INDIRECT DETECTION OF THE MARTIAN HELIUM CORONA

The ion composition mass spectrometer ASPERA on board the PHOBOS 2 spa cecraft detected particles with M/q=4 in the vicinity of Mars. A signi ficant difference between the measured particle velocity and the solar wind velocity suggests that these ions are of planetary origin, appar ently He+ from ionisation within the Martian helium corona. The partic les had typical energies of either more than 10 keV or about 500 eV. T he former correspond to ion pickup in the solar wind and the latter mi ght be ions extracted from the upper ionosphere by an electric field. The observed density of pickup He+ ions was 0.02 - 0.1 cm-3 and the He + density in the plasmasheet was of 0.2 - 0.7 cm-3. According to a rec ent model of the Martian neutral atmosphere [Moroz et al, 1991], the H e+ density could reach 0.2 cm-3 at the Phobos orbit. Such values give mass densities comparable to those of the solar wind. Thus, helium may play a role in the solar wind mass loading process near Mars.