MARS IONOSPHERE: A REVIEW OF EXPERIMENTAL RESULTS AND MODELING STUDIES

In this paper we review results from atmospheric and ionospheric experiments on the early planetary missions like the Mariners, Mars, and Viking 1 and 2 Orbiters/Landers. We then discuss the new results obtained from the two latest missions, namely, the Mars Global Surveyor (MGS) and Mars Express (MEX). The MGS had three ionospheric and atmospheric related experiments, namely, (1) the radio science experiment, which generated 5600 electron density profiles covering a major portion of sunspot cycle 23; (2) the magnetometer/electron reflectometer experiment, which very clearly answered the question about the presence or absence of Martian intrinsic magnetic field; and (3) the accelerometer experiment, which provided a large database of atmospheric density at various Martian locations during the aerobraking phases. The topside sounder on the MEX provided electron density profiles for altitudes above the primary ionospheric peak with a very high time resolution, thereby providing opportunity for exploring ionospheric conditions during events of rapid changes like solar flares. Unlike Venus, where simultaneous electron density, ion density, and magnetic field measurements were made, Mars lacks this kind of information. Consequently, most of our current understanding of Mars' plasma environment is based on theoretical models. We therefore review the various atmospheric and ionospheric models for Mars, which have been generated during the last 4 decades.