Statistical Properties of Field-Aligned Currents in the Plasma Sheet Boundary Layer

Field-aligned currents (FACs), also known as Birkeland currents, are the agents by which momentum and energy can be transferred to the ionosphere from solar wind and the magnetosphere, exhibiting a seasonal variation as that of ionospheric conductance at low altitude. By using magnetic field and plasma measurements from the Magntospheric Multiscale (MMS), we estimated the properties of the small-scale FACs in the plasma sheet boundary layer (PSBL) region. The occurrence rates of those FACs are larger near the midnight plane and near the flank region; they are also larger in the northern (summer) hemisphere than in the southern hemisphere, especially for the earthward FACs. Different distribution patterns as a function of plasma β are found for the Beam-type FACs and the Flow-type FACs (accompanied with observable perpendicular currents). The latter are closer to central plasma sheet (higher β) and their occurrence rate decreases linearly toward tail lobe (lower β), while the former mainly appear within the β range of 0.1 to 1. FAC magnitudes show little dependence on plasma β, while they would increase when approaching Earth generally. The occurrence rate and magnitude of FACs both increase from low to high geomagnetic activity, consistent with observation at ionospheric altitude. The main carriers for FACs in PSBL are thermal electrons, while cold electrons sometimes could also have contribution, especially under high geomagnetic activity. This study shows that FACs in the PSBL exhibit an asymmetry of occurrence rate between the northern and southern hemisphere and different signatures under low and high geomagnetic activity, which are consistent with FACs at ionospheric altitude. This demonstrates that FACs are significant in magnetosphere-ionosphere coupling and illustrates the possible ionospheric feedback effects to magnetosphere in the nightside.