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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
9
• 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
13
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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