Study of the solar wind deceleration upstream of the Martian terminator bow shock

Abstract

Solar wind plasma and magnetic data obtained near the Martian terminator bow shock by the TAUS energy spectrometer and the MAGMA magnetometer onboard the Phobos 2 spacecraft are analyzed. It is revealed that on average the solar wind stream is slowing down just upstream of the bow shock. Nearly inverse correlation is found between the values of the velocity decrease and the undisturbed solar wind density for the outbound (mostly quasi‐parallel) bow shock crossings, while for the inbound crossings (mostly quasi‐perpendicular) this correlation is observed only for the velocity decrease upstream of the shock foot. This result permits us to distinguish between the two possible reasons causing solar wind deceleration: (1) mass loading of the solar wind flow by planetary ions originating from the corona of Mars, and (2) solar wind protons reflected from the bow shock. The solar wind deceleration upstream of the bow shock foot turned to be approximately dawn‐dusk symmetric. On the basis of the revealed relation between the velocity decrease and upstream solar wind density, a coronal density profile is deduced which is in agreement with earlier results for the subsolar region. According to this profile, the density of the hot oxygen corona of Mars might be ∼ 3 times higher in the period of observations than the estimations of the “extreme” corona model suggests, even taking into account the possible contribution of the hydrogen corona to the solar wind deceleration effect.