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Keywords:

  • Mercury;
  • magnetic field;
  • magnetosphere;
  • plasma

[1] Since insertion of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft into orbit around Mercury on 18 March 2011, the probe's Magnetometer has routinely observed localized reductions of the magnetic field magnitude below the level predicted by a planetary dipole model corrected for magnetospheric magnetic fields. These magnetic depressions are observed on almost every orbit, and the latitude at which they are observed is local-time dependent. The depression signatures are indicators of the presence of enhanced plasma pressures, which inflate the magnetic field locally to maintain pressure balance, thus lowering the magnetic flux density. Mapping the magnetic depressions in local time and latitude provides insight into the plasma distribution near the planet, which complements that provided by MESSENGER's Fast Imaging Plasma Spectrometer. The spatial distribution shows that magnetic depressions are concentrated in two distinct regions, one near the equator on the nightside and another at high latitudes principally on the dayside. Here we focus on the nightside, equatorial pressure signatures, which we attribute to the magnetotail plasma sheet. The plasma-sheet pressures extend from dusk to dawn and are offset northward from the planetary geographic equator by about 10° in latitude, commensurate with the offset of the planetary dipole. The pressures associated with the plasma-sheet depressions range from 0.1 to 3 nPa and are systematically higher at dawn than at dusk. Proton gradient-curvature and convection drift in Mercury's dipole magnetic field with a dawn-to-dusk electric field result in low drift velocities near dawn, leading to systematically higher densities and pressures at dawn than at dusk, consistent with the observations.