3D Plasma Observations Near Mars

3‐dimensional (3D) proton velocity data measured by the ASPERA mass spectrometer onboard the Phobos‐2 spacecraft are presented. The mass spectrometer was capable of providing 3D plasma distribution data by scanning through the 4π space, but most of the time its own scanner was switched off, and only 2D data have been presented so far. Because the spacecraft was often spinning around its Sun‐pointing axis, the specific data treatment presented in this paper gives a possibility to restore the full angular coverage and obtain 3D vectors of the proton velocity near Mars for the first time. For the circular orbits analysed in this 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 taken into account in the gas dynamic model, are discussed.