THE MARS THERMOSPHERE-IONOSPHERE - PREDICTIONS FOR THE ARRIVAL OF PLANET-B

The primary science objective of the Planet-B mission to Mars is to st udy the Martian upper atmosphere-ionosphere system and its interaction with the solar wind. An improved knowledge of the Martian magnetic fi eld (whether it is induced or intrinsic) is needed, and will be provid ed by Planet-B. In addition, a proper characterization of the neutral thermosphere structure is essential to place the various plasma observ ations in context. The Neutral Mass Spectrometer (NMS) onboard Planet- B will provide the required neutral density information over the altit ude range of 150-500 km. Much can be learned in advance nf Planet-B da ta taking aa multi-dimensional thermosphere-ionosphere and MHD models are exercised to predict the Mars near-space environment that might be expected during the solar maximum conditions of Cycle 23 (1999-2001). Global model simulations of the Mars thermosphere-ionosphere system a re presented and analyzed in this paper. These Mars predictions pertai n to the time of Planet-B arrival in October 1999 (F10.7 similar to 20 0; Ls similar to 220). In particular, the National Center for Atmosphe ric Research (NCAR) Mars Thermosphere General Circulation Model (MTGCM ) is exercised to calculate thermospheric neutral densities (CO2, CO, N-2, O, Ar, O-2), photochemical ions (CO2+, O-2(+), O+ below 200 km), neutral temperatures, and 3-components winds over 70-300 km. Cases are run with and without dust loading of the lower atmosphere in order to examine the potential impacts of dust storms on the thermosphere-iono sphere structure. Significant dust-driven impacts are predicted in the lower thermosphere (100-120 km), but are less pronounced above 150 km . The ionospheric peak height changes greatly with the passage of a Ma rs global dust storm event. In addition, Martian dayside exobase tempe ratures are generally warmer during dusty periods, in accord with Mari ner 9 UVS data (Stewart et al., 1972). During the Planet-B mission, th e NMS team intends to use the MTGCM as a facility tool whose simulated output can be utilized to aid various investigations.