Thermal Tides at Mars: Observations of Thermospheric Densities by the MAVEN Accelerometer During Aerobraking

We utilize in situ accelerometer data from the Mars Atmosphere and Volatile Evolution (MAVEN) mission spacecraft collected during the February through April 2019 aerobraking campaign (AB2019). During aerobraking, the spacecraft periapsis altitude was lowered from ~151 km to ~132 km with mostly Southern latitudes being observed, and this provides the broadest global coverage (latitude, longitude, LST) at these altitudes. Previously, nine "Deep Dip" (DD) campaigns were performed when similar altitudes were reached, but each of these lasted approximately one week.

For our analysis, we separate each of MAVEN's orbits by inbound (pre-periapsis) and outbound (post-periapsis) trajectories with a focus on inbound as it provides the greatest signal-to-noise (S/N) data. In this work, we aim to determine if diurnal Kelvin wave features are observable in the equatorial regions as has been shown to be the case with pre-MAVEN spacecraft (e.g. Withers et al, 2003; Keating et al. 2008; Bougher et al., 2017), and compare these to similar observations from MAVEN during the DD2 and DD6 campaigns. Four constant altitude levels are utilized for this study: 135, 140, 145, and 150 km. For each campaign data set, a wave-fitting model is run (i.e., Fourier fits) to compute wave amplitudes and phases in mass density measurements, and we utilize least squares computation to determine scale heights and temperatures at each of the four fixed altitudes. This enables us to 1) investigate composite and harmonic thermal tide signatures, 2) calculate constant pressure level fields in the thermosphere to compare with corresponding tidal features lower in the atmosphere from MRO Mars Climate Sounder datasets, and ultimately, 3) to investigate thermal tidal evolution over multiple seasons.

Current results indicate 1) the same wave modes dominate at each altitude along the equatorial region (0-20S), 2) wave amplitudes dampen with increasing altitude, and 3) some wave mode phases migrate slightly in longitude, and often in LST. We also find the wave structure becomes increasingly chaotic at higher latitudes where S/N is poor. Initial comparison of the AB2019 constant pressure level measurements with corresponding MCS 0.02 Pa measurements near ~95 km are made, illustrating lower to upper atmosphere wave coupling.