Mechanisms of Field-Aligned Current Formation in Magnetic Reconnection

Satellite observations provide strong evidence for the generation of significant field-aligned currents (FACs) during magnetic reconnection. Reconnection of antiparallel magnetic field does not generate FACs in magnetohydrodynamics (MHD) due to coplanarity in MHD shocks. However, a guide magnetic field and a sheared velocity component are almost always present at the magnetopause and their absence is a singular case. It is illustrated that the presence of these noncoplanar fields requires FACs. Contrary to intuition, such currents are generated more efficiently for a small guide field and are more likely to be a result of the redistribution of already present FACs for large guide fields. It is demonstrated that moderate values of shear flow can generate significant ionospheric FACs. Similar to shear flow, the presence of Hall physics leads to significant FACs and we examine the scaling of these current with the ion inertia length.