MESSENGER observations of flux ropes and reconnection fronts: locations of the near tail reconnection site at Mercury

A statistical study of flux ropes and reconnection fronts (also called dipolarization fronts) base on MESSENGER magnetic field and plasma observations and its implication for the distribution of magnetic reconnection site in the Mercury's magnetotail has been performed in this research. We have surveyed the plasma sheet crossings at Mercury during three hot seasons of MESSENGER for flux ropes and reconnection fronts, which are believed to be highly related with magnetic reconnection. During the three hot season orbits. MESSENGER crossed the plasma sheet were mostly at a distance between - 2 RM and -3 RM, which is the region of previous determined location of Near Mercury Neutral Line. Thirty-nine flux ropes and 92 reconnection fronts were identified. The occurrence frequency distribution of flux ropes and reconnection fronts in the plasma sheet shows a clear dawn-dusk asymmetry with higher occurrence frequency on the dawn side than the dusk side. This suggests that magnetic reconnection in Mercury's magnetotail happens more frequently on the dawn side than dusk side plasma sheet. This is opposite to the observations in Earth's magnetotail showing magnetic reconnection more frequently occurs on the dusk side than the dawn side plasma sheet. The distribution of plasma sheet thickness shows that plasma sheet near the midnight is the thinnest part and does not show obvious asymmetry. Thus, the reasons that cause magnetic reconnection to preferentially occur on the dawnside of the magnetotail at Mercury may not be the plasma sheet thickness and require further study. The peak occurrence rates of flux ropes and reconnection fronts in Mercury's plasma sheet are 60 times higher than that of Earth's values, which we interpret to be due to the highly variable magnetospheric conditions at Mercury. Such higher occurrence rate of magnetic reconnection would generate more plasma flows in the dawnside plasma sheet than in the duskside. These plasma flows would mostly brake and initiate the substorm dipolarization on the postmidnight sector at Mercury rather than the premidnight susbtorm onset location at Earth.