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

Energy and momentum can be transferred from the magnetosphere to the ionosphere through field-aligned currents (FACs), which show seasonal variations due to seasonal variation of the ionospheric conductance. Using Magnetospheric Multiscale Mission observations, we investigate the characteristics of kinetic-scale (close to local ion inertial length) FACs in the plasma sheet boundary layer (PSBL), such as occurrence rate, magnitude, and carriers. The results show that: (1) the occurrence rates of FACs exhibit a trident distribution; they are also larger in the northern hemisphere than in the southern hemisphere, especially for the earthward FACs. (2) Distribution patterns vs. plasma beta are different for the beam-type and the flow-type FACs. Flow-type FACs are observed closer to the central plasma sheet (higher beta), and their occurrence rate decreases linearly toward the tail lobe (lower beta), while the beam-type FACs are mainly and equally distributed within 0.1 beta < 1.0. (3) The magnitude of FACs shows little dependence on beta, and generally increases closer to the Earth. (4) Occurrence rate and magnitude of FACs both increase during active periods, consistent with ionospheric observations. (5) The main carriers for FACs in the PSBL are thermal electrons, while cold electrons also contribute occasionally, especially during active periods. Hemispheric asymmetry of occurrence rate and dependence on geomagnetic activity of the kinetic-scale FACs in the PSBL are consistent with FACs at ionospheric altitudes. This fact demonstrates that kinetic-scale FACs are significant in magnetosphere-ionosphere coupling and illustrates the possible ionospheric feedback effects to the magnetosphere in the nightside.