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.

MGS electron density profiles: Analysis and modeling of peak altitudes

“…Dust absorbs solar energy, which causes the atmosphere to heat and expand, thus raising the altitude of the base of the thermosphere, and that of the ionospheric peak as well. (e.g., Kliore et al, 1972aKliore et al, ,b, 1973McElroy et al, 1977;Stewart and Hanson, 1982;Stewart, 1987;Zhang et al, 1990;Keating et al, 1998;Forbes and Hagan, 2000;Wang and Nielsen, 2003;Moudden and Forbes, 2008a). The Mariner 9 primary mission occurred as the great planetwide dust storm of 1971 was subsiding, and the RO profiles showed F 1 peak maxima that were higher by 20-30 km than those at similar SZAs at other times, including those from the Mariner 9 extended mission, and the Viking 1 and 2 missions.…”

“…The variability of the ionospheric peaks with planetocentric longitude has been explained as due to vertically propagating migrating and nonmigrating thermal tides forced by interactions with solar heating near the surface; some investigators have suggested that the differential heating is due to effects such as topography or dust loading (e.g., Forbes and Hagan, 2000;Wilson, 2002;Forbes et al, 2002;Nielsen, 2003, 2004a;Withers et al, 2003, Angelats i Coll et al, 2004, Forbes and Miyahara, 2006Moudden and Forbes, 2008a). Moudden and Forbes (2010) used the MGS and MRO aerobraking data in conjunction with a general circulation model to argue that planetary wave-tide interactions provide an alternative explanation for driving density variability in the aerobraking region in the lower thermosphere.…”

Longitudinal structures in Mars' upper atmosphere as observed by MAVEN/NGIMS

“…Many modeling studies have simulated DE1 and DE2 tides and revealed their importance in controlling the longitudinal structures in the thermosphere [e.g., Bougher et al, ; Angelats i Coll et al, ; Wilson, ; Forbes and Miyahara , ; Moudden and Forbes , , ; Medvedev et al , ]. Some of these studies have found that the density amplitudes caused by these tides are ~20% at Ls = 30 – 90° in the tropics [ Bougher et al, ; Angelats i Coll et al, ; Forbes and Miyahara , ].…”

“…Atmospheric tides, originating from the interaction of solar radiation with Mars' topography and the atmosphere, can create significant changes in the upper atmosphere density and dynamics [e.g., Keating et al, ; Forbes and Hagan , ; Forbes et al, ; Wilson, ; Bougher et al , ; Moudden and Forbes , , ; Withers et al , ; England et al, ]. Tides are planetary‐scale oscillations with periods that are harmonics of the solar day.…”

“…Modeling studies [e.g., Forbes and Hagan , ; Forbes et al, ; Wilson, ; Bougher et al, ; Angelats i Coll et al , ; Moudden and Forbes , , ] have demonstrated that DE1, DE2, and DE3 tides are most important in driving the longitudinal wave 2–4 patterns although semidiurnal tides may also be responsible. It is generally believed that stationary planetary waves forced by the topography are unable to directly propagate to thermospheric heights [e.g., Wilson, ].…”