Trends in Field-Aligned Currents from the Polar Regions to the Ring Current Region: 10 Years of Observations from Swarm and MMS

Using data from the Swarm and Magnetospheric Multiscale (MMS) missions collected between September 1, 2015, and May 1, 2024, we present a direct comparison of field-aligned currents (FACs) from the ring current (RC) region and polar regions for the first time through statistical analysis. Our analysis examines the response of FACs to different upstream interplanetary magnetic field (IMF) directions and solar wind (SW) flow velocity directions, the FAC distribution corresponding to various AE and SYM_H indices, and the overall trends of FAC current directions in both the polar and RC regions. Leveraging the extensive coverage provided by Swarm data, we conducted a seasonal analysis of how polar FACs respond to IMF B_X, B_Y, B_Z, as well as SW V_Z. The results show that the direction of SW flow velocity has a weak effect on polar FACs, while the IMF plays a significant role. The average results smooth out the seasonal differences indicate that polar FACs only show differences in response to the varying directions of B_Z. In contrast, RC region FACs are also significantly influenced by SW V_Z. Both increases in the AE and SYM_H indices are associated with enhanced FAC current densities, with polar FACs showing a better correlation with the AE index. Overall, the flow direction trends of FACs < 70° MLAT in both the polar and RC regions are similar, with stronger current densities observed in the Northern Hemisphere. However, current densities derived from simple coefficient calculations in the polar and RC regions do not directly match, showing a significant order-of-magnitude difference. Additionally, a marked hemispheric difference in FAC direction is observed in the 07:30-10:00 MLT, 60°-70° MLAT region in both the polar and RC regions.