Multi-instrument study of the upstream region near Mars' The Phobos 2 observations

Abstract : Multi-instrument data analysis of the observations made by the Phobos 2 spacecraft in the upstream Martian environment shows that the solar wind deceleration observed in elliptical and circular orbits and previously referred to as mass loading by planetary exosphere occurs mainly in the foreshock region. Variations of the bulk speed are accompanied by strong magnetic field perturbations. The flow and field perturbations are well correlated, indicating their Alfven wave origin. The data suggest that the observed disturbances are caused by large-amplitude Alfven waves propagating in the solar wind as well as by waves generated locally at the foreshock. It is shown that the foreshock boundary is not just a topological boundary separating the regions either magnetically connected or disconnected with the bow shock. The transition across the tangent line is often accompanied with a sharp rotation of the magnetic field, decrease of the magnetic field value, and increase of the proton number density and temperature. Although variations of the solar wind speed at R _>6000 km are mainly controlled by Alfven waves, at closer distances to the planet, where the solar wind slows down at 150-200 km s-•, perturbations of the field and velocity do not follow the relation common for Alfven waves. This strong deceleration can be attributed to mass loading on oxygen atmosphere.