# Interplanetary Magnetic Field Bx Effect on Field-Aligned Currents in Different Local Times
## 1 Introduction  
Field-aligned currents (FACs) flow along geomagnetic field lines and play a crucial role in the transfer of energy and momentum between the magnetosphere and the ionosphere [1, 2, 3]. On the dawn side, current sheets at higher latitudes (lower latitudes) flow downward into (upward out of) the ionosphere, forming R1 (R2) FACs. Conversely, on the dusk side, R1 (R2) FACs flow outward from (into) the ionosphere. The interplanetary magnetic field (IMF) significantly influences the spatial distribution and strength of FACs [4, 5, 6, 7, 8]. Among the three components of IMF, IMF Bx is dominant in shaping the development and characteristics of FACs. For a southward IMF (Bz \(<\) 0), both R1 and R2 FACs intensify and extend into lower latitudes. However, for a northward IMF (Bz \(>\) 0), R1 and R2 FACs weaken, and two additional current sheets, known as NBZ currents, emerge in the polar cap region. Moreover, IMF By drives dawn-dusk and hemispheric asymmetries in FACs. When IMF By is dominant, two current systems with opposite polarities form in the noontime cusp region, known as DPY FACs. The DPY FACs located on the polar and equatorward sides are referred to as midday R0 and midday R1 FACs, respectively. For positive (negative) IMF By, the midday R0 (R1) FACs flow outward from (into) the Northern Hemisphere (NH) and inward to (out of) the Southern Hemisphere (SH). To account for the combined effects of IMF By and Bx on FACs, the merging electric field Em has been proposed in the literature [9]. Em is regarded as significant in indicatingthe effectiveness of solar wind energy and momentum input into the magnetosphere. The intensities and latitudes of FACs have been shown to be well controlled by Em [5]. Notably, Em reflects the direction of IMF Bz but remains symmetric with respect to the polarity of IMF By. Recent studies, however, have highlighted local time and hemispheric asymmetries in the high-latitude current system in relation to the orientation of IMF By [10; 11]. These studies show that at dusk and noon, when the auroral electrojet is eastward, NH (SH) FACs are greater when IMF By \(<0\) (By \(>0\)). In contrast, at dawn, midnight, and noon, when the auroral electrojet is westward, NH (SH) FACs are more intense for IMF By \(>0\) (By \(<0\)). Additionally, when the auroral electrojet is eastward, FACs are positioned at a higher latitude for positive (negative) IMF By, whereas FACs associated with westward auroral electrojet exhibit the opposite trend. As outlined above, previous studies have primarily focused on the effects of IMF By and Bz on FACs characteristics. However, several studies have indicated that IMF Bx can significantly influence magnetospheric topology, the reconnection rate, and the potential difference across the ionosphere [12; 13]. As a result, the energy released from magnetospheric reconnection may alter the intensity and distribution of FACs. [PERSON] et al. (2024) [14] documented the impact of IMF Bx on the position and magnitude of the peak strength in the auroral electrojet. Specifically, within the midnight sector, the westward electrojet in the NH strengthens when IMF Bx is negative, while in the SH, the westward electrojet intensifies when IMF Bx is positive. This effect is more pronounced during the local winter season. Using multi-satellite and ground-based observations, [PERSON] et al. (2018) [15] showed that the polarity of IMF Bx can alter FACs and ionospheric equivalent currents by up to 10% under southward IMF Bx conditions. Their study relied on average distribution maps of auroral currents, sorted based on different IMF components. The simple average not accounting for the variation in current position may introduce errors. [PERSON] et al. (2002) [16] observed that under similar IMF By conditions with a southward IMF, auroral power in the hemisphere is higher for negative Bx values compared to positive Bx values. This asymmetry in auroral power, influenced by the polarity of Bx, is evident in the seasons of summer and winter. [PERSON] et al.