STUDY OF THE SOLAR-WIND DECELERATION UPSTREAM OF THE MARTIAN TERMINATOR BOW SHOCK

Solar wind plasma and magnetic data obtained near the Martian terminat or bow shock by the TAUS energy spectrometer and the MAGMA magnetomete r onboard the Phobos 2 spacecraft are analyzed. It is revealed that on average the solar wind stream is slowing down just upstream of the bo w shock. Nearly inverse correlation is found between the values of the velocity decrease and the undisturbed solar wind density for the outb ound (mostly quasi-parallel) bow shock crossings, while for the inboun d 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 so lar wind deceleration: (1) mass loading of the solar wind flow by plan etary ions originating from the corona of Mars, and (2) solar wind pro tons reflected from the bow shock. The solar wind deceleration upstrea m 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. Ac cording to this profile, the density of the hot oxygen corona of Mars might be similar to 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 s olar wind deceleration effect.