Birkeland Currents and Their Closure at Mercury

Abstract:

Signatures of Birkeland currents at Mercury have been identified in data returned from the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit about Mercury. Sunward magnetic perturbations over the northern polar region are consistent with field-aligned currents directed downward in the morning and upward in the evening. The average total current is typically 30 kA, with nominal current densities of 20 nA per square meter. Observations from orbits with minimum altitudes spanning 50 to 500 km are used to test whether the signature varies as the square root of the background magnetic field intensity, as is expected for Birkeland currents. The data also provide confirmation that currents close below the observation altitude, and measurements obtained from the lowest spacecraft periapsis distances set upper limits on the closure altitude of the currents. Initial modeling of current closure in a spherical shell model indicates that the currents may close within the planet itself, radially through the outermost layers of low electrical conductivity, and laterally at greater depth where the conductivity is presumed to be substantially higher as a result of greater temperatures in the semiconducting lower crust and upper mantle. We present results of an electrostatic solution in spherical geometry modified for a conductivity that increases with depth, consistent with expectations from radial temperature profiles given by thermal history models, to constrain the conductivity properties of the north polar region of Mercury.