# Interplanetary Magnetic Field Bx Effect on Field-Aligned Currents in Different Local Times
## 4 Discussion  
This study leverages over six years of observational data (2014-2020) from dual Swarm satellites to examine how the polarity of the IMF Bx, as characterized by IMF cone angles, affects the peak density and magnetic latitude of FACs. While previous research has highlighted the significant roles played by IMF By and Bx in shaping the polar current system (e.g., [4; 5; 6; 7; 8]), our focus is specifically on isolating the effect of IMF Bx. To minimize  
Figure 4: Differences in absolute peak FACp (**a**), FACe (**b**), and MLat between IMF cone angles \(\leq\) 45\({}^{\circ}\) (**c**) and \(\geq\)135\({}^{\circ}\) (**d**) as a function of MLT. The June solstice is marked by asterisks, and the December solstice is marked by circles. Data from the Northern Hemisphere are shown with blue lines, whereas those from the Southern Hemisphere are shown by red lines. Error bars denote the standard error of the mean. the influence of the other IMF components (By and Bz), we select IMF cone angles of \(\leq\)45\({}^{\circ}\) and \(\geq\)135\({}^{\circ}\), which correspond to periods dominated by IMF Bx \(>\) 0 and Bx \(<\) 0, respectively. To further validate the effectiveness of this classification method in isolating the influence of IMF Bx, Figure 5 shows the differences in IMF Bx, By, Bz, and Em between the two selected IMF cone angle ranges (\(\leq\)45\({}^{\circ}\) and \(\geq\)135\({}^{\circ}\)). The results reveal that IMF Bx ranges from 7.5 to 9.5 nT, IMF By ranges from \(-\)1.7 to \(-\)2.7 nT, IMF Bz remains within a \(\pm\)0.5 nT range, and Em fluctuates within a \(\pm\)0.3 mV/m range. The minimal differences in Bz and Em suggest that their effects are negligible and can be disregarded. [PERSON] et al. (2023) [24] reduced IMF \(|\)By\(|\) to less than 2 nT to minimize the influence of By, and while the present IMF By slightly exceeds 2.5 nT, it remains less than one-third of the magnitude of IMF Bx. Therefore, the observed variations in current density and latitude are predominantly driven by the influence of IMF Bx. The influence of IMF Bx on downward FACp in the polar regions is primarily concentrated in the dawn, midnight, and noon sectors, where westward PEJ predominates (as shown in Figure 4). In contrast, its effect on the upward FACp in sectors dominated by eastward PEJ is comparatively weaker. The strong influence of IMF Bx on downward FACp in the dawn and midnight sectors is more apparent and may be attributed to the alignment of IMF Bx in the noon-meridional direction. Our previous study ([PERSON] et al., 2024 [14]) indicated that the polarity of IMF Bx has varying effects on PEJ, with a stronger impact on westward PEJ than on eastward PEJ. For instance, the variations in eastward PEJ in the dawn sector are more pronounced than those in the westward PEJ, and the variations  
Figure 5: Differences in IMF Bx (**a**), B\({}_{y}\) (**b**), B\({}_{z}\) (**c**), and E\({}_{m}\) (**d**) between IMF cone angles \(\leq\) 45\({}^{\circ}\) and \(\geq\)135\({}^{\circ}\) as a function of MLT. The June solstice is indicated by an asterisk, while the December solstice is indicated by a circle. Blue lines represent data from the Northern Hemisphere, and red lines represent the Southern Hemisphere. Error bars denote the standard error of the mean. in westward PEJ are greater in the morning sector than in the dusk sector. This pattern is consistent with the observed behavior of FACp. Additionally, our earlier case study ([PERSON] et al., 2014 [2]) showed that when IMF Bx < 0, there is an enhancement of downward FACp in the high-latitude Northern Hemisphere at noon. These results align with those of the present study, further confirming that IMF Bx exerts a significant influence on downward FACp at noon. The impact of the IMF Bx component on FACp shows a hemispherical difference: in the NH, FAC intensity is greater when IMF Bx < 0, whereas in the SH, FAC intensity is greater when IMF Bx > 0.