3D PLASMA OBSERVATIONS NEAR MARS

3-dimensional (3D) proton velocity data measured by the ASPERA mass sp ectrometer onboard the Phobos-2 spacecraft am presented. The mass spec trometer was capable of providing 3D plasma distribution data by scann ing through the 4pi space, but most of the time its own scanner was sw itched off, and only 2D data have been presented so far. Because the s pacecraft was often spinning around its Sun-pointing axis, the specifi c data treatment presented in this paper gives a possibility to restor e the full angular coverage and obtain 3D vectors of the proton veloci ty near Mars for the first time. For the circular orbits analysed in t his work the flow was deflected at the bow shock and almost stopped at a boundary termed the magnetopause. The main features of the flow and the location of the boundary are compared with a gas-dynamic model of the flow. In general, the model and data are in good agreement. Other observed effects, possibly associated with reflected and pick-up ions , non-thermal proton population, and/or mass loading, which are not ta ken into account in the gas dynamic model, are discussed.