Exospheric Density Waves Observed in MAVEN NGIMS Data

Multiple recent studies have analyzed density perturbations found in the MAVEN Neutral Gas and Ion Mass Spectrometer data, assumed to be propagating gravity waves (Yigit et al. 2015, England et al. 2015, Terada et al. 2017). As gravity waves are understood to be ubiquitous in planetary atmospheres, their detection in the upper thermosphere is unsurprising. However, we examine 252 MAVEN passes through the Martian atmosphere where NGIMS data show wave-like density structures with amplitudes larger than 40% of the background density present above the nominal exobase, where the atmosphere transitions from collisional to ballistic. In this region of a planet's atmosphere the continuum physics typically used to describe gravity waves is not applicable. Here we present the profiles of these wave-like structures as a function of atmospheric column density rather than altitude, as this removes the dependence of altitude-density profiles on composition, scale height, and local solar time. This allows us to construct an average wave amplitude profile as a function of total atomic column density, which shows a sharp peak above the nominal exobase as defined by the O+CO₂ cross section and then a decaying amplitude with decreasing column density. As continuum models break down in this region of the atmosphere, we use molecular kinetic simulations (Leclercq et al. 2018) to describe the propagation of such features to these altitudes, as well as the causes of their eventual dissipation, as the methods of dissipation differ from the wave saturation that occurs at higher densities (e.g. D.C. Fritts 1984). Such density structures are of interest as the wave-like activity can affect the local heating rate (Charney and Drazin 1961, Hunsucker 1982) and possibly the escape rate (Walterscheid et al. 2013).