A modified Equivalent Source Dipole method to model partially distributed magnetic field measurements, with application to Mercury

Research areas:
J. Geophys. Res.
Hermean magnetic field measurements acquired over the northern hemisphere by the
MErcury Surface Space ENvironment GEochemistry, and Ranging (MESSENGER) spacecraft provide crucial
information on the magnetic field of the planet. We develop a new method, the Time Dependent Equivalent
Source Dipole, to model a planetary magnetic field and its secular variation over a limited spatial region.
Tests with synthetic data distributed on regular grids as well as at spacecraft positions show that our
modeled magnetic field can be upward or downward continued in an altitude range of ~300 to 1460 km
for regular grids and in a narrower range of 10 to 970 km for spacecraft positions. They also show that
the method is not sensitive to a very weak secular variation along MESSENGER orbits. We then model the
magnetic field of Mercury during the ~rst four individual sidereal days as measured by MESSENGER using
the modi~ed Equivalent Source Dipoles scheme and excluding the secular variation terms. We ~nd a
dominantly zonal field with small-scale nonaxisymmetric features corotating with the Sun in the Mercury
Body Fixed system and repeating under similar local time, suggestive of external origin. When modeling
the field during one complete solar day, these small-scale features decrease and the field becomes
more axisymmetric. The lack of any coherent nonaxisymmetric feature recovered by our method, which
was designed to allow for such small-scale structures, provides strong evidence for the large-scale and
close-to-axisymmetry structure of the internal magnetic field of Mercury.