Methane Ions Produced by Titan's Exosphere
Abstract
The main source of CH4+ above Titan's exobase is from pickup ions produced by its CH4+ exosphere. Such ions are predicted from a CH4+ exosphere model describing density, temperature and wind at the exobase. Ionizing the CH4+ exosphere forms CH4+ ions that are picked up by the motional electric field of the magnetosphere. A range of exosphere models is considered relative to the Sun, accounting for the 360-degree ram direction produced by Saturn's magnetospheric rotation. Exospheric densities increase in the equatorial or polar regions when exobase winds are zonal or meridianal. Pickup ions entering the thermosphere produce heating as they slow down, raising the exobase temperature in these places. Hot spots also occur in different places of the exobase depending on where Saturn's magnetospheric current sheet is relative to Titan's orbit, being above, below or within; dipolar magnetospheric magnetic fields will cause equatorial heating on the Saturn side of Titan, while equatorially confined magnetospheric magnetic fields (current sheet geometry) heating will be at north polar region when Titan is below current sheet and on south polar region when Titan is above current sheet. When the methane exosphere is exposed to the magnetosphere's sheet, its temperature may be as high as 190 K. The corresponding CH4+ pickup ion density peaks at about 2×10-3 cm-3 in the up-flow direction of Titan, 2000 km above its ionopause. Alternatively, if the magnetosphere is in a lobe state, the exosphere's temperature may be reduced to 110 K, reducing the peak to 10-6 cm-3. This CH4+ pickup ion density difference can be used by the CAPS ion instruments to determine if the magnetosphere is in the sheet or lobe state at Titan's orbit. Furthermore, there are enough CH4+ pickup ions measured to be consistent with classical exosphere theories but may not be enough to support hydrodynamic expansion models [Sittler et al., 2009]. References: Sittler et al., (2009), Methane Group Ions in Saturn's Outer Magnetosphere?, Eos Trans. AGU, 90(52), Fall Meet. Suppl., Abstract P11B-1232.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.P53C1880S
- Keywords:
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- 6281 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS Titan;
- 0328 ATMOSPHERIC COMPOSITION AND STRUCTURE Exosphere;
- 6026 PLANETARY SCIENCES: COMETS AND SMALL BODIES Ionospheres;
- 6008 PLANETARY SCIENCES: COMETS AND SMALL BODIES Composition