The Distributions and Temporal Variations of FACs During Substorms and Quiet Periods

Field-aligned currents (FACs) are an important link connecting the magnetosphere and ionosphere, closely related to phenomena such as particle upflowing, precipitations, and auroras. Based on long-term observations from FY-3E and Active Magnetosphere and Planetary Electrodynamics Response Experiment satellites, we statistically analyzed the density distribution and intensity variations of FAC during different substorm activities. Our results indicate that during high-SME periods, there is a dawn-dusk asymmetry in the FAC density distribution, with FACs located at lower latitudes on the duskside. The region I (R1) FAC intensifies on the nightside after normal substorm onsets, while the enhancement primarily occurs on the dayside after intense substorm onsets. The increased region II (R2) FAC shows a westward rotation during substorms. The total FAC intensity in the northern hemisphere and its increase with the SuperMag electrojets (SME) index is larger than those in the southern hemisphere. FAC intensity also exhibits yearly, seasonal, and UT variations. There are phase shifts in the yearly variation with the solar cycle under different SME indices. The seasonal and UT variations are more likely associated with the solar zenith angle of the geomagnetic poles rather than ionospheric conductivity. Our results provide a more detailed and comprehensive perspective on the overall variations and specific distributions of the FACs.