First measurements of plasma waves near Mars

HERE we report preliminary results from electic field measurements in the environment of Mars using the plasma-wave system on board the Soviet spacecraft Phobos 2. It also includes a Lang-muir probe which measured plasma densities. Electron-plasma oscillations observed upstream of the bow shock correspond to a solar-wind density of 2 cm^-3. The shock-foot boundary was crossed up to three times on each orbit. The shock ramp was detected at altitudes between 0.45 and 0.75 Mars radii (R_M) above the planetary surface. The density increased by about a factor of two at the ramp. The shock position, although variable, seemed to be consistent with previous measurements. The downstream mag-netosheath contained broadband electric-field noise below the plasma frequency. The boundary of the obstacle, or planetopause, was crossed at altitudes of the order of 0.28 R_M; the cold plasma density was highly variable within the planetopause and reached the unexpected value of 700 cm^-3 on the third orbit, at 0.25 R_m altitude. Bursts of waves with frequencies below the electron cyclotron frequency, possibly in the whistler mode, occur within the planetopause.