# Interplanetary Magnetic Field Bx Effect on Field-Aligned Currents in Different Local Times
## 2 Materials and Methods
To eliminate the ring current effect, the horizontal current intensity at \(\pm\)50\({}^{\circ}\) magnetic latitude (MLat) was set to zero, and the current intensity along the orbit was shifted by an equal amount [20; 21]. This study examined the six-year period from 17 April 2014 to 16 April 2020, using FAC observations from both the Swarm A and Swarm C satellites. To investigate the impacts of solar wind and IMF on the current system, OMNI solar wind and IMF data (NASA Goddard Space Flight Center, Greenbelt, MD, USA) at a 1 min resolution were employed. To align the time trajectories of FACs and IMF data, a 15 min moving average was applied to the OMNI data, and a 20 min delay was introduced to account for the propagation time from the bow shock to the ionosphere. The criteria for selecting dominant IMF Bx cases were based on the IMF cone angle, which is the angle between the direction of the IMFand the X-axis of the Geocentric Solar Magnetospheric (GSM) coordinate system, using the following formula: \(\theta=\arccos\frac{\mathrm{B_{x}}}{\sqrt{\mathrm{B_{2}^{2}+B_{y}^{2}+B_{z}^{2}}}}\). Specifically, \(\theta<45^{\circ}\) or \(\theta>135^{\circ}\) indicates that the IMF Bx component is primarily directed toward the Sun or the Earth. Therefore, in this study, we used an IMF cone angle \(\leq 45^{\circ}\) to represent periods with dominant positive IMF Bx values and an IMF cone angle \(\geq 135^{\circ}\) to represent periods with dominant negative IMF Bx values. Using this angle criterion helped us to isolate the IMF Bx effect from other components of the IMF, preventing the potential contamination of the results due to the Parker spiral configuration of the IMF. Figure 1 shows the distribution of auroral crossings, categorized by hemisphere, season, and local time. Local time is divided into four distinct intervals, 15-20, 21-02, 03-08, and 09-14 MLT, with each interval spanning six hours, consistent with the methodology described by [PERSON] et al. (2024) [11]. To examine seasonal variation, the data are grouped into two distinct periods: the June solstice (\(\pm 66\) days around 1 July) and the December solstice (\(\pm 66\) days around January 1). The orientation of the PEJ in the 09-14 MLT sector is influenced by IMF By orientation. Consequently, the 09-14 MLT sector is further divided into eastward and westward PEJs (positive and negative PEJs). There are a higher number of events in the NH, with each local time interval generally containing at least 100 occurrences. In contrast, the SH suffers from significant under-sampling in many high-latitude regions, mainly due to the large displacement of its geomagnetic pole. Such displacement is more pronounced in the SH, where the geographic and geomagnetic poles are more widely separated than in the NH. This spatial difference affects the detection and frequency of FAC events in the SH. However, each MLT sector in  
Figure 1: Observed FAC event numbers in the Northern and Southern Hemispheres for different local times and seasons under IMF cone angle \(\leq 45^{\circ}\) (i.e., IMF Bx \(>0\)) or \(\geq\)135\({}^{\circ}\) (i.e., IMF Bx \(<0\)) conditions. The Northern Hemisphere (**a**) and Southern Hemisphere (**b**) are shown from left to right. Black bars represent the June solstice (JuneS) with an IMF cone angle \(\leq 45^{\circ}\), gray bars denote the JuneS with an IMF cone angle \(\geq 135^{\circ}\), blue bars indicate the December solstice (DeceS) with an IMF cone angle \(\leq 45^{\circ}\), and green bars signify the DeceS with an IMF cone angle \(\geq 135^{\circ}\). The MLT segments are 15–20, 21–03, 03–08, and 09–14 MLT. The 09–14 MLT segment is subsequently subdivided according to eastward or westward PEJ. the SH still typically records more than 30 events, ensuring adequate statistical reliability for the analysis.