Asymmetric penetration of shocked solar wind down to 400 km altitudes at Mars

The penetration boundary of shocked solar wind (magnetosheath) into the Martian upper atmosphere is typically located at altitudes above 800km. However, magnetosheath plasma occasionally penetrates into low altitudes below 400km. Here we used Mars Global Surveyor magnetic field and electron observations from April 1999 to November 2006 to investigate the magnetosheath penetration events. We identified 1145 events and found that both solar wind dynamic pressure (P-dyn) and the orientation of the interplanetary magnetic field (IMF) control the occurrence of the events. The magnetosheath penetration events during low P-dyn periods tend to be distributed in low latitudes of the northern hemisphere or where the crustal magnetic field is weak, while the event locations are widely distributed in terms of the latitude under high P-dyn conditions. During low P-dyn periods, a remarkable feature is that the observational probability is approximately 2.4 times larger during periods of the away IMF sector than during the toward sector. The northern hemisphere during the away sector corresponds to the upward electric field hemisphere due to the convection of draping solar wind origin magnetic flux tubes. These results thus indicate that the magnetosheath penetrations into Martian upper atmosphere more often occur in the upward electric field hemisphere than the downward hemisphere during low P-dyn periods. Large-amplitude undulation excited by the Kelvin-Helmholtz instability in the upward electric field hemisphere is a candidate process to cause the asymmetric penetration during low P-dyn periods. Another possibility might be the mirror-mode instability by the asymmetric distribution of planetary pickup ions.