Structure and variability of the Martian MPB and Bow Shock from MGS MAG/ER observations

BERTUCCI, C.; MAZELLE, C.; ACUNA, M.

Congreso; 35th COSPAR Scientific Assembly; 2004

The observations of the MAG/ER (Magnetometer / Electron Reflectometer) investigation aboard Mars Global Surveyor (MGS) during the aerobraking phase around Mars allowed a deep characterization of the bow shock and the magnetic pileup boundary (MPB). The Martian bow shock crossings are identified by an increase in the magnetic field magnitude and in its fluctuation level, and a simultaneous increase in the superthermal fluxes, as the plasma is compressed and heated. All these crossings display a complex structure, even when the angle between the shock normal and the magnetic field is close to 90 degrees. This feature makes Mars' bow shock more similar to comets' than Venus'. On the other hand, the MPB is perhaps one of the most spectacular features in the Mars-Solar wind interaction. This boundary represents the outer edge of the magnetic barrier. 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. A recenty discovered MPB feature, the enhancement of the magnetic field line draping, allowed to identify this boundary also at Venus. In this talk we analyze the structure, location and variability of these two boundaries from MGS MAG/ER data and we discuss the influence of different factors as solar cycle, upstream solar wind conditions, and mass loading. The role of the exosphere / ionosphere in their nature and behavior is also studied.