Steady-state field-aligned currents at Mercury

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 similar to 30 kV from dayside magnetopause magnetic reconnection implies a net electrical conductance of similar to 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.