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

Abstract

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 N 2 and CH 4 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 × 10 5 cm −3 and 1.1 ± 0.9 × 10 5 cm −3 , and an energy deposition rate at the exobase of 1.1 ± 0.9 × 10 2 eV cm −3 s −1 and 3.9 ± 3.5 × 10 1 eV cm −3 s −1 for the hot N 2 and CH 4 populations, respectively. The energy deposition rate allowed us to roughly estimate escape rates for nitrogen of ≃7.7 ± 7.1 × 10 7 N cm −2 s −1 and for methane of ≃2.8 ± 2.1 × 10 7 CH 4 cm −2 s −1 . Interestingly, no suprathermal component was observed in the ingress‐ T 5 data.