Asymmetric Penetration of Shocked Solar Wind Down to 400-km Altitudes at Mars

Abstract:

The solar wind interaction with a planetary atmosphere produces the magnetic pile-up region near the planet. The Martian magnetic pile-up region prevents direct penetration of shocked solar wind (magnetosheath) plasma and the penetration boundary is typically located at altitudes above 800 km. Spacecraft measurements have shown, however, that magnetosheath plasma occasionally penetrates into low altitudes below 400 km. Here we used Mars Global Surveyor magnetic field and electron observations during the period from April 1999 to November 2006 to investigate the magnetosheath penetration events. We identified 240 events and found that both solar wind dynamic pressure (Psw) and the orientation of the interplanetary magnetic field (IMF) control the occurrence of the events. The magnetosheath penetration events during the low Psw periods tend to be distributed in low latitudes of the northern hemisphere where the crustal magnetic field is weak, while the event locations widely distributed in terms of the latitude under high Psw conditions. During the low Psw periods, a remarkable feature is that the observational probability is approximately 7 times larger during periods of the “away” IMF sector than during the “toward” sector. These results thus indicate that the magnetosheath penetrations into Martian upper atmosphere more often occur in the upward electric field hemisphere than in the downward electric field hemisphere. Large-amplitude undulation excited by the Kelvin-Helmholtz instability in the upward electric field hemisphere previously suggested from global hybrid simulation can be one of candidate processes to cause the asymmetric penetration during low Psw periods.