Mercury's Core Field: Beyond the Offset Axial Dipole

Since 2008, the core field of Mercury has been studied using MESSENGER satellite magnetic field data. It is now well-known to be highly axisymmetric at the longest wavelengths, but asymmetric about the geographic equator. Yet, structure in the field above spherical harmonic degree and order five (i.e. at wavelengths less than 3000 km) and nonzonal structure at any wavelength remain poorly understood. Addressing this issue is important because these higher-degree-and-order spherical-harmonic terms can offer clues as to the processes that drive Mercury's dynamo. Among the greatest difficulties when inverting satellite observations for a Mercury core-field model are the magnetic signatures of field-aligned currents that vary with solar wind conditions and local time, have spatial scales of O(1000 km) and increase with proximity to the planet. These signals are stronger than the residual core field after subtraction of an offset-dipole core field model together with magnetopause and magnetotail fields. Because the sources of these contributions lie within the region of data acquisition, and because they increase with decreasing altitude we cannot make use of traditional potential-field internal/external separation approaches. We take advantage of the local-time spatial structure of the field-aligned currents by fitting and subtracting low-degree vector spherical harmonic fields from data grouped by altitude above the planet's surface. The radial field component in the remaining nighttime data for quiet orbits shows large-scale patterns in the geographic reference frame, but no substantial structure in the local time frame. The intensity of these patterns increases with decreasing altitude, indicating that the core is their likely origin.