MARS MESOSPHERE AND THERMOSPHERE COUPLING - SEMIDIURNAL TIDES

The National Center for Atmospheric Research thermosphere ionosphere g eneral circulation model for Earth has been modified to examine the th ree-dimensional structure and circulation of the upper atmosphere of M ars (MTGCM) (Bougher et al., 1988b, 1990). Recent examination of Marin er 9 UVS airglow measurements taken during a global dust storm provide evidence of large temperature variations and atomic oxygen distributi ons uncorrelated with solar activity and the corresponding MTGCM in si tu driven winds. We suspect significant forcing of the thermosphere fr om below as a result of upward propagating gravity waves or tides gene rated by solar heating of airborne dust (Stewart et al., 1992). The ef fects of upward propagating tides are introduced into the MTGCM by app ropriately specifying its lower boundary condition according to classi cal tidal theory. We initially adapt the terrestrial scheme used by Fe sen 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 (approximately 100 k m) are specified for a range of possible lower atmosphere dust conditi ons. MTGCM simulations contrasting tidally driven fields with solar-on ly forced ones show a dramatic change in the horizontal and vertical w ind patterns, whereby the global temperature and oxygen distributions are also modified significantly. This semidiurnal component predominat es below 135 km, while the in situ solar-driven diurnal component is l argely dominant above. Constructive interference serves to enhance mid afternoon exospheric temperatures toward Mariner 9 observed values. Th e thermospheric response and the altitude of penetration of these semi diurnal tides is found to be much greater during solar minimum periods when dissipation due to viscosity and thermal conductivity is diminis hed from that at solar maximum. Finally, the Martian response during d usty periods is predicted to be much larger than that typically observ ed for Earth. Martian dust-driven tides, especially during solar minim um time periods, cannot be ignored when addressing the Mars thermosphe ric structure and dynamics.