Density waves in Titan's upper atmosphere

Abstract

Analysis of the Cassini Ion Neutral Mass Spectrometer data reveals the omnipresence of density waves in various constituents of Titan's upper atmosphere, with quasi‐periodical structures visible for N2, CH4,29N2, and some of the minor constituents. The N2 amplitude lies in the range of ≈4%–16%with a mean of ≈8%. Compositional variation is clearly seen as a sequence of decreasing amplitude with increasing scale height. The observed vertical variation of amplitude implies significant wave dissipation in different constituents, possibly contributed by molecular viscosity for N2but by both molecular viscosity and binary diffusion for the others. A wave train with near horizontally propagating wave energy and characterized by a wavelength of ≈730 km and a wave period of ≈10 h is found to best reproduce various aspects of the observations in a globally averaged sense. Some horizontal and seasonal trends in wave activity are identified, suggesting a connection between the mechanism driving the overall variability in the background atmosphere and the mechanism driving the waves. No clear association of wave activity with magnetospheric particle precipitation can be identified from the data.

Titan's Variable Ionosphere During the T118 and T119 Cassini Flybys

“…Neutral density wave structures are observed in the INMS data during these flybys (not shown), which could also affect the plasma density. The vertical extent (∼100 km) of the two distinct peaks in electron density during T118 are of typical wavelength for neutral waves (Cui et al, ; Müller‐Wodarg et al, ). Also, the fact that the magnetospheric ram direction and the direction to the Sun are separated for these flybys (at 02 hr SLT) could result in the neutral wind direction and the magnetic forcing direction being quite different and lead to flow shear and/or stagnation layers around an altitude of ∼1,300 km (Cravens et al, ).…”

“…10.1029/2018GL078436 during T118 are of typical wavelength for neutral waves (Cui et al, 2014;Müller-Wodarg et al, 2006). Also, the fact that the magnetospheric ram direction and the direction to the Sun are separated for these flybys (at 02 hr SLT) could result in the neutral wind direction and the magnetic forcing direction being quite different and lead to flow shear and/or stagnation layers around an altitude of ∼1,300 km (Cravens et al, 2010).…”

Ionization Efficiency in the Dayside Martian Upper Atmosphere

“…This leaves a clean selection of orbits where secondary ionization is predominantly caused by photoelectrons. The exclusion of "contaminated" orbits is implemented by requiring that the relative amplitude in ionization efficiency does not exceed 5%, where the relative amplitude is determined following our previous procedure used for calculating the wave amplitude in Titan's upper atmosphere (Cui et al 2013(Cui et al , 2014. These orbits are evenly distributed in longitude but concentrated around the equator, without evidence for clustering near strong magnetic anomalies (Acuna et al 1999).…”

Solar cycle variations in ion composition in the dayside ionosphere of Titan

“…The densities of some minor neutral species can be derived directly from INMS measurements [ Waite et al ., , ; Magee et al ., ; Cui et al ., , ] whereas others are deduced from combined ion‐neutral chemistry models [ Toublanc et al ., ; Keller et al ., ; Vuitton et al ., , ; Krasnopolsky, , ; Bell et al ., ; Müller‐Wodarg et al ., ]. These minor neutral species are particularly important for the production of higher‐mass ion species, but information on their dependence on solar activity has not been studied yet.…”