Conditions in the Martian ionosphere/atmosphere from a comparison of a thermospheric model with radio occultation data

In this paper a possibility to find out some effect of dipole-like intrinsic magnetic field on the plasma scale height at altitudes 180-240 km in the Martian ionosphere, that is above the topside boundary of the chemically controlled region predicted by existing models of the magnetic field-free ionosphere, is examined. It is assumed that the effect could be detected if the relationship between plasma and neutral atmosphere scale heights which is expected under the photochemical equilibrium conditions in the multicomponent atmosphere remains valid at altitude region 180-240 km. The plasma scale heights assessed from the data collected during the Mariner 9 and Viking 1 sessions are statistically analyzed and its trends with the solar zenith angle and F-10.7 flux are evaluated A half of statistically averaged scale height presented as a function of the solar zenith angle (SZA) and F-10.7 flux is compared with the neutral atmosphere scale heights, calculated from the model of Izakov and Roste (1996) for the corresponding SZA and F-10.7 fluxes. Since the conditions for the radio occultation experiments during the Mariner 9 and Viking 1 sessions are different a ratio of assessed plasma scale height to the appropriate neutral atmosphere scale height suits better for comparison than the average scale heights itself does. Employing such ratios the effect of dust content within the lower atmosphere on the thermal balance within the topside atmosphere at altitude 180-200 km is estimated. (C) 1998 Elsevier Science Ltd. All rights reserved.