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Keywords:

  • MESSENGER Magnetometer;
  • Mercury's exosphere;
  • Mercury's magnetic field;
  • Mercury's northern cusp

[1] The magnetic cusp of a planetary magnetosphere allows solar wind plasma to gain access to the planet's magnetosphere and, for Mercury, the surface. From measurements by the MESSENGER Magnetometer we have characterized the magnetic field in the northern cusp region of Mercury. The first six months of orbital measurements indicate a mean latitudinal extent of the cusp of ∼11°, and a mean local time extent of 4.5 hrs, at spacecraft altitudes. From the average magnetic pressure deficit in the cusp, we estimate that (1.1 ± 0.6) × 1024 protons s−1 bombard the surface over an area of (5.2 ± 1.6) × 1011 m2near the northern cusp. Plasma pressures in the cusp are 40% higher when the interplanetary magnetic field (IMF) is anti-sunward than when it is sunward. The influence of the IMF direction does not overcome the north-south asymmetry of Mercury's internal field, and particle flux to the surface near the southern cusp is predicted to be a factor of 4 greater than in the north. The higher particle flux impacting the surface in the south should lead to a greater exospheric source from the south and a higher rate of space weathering than in the area of the northern cusp.