Topographic connections with density waves in Mars' aerobraking regime

Abstract

Atmospheric densities derived from measured acceleration of several Mars orbiters reveal large amplitude Sun‐synchronous longitudinal density variations at altitudes of about 100–160 km. These variations are associated with vertically‐propagating nonmigrating solar thermal tides that are excited near Mars' surface and propagate into the thermosphere. Excitation of these waves is associated with the zonally asymmetric component of the near‐surface heating distribution and is commonly attributed to topographic modulation of solar heating. However, there are other possible contributors to the excitation of nonmigrating tides, including zonal variations in surface properties and wave‐wave nonlinear interactions, whose relative contributions remain unexplored. In this study we use a general circulation model in combination with the Mars Global Surveyor accelerometer measurements to isolate the different waves responsible for the observed density structures. While it is evident that topography accounts for most of the waves' amplitudes, the surface thermal inertia and albedo are found to have a nonnegligible contribution to the eastward‐propagating diurnal tide with zonal wave number s = −1 and the standing s = 0 diurnal tide. Previous studies have reported the dominance of wave numbers 2 and 3 in the resulting Sun‐synchronous aerobraking density maps. We show that wave number 1 has a comparable amplitude and is mostly the result of mapping the diurnal standing wave into a near Sun‐synchronous orbit. While most of the nonmigrating tides at thermospheric altitudes are generated by an interaction between the migrating solar radiation and the dominant topography harmonics, we also find that wave‐wave interaction is necessary to explain some of the observed density features.