# Interplanetary Magnetic Field Bx Effect on Field-Aligned Currents in Different Local Times
## 5 Conclusions  
This study examines the impact of the IMF cone angle (i.e., Bx) on the maximum strengths and positions of FACs in both hemispheres, utilizing magnetic field observations of dual Swarm satellites, covering 2004 to 2020. Our results reveal substantial variations in the FAC response to IMF Bx, influenced by local time, season, and hemisphere. The main findings of our study are summarized as follows:  
1. In the NH, polar-side FACs (FACp) are larger for IMF Bx < 0 compared to those for IMF Bx > 0, whereas the opposite trend is observed in the SH. This IMF Bx effect is most pronounced in the MLT sectors of 03-08, 21-02, and 09-14, when the westward auroral electrojet dominates. During these times, FACp flows downward, while the auroral electrojet flows westward. 2. The equatorward FAC (FACe) exhibits a weaker response to IMF Bx than FACp at dawn, dusk, and midnight (21-02 MLT). However, in the 09-14 MLT sector, the response of FACe to IMF Bx is opposite to that of FACp, which aligns with the behavior of the auroral electrojet. 3. The influence of IMF Bx on FACs shows distinct seasonal variations. During local summer, the effect is more pronounced around noon and midnight due to higher ionospheric conductivity from solar illumination. In contrast, the Bx effect is stronger around dawn during local winter, influenced by the magnetic field geometry and particle precipitation, both of which are modulated by IMF Bx. 4. The impact of IMF Bx on FAC latitude is most pronounced along the noon-midnight meridian. During local times dominated by westward (eastward) auroral electrojets, FACs in the Northern Hemisphere (NH) are positioned at higher latitudes when IMF Bx > 0 (<0) compared to when IMF Bx < 0 (>0). In contrast, the opposite pattern is observed for FACs in the Southern Hemisphere (SH). This behavior is consistent with changes in the latitude of the auroral electrojet. To our knowledge, FACs have been studied primarily as a function of the IMF Bx and By components; however, the influence of the IMF Bx component has not been thoroughly explored. This study presents compelling satellite observations, demonstrating that the IMF Bx component plays a crucial role in modulating FACs concerning hemisphere, local time, and seasonal variations. It unveils a novel pathway for solar wind energy transfer into the magnetosphere, marking a significant advancement in our understanding of the energy coupling process in geospace. Conceptualization and methodology, Y.S. and H.W. ; software and validation, Y.S. ; formal analysis, Y.S. and H.W. ; investigation and data curation, Y.S. ; resources, H.W. ; writing--original draft, Y.S. ; writing--review and editing, H.W. ; visualization, Y.S. ; supervision, H.W. ; project administration and funding acquisition, H.W. All authors have read and agreed to the published version of the manuscript. The authors are grateful for the support of the National Natural Science Foundation of China (42374200), the National Key Research and Development Program (2022 YFF0503700), and the National Natural Science Foundation of the China Basic Science Center (42188101). Swarm field-aligned currents are available at [[https://Swarm-diss.eo.esa.int/#swarm/Level2](https://Swarm-diss.eo.esa.int/#swarm/Level2) daily/Latest_baselines/FAC/TMS/Sat_AC]([https://Swarm-diss.eo.esa.int/#swarm/Level2](https://Swarm-diss.eo.esa.int/#swarm/Level2) daily/Latest_baselines/FAC/TMS/Sat_AC) accessed on 1 August 2023. Swarm and polar electrojets data are available at [[https://Swarm-diss.eo.esa.int/#swarm/Level2](https://Swarm-diss.eo.esa.int/#swarm/Level2)]([https://Swarm-diss.eo.esa.int/#swarm/Level2](https://Swarm-diss.eo.esa.int/#swarm/Level2)) daily/Latest_baselines/AEJ/LPL accessed on 1 August 2023. Solar wind, interplanetary magnetic field, and Dst index data are sourced from OMNIWeb ([[https://omniweb.gsfc.nasa.gov](https://omniweb.gsfc.nasa.gov)]([https://omniweb.gsfc.nasa.gov](https://omniweb.gsfc.nasa.gov))) accessed on 1 August 2023.