Mars mesosphere and thermosphere coupling: Semidiurnal tides

Abstract

The National Center for Atmospheric Research thermosphere ionosphere general circulation model for Earth has been modified to examine the three‐dimensional structure and circulation of the upper atmosphere of Mars (MTGCM) (Bougher et al., 1988b, 1990). Recent examination of Mariner 9 UVS airglow measurements taken during a global dust storm provide evidence of large temperature variations and atomic oxygen distributions uncorrelated with solar activity and the corresponding MTGCM in situ driven winds. We suspect significant forcing of the thermosphere from below as a result of upward propagating gravity waves or tides generated by solar heating of airborne dust (Stewart et al., 1992). The effects of upward propagating tides are introduced into the MTGCM by appropriately specifying its lower boundary condition according to classical tidal theory. We initially adapt the terrestrial scheme used by Fesen et al. (1986) to a Mars model appropriate to Mariner 9 near‐solar‐minimum conditions. Estimates of the amplitude and phase of the likely dominant semidiurnal (2,2) mode at the mesopause (∼100 km) are specified for a range of possible lower atmosphere dust conditions. MTGCM simulations contrasting tidally driven fields with solar‐only forced ones show a dramatic change in the horizontal and vertical wind patterns, whereby the global temperature and oxygen distributions are also modified significantly. This semidiurnal component predominates below 135 km, while the in situ solar‐driven diurnal component is largely dominant above. Constructive interference serves to enhance midafternoon exospheric temperatures toward Mariner 9 observed values. The thermospheric response and the altitude of penetration of these semidiurnal tides is found to be much greater during solar minimum periods when dissipation due to viscosity and thermal conductivity is diminished from that at solar maximum. Finally, the Martian response during dusty periods is predicted to be much larger than that typically observed for Earth. Martian dust‐driven tides, especially during solar minimum time periods, cannot be ignored when addressing the Mars thermospheric structure and dynamics.