SOLAR-WIND MEASUREMENTS NEAR MARS AND THEIR IMPLICATION IN THE RED-PLANET ENVIRONMENT

The Phobos 2 spacecraft was operational around Mars for almost two mon ths. From 29 January to 27 March 1989, the Martian bow shock was cross ed at least 200 times and the upstream regions extensively probed. The solar wind density and velocity deduced from the a.c. electric-field measurements performed with the PWS plasma and wave experiment are com pared with those of the ASPERA 3-D plasma composition experiment. A re asonably good agreement is found between the 12 min averages of the pl asma density numbers derived from PWS and ASPERA (considering the poor precision of the instruments): they differ by at most a factor of fou r. The solar wind density is found to decrease when the flow speed inc reases, in agreement with the stream structure of the solar wind, whic h results from heliomagnetic latitude dependencies of solar wind param eters. The solar wind ram pressure, which is essential to study the so lar wind-Mars interaction, is also estimated : the absence of a clear relationship between this dynamic pressure and the bow shock location is confirmed. The bow shock observations performed during very high so lar activity db not support the hypothesis that Mars possesses a signi ficant intrinsic magnetic field.