The structure of the magnetic pileup boundary at unmagnetized bodies.

BERTUCCI, C.; MAZELLE, C.; RUSSELL, C.; SLAVIN, J.; ACUNA, M.

Congreso; 35th COSPAR Scientific Assembly; 2004

The absence of global-scale, dynamo-generated magnetic fields and the presence of an atmosphere at Mars, Venus and comets make their solar wind interactions very similar. The major features of these interactions are the formation of a magnetic barrier in front of a highly conducting obstacle on the dayside and an induced magnetic tail on the nightside. The outer edge of the magnetic barrier is the Magnetic Pileup Boundary (MPB). At Mars and comets, the MPB has been identified by very clear observational signatures, including a strong gradient in the magnetic field magnitude accompanied by a decrease in the magnetic field fluctuations and a drastic decrease in the solar wind electron and proton densities, as exospheric-induced ions become dominant. Recently, the enhancement of the magnetic field draping has been confirmed to be another distinctive signature of the MPB. This led to the identification of this boundary at Venus for the first time. We study the magnetic structure of the magnetic pileup boundary at Mars and Venus by performing minimum variance analysis on Mars Global Surveyor and Pioneer Venus magnetic field data. For each crossing, the normal vector to the current sheet is very well defined. At Mars, its direction is in very good agreement with the normal vector deduced from a fit of the Martian MPB. We compare its thickness with physical scales, and study its temporal variations. The results suggest that this well-defined plasma boundary resembles to a tangential discontinuity in the strict MHD sense. However, its nature seems to be related to the multi-ionic nature of the interaction of the solar wind with these objects. Finally, these results are compared with previous analysis at the MPB of comets and at the tail boundary of Venus.