MESSENGER observations of fast plasma flows in Mercury's magnetotail

We present the first observation of mean fast plasma flows in Mercury's magnetotail. Mercury experiences substorm activity phenomenologically similar to Earth's, however, it has not been possible to detect fast flows in the plasma sheet due to the Fast Imaging Plasma Spectrometer (FIPS) field of view being occluded by MESSENGER's sunshade and spacecraft body. In these incomplete plasma distributions, however, sufficiently low Mach number flows would impart an asymmetry in the partial plasma distribution, even if the flow directions were outside the field of view. We combine FIPS observations from 387 intervals containing magnetotail dipolarizations to mitigate these instrument limitations. By taking advantage of the variations in spacecraft pointing during these intervals, we construct composite plasma distributions from which mean flows are determined. We find that dipolarizations at Mercury are embedded within fast sunward flows with an averaged speed of 300 km/s. This flow speed implies a convective electric field of 11 mV/m and a magnetic flux transport of 0.06 MWb associated with dipolarizations, suggesting multiple ( 10) dipolarizations are required to unload Mercury's magnetotail during substorm expansion. By comparison, we find the mean sunward flow during non-dipolarization intervals to be 50 km/s, corresponding to a cross-tail potential of 23 kV.