Waves and horizontal structures in Titan's thermosphere

Abstract

[1] The Ion Neutral Mass Spectrometer (INMS) on board the Cassini spacecraft carried out in situ measurements of neutral gas composition above 1025 km altitude in Titan's atmosphere during its flybys in October 2004 (TA) and April 2005 (T5). Strong perturbations are present in the N 2 and CH 4 densities which we interpret as vertically propagating waves. Typical vertical wavelengths range from 170 to 360 km with density and pressure amplitudes reaching 4-12% of the background values and temperature amplitudes of 5-10 K. Amplitudes over our sampled height range, 1025 (T5) or 1176 (TA) to 1600 km, remain roughly constant, implying that the exponential increase in wave amplitudes with height due to the decrease of density is offset by damping. This finding allows us to constrain the wave periods to values in the order of hours. Estimates of wave-induced acceleration of the background thermosphere suggest that the waves we observe could deposit considerable momentum in Titan's thermosphere, thereby coupling the dynamics of the upper atmosphere with that of the middle atmosphere. In addition, we infer latitudinal structures in Titan's thermosphere with a factor of 3-4 increase of mass densities from pole to equator in the northern hemisphere. A preliminary evaluation of local time variations suggests densities and thermospheric temperatures to be largest near dusk, contradicting expectations for a thermosphere driven energetically and dynamically primarily by solar EUV. From the latitudinal density gradients we derived zonal wind speeds of around 245 ± 50 ms À1 , implying that Titan's thermosphere, like its stratosphere, could be superrotating. Our analyses were based on the TA and TS flybys only, and future Cassini Titan flybys could either support or invalidate our findings.

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

“…A periodic variation of the density about the best fit profiles reveal the presence of vertical and probably horizontal waves in Titan's upper atmosphere [ Waite et al , 2005b; Yelle et al , 2006]. These waves, originating from complex dynamics, are described in more detail by Müller‐Wodarg et al [2006]. The wave pattern appeared to be stronger in the case of T 5 as compared to T A and T B .…”

“…Other species such as C 3 H 8 , C 4 H 2 , HCN, HC 3 N, C 6 H 6 , and C 2 N 2 were identified at less than 5 ppm. In addition, large spatial scale (≃180 km) and large‐amplitude (≃10 K) atmospheric waves were observed between 1174 and 1500 km [ Müller‐Wodarg et al , 2006].…”

Titan's ionosphere in the magnetosheath: Cassini RPWS results during the T32 flyby

“…2 reveals relative velocities (with respect to the spacecraft velocity) up to a few km/s, which may correspond to ion flows (and thus electrodynamic processes) but also neutral winds. However, Müller-Wodarg et al (2006 inferred horizontal neutral winds, based on the Ion Neutral Mass Spectrometer data (Waite et al, 2004), up to only 300 m/s. The total ion energy W i,eff (kinetic + thermal energies; Fig.…”

Neutral Atmospheres

“…Cassini observations in Titan's mesosphere (500-1000 km) region are too scarce to constrain dynamics, so little is known of winds at 1000 km, the bottom boundary of the region systematically examined by the Ion Neutral Mass Spectrometer (INMS). Further observations from Cassini during the extended mission will help further constrain dynamics on Titan and assess the role of waves identified in the Cassini and Huygens data (Fulchignoni et al 2005;Mueller-Wodarg et al 2006b).…”