On the momentum transfer of the solar wind to the Martian topside ionosphere

Hot plasma measurements from the Soviet Phobos‐2 spacecraft in the Martian magnetosphere suggests that the solar wind interaction with Mars is cometary‐like, with mass loading of the solar wind and ion pick‐up occurring also outside the subsolar bow‐shock. The interaction is characterized by a pronounced decrease of the solar wind speed inside what has teen termed the mass‐loading boundary (MLB). Well outside the MLB, the ion pick‐up process acts in a “normal” sense. There ions pick up approximately the solar wind velocity‐independent of mass. Inside the MLB, the momentum loss of solar wind ions is more pronounced—heavy ions of Martian origin taking up most of the solar wind ion momentum flux. The heavy mass‐loading of solar wind ions in the innermost part of the Martian boundary layer (near the “magnetopause”) leads to a “loaded” ion pick‐up. The process can be understood as internal loading of an MHD‐dynamo, propelled by a driver plasma ‐ the solar wind. The ASPERA ion composition and momentum data is consistent with such a pick‐up process. Inside the “magnetopause” ions of Martian origin are accelerated up to energies close to those of the solar wind protons. We propose two types of acceleration processes, one similar to that acting within the Earth's auroral acceleration region (acting in the presence of an ambient magnetic field), another a pick‐up process acting within a limited spatial region.