Structure of the magnetic pileup boundary at Mars and Venus -: art. no. A01209

[1] The magnetic pileup boundary (MPB) is a sharp, thin, and well-defined plasma boundary located between the bow shock and the inner ionospheric boundary at comets, Mars, and Venus. This boundary separates the magnetosheath, a region of low magnetic fields with a conspicuous wave activity, from the magnetic pileup region dominated by strong, highly organized magnetic fields as a result of the pileup and draping of the interplanetary magnetic field. In the present paper we study the magnetic structure of the magnetic pileup boundary at Mars and Venus by means of the technique of minimum variance of the magnetic field. For each one of the crossings analyzed, we obtain a very well defined minimum variance vector. At Mars the direction of this vector agrees with the normal to the MPB fit obtained from Mars Global Surveyor crossings. The results confirm that the MPB is a well-defined plasma boundary. According to empirical criteria based on minimum variance analysis results, the Martian and Venusian MPB crossings would resemble an MHD tangential discontinuity rather than a rotational discontinuity. However, spacecraft observations suggest that the nature of the MPB could be far more complex. We compare our results with similar studies at the MPB of comets and the magnetic tail boundary of Titan, and we discuss the nature of the boundary from a general perspective.