Structure and Variability of Low-Energy Ions in Mercury's Magnetosphere: Initial Results from BepiColombo Mio MIA Observations
Résumé
We present initial results from low-energy ion measurements by the Mercury Ion Analyzer (MIA) on board BepiColombo Mio during the first and second Mercury flybys. The orbital configurations of the two flybys were very similar, but the ion properties observed by MIA are significantly different presumably because of different upstream solar wind conditions. Specifically, the ion energies are generally lower during the first flyby than the second flyby, suggesting slower solar wind conditions in the former case. The ion energy spectra obtained during the first flyby suggest the presence of a relatively cold dense ion component in the midnight magnetotail, sources and transport mechanisms of which remain elusive. Additionally, by utilizing an MIA data product that is originally designed to separate ion directions according to the spacecraft spin phase, we derive bonus data of high-time resolution (>~1 s) ion flux measurements from non-spinning observations during the cruise phase. Such high-time resolution measurements could be useful to investigate boundary dynamics, ion kinetics, etc. These flyby observations with limited capabilities suggest rich dynamics of low-energy ions in Mercury's magnetosphere, more complete views of which will be derived from future in-orbit observations of full 3-dimensional ion velocity distribution functions by MIA, along with ion composition and magnetic field measurements by other instruments on Mio.