Impact of Solar Wind Dynamic Pressure on Polar Electrojets and Large- and Small-Scale Field-Aligned Currents

Abstract

This study examines the impact of the solar wind dynamic pressure (Pd) on the peak current density and latitude of polar electrojets (PEJs), large-scale field-aligned currents (LSFACs), and small-scale FACs (SSFACs) in various local times, seasons, and hemispheres, using Swarm observations during 2014 to 2020. The different Pd effects with enhanced solar wind mass density (Nsw effect) or with enhanced solar wind velocity (Vsw effect) are differentiated. LSFACs and PEJs show pronounced hemispheric and seasonal differences around noontime, where summer variations are more pronounced than winter, due to higher solar EUV conductivity. Increased Pd typically enhances LSFACs, except at midnight when opposing effects from Nsw and Vsw exert on poleward-side FACs. The impact of Vsw on FACp surpasses that of Nsw mostly except for midnight. In contrast, the Nsw impacts on equatorward-side FACs and SSFACs are mostly stronger than the Vsw effect except for the noontime. PEJs strengthen with increasing Vsw effects more efficiently than with increasing Nsw effects. Additionally, a higher Pd shifts PEJs and SSFACs equatorward, with Vsw effects being more prominent than Nsw effects, except for midnight SSFACs.

1. Introduction

Auroral current systems, such as polar electrojets, PEJs, and field-aligned currents, FACs, are key elements in the coupling processes between the magnetosphere and ionosphere. The average FAC and PEJ patterns show clear variations with local time, season, and hemisphere, responding dynamically to the interplanetary magnetic field (IMF), as well as solar and geomagnetic activities. These variations have been extensively studied through observations from ground magnetometers, rockets, and satellites, as well as theoretical simulations [1,2,3].

FACs, flowing along the geomagnetic field line, serve as one of the primary mediums for transporting energy, momentum, and mass in the magnetosphere and ionosphere [4,5,6,7,8,9,10]. Region 1 (2) FACs flow into (out of) the ionosphere at dawn, while flowing out of (into) the ionosphere at dusk, termed as R1 and R2 FACs. The R1 FACs are generated from the magnetopause and magnetotail lobe, whereas R2 FACs are associated with inner magnetospheric ring current [11]. Additionally, during northward IMF conditions, a pair of FACs called NBZ FACs emerge in the area of the polar cap, showing opposite polarity to R1 FACs and linking to lobe reconnection on the nightside [7]. Other configurations, such as DPY (Dayside Polar cap Y) FACs in the cusp region, exhibit flow directions that vary depending on the orientation of the IMF By component and the hemisphere [12]. When the IMF Bx is strong and directed sunward, a new pair of FACs forms at noon, with the poleward component flowing downward and the equatorward component flowing upward [9].

PEJs, flowing perpendicular to both electromagnetic fields in the E region in the ionosphere, similarly exhibit distinct local time and seasonal variations. Eastward PEJs are stronger in summer at dusk, while westward PEJs at dawn show weaker seasonal variation [3,13]. Using Swarm satellite observations, during local summer, the northern PEJs were reported to be stronger than the southern ones. This hemispheric difference becomes more pronounced during low geomagnetic activity, primarily because of variations in ionospheric conductivity and geomagnetic field strength [14,15]. The polarity of IMF By also introduces significant longitudinal, hemispheric, and local time asymmetries in the strength and position of FACs and PEJs [10,16,17].

The intensities of both FACs and PEJs are largely influenced by merging the electric fields, Em = ${\displaystyle \frac{1}{3000}}{Vsw}^{{\displaystyle \frac{4}{3}}}{B_t}^{{\displaystyle \frac{2}{3}}}{sin}^{{\displaystyle \frac{8}{3}}}{\displaystyle \frac{\theta }{2}}$; here, Vsw denotes the solar wind velocity (km/s), and $B_t=\sqrt{B_y^2+B_z^2}$, $\theta =\mathit{arctan}{\displaystyle \frac{B_y}{B_z}}$. The factor 1/3000 is used to make sure the electric field is in mV/m [18]. Em accounts for combined effects from Vsw, IMF Bz, and IMF By [6,19,20]. One can notice that the Em has no term of solar wind mass density, Nsw. However, Shue and Kamide [21] identified an important relationship between Nsw and auroral current intensities (as indicated by AL and AU indices), with stronger relationships under southward IMF conditions. Their study also revealed that Nsw effects dominate in westward currents for southward IMF and in eastward currents for northward IMF. However, they did not examine how Nsw affects hemispheric or local time variations in auroral currents. More recently, Zhong et al. [22] explored local time differences in auroral current responses to interplanetary shocks, with the largest response of the eastward PEJs (westward PEJs) at dusk (dawn) during interplanetary shocks. Their work highlighted the immediate response of PEJs to a sudden jump in solar wind dynamic pressure Pd, thus having nearly simultaneous solar wind effects. Yet, they did not differentiate between the effects of Vsw and Nsw.

Previous studies have focused on the relationship between FACs and solar wind dynamic pressure (Pd = NswVsw²). Iijima and Potemra [23] found a good correlation between R1 FACs and Pd, but they did not show the local time variation in the Pd effect. Nakano et al. [24] observed that R2 FACs exhibited dependence on Pd during geomagnetic storms, with weaker correlations during quiet periods. This dependence is attributed to changes in the geometric relationship between the Pd gradient and the magnetospheric thermal pressure gradient. Wang et al. also showed a strong correlation between Pd and FAC peak densities around noontime during geomagnetic storms, although they did not investigate Pd effects across different local time sectors or distinguish the effects of Vsw and Nsw. Wing et al. [25] investigated the relationship between daytime FAC densities and solar wind variations. They found that around noon, R1 currents are primarily influenced by Vsw, while R2 currents are more affected by Nsw. However, large-scale FACs in the dawn–dusk sectors were more sensitive to Em than to Pd. Their study, however, was limited by the DMSP satellite’s orbit, which did not cover all local time sectors, particularly during the nighttime. Furthermore, previous studies on Pd have focused on large-scale FACs (LSFACs), with little attention given to small-scale FACs (SSFACs), which carry most of the energy from the magnetosphere to the ionosphere.

FACs manifest over a broad range of horizontal scales, from less than a kilometer to up to 1000 km [4]. The intensity of SSFACs is significantly higher than that of LSFACs. SSFACs are primarily driven by kinetic Alfvén waves, whereas LSFACs can be considered quasi-static. SSFACs are most commonly observed at noon, with a secondary peak around midnight. These peaks are associated with cusp and substorm activities, respectively [26]. High-amplitude SSFACs are more frequent during local summer and lighter conditions [27]. Additionally, the occurrence rate of SSFACs is enhanced on the nightside during magnetically active periods [28]. The intensity of SSFACs increases with a higher Kp index, and their locations shift to lower latitudes around noon and midnight, respectively, due to the expansion of the auroral oval [27].

This work aims to investigate in detail the influence of Pd on the peak strength and latitude of PEJs, large-scale FACs, and small-scale FACs (10–150 km wavelength) across four local times, three seasons, and two hemispheres, using data from Swarm satellite constellations. The data procession methodology is presented in Section 2, followed by statistical analyses in Section 3. Discussions and conclusions, based on previous studies, are given in Section 4 and Section 5, respectively.

2. Materials and Methods

The European Space Agency launched three Swarm satellites to near-polar orbits in November 2013. The final orbital constellation was completed in April 2014. Swarms A and C fly side by side in a 1.4° longitudinal separation at an altitude of around 450 km. Swarm B operates at around 510 km altitude. The Swarms covered a full local time sector within approximately 133 days.

LSFACs are calculated by using Ampère’s integral law, $j_{\parallel }={\displaystyle \frac{1}{{\mu }_{0}Asin\left(i\right)}}\oint Bdl$, utilizing Swarm A/C vector magnetic field measurements. The line integral is performed along a closed path around 4 points of measurement [29]. In the above equation, B is the horizontal disturbance magnetic field after the removal of the background magnetic field that is determined by the CHAOS-6 model, dl is the line element, A is the area formed by four observation points, μ₀ is vacuum permeability, and i is the inclination angle of the geomagnetic field. LSFACs are 20 s low-pass-filtered in order to suppress an SSFAC structure that is smaller than 150 km [30]. This is because, according to the sampling theorem, the signal should not exhibit spatial variations with wavelengths shorter than twice the distance between any two measurement points. We assume that spatial structures exhibit comparable wavelengths in both the along-track and cross-track directions. To meet this requirement, we applied a low-pass filter with a 20 s cutoff period to the magnetic field residuals. This corresponds to an along-track wavelength of approximately 150 km. With this approach, spatial aliasing effects should be minimized, particularly at auroral latitudes. Positive (negative) values represent FACs flowing out of (into) the ionosphere. LSFACs with an absolute density weaker than 50 µA/m² are utilized in the present work to avoid false detections.

We also looked at the SSFACs (10–150 km wavelengths). SSFACs are calculated from 1 Hz Swarm A vector magnetic data. Here, SSFACs are calculated using $j_{\parallel }={\displaystyle \frac{1}{{\mu }_0{v}_{x}sin\left(i\right)}}{\displaystyle \frac{\partial B_y}{\partial t}}$; here, the disturbed magnetic field $B_y$ represents the magnetic eastward component that is aligned with FACs, and $v_x$ is a velocity component that is perpendicular to FACs [7]. We use the proxy intensity of SSFACs by calculating the root mean square of SSFACs at an interval of 11 s along the satellite orbit, in a similar way to our previous procession method [9].

The PEJs were calculated from Swarm A scalar magnetic field data using the line current method [31,32]. This method was checked through comparison with ground-based observations [33]. Several infinite line currents are placed in the ionospheric E layer with 1° latitude apart to approximate PEJs. Each line current strength is inverted using a least-square fitting method from observed scalar field residuals. The positive (negative) value denotes eastward (westward) electrojets. Absolute peak values need to be within 0.03 A/m and 3 A/m. An apex latitude was adopted for FACs and quasi-dipole latitude was adopted for PEJs. In this way, we compare FACs and PEJs in the same magnetic coordinate system in the ionospheric E region.

This study utilizes PEJs, SSFACs, and LSFACs data from 17 April 2014 to 16 April 2020. Thereafter, FAC data are unreliable due to changes in the satellite orbits. The eastward (westward) PEJ peaks are identified for each auroral crossing in the range of 15–20 magnetic local time (MLT, 21-02 MLT, and 03-08 MLT). Around 09-14 MLT, the polarity of PEJs is controlled by the orientation of IMF By. Consequently, both eastward and westward PEJ peaks are picked out.

The number of orbits in the polar region is presented in Figure 1, in two hemispheres, three seasons, and four local time sectors. Figure 1a,b show the low and high Pd (Pd ≤ 2 nPa and Pd > 2 nPa), Figure 1c,d show the low and high Nsw (Nsw ≤ 5 cm⁻³ and Nsw > 5 cm⁻³), and Figure 1e,f show the slow and fast Vsw (Vsw ≤ 400 km/s and Vsw > 400 km/s). The selected thresholds take into account whether the sample size meets the requirements for statistical research. The criteria for determining high and low Pd are the same as those used by Cheng et al. [34]. The solar wind parameters are averaged over 10–20 min preceding peak PEJ measurements to take into account the propagation time from the bow shock to the ionosphere [17]. Overall, a more favorable situation can be found in the Northern Hemisphere (Figure 1a,c,e), typically exceeding 600 events in each bin. In contrast, the southern high magnetic latitudes (Figure 1b,d,f) are undersampled because of the substantial offset of the geomagnetic and geographic poles. The sampling limitation is particularly evident at noontime when some FACs are situated within the area of the polar cap. Despite this, the number of events generally exceeds 300 across each MLT bin, guaranteeing statistical reliability.

3. Observations

For a superposed analysis of location, FACs and PEJs are categorized into 2° ∆MLat bins. The central latitude, ∆MLat = 0°, represents the latitude of peak PEJs for each auroral crossing. Average profiles of FACs and PEJs centered around the central latitude are computed. These latitudinal profiles are compiled for different local time sectors and Lloyd seasons. June solstice (December solstice) occurs 66 days before and after 1 July (1 January), while 33 days before and after 1 April and 1 October are the equinox. This method tends to avoid unexpected smearing, which might result from simple averaging of current densities.

The mean latitudinal profiles of FACs and PEJs for low and high Pd conditions are shown in Figure 2 and Figure 3; here, 0° ΔMLat denotes the latitude of peak PEJs. A positive (negative) latitude denotes an MLat closer to (farther away from) the geomagnetic pole. The figures are within ±10° ∆MLat in an MLat interval of 2°. There are five curves in each figure in different colors, which represent various local times. Figure 2a–f and Figure 3a–f illustrate PEJs, and Figure 2g–l and Figure 3g–l represent LSFACs separately for different seasons and hemispheres.

It can be seen that eastward PEJs dominate at dusk (purple), and westward PEJs dominate at midnight as well as at dawn (black and blue, Figure 2a–f and Figure 3a–f). The PEJs flow eastward (orange) or westward (green) in the noon sector, in response to IMF By orientation. In the polar region, sunward returning currents emerge. In the 15–20 MLT (Figure 2g–l and Figure 3g–l), the upward LSFAC (purple) was located on the poleward side, and the downward LSFAC was positioned on the equatorward side of 0° ∆MLat, representing R1 and R2 LSFACs and eastward PEJs on the dusk side. At 21-02 MLT, there is a downward LSFAC (black) at higher latitudes and an upward LSFAC at lower latitudes of westward PEJs. Around 21-03 MLT, the normal classification of R1 and R2 LSFACs is not applicable. Three sheets of FACs might appear around the Harang discontinuity, with upward FACs in the center being the largest. At dawn, the normal downward R1 and upward R2 LSFACs emerge (blue). Around noontime, the mid-day R0 and R1 FACs in the area of the polar cap can be observed. The polarities of these currents are affected by the orientation of IMF By.

The amplitudes of SSFACs (Figure 2m–r and Figure 3m–r) are considerably larger than those of the LSFACs (Figure 2g–l and Figure 3g–l), indicating that energy transport is primarily governed by smaller-scale FACs. On both dawn and dusk sides, SSFAC peaks are observed at 2° ΔMLat, which is near the peak of large-scale R1 FACs. In contrast, most midnight SSFAC peaks are found at 0° ΔMLat, positioned between the peaks of upward and downward LSFACs. During noontime, when the PEJs are positive, these peaks typically shift to 2° ΔMLat, which is closer to the peak of upward LSFACs. However, in the case of negative PEJs, SSFAC peaks may occur either at 0° ΔMLat (between the upward and downward LSFAC peaks) or at −2° ΔMLat (closer to the peak of upward LSFACs). The distribution patterns of PEJs, LSFACs, and SSFACs as a function of ΔMLat at low and high levels of Nsw and Vsw resemble those shown in Figure 2 and Figure 3.

Therefore, when the PEJs are negative, compared to when they are positive, the SSFAC peaks are located at lower latitudes, displaying characteristics related to the IMF polarity. Based on data from the AKEBONO satellite, Fukunishi et al. (1991) [35] found a one-to-one correspondence between SSFACs and localized electron precipitation events. Previous studies have shown that the contribution of downward precipitating electrons to FACs typically exceeds that of upward ionospheric ions by an order of magnitude (e.g., Cattell et al., 1979 [36]). Based on these findings, SSFACs may be associated with intense electron precipitation, and their peak density is therefore closer to the upward FAC region.

Figure 4 illustrates the absolute values of peak polar electrojets (PEJs), the poleward component of FACs (FACp), and the equatorward component of FACs (FACe), as well as SSFACs during JuneS (marked with asterisks), EquiX (indicated with pentagrams), and DeceS (represented by circles) for Pd ≤ 2 nPa. The orange dashed lines indicate the Northern Hemisphere. The black dashed lines represent the Southern Hemisphere. The subfigures represent 15-20, 21-02, 03-08, and 09-14 MLT with positive and negative PEJs. For comparison, the variations in averaged Em, conductivity, and Pd are also shown. The sunlit ionospheric Hall conductivity is calculated for every peak PEJ [37]. It should be noted that there is an empirical model of conductivity due to auroral electron precipitation, which is based on the average electron energy and flux [38]. The Global Airglow Model (GLOW) could also output the ionospheric conductivity with the inputs of precipitation electron fluxes at the tipside ionosphere [39,40]. These models could be utilized in future work using combined DMSP observations. It can be seen that the average merging electric field representing the solar wind–magnetosphere energy input remains below 1.5 mV/m, which indicates a general quiet study period. Pd is approximately 1.25 nPa across the five MLT sectors.

On the dusk side, it is clear that PEJs (Figure 4a), large-scale FACp (Figure 4f), and SSFACs (Figure 4p) show larger densities in the summer when compared to winter. This trend aligns generally with the summer–winter variations in the ionospheric conductivity (Figure 4z). One can notice that the conductivity changes are not fully consistent with those of FACp. For example, the hemispheric differences in FACp during DeceS are relatively small, while the southern conductivity is significantly larger than the northern one. This difference might be due to the effect of Em. Em is larger in local winter when compared to local summer, potentially diminishing conductivity impact.

The large-scale FACe (downward R2 FACs, Figure 4k) displays a somewhat contrasting or negligible seasonal variation, with winter weakly surpassing summer. This may be associated with the Em behavior (Figure 4v), which is larger in winter than in summer. Another contributing factor could be the Pedersen current in the polar cap [41]. The Pedersen current in the winter polar cap is weak, resulting in a significant closure of the duskside R1 by R2 FACs. In contrast, the ionospheric Pedersen currents in the summer polar cap are strong, resulting in an important closure path between dawnside and duskside R1 FACs.

In the midnight sector (21-02 MLT), the seasonal variation in PEJs (Figure 4b) is inversely correlated with hemispheric fluxtube-integrated conductivity (Figure 4A, green). This finding is consistent with our previous result, i.e., nighttime westward PEJs are inversely related to the fluxtube-integrated ionospheric conductivity [42]. However, the seasonal changes in FACp (downward FACs, Figure 4g) and SSFACs (Figure 4q) can primarily be due to the ionospheric conductivity induced by sunlight (Figure 4A). Notably, FACp exhibits a larger difference in JuneS compared to DeceS, even though the ionospheric conductivity exhibits a larger difference in DeceS (see Figure 4q). This discrepancy may arise from the influence of Em, which has a weaker hemispheric discrepancy in DeceS. The relatively small summer–winter difference in FACe (Figure 4l) may be related to the total conductivity, resulting from particle precipitation and solar illumination. Notably, the nighttime energetic electron precipitation in the upward FAC region can be suppressed by sunlight, because of the feedback effect of the ionosphere [28]. This ionospheric feedback may likely account for minimal sunlight impact on the nighttime upward FAC density, a finding confirming previous results [6,17]. Conversely, the downward FACp (Figure 4g) is weakly influenced by the energetic electron precipitation, but relies more on sunlight, making it larger in summer with larger conductivity.

In the dawn side (03-08 MLT), the seasonal variation trend of PEJs (Figure 4c) in both hemispheres is consistent with that of Em (Figure 4w). The seasonal variations in FACp, FACe, and SSFACs (Figure 4h,m,r) are also aligned with conductivity (Figure 4B). However, FACe is similar in both hemispheres in DeceS, whereas the conductivity is significantly higher in the Southern Hemisphere. In the noontime (09-14 MLT), PEJs, FACe, FACp, and SSFACs are generally stronger during summer than winter, highlighting the significant effect of ionospheric conductivity. However, notable discrepancies exist in the seasonal variations in FACs and ionospheric conductivity. These divergences may arise from the nearly opposite seasonal variations in Em and ionospheric conductivity. In addition, the empirical models of ionospheric conductivity might not fully capture real ionospheric conductivity conditions. Our analysis also did not account for the influence of energetic particle precipitation, further complicating the variation in FACs. In spite of these factors, in the dawn and noon, and in two hemispheres, FACs are typically stronger in summer compared to winter, underscoring the important roles of sunlight-induced conductivity.

Figure 5 shows the absolute density of peak PEJs, large-scale FACp, FACe, and SSFACs for Pd > 2 nPa. The mean Em remains weaker than 2 mV/m, suggesting a generally quiet northward IMF during this period. The average Pd is approximately 3 nPa across the five MLT sectors. The seasonal, local time, and hemispheric differences in the auroral current systems for Pd > 2 nPa are largely similar to those for Pd ≤ 2 nPa, and thus are not described here in detail.

Before examining the differences in PEJs and FACs under low and high Pd levels, we first assess the differences in external solar conditions and ionospheric conductivity between the two Pd levels. Figure 6 illustrates the differences in Em, ionospheric conductivity, and dynamic pressure for Pd ≥ 2 nPa compared to Pd < 2 nPa. The absolute variation in Em is less than 0.7 mV/m, and the difference in ionospheric conductivity is under 0.5 S. In contrast, the difference in dynamic pressure exceeds 1.5 nPa. This indicates that the effect of Em and conductivity can almost be ignored, and the discrepancy comes mainly from Pd.

Figure 7 displays the differences in the absolute densities of peak FACp (7a–e) and FACe (7p–t) between Pd > 2 nPa and Pd ≤ 2 nPa, denoted as ΔFACp and ΔFACe (i.e., Figure 5f–j minus Figure 4f–j and Figure 5k–o minus Figure 5k–o). A positive ΔFAC indicates stronger FACs when Pd > 2 nPa compared to when Pd ≤ 2 nPa, while a negative ΔFAC indicates the reverse. To further distinguish the effects of Pd associated with increased Vsw versus increased Nsw, Figure 7f–j,u–y show the differences in FACp and FACe between high and low Nsw conditions (Nsw > 5 cm⁻³ minus Nsw ≤ 5 cm⁻³). Meanwhile, Figure 7k–o,z–D illustrate the differences between high and low Vsw conditions (Vsw > 400 km/s minus Vsw ≤ 400 km/s). A positive ΔFACs in these figures means that FACs are more pronounced at larger levels of Nsw or Vsw, whereas a negative ΔFACs means the reverse condition.

On the dusk side (Figure 7a,p), an increase in Pd results in a notable enhancement in both FACp and FACe. The hemispheric and seasonal differences are generally weak. For FACp and FACe, the June solstice and equinox show a slightly larger increase compared to the December solstice. During the June solstice and equinox, this increase is a little more pronounced in the Southern than in the Northern Hemisphere. However, during the December solstice, the northern FACp is more intense than the southern one, while FACe remains similar between the two hemispheres. Comparing Figure 7f,k, it is clear that during the equinox and December solstice, Vsw has a more significant influence than Nsw, whereas during the June solstice, Nsw exerts a greater impact. Figure 7u,z further demonstrate that Nsw has a larger influence than Vsw.

At midnight, the impact of dynamic pressure on FACp (Figure 7b) is less pronounced when compared to other local times. The northern FACp shows a weak increase at the equinox, while the southern FACp intensifies at the December solstice. On the other hand, the increasing Pd significantly enhances FACe (Figure 7q), with the summer hemisphere showing a slightly stronger response than the winter hemisphere; during the equinox, the response is similar in both hemispheres. Comparing Figure 7g,l, an increase in Nsw correlates with an increase in FACp, whereas an increase in Vsw results in a decrease in FACp, leading to an overall insignificant response in FACp to Pd. In addition, Figure 7v,A show that the impact of Nsw on FACe is significantly greater than that of Vsw, indicating that the Pd effect on FACe primarily arises from Nsw.

On the dawn side, the increase in Pd similarly enhances both FACp (Figure 7c) and FACe (Figure 7r). The response for FACp (Figure 7c) is strongest during the June solstice, with the Northern Hemisphere showing a higher response than the Southern Hemisphere during the equinox and December solstice; however, the hemispheric responses are similar during the June solstice. For FACe (Figure 7r), the Northern Hemisphere exhibits a notably stronger response during the June solstice compared to other seasons, while the Southern Hemisphere responds more during the equinox. In the June solstice, the Northern Hemisphere’s response is stronger than that of the Southern Hemisphere, whereas the responses in the other two seasons are relatively comparable. When comparing Figure 7h,m, this indicates that Vsw has a much greater impact on the FACp than Nsw, except for the southern FACp at the equinox and June solstice. When comparing Figure 7w,B, it shows that the influence of Nsw on FACe is greater than that of Vsw.

Around noon during the eastward PEJ period, both ΔFACp and ΔFACe (Figure 7d,s) exhibit stronger responses in local summer compared to local winter. Figure 7i,n indicate that the effect of ΔVsw closely aligns with the variations in ΔFACp. However, the impact of ΔNsw becomes significant in local winter. During the westward PEJ period around noon, the response of FACp (Figure 7e) is more intense in local summer when compared to local winter. However, FACe (Figure 7t) shows a stronger response during the June solstice, with the Northern Hemisphere displaying a more robust reaction than the Southern Hemisphere. A comparison of Figure 7j,o suggests that Vsw primarily influences FACp, except during the June solstice in the Southern Hemisphere. Similarly, Figure 7y,D reveal that Vsw is the more dominant factor for FACe, again with the exception of the June solstice in the Southern Hemisphere.

Figure 8 illustrates the effect of Pd on PEJs and SSFACs in a format similar to Figure 7. In the dusk sector, the increase in Pd results in a corresponding rise in PEJs (Figure 8a), with the most pronounced increase occurring in local summer. In this case, Vsw has a more significant influence than Nsw (comparing Figure 8f,k). For SSFACs (Figure 8p), increasing Pd also strengthens SSFACs, most notably in the June solstice than in the December solstice. SSFACs are higher in the Northern Hemisphere during the June solstice and equinox, whereas the December solstice shows similar values in both hemispheres. During the June solstice, Nsw exerts a stronger influence, whereas both Nsw and Vsw contribute significantly during the other two seasons.

At midnight, an increase in Pd intensifies PEJs (Figure 8b) more significantly than the dusk side. The influence during the December solstice is lower than in the other two seasons, exhibiting minimal hemispheric differences. It is evident that Vsw has a greater impact than Nsw (see Figure 8f,k). Pd also positively affects SSFACs (Figure 8p), with the strongest influence during the local summer; this effect primarily arises from Nsw (Figure 8v). Vsw further contributes to the seasonal differences in SSFACs (Figure 8A).

On the dawn side, the positive impact of Pd on PEJs (Figure 8c) is similar to midnight but greater than on the dusk side. Seasonal variations are comparable to those at midnight, with minimal hemispheric differences, primarily driven by Vsw (Figure 8m). An increase in Pd results in a rise in SSFACs (Figure 8r), most notably during the local summer, mainly due to the Nsw effect (Figure 8w).

At noon (Figure 8d,e), the influence of Pd is weaker on average than other local time sectors, but larger in summer compared to winter, which is primarily related to Vsw enhancement (Figure 8n,o). However, the influence of Pd on SSFACs remains strong at noon and is also larger in summer compared to winter, with both Vsw and Nsw playing roles (Figure 8x,C). Notably, at noon with westward PEJs (Figure 8e), Vsw exhibits a counteracting effect in the Southern Hemisphere at the equinox.

In summary, the impact of dynamic pressure on FACp and FACe varies across hemispheres and seasons at different local times. These significant variations in ΔFACs might not be attributed to the minor changes observed in Em or ionospheric conductivity; instead, they are primarily driven by changes in dynamic pressure. Generally, increased Pd strengthens both large-scale FACp and FACe in most local times, except for FACp at midnight. We further differentiated the relative influences of enhanced solar wind mass density and velocity. In the dawn and dusk sectors, FACp (R1 FACs) is primarily influenced by the effects of Vsw. However, during the midnight period, there is an opposing effect of Nsw and Vsw, which offsets the influence of Pd on FACp. Around noontime, the impact of Vsw on FACp is more pronounced than Nsw. Differently, in the dawn, dusk, and midnight periods, the Pd effect on FACe primarily arises from Nsw rather than Vsw. Conversely, during the noontime, in addition to Nsw, Vsw becomes significant, especially in the Northern Hemisphere. The increased dynamic pressure also strengthens PEJs and SSFACs. PEJs are primarily influenced by dynamic pressure with enhanced Vsw. However, SSFACs are affected mostly by dynamic pressure with enhanced Nsw, with enhanced Vsw further contributing to their seasonal differences.

Figure 9a–e illustrate the latitude variations in peak PEJs under different dynamic pressure conditions, highlighting the different effects from enhanced Nsw and Vsw. As Pd increases, the peak PEJs tend to shift toward lower latitudes, showing minimal hemispheric and seasonal differences. At noon, the shift in the Northern Hemisphere is slightly larger when compared to the Southern Hemisphere, except during the equinox for eastward PEJs. Clearly, the influence of Pd is primarily due to the enhanced Vsw effect (Figure 9k–o), while the impact from enhanced Nsw is relatively weak (Figure 9f–j).

The shifts in the latitudes of peak SSFACs under varying Pd conditions are illustrated in Figure 9p–t. During dawn and dusk (Figure 9p,r), the peak SSFACs move toward lower latitudes as Pd increases, primarily due to the influence of Vsw. At midnight, with the exception of the December solstice in the Southern Hemisphere, the peak SSFACs generally shift toward lower latitudes, which might be attributed to the Nsw effect (Figure 9v,q show similar patterns). At noon, the SSFACs typically shift toward lower latitudes, influenced mainly by Vsw, except for the June solstice cases for the eastward (westward) PEJs in the Northern (Southern) Hemisphere.

4. Discussion

We examine the effects of Pd on the peak current density and location of auroral electrojets and large- and small-scale FACs using Swarm satellite data covering 2014 to 2020. Our findings indicate that Pd influences both the strength and latitude of the auroral current system, with differences across local time, seasons, and hemispheres. Additionally, we distinguish the effects of Nsw and Vsw, which display marked dependence on these factors but vary at different local times.

The effects of Pd on PEJs and LSFACs show pronounced hemispheric and seasonal variations, particularly around noon, where they are generally stronger in the summer when compared with winter hemispheres. This trend may relate to higher solar EUV conductivity during the summer at noontime. Moreover, dynamic pressure influences SSFACs across nearly all local time sectors, being larger in summer than in winter, and highlighting the relationship between SSFAC responses and ionospheric conductivity.

4.1. Local Time Differences in Pd Effect on LSFACs

Generally, the increased Pd strengthens large-scale FACp in most local time sectors, with the exception of midnight. However, for FACp and FACe, the elements (Nsw or Vsw effect) that play a dominant role in the influence of Pd are not completely the same in various local time sectors. In the dawn–dusk sectors, FACp (i.e., R1 FACs) is primarily influenced by the effects of Vsw, while the Nsw effect plays a dominant role in a few cases. However, during the midnight period, there is an opposing effect of Nsw and Vsw, which together weaken the overall influence of Pd on FACp. At noontime, the impact of Vsw on FACp is more pronounced than Nsw, especially during local summer. Different from FACp, the prominent Pd effect on FACe primarily arises from the Nsw effect rather than from the Vsw effect in the dawn, dusk, and midnight sectors. Conversely, around noontime, the Vsw effect becomes more significant, especially in the Northern Hemisphere.

Our results confirm previous work that large-scale R1 FACs at dawn and dusk increased with dynamic pressure [23,43]. The Pd effect in the dawn and dusk periods is mainly due to the Vsw effect, which could be explained by two reasons. First, dawn- and duskside R1 FACs originate at the magnetopause boundary; an increase in magnetosheath speed results in greater velocity shear and voltage drop across this boundary, leading to larger R1 FACs. Second, increased Vsw during southward IMF generates a stronger merging electric field, which enhances the convection electric field. Since R1 currents are directly influenced by the solar wind electric field, they exhibit a significant increase. Additionally, we observed that as Nsw increases, FACp also tends to rise, in some cases surpassing the contributions from Vsw, particularly during the June solstice in the dawn–dusk sector of the Southern Hemisphere and during the Equinox in the dusk sector. This is because an increase in the magnetosheath or low-latitude boundary layer (LLBL) plasma density raises the number of R1 current carriers, subsequently increasing R1 FACs. Previous studies have shown that the plasma density and velocity in the magnetosheath and LLBL are positively correlated with the solar wind Nsw and Vsw, respectively [43].

Conversely, for both dawnside and duskside R2 FACs, the positive impact of dynamic pressure primarily stems from solar wind mass density. It is well known that R2 FACs mainly originate from the central plasma sheet (CPS) and boundary plasma sheet (BPS) on the dawn and dusk sides, respectively, which are predominantly situated on closed field lines. The solar wind particles could enter along the magnetosphere flanks and be convected from the nightside to the dayside, thereby altering the plasma pressure gradient in the closed field line regions. Since R2 FACs arise from the ring current pressure gradient, they are expected to be primarily affected by Nsw. Nakano et al. [24] found that R2 FACs exhibited dependence on solar wind dynamic pressure during geomagnetic storms, with weaker correlations during quiet periods. Our work shows that a strong Pd effect on R2 FACs exists in both dawn and dusk sectors during quiet time.

Wing et al. [25] demonstrated that Vsw could influence R1 FACs on both the dawn and dusk sides, while Nsw could not have a significant effect on R1 FACs. This finding is generally consistent with our results. However, since their study did not differentiate between seasons and hemispheres, direct comparisons are challenging. Wing et al. [37] also found that R2 FACs at dawn and dusk were best correlated with Em but showed a weak correlation with Nsw. In contrast, our work indicates that Nsw plays a dominant role in the effect of dynamic pressure on R2 FACs in the dawn and dusk sectors.

At noontime, the influence of solar wind dynamic pressure on FACp and FACe is most significant in local summer, which comes mainly from the Vsw effect. The reason is that the change in Vsw leads to changes in the SW dynamic pressure and merging rate, which, in turn, reconfigures magnetospheric FACs. Wing et al. [25] showed that the mid-day R1 FACs generally correlated best with Nsw, which was attributed to the fact that at the subsolar stagnation point, magnetosheath density was highest near noon. Consequently, this density significantly affects mid-day FACs. The discrepancies between our work and that of Wing et al. [25] may stem from differing study periods; their research includes both quiet and active periods, whereas our focus is primarily on quiet times.

At midnight, the influence of solar wind dynamic pressure on FACp is relatively weak compared to other time periods. This is mainly due to the mutual cancelation of the effects of Nsw and Vsw: an increase in Nsw leads to a rise in FACp, while an increase in Vsw results in a decrease in FACe. One possible explanation is that higher Vsw causes a quicker loss of nighttime plasma, reducing the density of FAC carriers in the nighttime magnetospheric source region and thereby weakening FACp. In contrast, dynamic pressure significantly impacts FACe, primarily due to the dominant role of the Nsw effect. We propose that the increase in Nsw might elevate the inner magnetosphere pressure gradient, thereby enhancing FACe. Shue et al. [44] examined solar wind mass density and velocity effect on aurora intensity separately and found that the density effect on auroral brightness was primarily confined to the nightside, thus increasing upward FACe around midnight. This aspect is consistent with our results. Furthermore, the different responses of FACp and FACe to dynamic pressure indicate an imbalance between the two current systems, suggesting that FACe cannot fully close through FACp in the meridional direction. This imbalanced upward FACe under high Pd conditions might instead be supplied mostly by the remote closure current with R1 FACs in the dawn sector [40].

4.2. Local Time Differences in Pd Effect on PEJs and SSFACs

For PEJs, an increase in dynamic pressure leads to a corresponding rise in the current density. In this context, solar wind velocity has a more pronounced effect than solar wind mass density. This aligns with previous research indicating that PEJ intensity is primarily influenced by Em, which incorporates the effects of Vsw [6,19,20]. Shue and Kamide [21] identified a positive correlation between solar wind density and auroral current intensities, as indicated by the AL and AU indices, with stronger correlations observed under southward IMF conditions. However, they did not investigate the effects of Vsw on AL and AU. In contrast, our findings suggest that Nsw has a weaker impact on PEJs when compared to Vsw. This discrepancy may arise because our work is statistical, whereas theirs is case-based. Zhong et al. [22] demonstrated that the convection electric field contributes to PEJs at dawn, dusk, and nighttime, which is consistent with our results. Our findings indicate that the impact of dynamic pressure is, on average, greater during dawn, dusk, and midnight when compared to noontime. Zhong et al. [22] also noted that following a sudden increase in Pd, the strongest rise in PEJs occurs on the dawn and dusk sides. It should be noted that their research addresses the immediate response of auroral electrojets to increases in dynamic pressure, while our focus is on the variations in PEJ intensity under differing solar dynamic pressure conditions.

For small-scale FACs, an increase in Pd also results in a rise in SSFAC density. However, the effects of Nsw and Vsw on SSFACs differ from those on LSFACs. Nsw significantly influences SSFACs across nearly all local times, seasons, and hemispheres, with Vsw contributing positively in certain scenarios. For example, around noon, during periods of eastward PEJs, the impact of Vsw is nearly equivalent to that of Nsw in the local summer.

4.3. Pd Effect on Auroral Current Latitudes

Our study demonstrates that the increasing Pd causes PEJs to shift equatorward. Previous studies have reported that increases or decreases in solar wind dynamic pressure lead to corresponding changes in magnetospheric size [45]. An increase in solar wind dynamic pressure enhances the coupling efficiency between the solar wind and the magnetosphere, thereby creating more pile-up of open fluxes, and expanding the polar cap region [46]. Since PEJs and LSFACs can represent the primary positions of the auroral oval, both of them tend to move further equatorward under high Pd conditions.

Separate analyses of the effects of solar wind velocity and solar wind mass density reveal that enhanced Vsw is more effective in causing equatorward shifts in PEJ latitudes than the Nsw effect. This finding is consistent with previous research, indicating a strong correlation between Em and PEJ latitudes [16]. The reason for this is that an increase in Vsw results in changes to both solar wind dynamic pressure and merging rates, which together promote the equatorward movement of the auroral oval. Additionally, the present study shows that enhanced Pd results in the equatorward shift in SSFACs, with the Vsw effect being more prominent than Nsw in most seasons and hemispheres, except in the midnight sector, where Nsw appears to play a more significant role. This aligns with the findings of Shue et al. [44], who suggested that enhanced dynamic pressure associated with increased velocity was more effective in compressing the dayside magnetosphere, while dynamic pressure associated with increased mass density could compress the nightside magnetosphere, leading to particle precipitation and auroras at night [47]. Moreover, our research did not find significant hemispheric and seasonal differences in the locations of PEJ currents, as well as SSFACs at dawn and dusk. However, there are significant hemispheric and seasonal variations in SSFACs in the noon–midnight sectors, indicating the complexity of SSFAC distribution in response to Pd during these two time periods.

5. Summary

In this work, we study the effects of solar wind Pd on the peak current density and location of PEJs, LSFACs, and SSFACs using Swarm A/C satellite observations spanning from 2014 to 2020. Additionally, we distinguish the roles of solar wind Nsw and Vsw in shaping the auroral current system.

• The effects of Pd on LSFACs and PEJs show pronounced hemispheric and seasonal variations, particularly around noon, where they are generally stronger in the summer compared to the winter hemisphere. This trend may relate to higher solar EUV conductivity during the summer at noontime. Moreover, Pd influences SSFACs across nearly all local time sectors, which is more obvious in summer compared to winter, highlighting the relationship between SSFAC responses and ionospheric conductivity.
• The increased Pd typically enhances large-scale R1 FACs in most local times, with the exception of midnight. At dawn and dusk, R1 FACs are influenced by both Vsw and Nsw; however, during midnight, the opposing effects of these factors weaken the overall influence of Pd. At noon, the effect of Vsw becomes more dominant, particularly in the local summer. Unlike R1 FACs, the influence of Pd on FACe is primarily attributed to Nsw, especially during dawn, dusk, and midnight, while at noon, Vsw plays a crucial role.
• Turning to the effects on PEJs, the increase in Pd leads to a rise in current density, with Vsw exerting a more substantial effect than Nsw. The Pd impacts are generally more pronounced during dawn, dusk, and midnight when compared to noontime.
• For SSFACs, an increase in Pd also results in higher current densities, but the contributions of Nsw and Vsw differ from those affecting LSFACs. Notably, Nsw has a significant impact on SSFACs across various seasons and hemispheres, with Vsw playing a crucial role in specific scenarios.
• Higher Pd causes PEJs to shift more equatorward. The enhanced Vsw is more effective in driving equatorward shifts in PEJs and SSFACs (except for during the midnight period) than Nsw. While we found no significant hemispheric or seasonal differences in the locations of PEJs, SSFACs exhibit noticeable variations around noon and midnight, emphasizing the complexity of their distribution in response to Pd in these local time sectors.