Physics
Scientific paper
Jul 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010jgra..11507312g&link_type=abstract
Journal of Geophysical Research, Volume 115, Issue A7, CiteID A07312
Physics
9
Ionosphere: Planetary Ionospheres (5435, 5729, 6026), Ionosphere: Ionization Processes (7823), Ionosphere: Solar Radiation And Cosmic Ray Effects, Ionosphere: Particle Precipitation, Planetary Sciences: Solar System Objects: Titan
Scientific paper
We analyze a multi-instrumental data set from four Titan encounters by the Cassini spacecraft to investigate in detail the formation of the ionosphere. The data set includes observations of thermospheric and ionospheric species and suprathermal electrons. A model describing the solar and electron energy deposition is used as an organizing element of the Cassini data set. We first compare the calculated secondary electron production rates with the rates inferred from suprathermal electron intensity measurements. We then calculate an effective electron dissociative recombination coefficient, applying three different approaches to the Cassini data set. Our findings are threefold: (1) The effective recombination coefficient derived under sunlit conditions in the deep ionosphere (<1200 km) is found to be independent of solar zenith angle and flyby. Its value ranges from 6.9 × 10-7 cm3 s-1 at 1200 km to 5.9 × 10-6 cm3 s-1 at 970 km at 500 K. (2) The presence of an additional, minor source of ionization is revealed when the solar contribution is weak enough. The contribution by this non-solar source—energetic electrons most probably of magnetospheric origin—becomes apparent for secondary electron production rates, due to solar illumination alone, close to or smaller than about 3 × 10-1 cm-3 s-1. Such a threshold is reached near the solar terminator below the main solar-driven electron production peak (<1050 km). (3) Our ability to model the electron density in the deep ionosphere is very limited. Our findings highlight the need for more laboratory measurements of electron dissociative recombination coefficients for heavy ion species at high electron temperatures (especially near 500 K).
Coates Andrew
Cui Jun
Galand Marina
Vuitton Veronique
Wahlund Jan-Erik
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