Electrons at Saturn’s moons: selected CAPS-ELS resultsA.J. Coates1,2. G.H. Jones1,2, C.S.Arridge1,2, A. Wellbrock1,2, G.R. Lewis1,2,

D.T. Young3, F.J. Crary3, J.H. Waite Jr.3, R.E. Johnson4, T.W. Hill5 1. MSSL-UCL, UK 2. Centre for Planetary Sciences at UCL/Birkbeck, UK3. SwRI, USA4. University of Virginia, USA 5. Rice University, USA

CAPS instrument

Sensor Measures Energy range (eV/q)

Energy resolution (E/E, %)

Angle range ()

Angle bin ()

Ion mass spectrometer

(IMS)

Ion mass, energy and

direction

1-50,000 17 160x8 20x8

Ion beam spectrometer

(IBS)

Narrow ion beams;

energy and direction

1-50,000 1.4 150x1.4 1.5x1.4

Electron spectrometer

(ELS)

Electron energy and

direction

0.6-28,000 17 160x5 20x5

•Three sensors + DPU, actuatorThree sensors + DPU, actuator

•Young et al., 2004Young et al., 2004

•ELS: Coates et al., 1992, Linder et al., 1998, ELS: Coates et al., 1992, Linder et al., 1998, Lewis et al., 2008, 2010Lewis et al., 2008, 2010

Titan - Ionospheric plasma in the tail

Ionospheric photoelectrons

e

i

e

T9 encounter

•Interval 1 – ionospheric photoelectrons at 6.8-5.4 RT in tail: magnetic connection to sunlit ionosphere; plasma escape

•Interval 2 – ionospheric & magnetospheric plasma; light ions

•Role of ambipolar electric field in escape – similar to Earth’s polar wind – & lower mass from higher altitude

•Coates et al, GRL 2007a, PhTrRS A 2009, PSS in press 2011, Wellbrock et al in prep; Wei et al., GRL 2007

•Seen on other encounters e.g. T15 (Wellbrock et al., paper in prep), T55-59 (Edberg et al., PSS 2011)

•Process seen at Mars, Venus; Frahm et al, 06,07, Coates et al, 2008, 2011 (PSS in press)

T75 – tail encounter (c.f. T9)

e

i1 2 3

Intermittent spectra showing photoelectron peak in tail regions 1 & 2 – plasma from dayside ionosphere

MAG data consistent – Wei et al 2011

T63 – tail encounter

e

i

Intermittent spectra showing photoelectron peak in tail regions 1 & 2 – plasma from dayside ionosphere

1 2

MAG data consistent – Wei et al 2011

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Negative ions in Titan’s ionosphere - Unexpected! - Ram direction - Near closest approachUse ram velocity as a mass spectrometer mamu~5.32EeV

Confirmed in further low altitude encounters...

T16 T17

T18 T19Originally seen on TA in 2004…

Coates et al, 2007b, GRL

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• Data from CAPS electron spectrometer (ELS) at Titan• E/E=16.7%, scanned through ram direction• Always see negative ion population < 1400km• Maximum mass 13,800 amu/q on this encounter (T16)

Coates et al., Faraday Discussions, 2010

Heavy neutrals and positive ions: Waite et al, Science 2007, Crary et al. PSS 2009)

Coates et al, GRL 2007b, PSS 2009, PhTrRSA 2009, Farad. Disc. 2010. Sittler et al PSS 2009, Michael et al PSS 2011

Also Wellbrock et al talk

8Waite et al., 2007

Aerosols (UVIS, solar

occultation)

Liang et al, 2007

Titan’s ionosphere: hydrocarbon & nitrile rich

Source of Titan’s haze - tholins

Unexpected Negative ions:

•In ram direction at <1400km

•Heaviest (up to 13,800 amu/q) at lowest altitudes

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• First chemical model including negative ions (low mass), c.f. ELS data at 1015 km (T40) (Vuitton et al., PSS 2009)

• Hypothesized PAHs (Waite et al., 2007, Coates et al., 2007) & fullerenes (Sittler et al., 2010)• Negative ions confirmed by RPWS-LP (Wahlund et al, 2010)• Work continuing on composition (Vuitton et al), agglomeration (Lavvas et al), tholins in lab (Horst

et al) – Titan meeting, St Jacut, mass analysis (Wellbrock et al)

Production processes:•Several considered and rates estimated•Mainly dissociative electron attachment

Loss processes:•Several considered and rates estimated•Mainly associative detachment•Some photodetachment

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E3

Negative ions in the plume

Charged nanograinsJones et al., GRL, 2009

Coates et al., Icarus 2010, Faraday Discussions 2010

Unexpected discoveries from CAPS-ELS at Enceladus

See also Hill et al talk, Kanani et al poster, Tokar et al 2009 GRL, Gurnett et al GRL 2011

Rhea’s O2 and CO2 atmosphere – from INMS and CAPSTeolis, B.D., et al., Science, 2010

In-situ neutral atmosphere measurements (INMS)

Negative and positive ions picked up from atmosphere pinpoint near-surface source (CAPS)

Also surface charging at Rhea (Jones et al. talk) and Hyperion (Nordheim et al MAPS meeting)

Enceladus’ auroral spotPryor, Rymer et al., Nature 2011

Variability a monitor of emission

Field aligned electrons a key observation

Ganymede and Europa: JUICE• Weak, O2/H2O atmospheres

• Ganymede magnetic field• Ionospheres present

Need to measure:• Upstream plasma conditions key for interaction – e.g.

cold, hot electrons• Photoelectrons to trace magnetic field to ionospheres• Search for suspected negative ions at Europa, (e.g.

Cl-, Vollwerk et al, 2001), also Ganymede

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Johnson et al 2003

Khurana et al 1996

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Atmosphere? Ionosphere? Magnetic field Interaction

Titan Dense, N2/CH4 Yes No Subsonic

Enceladus Plume, H2O products

Plume No Subsonic

Main rings Weak, O2 Weak No Subsonic

Rhea Weak, O2 and CO2

Weak No Subsonic

Ganymede Weak, O2, H2O Weaker Yes Subsonic

Europa Weak, O2, H2O Weak No Subsonic

Conclusions

• Good electron measurements important at outer planet moons– Plasma environment– Ionisation effects– Photoelectrons – trace field connection– Surface charging– In-situ measurements of fleld aligned flux, Alfven wing structure –

producing auroral spot

• Additional species in electron data– Negative ion measurements important also – composition, pickup ions– Charged nanograins

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