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

Abstract

[1] 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 Â 10 5 cm À3 and 1.1 ± 0.9 Â 10 5 cm À3 , and an energy deposition rate at the exobase of 1.1 ± 0.9 Â 10 2 eV cm À3 s À1 and 3.9 ± 3.5 Â 10 1 eV cm À3 s À1 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 Â 10 7 N cm À2 s À1 and for methane of '2.8 ± 2.1 Â 10 7 CH 4 cm À2 s À1 . Interestingly, no suprathermal component was observed in the ingress-T 5 data.

First detection of hydrogen isocyanide (HNC) in Titan’s atmosphere

“…For altitudes lower than z 0 , the mixing ratio is taken as zero. Taking into account the high molecular diffusion above the homopause at ∼1200 km (D = 0.5 −−1×10 10 cm 2 s −1 , De La Haye et al 2007;Yelle et al 2008), we also assumed a constant mixing ratio above the homopause z h = 1200 km. With this model, the best fit is obtained Fig.…”

“…The thermal profile used for our computation is a combination of (i) the temperatures measured by Huygens/HASI (Fulchignoni et al 2005) in the troposphere at altitudes between 0-140 km; (ii) the disk-averaged Cassini/CIRS stratospheric temperatures (Vinatier et al 2010) at altitudes between 140-500 km; (iii) the Cassini/INMS retrieved temperatures (i.e. 155 K in average, De La Haye et al 2007) at altitudes between 1000-1500 km; (iv) a decreasing temperature from 165 K to 155 K at altitudes between 500 and 1000 km. Our modeling results, presented below, indicate that the HNC(6-5) line is optically thin (opacity τ ∼ 0.12), therefore results are not too sensitive to the precise temperature profile.…”

Saturn Plasma Sources and Associated Transport Processes

“…Several processes have been proposed as sources from Titan including thermal escape Bell et al, 2011), chemically-induced escape (De La Haye et al, 2007), slow hydrodynamic escape (Strobel 2008(Strobel , 2009Yelle et al, 2008), pick-up ion loss and ionospheric outflow (Ledvina et al, 2005;Wahlund et al, 2005;Sillanpaa et al, 2006;Ma et al, 2006;Hartle et al, 2006;Coates et al. 2007a;Edberg et al, 2011;Westlake et al, 2012;Coates et al, 2012) and plasma-induced atmospheric sputtering (Shematovich et al, 2003;Michael et al, 2005;De La Haye et al, 2007). …”

The Structure of Titan’s N2 and CH4 Coronae

“…On Titan, reliable measurements of corona N 2 and CH 4 made by the Ion Neutral Mass Spectrometer (INMS) on board Cassiniextend up to an altitude of 2000km (De La Haye et al 2007a), whereas measurements of corona H 2 extend up to at least 7000km (Cui et al , 2011. Existing studies revealed no remarkable deviation in exobase temperature of H 2 from the mean temperature in the upper atmosphere (Cui et al , 2011), implying that a hot H 2 population does not exist.…”

“…Existing studies revealed no remarkable deviation in exobase temperature of H 2 from the mean temperature in the upper atmosphere (Cui et al , 2011), implying that a hot H 2 population does not exist. For N 2 and CH 4 , the analysis of the TA, TB, and T5 INMS data by De La Haye et al (2007a) revealed the presence of suprathermal coronae for both constituents with a typical temperature enhancement of ∼20-70K over the mean state below the exobase. The above distinction calls for certain heating mechanisms to be operative for N 2 and CH 4 , but not for H 2 near Titan's exobase.…”