Large-Amplitude Oscillatory Motion in Mercury's Cross-tail Current Sheet

Earlier MESSENGER studies on reconnection-driven space plasma phenomena (e.g. flux ropes formation, dipolarization and substorm) at Mercury had revealed a structurally complex and highly dynamic cross-tail current sheet. Here, we report and analyze MESSENGER observations of large-amplitude oscillatory motions of Mercury's cross-tail current sheet, which are characterized by multiple reversals of B_X (i.e. current sheet crossings) and decrease in magnetic field intensity. We surveyed four years of MESSENGER magnetic field data and identified intervals of multiple large-amplitude current sheet crossings (i.e. oscillatory motion of Mercury's current sheet). We then performed minimum variance analysis (MVA) on each current sheet crossings (i.e. half waveform) to determine the current sheet normal. Our results show that the current sheet normal vectors points predominantly in the y and z-directions, with the y-component of the vector reversing polarity between adjacent half waveforms of the oscillation. Our results are consistent with earlier terrestrial studies, which suggest that these current sheet waves most likely originate from the center of the current sheet and propagate in the cross-tail direction. Statistical analysis of the wave properties also shows that the oscillatory motion of Mercury's current sheet has wave periods and amplitude ranging from 5-10s and 20-50 nT. We also correlate the occurrence of multiple current sheet crossings with solar wind IMF and Mercury's magnetotail activity level to determine if these waves are driven internally (or externally). Possible excitation mechanism of these current sheet oscillatory motions will also be discussed.