Abstract
Cassini Ion and Neutral Mass Spectrometer data in Titan's upper atmosphere and exosphere: Observation of a suprathermal corona
Southwest Research Institute, San Antonio, Texas, USA
Southwest Research Institute, San Antonio, Texas, USA
Astronomy Department, University of Virginia, Charlottesville, Virginia, USA
Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas, USA
Space Sciences Laboratory, University of California, Berkeley, California, USA
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
Southwest Research Institute, San Antonio, Texas, USA
Southwest Research Institute, San Antonio, Texas, USA
Service d'Aéronomie du CNRS/IPSL, Paris, France
Civil Engineering, Indian Institute of Technology, Kanpur, India
Center for Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas, USA
The neutral nitrogen and methane measurements made by Ion and Neutral Mass Spectrometer during Cassini flybys T A , T B , and T 5 in Titan's upper atmosphere and exosphere are presented. Large horizontal variations are observed in the total density, recorded
to be twice as large during T A as during T 5. Comparison between the atmospheric and exospheric data show evidence for the presence of a significant population of suprathermal
molecules. Using a diffusion model to simultaneously fit the N2 and CH4 density profiles below 1500 km, the atmospheric structure parameters are determined, taking into account recent changes in
the calibration parameters. The best fits are obtained for isothermal profiles with values 152.8 ± 4.6 K for T A , 149.0 ± 9.2 K for T B , and 157.4 ± 4.9 K for T 5, suggesting a temperature
5 K warmer at night than at dusk, a trend opposite to that determined by solar-driven models. Using standard exospheric theory
and a Maxwellian exobase distribution, a temperature of 20 to 70 K higher would be necessary to fit the T A , T B , and egress-T 5 data above 1500 km. The suprathermal component of the corona was fit with various exobase energy distributions, using a method
based on the Liouville theorem. This gave a density of suprathermals at the exobase of 4.4 ± 5.1 × 105 cm−3 and 1.1 ± 0.9 × 105 cm−3, and an energy deposition rate at the exobase of 1.1 ± 0.9 × 102 eV cm−3 s−1 and 3.9 ± 3.5 × 101 eV cm−3 s−1 for the hot N2 and CH4 populations, respectively. The energy deposition rate allowed us to roughly estimate escape rates for nitrogen of
7.7 ± 7.1 × 107 N cm−2 s−1 and for methane of
2.8 ± 2.1 × 107 CH4 cm−2 s−1. Interestingly, no suprathermal component was observed in the ingress-T 5 data.
Received 12 December 2006; accepted 3 April 2007; published 27 July 2007.
Citation: (2007), Cassini Ion and Neutral Mass Spectrometer data in Titan's upper atmosphere and exosphere: Observation of a suprathermal corona, J. Geophys. Res., 112, A07309, doi:10.1029/2006JA012222.
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