Steady-state field-aligned currents at Mercury

Abstract

Magnetic field observations acquired in orbit about Mercury by the MESSENGER spacecraft demonstrate the presence in the planet's northern hemisphere of Birkeland currents that flow to low altitudes. Currents of density 10-30 nA/m 2 flow downward at dawn and upward at dusk. Total currents are typically 20-40 kA and exceed 200 kA during disturbed conditions. The current density and total current are two orders of magnitude lower than at Earth. An electric potential of ~30 kV from dayside magnetopause magnetic reconnection implies a net electrical conductance of ~1 S. A spherical-shell conductance model indicates closure of current radially through the low-conductivity layers near the surface and by lateral flow from dawn to dusk through more conductive material at depth. Key Points Steady-state Birkeland currents are present at MercuryThe currents are 100 times weaker than at Earth and of only Region 1 polarityThe currents close through the interior of the planet