Dawn-dusk asymmetries in the magnetopause and ring current.

Abstract:

Motion of charged particles in the Earth's magnetosphere sets up a system of currents. Current continuity requires that these currents are closed, either locally or via other current systems. We have investigated whether magnetopause surface currents can contribute to ring current closure. Using 10 years of measurements from the Cluster constellation of spacecraft, we calculated the magnetopause current density for a large number of flank magnetopause traversals. For each event, we also consulted sectorial ring current indices, derived from SuperMAG - a large constellation of ground based magnetometer stations. SuperMAG results show a significant and persistent dawn-dusk asymmetry in ground magnetic perturbations which indicates a more intense ring current on the dusk side. The asymmetries become more pronounced during disturbed magnetospheric conditions, indicating an increased divergence of the current and closure through other current systems. A similar response to geomagnetic activity is also observed at the magnetopause. Duskside magnetopause current densities are generally higher than their dawnside counterparts, and the magnetopause asymmetry becomes more pronounced during disturbed conditions. Although the two current systems are related to different processes - gradient drift of energetic plasma sheet particles for the ring current and a surface current due to differential motion of ions and electrons inside the magnetopause interface for the magnetopause current - the results demonstrate a mutual relation between the two current systems.