of the more recent imaging results (Mauk et af., 1998a). In particular, it has been discovered that the hot plasmas that constitute the ring current populations just outside the IO torus have been significantly depleted (factor of 5 in energy density) between the time of the Voyager 1 (Broadfoot et al., 1979) encounter (1979) and the time of the Galileo encounter (1995) of the IO torus regions. Since this hot plasma population is the very one thought by Voyager researchers to be responsible for the intense aurora1 emissions, it is natural to speculate that this change in Jupiter’s magnetosphere may be at least partly reflected in the changing conclusions regarding the magnetospheric source regions and charged particle populations responsible for Jupiter’s more intense aurora1 emissions with the former Voyager era more dominated by energetic particles and the present aurora1 conditions of the Galileo era more dominated by field-aligned electron acceleration.

The extensive data sets of aurora1 images at both infrared and ultraviolet wavelengths continue to provide our best source of information about aurora1 morphology and its temporal variability. This point is well illustrated by the three STIS images of Jupiter’s northern aurora in Figure 2. The three images are taken in the bandpass 130 to 170 nm and represent three different central meridian longitude (CML) positions. From these images it appears that the variation in the aurora as a function of planetary rotation is predominantly due to the geometric viewing effects imposed as a result of the 1 l-degree offset between the rotation axis and the axis of symmetry of the magnetic field about which the aurora1 oval is generally centered. As this figure clearly shows, the dawn flank of the aurora1 oval appears to be largely a discrete arc, while the dusk flank is dominated by a large diffuse emission feature that extends into the polar cap. Similar morphological features are revealed in images of FUV emissions acquired with the HST FOC (Prange et al.. 1998) and Galileo images from SSI (Vasavada et al., 1999). In the SSI study the primary arc between hIII 150 and 190 was imaged as it rotated from dusk to dawn. The morphology changes from a wide, multiply-branched arc at dusk to a single thin arc at dawn. This result is completely consistent with the imaging results from the dayside and clearly suggest that the features are fixed in solar local time and do not rotate with System III longitude.

Three studies have been carried out to simulate the aurora1 forms with simple multi-component models of the aurora. The first such study was performed by Satoh er al. ( 1996), who used a general inverse methodology applied to individual pixel intensities in the sequence of infrared images of H3+ emissions to construct a model of the aurora (Figure 3). The base function is an aurora1 oval that maps to 30 RJ using the 06 magnetic field model. The model includes a combination of an intensity anomaly fixed in System III longitude and an enhancement in the afternoon sector fixed in local time. In addition, there is a region of weaker emission outside the aurora1 oval that extends to both lower ad higher latitudes at all longitudes. This combination of features gives the best agreement between the simulation and the observed images.

J. H. Wait, c’, r,/

Fig 2. Three views of Jupiter’s northern aurora acquired in 1997 and 1998 with the HSTiSTIS UV-MAMA p F25 SRF?. The bandpass is -130-170 nm. The STIS point spread function is -0.08 arc sec. which is equivalent lo

Oil Jupiter. The STIS spatial resolution is similar to that of WFPC 2 (although WFPC 2 under samples the pail fun, ction) and roughly two times better than that of the FOC. STIS exposure times were generally shorter than WF ‘PC2 (typically 400-700 s) and FOC (typically 700- 1000 s), thus reducing the effect of blurring due to planetar) and allowing the detection of faint or previously unresolved emission features. The effect of planetary rotation on ri can be seen by comparing the top panel (exposure time = 100 s) with the bottom panel (exposure time = 600

exp osure time for the middle image was 300 s.)

llus filter -300 km It spread those of

f rotation esolution s). (The

Fig. 3. Model of Jupiter’s northern H3+ aurora. showing a broad, diffuse emission enhancement in the polar cap and a bright arc extending from midnight to noon along the aurora1 oval and slightly equatorward of it. The diffuse polar cap emission is fixed in local time (LT); the bright arc is fixed in System III (S3) longitude. Also shown in the upper right is a comparison of the model with the observed H3+ aurora and, in the upper left, a schematic illustrating the principal emission features. These include, in addition to the main oval, bright arc. and polar cap emissions, a “collar” of diffuse emissions extending equatorward from the main oval approximately to the IO footprint. (Figure adapted from Satoh et al., ( 1996).)

HST WFPC2 images of ultraviolet emissions in the northern aurora1 zone have been modeled by Grodent et al.

( 1997) (Figure 4). The spatial resolution in these images is higher than that in the IRTF images analyzed by Satoh et af. (1996) and the more detailed emission structure that they require reveals additional complexity in terms of selecting the placement of diffuse regions and arc structures to match the four WFPCZ images shown in the figure. In addition, the low signal to noise of the images is not amenable to the generalized inverse method, thus introducing greater subjectivity into the analysis. Despite some differences, the two models nonetheless yield similar pictures of jovian aurora1 morphology. Grodent et al. and Satoh et al. conclude that in addition to a slight shift from the nominal positions of the 06 plus current sheet model with a nominal mapping to 30 RJ, a better fit is obtained by adding two anomalies. The main one is a thin arc fixed in System III longitude with a maximum brightness near 230 degrees, which is closely correlated with a “morning” (as viewed at CMLs between 115 and 220 degrees, which are those best seen by Earth based observation due to the magnetic field tilt with respect to the rotation axis) arc (ARC3 in the nomenclature of Grodent et al.) that appears in many of the Grodent et al. model tits. The second feature is a weaker diffise anomaly fixed in local time in the afternoon sector and generally associated with “afternoon” arcs (ARCl, ARC2) and a localized diffise emission zone (GLOW2 in the nomenclature of Grodent et al. ).

Fig.

Grodent et al., 1997

4. WFPCZ images of Jupiter’s northern ultraviolet aurora (A. D, G. J) compared with model-generated images (B. E. H. -

K). (The numbers in the upper right-hand comer of the WFPC2 images refer to the HST file designatton.) The model, which is described by Grodent er al. (1997), reproduces the observed features quite well. The four sketches on the right (C, F, I. L) illustrate schematically the different features and structures incorporated in the model to reproduce the observed aurora1 morphology. The “morning” arc referred to in the text (ARCI) is in red; the two “afternoon” arcs (ARC], ARC2) are in yellow and green. The colored bar graphs inset in each sketch show the intensity of each emission feature, and the straight red line indicates the CML. (From Grodent et a/.. 1997.)

,Multisprctral Obwrvatmn\ of Jupiter’s Aurora 14hl

More recently, Satoh and Connerney (1999) have revisited the aurora1 morphology using improved imaging. They find the most intense emission is found between the 12 RJ and 30 RJ ovals. Longitudes of the peak emission (h[II-260” in the north and -1 30° in the south) coincide with those of the minimum surface field magnitude in the VIP4 model. Rapid pitch angle scattering of aurora1 particles in the source region may be responsible for this similarity. This zone accounts for approximately 25% of the total aurora1 emission. Significant emission also exists between the 8 RJ and 12 RJ ovals. Longitudes of the peak emission @III-215” in the north and -25” in the south) are in good agreement with those of Satoh et al. ‘s (1996) System-III anomalies. This indicates the existence of relatively slow pitch angle scattering of drifting electrons and consequent “windshield wiper” effect in the corresponding magnetosphric source region. This zone accounts for approximately 20% of the total aurora1 emission. Only weak emission, except that confined to the instantaneous foot of the IO Flux Tube, is found between 6 RJ and 8 RJ (approximately 10% of the total aurora1 emission). This result is consistent with Mauk et al. ‘.s ( 1996) analysis of the Voyager 1 LECP instrument data, in which the intense aurora due to electron precipitation is predicted outward of 12 RJ radial distance. Local time brightening is found for emissions poleward of the 12 RJ oval. More studies are needed to understand the behavior and/or the origin of the emission patches seen within the polar cap.

The morphological features revealed in recent FOC (Prangt et al., 1998), WFPC2 (Clarke et al., 1998b), and STIS images are generaily consistent with the models based on the analyses of the infrared and earlier WFPC2 ultraviolet images. The general components of the evolving model as described in this paper and indicated in Figure 5 are: 1) an aurora1 oval with a boundary that magnetically maps to lo-30 RJ in the magnetosphere (Vasavada et al., 1999; Clarke et al.. 1998b; Satoh et al., 1996), 2) a local time enhancement in the dusk sector that is diffuse and extends well into the polar cap (Vasavada et al., 1999; Clarke et ai., 1998b), 3) a bright arc in the dawn sector (Clarke er al., 1998b; Vasavada et al., 1999) that is enhanced by a series of bright storms that periodically appear in the morning sector (at a longitude of 200 to 260 degrees) and disappear within a Jupiter rotation (<IO hrs) (i.e., Gerard et al., 1994; Ballester et al., 1996; Clarke et al., 1998b), 4) an IO hot spot with a trailing tail (Prange et al., 1998; Clarke et al., 1998a, b), and 5) a diffuse region between the oval and the latitude that maps magnetically to IO’S orbit time (Satoh ef al., 1996; Prange el al., 1998; Clarke ef al., 1998a). Recent STIS images (cf. Figure 3) suggest that this region of diffuse emission occasionally brightens with some combinations of SIB longitude and local time and may be related to changes in the plasma wave activity in the IO torus (Rezeau et al., 1997). The STIS images also show, in addition to IFT-related emission, emissions associated with the footprint of field lines mapping to the orbit of Ganymede (Clarke et al., 1998a).

Although the large-scale dynamics of Jupiter’s magnetosphere appear to be substantially dominated by planetary rotation out to distances as far as 125 RJ (Kane et al., 1995), anti-corotational convective flows resulting from solar wind coupling have been deduced from in-situ measurements (Cowley et al., 1993; 1996; Hawkins et al., 1998); and various phenomena have been identified that show--or may show-evidence of solar wind control. These phenomena include radio emissions at hectometer (Desch and Barrow, 1984) and decimeter (Bolton ef al., 1989) wavelengths and increases in the brightness of the H3+ aurora that have been correlated with increases in solar wind dynamic pressure (Baron et al.. 1996). Enhancements in the intensity of ultraviolet aurora1 emissions, such as that reported by Prange et al. (1993) and Gerard et al. (1994), may also be evidence of solar wind influence; however, interpretations of the I-IV brightenings in terms of internal magnetospheric processes are equally (Prange et al., 1993 )--or more (Ballester et al., 1996; Clarke et al., 1998b)-plausible.

The organization of aurora1 emissions in System III longitude and local time and their temporal variability furnish important clues as to the configuration and dynamical behavior of Jupiter’s magnetosphere. However, as indicated by the uncertainty in the interpretation of the ultraviolet aurora1 brightening, it can be difftcult to determine whether aurora1 features, such as those summarized above, are to be understood as signatures of internal, rotation- driven processes or aa the effects of solar wind influence. Extended temporal coverage at multiple wavelengths and with high spatial and temporal resolution permits longitude dependencies indicative of rotation-controlled processes to be distinguished from local time effects and episodic phenomena that are perhaps indicative of solar wind-controlled processes. To successfully interpret the emissions as signatures of large-scale magnetospheric processes, however, it is necessary to obtain simultaneous or near-simultaneous in-situ data on the solar wind upstream of Jupiter and, ideally, within the jovian magnetosphere as well. For example, the availability of Ulysses solar wind measurements permitted correlation of brightness enhancements of the H3+ aurora with increased solar wind ram pressure (Baron et al., 1996) and the tentative association of an intensification of the UV aurora with a CME and its associated shock (Prange et al., 1993). Science planning is currently under way for the Cassini flyby of Jupiter in December 2000, which will afford a unique opportunity for coordinated in-situ solar wind measurements with the Cassini fields-and-particles instruments and observations of aurora1 morphology with the

------ Evening Enhancement/ Polar Cap Arcs

- Foot of IFT (5.9 RJ)

Dawn _f Enhancement

Ion Aurora <12 RJ

Fig. 5. Conceptual illustration showing the main features of Jupiter’s northern ultraviolet aurora.

HST, AXAF, IRFT, and other Earth-based telescopes. Not only will solar wind data be obtained during the encounter, but the Cassini photon imaging experiments (WE, KS, and VIMS) will also acquire spectral and morphological data on jovian aurora1 emissions at local times not observable from Earth (e.g., on the night side) and simultaneous imaging from the Cassini spacecraft and Earth will provide a stereoscopic view of Jupiter’s aurora that will help distinguish local time from longitudinal effects; a process that began using the Galileo SSI data set by Vasavada et al. (1999). In addition, Cassini’s MIMI investigation will image energetic neutral atom (ENA) emissions From the inner and middle magnetosphere (Mauk et al., 1998b), which can be used to monitor the charge exchange of energetic ions in the 10 plasma torus region which will also monitor the energetic ions streaming in from the outer magnetosphere and its interaction with the hot plasma in the torus. Cassini ENA imaging can be correlated with solar wind data and with aurora1 imaging to study the response of Jupiter’s inner and middle magnetosphere (5-30 RJ) to changes in the interplanetary environment and to relate this response to variations in the intensity and distribution of the aurora1 emissions.

Valuable tools for the interpretation of the aurora1 emissions as diagnostic signatures of magnetospheric processes are provided by empirical models of magnetospheric plasma distributions and flows (e.g., Cheng and Krimigis, 1989; Cowley et al., 1996; Hawkins et al., 1998) and by global MHD simulations of magnetospheric dynamics (Ogino et al., 1998). To illustrate the utility of such models in elucidating the relationship between global magnetospheric dynamics and the distribution and variability of the aurora1 emissions, we show in Figure 6 representative results of a recent MHD simulation by Ogino, Walker, and Kivelson of the solar wind interaction with Jupiter’s magnetosphere. Both panels show plasma flows and pressures in the equatorial plane for a nominal

(T=625 hours, pv2=0.18 nPa)

Northward IMF

-240

Southward IMF

120 0 -240

WV

Fig. 6. Flow patterns and plasma pressure in the equatorial plane of Jupiter’s magnetosphere under conditions of both northward and southward IMF. Solar wind speed and pressure are 300 km s“ and 0.18 nPa, respectively. Comparison of the northward IMF case with the southward case illustrates the effect of solar wind coupling on the magnetospheric flow pattern. (Because Jupiter’s magnetic tield is directed southward, merging is expected to occur when the solar wind has a northward Bz component. At Earth. the situation is reversed.) The MHD model that generated these plots is described by Ogino et al. (1998).

1464 J. H. Waite et rrl

solar wind speed of 300 km s-l and dynamic pressure of 0.18 nanopascal. Plasma pressures are indicated by the color contours; the arrows indicate ion flow velocities. The two panels illustrate magnetospheric flow patterns under conditions of both southward (bottom panel) and northward (top panel) IMF. When Bz is southward (i.e., aligned with the jovian magnetic field), reconnection does not occur, and the flow is rotationally dominated out to 160 RJ. In the case of northward IMF, however, reconnection will occur and the model reveals significant modification of the flow pattern, with vertical motions in the afternoon sector and along the duskside flank of the magnetotail, a stagnation region on the dawn flank, and sunward convection with strong shears at the dayside magnetopause boundary. It is plausible (as indicated by field line mapping with the MHD model of Walker (private communication, 1999)), that these features are linked to the afternoon brightenings that show a diffise breakup, the occasional bright morning storms, and the diffuse emissions in the dusk sector polar cap region, respectively. In addition, the corotation shear is different for the case of northward Bz occurring in the afternoon and nightside near 20 RJ and moving out to the magnetopause boundary on the dayside.

The MHD model of Ogino et al. (1998) does not include the region of the IO plasma torus, which is a source region for the discrete emission at the magnetic footprint of the IO flux tube (IFT) and the associated faint trailing emission, both of which can be clearly seen in the top and bottom STIS images presented in Figure 2. While the discrete feature clearly results from particle precipitation along field-aligned currents driven by the interaction of IO’S ionosphere with the plasma torus (cf., Connemey et al., 1993), the trailing emission has not been explained. It may result from the scattering of ions produced by plasma waves generated in the ion pickup process.

Another aurora1 feature occurring at latitudes that map to the IO torus region is the diffuse emission region evident in the bottom panel of Figure 2 between the main oval and the IFT footprint and its trailing emission. Two brighter emission patches can be seen within this region, and the entire feature is associated with a broadening of the main oval in the afternoon-dusk sector that extends into the polar cap. It is possible that the occasional brightenings observed within the diffise emission region are related to dynamic charged particle events such as those observed with the Galileo EDP detector in Jupiter’s inner magnetosphere (Mauk et al., 1997; 1998~). These events are inward radial injections (1 to several RJ) of hot plasmas (>20 keV ions and electrons) over azimuthal sectors that span several degrees to >60 degrees. While the injections are observed most prominently at -1 I- 12 RJ, the distribution of observed events is broad, extending from ~9 RJ to -27 RJ, and encompasses at least a portion of the regions thought to map to the most intense aurora1 emission ring. At Earth, such injections are accompanied by enhanced precipitation of charged particles into the upper atmosphere. Thus the injections detected with the EDP could be related to some of the dynamic variability observed in the diffise emission region equatorward of Jupiter’s main oval (thought to be caused by ion precipitation (Mauk et al., 1996)). They may also be related to some variability observed within the main aurora1 oval itself (thought to be caused by electron precipitation). There appears to be no local time preference for the occurrence of the jovian injection events, which are probably not coupled directly to variations in the solar wind.

ANALYSIS OF AURORAL SPECTRA

In the previous section we indicated that multispectral observations of aurora1 morphology can be interpreted in the context of in-situ solar wind and magnetospheric plasma measurements and in the context of models of large-scale magnetospheric plasma flows to establish the relationship between global dynamics of Jupiter’s magnetosphere and the observed emission patterns and variability. In this section, we show how aurora1 spectra provide information about the identity of the precipitating energetic particles and are used to constrain models of the thermal structure of Jupiter’s aurora1 atmosphere.

One of the key outstanding questions about Jupiter’s aurora is the identity of the particles. In the case of Earth, aurora] emissions are excited principally by precipitating electrons, which generate both discrete and diffuse emissions. Discrete emissions are caused by energetic electrons accelerated as the consequence of field-aligned currents and the need to maintain current continuity while at the same time communicating momentum transfer over large distances between the ionosphere and magnetosphere (Knight, 1973); diffuse emissions result from the pitch-angle scattering of electrons in the near-equatorial regions. Diffuse emissions are also produced in regions that map to the radiation belts by energetic ions scattered by wave-particle processes into the atmospheric loss cone.

Both electrons and ions are thought to be involved in the generation of aurora1 emissions at Jupiter; the question is: “what are the dominant processes and where do they occur. 3” Strong radial changes in the phase space distribution of heavy ions observed by Voyager indicated that energetic ions are lost inside 12 RJ as they move inward from the

Multispectral Observations of Jupiter’!, Auroru I465

outer magnetosphere. This was the first suggestion of heavy ion precipitation as a mechanism for the production of the jovian aurora (Gehrels and Stone, 1983). The energetic ions are composed of oxygen and sulfur, which presumably originate in the IO plasma torus, are convected outward and energized in the outer jovian magnetosphere (see, for example, Goertz (I 978); Pontius and Hill (I 989); Cheng (1990); Barbosa (I 994)). and then diffuse back inward to the vicinity of the outer IO plasma torus, where they may be scattered into the atmosphere by wave-particle processes (cf. Thome, 1983); a process observed on at least one occasion by Ulysses (Rezeau et al., 1997). Further evidence for the role of heavy ion precipitation in the generation of Jupiter’s aurora was furnished by the discovery of soft x-ray emissions from Jupiter’s aurora1 regions (Metzger et al.. 1983). Metzger et al. argued on the basis of production efftciency that the soft x-rays originated from heavy ion precipitation and not from electron bremsstrahlung. This conclusion was supported by an analysis of ROSAT PSPC observations of the x-ray spectra (see Figure 7) that demonstrated that line emission spectra from S and 0 give a better fit to the x-ray spectra than the continuum spectrum of electron bremsstrahlung (Waite et al.. 1994).

This result should be robustly confirmed by AXAF observations of Jupiter in 1999. As suggested by the Voyager energetic heavy ion and hot plasma measurements (Gehrels and Stone, 1983; Mauk et al., 1996), heavy ion precipitation may be predominantly confined to latitudes that map to magnetospheric regions near 12 R J.

Energetic heavy ions should produce characteristic emissions in the ultraviolet portion of the spectrum as well at x- ray wavelengths. However, all attempts to detect such emissions to date have been unsuccessfd. We illustrate this point with a high-resolution spectrum near the expected 01 1304A emission (see Figure 8). We have superimposed

ROSAT PSPC ENERGY SPECTRUM

L --

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0 20-- 0

10 ‘--

-I I 1 T

BEST TWO LINE MODEL FIT 0.01 keV WIDTH AT 0.2 keV 0.7 keV WIDTH AT 0.9 keV x2 = 17

- - - - BEST BREMSSTRAHLUNG FIT T = 0.47 keV ;c2 = 35

10

PHOTON ENERGY (KeV)

Fig. 7. Plot showing best tits of two model energy spectra to the jovian x-ray spectrum measured with the ROSAT PSPC (Waite er al., 1994; Cravens et al., 1995). The fit for the two line (S+ and O+) model is better than that of the bremsstrahlung model, supporting the view that precipitating heavy ions are responsible for the jovian x-ray aurora.

c3 E m

5,000

GHRS .--___ Propagated Error . . . . . . . . . . . . . . . . . . Model H2 Fit

-.-._._ Model 01 130.4 nm Profile (12.9kR; 1 .O nm)

129 130

WAVELENGTH (nm)

131 Trafton et al., 1998

Fig. 8. Plot of emission intensity vs. wavelength for H-J band emissions. The observed spectrum is plotted as a solid line and compared with two model spectra, a best-fit band spectrum for emissions excited by energetic electron impact (dotted line) and 01 130.4 nm signal expected from O+ precipitation (dot-dashed) line. The absence of the 130.4 nm feature in the observed spectrum indicates that precipitating oxygen ions do not contribute significantly to Jupiter’s ultraviolet aurora. (From Trafton er al., 1998.)

over the H2 Lyman and Werner band emissions a model feature that represents the intensity of the 01 1304A emission line expected from energetic ion precipitation. To generate that feature, we assumed that the observed H2 band intensities were produced by primary ions and resulting secondary electrons created by a purely heavy ion aurora. The model feature should clearly be seen in the spectrum. It is not, however, demonstrating that the aurora1 ultraviolet emission cannot be explained by ion precipitation alone. Electron precipitation must also be involved, in addition to the ion precipitation suggested by both sot? x-ray observations and in-situ energetic ion observations. By our estimates, which are conservative, electron-induced emissions dominate at least 2 to 1 for the region of the main oval and poleward of that boundary (the region so far sampled in our spectral analysis, Trafion ef al., ( 1998)). Although, the reader should be cautioned that an exhaustive study of all aurora1 regions has not been carried out to date.

Another major question that spectral data can help answer concerns the thermal structure of Jupiter’s high-latitude upper atmosphere. The temperature structure of Jupiter’s equatorial thermosphere has been determined from temperature and pressure data acquired by the AS1 experiment on the Galileo Probe (Seiff et al., 1998). Unfortunately, however, there have been no corresponding in-situ measurements at high-latitudes. Thus our understanding of the structure of Jupiter’s auroral-zone atmosphere is heavily dependent upon atmospheric models. for which aurora1 spectral data provide constraints regarding heating and cooling rates as well as constraints on the altitudes at which the precipitating particles deposit their energy (and, hence, on the energy of the particles). Methane and acetylene as well as other hydrocarbons form a layer at the base of the thermosphere: the altitude distribution of these species delineates the atmospheric mixing layer boundary in the upper atmosphere (methane is

heavier than H2, and its abundance falls off rapidly with altitude above the mixing layer boundary). The hydrocarbons are strong infrared coolers that can radiate away the immense heating produced by aurora1 particle precipitation. In the upper thermosphere. the only infrared cooler is the ionospheric constituent H3+. Because the density of H3+ is relatively low (The observed H3+ emissions account for only a few percent of the input power needed to explain the ultraviolet emissions (Grodent et al., 1997)). Most of the heat deposited by aurora1 particles must be ultimately conducted downward into the methane layer to cool to space, thus producing a hot exosphere whose temperature is controlled by the distance between the aurora1 heating layer and the altitude of infrared coolers. Some of the aurora1 heat is redistributed by thermospheric dynamics (Waite et al., 1983, Achilleos et al.. 1998), but ultimately the heat must be conducted down into the methane layer to be lost to space, as is indicated by the huge emission flux of the observed methane infrared features (Drossart et al., 1993).

We can use aurora1 emissions at different wavelengths to probe the different regions of the atmosphere. Infrared CH4 and C2H2 emissions dominate near the steep thermocline. where the downward-conducted heat comes into contact with the increasing densities of the hydrocarbon coolers. Voyager IRIS observations, recently reanalyzed by Drossart et al. (1993), set the slope of thermal gradient in the thermocline at values exceeding 1K per km. Kostiuk et al., (1993) get similar results with the differences being indicative of variability and our present level of uncertainty. Above this altitude, the region of maximum heating from aurora1 particle deposition is also the region of maximum H2 band emission (cf. Waite et al.. 1983). CI;ven the atmospheric temperature structure inferred from the IRIS data. the relative altitude of the H2 emitting layer- and thus the energy of the incoming particles-can be estimated by determining the H2 ro-vibrational temperature. According to the analysis of Kim et al. (1997), uncertanties in the methane structure in the aurora1 region result in altitude errors of 50-80 kms and to uncertainties in temperature of 50 to 150K depending on the signal to noise ratio of the spectra. Nonetheless, they are an important diagnostic tool of the thermal structure as a function of pressure (altitude) in the aurora1 region pending a future in situ study by an atmospheric probe. The bottom panel of Figure 9 shows a fit between a Werner band spectrum obtained with the HST GHRS and a synthetic spectrum used to determine the H2 ro-vibrational temperature (Trafton et al., 1998). The excellent fit obtained between the observed and synthetic spectrum indicates that the calculated ro-vibrational temperature used in modeling the H2 emission is reasonable and thus can be used with confidence in modeling the structure of Jupiter’s aurora1 atmosphere. Although there is significant uncertainty associated with the fits, the 90% confidence interval is smaller than the observed variations in time and space (Trafion ef al., 1994, 1998; Kim et al., 1997). The upper panel of Figure 9 shows a plot of aurora1 emission intensity as a function of H2 temperature, the values for which range from 200 to 1000 K, with an average of 400- 600 K. According to Kim et al. (1995; 1997), the brightest emissions appear to occur at the lower temperatures, which led them to speculate that the most intense UV auroras occur at lower altitudes (i.e., where the H2 temperature is lower). However, the statistics are very poor (as can be seen in the figure) and an alternative explanation has been suggested by Trafion et al. (1998), who note that cooling by aurorally generated C2H2 could also account for the low temperatures associated with the bright emissions. Finally, the H3+ temperature can be used to probe the upper thermosphere. Fits by Drossart et al. (1993) of CFHT FTS data indicate that the ro- vibrational determination of the H3+ temperature and the kinetic temperature are the same. Thus H3+ is hot, near IOOOK at its peak of emission, and appears to be nearly in local thermodynamic equilibrium (LTE), even though non-LTE effects are important (Kim et al., 1992).

One final piece of the vertical structure puzzle is provided by recent determination of the altitude of the visible aurora1 emissions from Galileo SSI observations (Vasavada et al.. 1999). Because of the instrument’s excellent resolution and the fortuitous passage of the aurora1 arc over the limb, an absolute altitude of 245 +30 km for the emissions above the 1 bar pressure level was determined from the observations. This puts additional constraints on atmospheric models that are difficult to reconcile with the observations at other wavelengths, which indicate that the emission layer is located at higher altitudes (cf. the analysis of IDE data by Gladstone and Skinner (1989), which places the H2 band emission peak at -290 km (for 95 keV electrons), or Clarke et al. (1996), who cite an altitude of 400-700 km for the base altitude of the FUV emissions, or Prange et al. (I 998) who cite a range of altitude for different features). The differences observed are partially due to variations in the aurora as can be seen in the comparison of visible limb images observed by Vasavada et al. (1999). However, it is also true that Galileo’s unique viewing geometry allowed the emission altitude to be determined with greater accuracy than is possible with the Earth-based observations. The latter explanation is a valid point given the WFPC2 spatial resolution of -300 km. Vasavada et al. (1999) report consistent emission peak heights of 245 km, but observe a variance in the full width of the emission intensity at half maximum (between 120 and 460 km-depending on the image) with an extended skirt in at least one case that extends to >3500 km.

146X J. H. Waite YIN/.

300 I S 1 I I ” I I “I 1

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-

200 - +I-

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0 I I I , I I L 1 I I I I I1

0 200 400 600 800 1000

Temperature (K)

2000

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1252 1258 1264 1270 1276

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Fig. 9. Plots of wavelength vs. intensity for H2 Werner band emissions (bottom panel) and of temperature vs. intensity for Lyman and Werner band emissions (top panel). In the bottom panel, intensity as observed with the HST GHRS is compared with a synthetic spectrum. The goodness of tit indicates that the H2 ro-vibrational temperature is well determined. In the top panel are data from GHRS observations in 1993 (triangles) and 1995 (diamonds) by Kim et al. (1995; 1997). and in 1997 (squares) by Tratton et al. (unpublished data). A statistically significant correlation of emission intensity with temperature is not apparent.

We are currently revising our one-dimensional model of Jupiter’s aurora1 atmosphere in an attempt to achieve a more consistent tit with the constraints imposed by both the multispectral observations of jovian aurora] emissions and recent in-situ Galileo data on energetic electron distributions in the inner jovian magnetosphere. The principal components of the model are a neutral temperature profile derived from the Galileo ASJ data used only as an internal condition (Seiff et al., 1998); a one-dimensional thermal conduction model that includes particle heating and CHq, C2H2, and H3+ cooling; and a coupled, two-stream electron transport model used in determining the heating rates. (For a description of the basic thermal conduction model, see Waite et al. ( 1988) and Grodent et al. (1999).) The boundary conditions for the two-stream electron code are set by the electron energy distribution derived from Galileo EPD measurements in the magnetospheric regions between 10 and 25 RJ. This distribution is a kappa distribution (B. Bhattacharya and R. M. Thome. personal communication, 1998).

Representative model profiles for Eg = 15 keV, K = 2.1, and QO consistent with a total energy flux of 20 ergs cm-2 S-J are shown in Figure 10 and the model parameters are shown in Table 1. This energy input function gives an altitude for the emission peak of -190 km, which is somewhat below the Galileo SSI observations. The major altitude difference of maximum emission intensity between this study and earlier studies (Waite et al.. 1983; Rego et al., 1994) is due to the probe devised thermal structure in the lower atmosphere and the high energy of the incoming electrons. Agreement between the model and observed values is generally good, with two exceptions. First, the calculated H2 band emission intensities are lower than the averaged intensities of observed Lyman and Werner band emissions by approximately a factor of four. Second, the model-generated H2 and H3+ ro-vibrational temperatures are also lower than observed. These discrepancies suggest that the energy input into Jupiter’s aurora1 atmosphere is greater (by a factor of four) than that inferred from the Galileo EDP data and that increasing the energy input function in the model will yield improved fits to the observations. However, the differences between the model and the observations can only partially be accounted for in terms of low aurora1 particle energy input. In particular, the lower-than-observed ro-vibrational temperatures may be indicative of a problem with the assumed methane density-versus-pressure profile relative to the auroral emission altitude or perhaps of some other problem with the description of CH4 cooling in the aurora1 atmosphere. A more likely explanation, however, is that the higher observed temperatures are produced by a soft Maxwellian component of the precipitating energetic electron flux, a component whose input power is at present not well-constrained in our model. This soft particle component deposits its energy well above the methane cooling layer, thus producing the high thermospheric temperatures despite its relatively low energy flux. Fortunately, the needed constraints on the power input by this population are recently available from Galileo UVS observations of emissions in the EUV regime (Ajello et al., 1998). That is, because EUV emissions from H2 are strongly self-absorbed, the observed emissions are excited at relatively high altitudes and thus provide a sensitive probe of the region where the maximum energy deposition by the aurora1 soft particle population occurs. While the Galileo UVS observations of the self-absorbed H2 EUV emissions have low spectral and spatial resolution and are thus somewhat difficult to interpret. the soon-to-be-launched FUSE mission ought to provide excellent new data at sub-Angstrom resolution.

Preliminary indications from this self absorption analysis of H2 emissions are best resolved by using softer particle input energies as reported by Ajello. (private communication, 1999). Furthermore, the broad range of emission heights (FWHM 120 to 460 km) observed by Vasavada et al., (1999), led us to consider an additional case with a multi component electron energy distribution (three kappa functions) one with a representative energy of 15 keV and an energy flux of 20ergs cm-&-t one with a characteristic energy of 30 keV and 30 ergs cm-2s1 and a very soft component at 600eV with an energy flux of 50ergs cm-&-l. This electron energy distribution produces an emission that starts at 210 km and extends to almost 2000 km, reminiscent of the extended distribution of emission seen by (Vasavada et al.. 1999). The model parameters shown also in Figure 11 and Table 1 are in excellent agreement with the cumulative set of spectra at various wavelengths. However, to date the spectra have been taken at different time and places and represent only a composite picture of the aurora1 thermal structure that agrees reasonably well with modeling. Furthermore, the convoluted nature of the required electron energy distribution is only poorly linked to observations with the exception of the EPD data and most likely this electron energy distribution is also a composite that represents more than one region of magnetosphere input. Nonetheless, this is a good first step that can be improved upon by simultaneous measurements of the same region in multiple wavelengths and improved 3-dimensional modeling of the thermal structure.

CONCLUSION

Since its discovery two decades ago, Jupiter’s aurora has been observed at x-ray, ultraviolet. visible, and infrared wavelengths, with each wavelength regime illuminating a different aspect of jovian aurora1 processes and the response of Jupiter’s upper atmosphere to aurora1 energy input. Aurora1 imaging reveals the morphology of the

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emissions and provides information about their variability in space and time. Interpreted in the context of magnetic field models, which they also help to constrain, aurora1 images are used to map emission regions and features to particular magnetospheric source regions. Such mapping, along with the imaging information on aurora1 morphology and dynamics, provides a global context for the interpretation of in-situ charged particle data and is important for our understanding both of the acceleration, transport, and loss of charged particles in the jovian magnetosphere and of the large-scale influences-planetary rotation, solar wind coupling-to which these processes are subject. With regard to the nature of the macroscopic influences on Jovian magnetospheric and aurora1 processes, empirical and MHD magnetospheric models of Jupiter’s magnetosphere offer particularly valuable interpretive tools for relating aurora1 observations to changing magnetospheric configurations and dynamics.

In addition to images showing the distribution and variability of Jupiter’s aurora, numerous spectra of jovian aurora1 emissions have also been obtained, principally in the ultraviolet and infrared wavelength regimes. Such spectral data, from which the temperature of the emitting gas can be derived, are an important source of information about atmospheric heating and cooling and, in the absence of in-situ measurements, provide needed constraints on models of the thermal structure of Jupiter’s high-latitude upper atmosphere. Further, from the analysis of aurora1 spectra can be extracted information both about the energy of the incident particles and about the particles’ identity. For example, the “color ratio” (i.e., the relative absorption of H2 emissions in two different wavelength ranges by hydrocarbons) has been used to infer the depth to which the precipitating particles penetrate and hence the particle energy (cf. Yung et al., 1982; Livengood et al., 1990), while, as we have seen, analysis of H2 spectra obtained with the HST GHRS has shown that O+ precipitation makes only a minor contribution to the ultraviolet aurora (Trafion et al.. 1998). On the other hand, the dominant role of heavy ion precipitation in the generation of Jupiter’s x-ray aurora is expected to be confirmed by spectrographic observations to be made later this year with the AXAF ACIS-S instrument.

Twenty years of observations have provided progressively more detailed information about the phenomenology of the jovian aurora and have required revision of some earlier views based on the initial analysis of Voyager data (e.g., regarding the mapping of the main oval to the orbit of lo or the role of ions as the dominant aurora1 particle). Despite much progress, however, many basic questions remain unanswered. and the observational details have yet

to be integrated into a coherent picture of Jupiter’s dynamic aurora and of the magnetospheric processes that drive it. The outlines of such a picture are beginning to emerge, however, and its completion and refinement are the principal tasks for the third decade of jovian aurora1 studies. HST observations of Jupiter’s aurora continue, detailed analysis of Galileo imaging and fields-and-particles data has only just begun, ground based infrared observations are revealing new information about the dynamics of the thermosphere, and the development of sophisticated magnetospheric models is well under way. These efforts, together with availability in the near future of high-resolution spectral data from AXAF and FUSE and the planned Cassini Jupiter flyby with a coordinated observing campaign, promise that the next ten years will be exciting ones indeed for students of the jovian aurora.

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