Responses of the field-aligned currents in the plasma sheet boundary layer to a geomagnetic storm

Geomagnetic storms can result in large magnetic field disturbances and intense currents in the magnetosphere and even on the ground. As an important medium of momentum and energy transport among the solar wind, magnetosphere, and ionosphere, field-aligned currents (FACs) can also be strengthened in storm times. This study shows the responses of FACs in the plasma sheet boundary layer (PSBL) observed by the Magnetospheric Multiscale (MMS) spacecraft in different phases of a large storm that lasted from May 27, 2017, to May 29, 2017. Most of the FACs were carried by electrons, and several FACs in the storm time also contained sufficient ion FACs. The FAC magnitudes were larger in the storm than in the quiet period, and those in the main phase were the strongest. In this case, the direction of the FACs in the main phase showed no preference for tailward or earthward, whereas the direction of the FACs in the recovery phase was mostly tailward. The results suggest that the FACs in the PSBL are closely related to the storm and could be driven by activities in the tail region, where the energy transported from the solar wind to the magnetosphere is stored and released as the storm is evolving. Thus, the FACs are an important medium of energy transport between the tail and the ionosphere, and the PSBL is a significant magnetosphere-ionosphere coupling region in the nightside.