Cassini Ion and Neutral Mass Spectrometer data in Titan's upper atmosphere and exosphere: Observation of a suprathermal corona

摘要

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.