An Intense Thermospheric Jet on Titan

Doppler wind maps. Line-of-sight Doppler shifts (m/s) measured in CH3CN, CH3CCH, DCN, HCN (main line of the hyperfine triplet at 354.5055 GHz and satellite lines at 354.5039 and 354.5075 GHz), HC3N, and HNC. Right Ascensions (RA) and declinations are shown in terms of Titan’s apparent radius. Ellipses show the synthetized beams. White areas correspond to regions where emission signals are too faint for reliable measurements to be performed. Only CH3CN emission makes it possible to measure winds throughout the disk. Some measurements within the disk are also possible using the main HCN 354.5055 GHz line, that appears in absorption in nadir geometry (see Supplementary Figure 2). Doppler shifts in the various species clearly indicate a prograde wind regime, but with different structure for CH3CN vs HNC, HCN and HC3N. Winds in CH3CCH are mostly measurable outside of the low-latitude regions. Winds are only marginally detected from DCN emission, with 30-50 m/s error bars. See Supplementary Information for details.

Winds in Titan's lower and middle atmosphere have been determined by a variety of techniques.

These techniques include direct measurements from the Huygens Probe over 0-150 km, Doppler shifts of molecular spectral lines in the optical, thermal infrared and mm ranges, probing altogether the ~100-450 km altitude range, and inferences from thermal field over 10 mbar - 10 -3 mbar (i.e. ~100-500 km) and from central flashes in stellar occultation curves. These measurements predominantly indicated strong prograde winds, reaching maximum speeds of ~150-200 m/s in the upper stratosphere, with important latitudinal and seasonal variations.

However, these observations provided incomplete atmospheric sounding; in particular, the wind regime in Titan's upper mesosphere and thermosphere (500- 1200 km) has remained unconstrained so far. Here we report direct wind measurements based on Doppler shifts of six molecular species observed with ALMA. We show that unlike expectations, strong prograde winds extend up to the thermosphere, with the circulation progressively turning into an equatorial jet regime as altitude increases, reaching ~340 m/s at 1000 km. We suggest that these winds may represent the dynamical response of forcing by waves launched at upper stratospheric/mesospheric levels and/or magnetospheric-ionospheric interaction. We also demonstrate that the HNC distribution is restricted to Titan's thermosphere above ~870 km altitude.

E. Lellouch, M.A. Gurwell, R. Moreno, S. Vinatier, D.F. Strobel, A. Moullet, B. Butler, L. Lara, T. Hidayat, E. Villard
(Submitted on 28 Mar 2019)

Comments: Accepted for publication
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1903.12116 [astro-ph.EP] (or arXiv:1903.12116v1 [astro-ph.EP] for this version)
Submission history
From: Emmanuel Lellouch
[v1] Thu, 28 Mar 2019 17:01:33 UTC (3,201 KB)
https://arxiv.org/abs/1903.12116
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