Mercury’s Internal Magnetic Field: Results from MESSENGER’s Low-altitude Campaign
Tuesday, 16 December 2014
Magnetic field measurements made by the MESSENGER spacecraft in orbit around Mercury have shown that, to first order, Mercury’s internal field can be described by an axially aligned dipole, offset by 479 km north of the geographic equator (the offset axial dipole, hereafter OAD). Near-periapsis MESSENGER magnetic field measurements at altitudes less than 200 km have been obtained since April 2014. We use these observations, together with higher altitude data from orbits that have been characterized with low magnetic activity , to identify non-OAD internal field structure and to establish whether it is of crustal and/or core origin. Magnetospheric models developed with MESSENGER data allow estimated contributions from magnetopause, magnetotail, and OAD fields to be subtracted from vector magnetic field measurements, and the sources of residual signatures to be examined. For measurements made at spacecraft altitudes above 200 km, determining the magnitude and sources of additional regional and global-scale contributions to the internal field has been challenging because of MESSENGER’s orbit geometry and because the residuals are dominated by additional external fields that are organized in the local time frame and that vary with magnetospheric activity. After accounting for the large-scale magnetospheric fields, any additional external field contributions to the residual fields are estimated empirically in the local time frame. We investigate crustal and core contributions to the remaining signals, in particular to the low altitude signals, by examining repeatability in the body-fixed frame and using global (spherical harmonic) and local (equivalent source dipole) basis functions with regularization. Crustal sources associated with large-scale regional geological provinces such as the northern lowlands, the northern rise, and major impact basins are investigated and equivalent spherical harmonic core/crustal field spectra computed.