Mercury's surface magnetic field determined from proton‐reflection magnetometry

Abstract

Solar wind protons observed by the MESSENGER spacecraft in orbit about Mercury exhibit signatures of precipitation loss to Mercury's surface. We apply proton‐reflection magnetometry to sense Mercury's surface magnetic field intensity in the planet's northern and southern hemispheres. The results are consistent with a dipole field offset to the north and show that the technique may be used to resolve regional‐scale fields at the surface. The proton loss cones indicate persistent ion precipitation to the surface in the northern magnetospheric cusp region and in the southern hemisphere at low nightside latitudes. The latter observation implies that most of the surface in Mercury's southern hemisphere is continuously bombarded by plasma, in contrast with the premise that the global magnetic field largely protects the planetary surface from the solar wind. Key Points Mercury's offset dipole is confirmed at the surface using proton reflectometry Loss cones are oberved in northern cusp and on low‐southern‐latitude nightside Mercury's southern hemisphere is bombarded by the solar wind and plasma sheet