INDIRECT DETECTION OF THE MARTIAN HELIUM CORONA

The ion composition mass spectrometer ASPERA on board the PHOBOS 2 spacecraft detected particles with M/q=4 in the vicinity of Mars. A significant difference between the measured particle velocity and the solar wind velocity suggests that these ions are of planetary origin, apparently He+ from ionisation within the Martian helium corona. The particles had typical energies of either more than 10 keV or about 500 eV. The former correspond to ion pickup in the solar wind and the latter might 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 recent model of the Martian neutral atmosphere [Moroz et al, 1991], the He+ 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.