A MHD mechanism for the generation of the meridional current system during substorm expansion phase

In this study we propose a MHD mechanism for the generation of the meridional current system (MCS) during the substorm expansion phase (EP). The upward field-aligned current (FAC) intensifies as the dawn-dusk plasma pressure gradient enhances in the Harang region during the substorm EP. As a consequence of this intensification, the plasma convection pattern changes, with eastward flow accelerations observed in both the ionosphere and midmagnetotail during early substorm EP. The increasing eastward flow is associated with an inertial current in the midtail whose divergence give rise to a downward FAC poleward of the upward FAC region, constituting a MCS configuration. Theoretical calculations based upon the mechanism shows consistency with many observed features of the MCS. We further discuss how the present work relates to the large-scale FAC system and the interactions among its various components, with a particular emphasis on the MCS contribution to the overall substorm current closure.