Modeling Mercuryʹs internal magnetic field with smooth inversions

Three-component vector magnetic field observations from MESSENGERʹs first flyby of Mercury have confirmed the presence of an internal field, along with external fields due to magnetospheric current systems. We use techniques from inverse theory to investigate structure in Mercuryʹs internal magnetic field permitted by the Mariner 10 and MESSENGER flyby data and recoverable from orbit. We correct the flyby observations using external fields predicted by a parameterized magnetospheric model, and we include noise contributions from long-wavelength uncertainties in the external field and from un-modeled short-wavelength features. The internal field is represented by a spherical harmonic expansion to degree and order 8, with regularization constraints applied to the power spectrum. Latitudinal and longitudinal structure in the field is required in order to fit the data. Enhanced radial magnetic field in the region of the Mariner 10 and MESSENGER flyby latitudes is seen. Contributions to the internal field predicted by Aharonson and others for a long-wavelength crustal field are present, but the field is dominated by the g01 term rather than the predicted g03 term. Observations from MESSENGERʹs second and third flybys will provide additional low-altitude coverage at equatorial latitudes, while the orbital phase of the mission will provide high-latitude, northern-hemisphere coverage. We investigate recovery of three simulated core fields using the cumulative data coverage that will be obtained from the three MESSENGER flybys and MESSENGERʹs orbital phase, under the assumption that long-wavelength external fields can be modeled and removed. This provides a best-case scenario for internal field modeling. The results show excellent recovery of the dipole field and of field structure at mid to high northern latitudes with spatial scales equivalent to degree and order 10, providing encouraging results for the identification and characterization of core fields.