PROTON FLOW IN THE MARTIAN MAGNETOSHEATH

The Automatic Space Plasma Experiment with a Rotating Analyzer (ASPERA ) measurements on board the Phobos 2 spacecraft gave, for the first ti me, a three-dimensional (3-D) picture of the proton flow around Mars. The measurements from the circular orbits of Phobos 2 are well suited to study the bow shock at the terminator region, the nightside magneto sheath, and the tail region. Moreover, measurements from the elliptica l orbits offer dayside magnetosheath data. In this work, all circular orbits (11) where there was enough information for 3-D velocity calcul ations are analysed. The solar wind deflection at the bow shock and th e disappearance of the flow near the optical shadow of Mars are found to be typical features on all circular orbits. A dawn-dusk asymmetry i s detected in many cases as well. When the results are compared to a g asdynamic model, the locations of the observed boundaries and the gene ral behavior of the flow are found to be quite consistent with the mod el. The region where proton particle flux decreases significantly, ref erred to as a magnetopause near the optical shadow of Mars, was typica lly found near the magnetic field maximum. The magnetopause was thus i nside the so-called magnetic tail boundary, which is defined to be at the broad magnetic minimum between the bow shock and the central curre nt sheet. The magnetic tail boundary may be related to the O+ pick-up ions because the mass loading boundary also lies between the shock and the magnetopause. Because the proton flow may behave differently in t he dayside than in the nightside magnetosheath, the 3-D velocities are calculated on two elliptical orbits as well. However, in these cases the nature of the flow is not possible to determine as reliably as nea r the terminator.