 |
| Titel |
Methane Group Ions Produced by Titan's Exosphere and Ionosphere |
| VerfasserIn |
Edward Sittler, Richard Hartle, David Simpson, Menelaos Sarantos, John Cooper, Ashraf Ali, Alexander Lipatov |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250099566
|
| Publikation (Nr.) |
 EGU/EGU2014-15362.pdf |
|
|
|
|
|
| Zusammenfassung |
| We will be presenting results of methane ions that can be injected into Saturn’s
magnetosphere as pick up ions from Titan’s exosphere and outflowing methonium ions
CH5+, the HCNH+ ion and the ethenium ions C2H5+ from Titan’s ionosphere.
Ionospheric outflows have been seen during the T9 flyby (Sittler et al., 2010), and the T63
and T75 flybys (Coates et al., 2012) where source rates to magnetosphere can be
significant ~ 4.0x1024 ions/s. When methane pickup ions are born within Titan’s
exosphere and convective electric field points outward these ions will populate
Saturn’s magnetosphere, while inward convective electric field (Saturn side for dipolar
magnetospheric fields) will heat the upper atmosphere and exosphere. Using 1D exosphere
Westlake et al. (2011) found that the exosphere was hotter and more extended when
Titan was within Saturn’s sheet, while in lobe like regions of magnetosphere the
exosphere is cooler. Using a 3D exosphere model, which can include winds and
asymmetric heating at exobase to model methane pickup ion densities; we estimate that
when within Saturn’s sheet the exobase T ~ 190° K and the estimated density is
~ 2x10-3 ions/cm3 which are observable, while in lobe like regions exosphere
T ~ 110° K and densities ~ 10-6 ions/cm3 not observable. The heating from
methane pickup ions can be complex depending upon magnetic field geometry, dipolar
(heating on Saturn side) and disc geometry (below sheet north polar heating and above
sheet south polar heating). This CH4+ pickup ion density difference we estimate
can be used by the CAPS ion instruments to determine if the magnetosphere is
in the sheet (also plasma sheet usually dominated by water group ions with O+
ions) or lobe state (light ions H+/H2+ dominating the composition). We find CH4+
pickup ions for T36 and T39 flybs when Titan is within Saturn’s magntospheric
sheet, while during T41 when within lobe regions of Saturn’s magnetosphere CH4+
pickup ions were not observed. But for T41 the magnetic field was equatorially
confined so during approach from within Titan’s wake Cassini was magnetically
connected and ionospheric outflows like T9 were observed. We will discuss the
compositional signatures in the CAPS IMS data and the likely chemistry of Titan’s
ionosphere.
References:
Sittler, E.C. Jr., et al., (2010), Saturn’s Magnetospheric Interaction with Titan as
Defined by Cassini Encounters T9 and T18: New Results, Planet. Space Sci., 58,
327-350.
Coates, A.J., et al., (2012), Cassini in Titan’s tail: CAPS observations of plasma escape, J.
Geophys. Res., 117, A05324, doi:10.1029/2012JA017595.
Westlake, J. H., et al., (2011), Titan’s thermospheric response to various plasma
environments, JGR, 116, A03318, doi:10.1029/2010JA016251. |
|
|
|
|
|
|