Non-migrating tides connections with topography and surface properties

Atmospheric densities derived from measured acceleration of several Mars orbiters reveal large amplitude Sun-synchronous longitudinal density variations at altitudes of about 100-160km (see Figure 1). 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 non-migrating 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. We sought in this work to determine the specific origins of D0, DE1 and DE2, as well as other waves that make secondary contributions to the wave-1, 2, and -3 density structures. This was done by performing numerical experiments wherein we sequentially added different zonal wavenumber components to the specification of topography, and considered separately the effects of surface properties such as thermal inertia and albedo. The diurnal tides D0, DE1 and DE2 are found to be primarily generated by the interaction between the 24-hour harmonic of solar radiation interacting with the wave-1, -2, and -3 components of topography, which bear interesting similarities to the corresponding density structures in the thermosphere. Each of these interactions also produces a second wave, i.e., the westward-propagating diurnal tides DW2, DW3 and DW4. These waves have much shorter vertical wavelengths (i.e., < 30 km) than their eastward-propagating counterparts, are much more susceptible to dissipation as they propagate from the lower atmosphere into the thermosphere, and Figure 1: Density in kg/km, (a) measured by MGS and (b) simulated by GMMM at 120 km and 15 LST.