Position and shape of the Martian bow shock: the Phobos 2 plasma wave system observations

Abstract

We have derived a Martian bow shock model using all the data that are available from the plasma wave system on the Phobos 2 spacecraft. In contrast with a previous study, performed with a limited data set of 26 shock encounters, we have considered about 120 shock crossings, including four new distant downstream shocks. Thus, the shock position is determined accurately at the nose (four inbound shocks with the elliptical orbits), at the terminator (more than 110 inbound and outbound shocks in the circular orbits), and far downstream (four outbound shocks with the elliptical orbits). Emphasis is placed upon comparisons with the results of earlier studies derived from smaller data sets, data sets that come from other experiments and/or other missions. The location of the bow shock of Mars seems to respond to variations in solar activity, as is the case for Venus. On the other hand, the subsolar stand-off distance appears to vary slightly with the solar wind dynamic pressure: the greater the subsolar shock distance is from the planet, the lower the solar wind dynamic pressure. Assuming a sixth-root dependence. an upper limit of a possible Martian magnetic moment of 2.2 × 10 12Tm 3 has been obtained. Finally, the shock is closer to Mars on the dawn side than on the dusk side at the terminator: the angle between the local bow shock normal and the upstream magnetic field controls this asymmetry.