Effect of interplanetary magnetic field B_x on the polar electrojets as observed by CHAMP and Swarm satellites

1.   Introduction

The polar electrojets (PEJs) constitute important components of high-latitude current systems. PEJs stream within the ionospheric E region perpendicular to both the electric and magnetic fields. Observations from ground-based magnetometers, rockets, satellites, and comprehensive model simulations, demonstrate that PEJs exhibit variations in local time, seasons, and hemispheres in response to changes in the interplanetary magnetic field (IMF) (e.g. Gjerloev and Hoffman, 2014; Guo JP et al., 2014; Huang T et al., 2017; Laundal et al., 2018a, b; Zhong YF et al., 2022; Wang H and Lühr, 2023, 2024; Wang YB, 2023).

Among the three components of IMF, IMF B_z in the north−south direction stands out as the primary factor influencing the development and characteristics of the polar current system. While the direct influence of IMF B_z on PEJ might be less pronounced than field-aligned currents (FACs), it can indirectly affect the strength and behavior of a PEJ by modulating the polar electric field and ionospheric dynamics (e.g. Huang T et al., 2017). PEJs show stronger intensity under southward IMF B_z compared to northward IMF B_z (e.g. Wang H et al., 2008; Huang T et al., 2017).

IMF B_y in the dawn−dusk direction causes dawn−dusk and hemispheric asymmetry in PEJs. Around noontime in the polar region, prominent DPY (Dayside Polar cap Y) PEJs emerge with their flowing direction and intensity controlled by the orientation of the IMF B_y (e.g. Friis-Christensen and Wilhjelm, 1975). The merging electric field, E_m, considers the collective influence of IMF B_z and B_y on the solar wind electric fields that effectively drive the magnetosphere−ionosphere interactions. The intensity of prominent PEJs was found to be controlled by the merging electric field (e.g. Huang T et al., 2017). Note that E_m considers the direction of IMF B_z, while remaining symmetric regarding the polarity of IMF B_y.

However, recent work has revealed local time and hemispheric asymmetry in the auroral current system in relation to the orientation of IMF B_y (Wang H and Lühr, 2023). Generally, they found that the opposite signs of dipole tilt angle (the angle between the Earth's dipole axis and the Sun–Earth connection) and IMF B_y tend to enhance the westward PEJ in the dawn and midnight sectors, supporting the finding of Reistad et al. (2020). At dawn, this clear IMF B_y effect is roughly equal in both winter and summer hemispheres; however, at midnight, the effect is relatively stronger in the local winter hemisphere. At noon, the influence of IMF B_y is basically consistent with the dependence of DPY current on IMF B_y polarity, and the impact is stronger in the local summer hemisphere. At dusk, the effect of IMF B_y on the eastward PEJ is significantly weaker than that observed in the other local time sectors. Furthermore, the positions of the PEJ in the Northern Hemisphere (NH) and Southern Hemisphere (SH) are not conjugate; instead, their positions are related to the orientation of IMF B_y. For the eastward and westward PEJs, the variation of hemispheric latitude displacement of PEJ with IMF B_y polarity is completely opposite.

Unlike IMF B_y and B_z, IMF B_x does not appear to influence PEJ intensity. However, variations in IMF B_x are known to correlate with auroral intensity; thus, it might be that IMF B_x contributes to changes in the PEJ pattern. Elphinstone et al. (1990) found that NH aurora at dawn and dusk was weaker for IMF B_x > 0 than B_x < 0 under northward IMF condition. Shue et al. (2002) reported that the NH nighttime auroral power was higher for IMF B_x < 0 than for IMF B_x > 0 under similar IMF B_y conditions when IMF is southward. This specific IMF B_x effect was significant in both summer and winter. For northward IMF, such IMF B_x asymmetry was not observed due to weak aurora intensity. They concluded that hemispheric auroral power reached its highest level when all the components of IMF were negative. Reistad et al. (2014) found that the duskside aurora in the winter NH was brighter for IMF B_x < 0, while SH aurora was brighter for IMF B_x > 0. This distinctive impact of IMF B_x was clarified as due to asymmetric dayside reconnection — a northward shift of the dayside reconnection line for IMF B_x > 0 and a southward shift for IMF B_x < 0. As a consequence, the tail plasma sheet undergoes a reverse northward−southward directional shift (Cowley, 1981; Hoilijoki et al., 2014). Moreover, Kubyshkina et al. (2018) revealed that IMF B_x is important for the more frequent occurrence of substorms, based on Kivelson and Hughes (1990)’s theory regarding a lower threshold of substorm instability in a curved current sheet.

Through the analysis of a typical case of a magnetic storm event, Wang H et al. (2014) showed that, with a strong sunward IMF B_x, a new pair of strong field-aligned currents emerge at noon, with the poleward part flowing downward and the equatorward part flowing upward. By using multi-satellite and ground based observations, Laundal et al. (2018b) showed that the sign of IMF B_x changes FACs and ionospheric equivalent current by no more than 10% under southward IMF B_z conditions. Their work relied on average distribution maps of satellite-observed currents, sorted by IMF components. Additionally, their work extensively compared FACs to the findings of Reistad et al. (2014), but overlooked examination of PEJs in different local times and hemispheres. On the other hand, Kubyshkina et al. (2023) highlighted the obvious asymmetric polarity-dependent effects of IMF B_x, attributed to the dipole tilt effect on magnetotail stability and nightside reconnection. In winter, IMF B_x > 0 suppresses AL index while B_x < 0 increases AL under similar driving conditions. Their work focused primarily on exploring the influence of nighttime IMF B_x, neglecting investigation into its impact on the dayside. In summary, the local time and hemispheric differences in the impact of IMF B_x on PEJs have not been comprehensively investigated in the literature; further analysis is thus required to address these gaps.

This study is the first to provide a detailed investigation of IMF B_x effects on PEJ strength and latitude for different local times, seasons, and hemispheres, using observations from CHAMP and Swarm satellites. In the following section the data sets we studied and our methods are described. The statistical analysis of observations is presented in Section 3. A discussion and summary in the context of previous findings is presented in Sections 4 and 5.

2.   Data: Sources and Processing Methods

The three European Space Agency Swarm satellites were launched on November 22, 2013, into a near-polar orbit with an inclination of 87.5°. The final orbit constellation was achieved on 17 April 2014. Swarm-A and Swarm-C fly side-by-side with a separation of 1.4° in longitude at about 450 km altitude; Swarm B flies at about 510 km with a slightly higher inclination. CHAMP was launched into a near-polar orbit on July 15, 2000 (87.3° inclination, 450 km altitude, orbital period of 93 min; Reigber et al., 2002) the mission lasted until September 2010.

PEJ strengths and positions were derived using scalar magnetic field data. Olsen (1996) first introduced the line current method for estimating the strength of an ionospheric PEJ. This method was later applied to CHAMP data, validated against ground-based observations (Ritter et al., 2004), and extended to Swarm data by Aakjær et al. (2016). The auroral electrojet is represented by a series of infinite line currents in the Earth’s ionospheric E layer (about 115 km), each spaced 1° apart. The observed total field residuals undergo inversion via a least-squares fitting approach to derive the strength of each line current. This study is focused on estimating the zonal component of the PEJ, where positive values indicate eastward currents and negative values signify westward electrojet. To prevent false detections, peak absolute values need to fall within the range of 0.03 A/m to 3 A/m. In this analysis, a quasi-dipole (QD) coordinate system has been used (e.g., Richmond, 1995; Emmert et al., 2010).

This study utilizes CHAMP data from July 26, 2000 to September 4, 2010 and Swarm-A PEJ estimates from 17 April 2014 to 16 April 2020 (in total, more than 16 years of data). We select eastward PEJ peaks in the 15−20 MLT, and westward PEJ peaks in the 21−02 MLT and 03−08 MLT sectors from each auroral crossing. In the noontime, 09−14 MLT, both eastward and westward PEJ peaks are selected, because the direction of DPY current is related to the sign of IMF B_y. In this study we focus on the peak current density and location of the PEJs for several reasons. The PEJs represent a concentrated and intense current system within the auroral region. Typically, field-aligned currents flank PEJ peaks on both their poleward and equatorward sides. Thus, the PEJ peak location serves as a focal point representing the central region of auroral and field-aligned currents. The peak density of a PEJ frequently acts as an indicator of the magnitude of high latitude magnetic activity. Monitoring both peak density and location of a PEJ provides insights into geomagnetic activity and disturbances within the high latitude ionosphere.

The data are divided for two Lloyd seasons: June solstice (66 days before and after 1 July) and Dec. solstice (66 days before and after 1 January). As utilized in previous work (e.g. Huang T et al., 2015), the specific criterion used to select dominant IMF B_x events involves the IMF cone angle, calculated as the angle between the IMF vector and the Sun−Earth direction. It is calculated as

When the IMF cone angle is less than 30° or exceeds 150°, it signifies that IMF B_x component predominantly points toward the Sun or toward the Earth. Employing this angle criterion allows us to separate IMF B_x effect from other components of IMF, preventing potential contamination of results because of the Parker spiral configuration of the IMF.

The event numbers of auroral crossings for different IMF B_x cone angle are shown in Figure 1, categorized by local time, season, and hemisphere. This study focuses on periods with E_m ≤ 3 mV/m since there are fewer events with IMF cone angle ≤30° and ≥150° during high E_m periods. Overall, the situation is more favorable for IMF B_x cone angle between 30° and 150°. Nevertheless, across each MLT bin, the number of events generally surpasses 50. However, despite our having an adequate number of samples in each local bin, the uneven distribution of events might impact the results. Notably, we have removed outliers from our analysis, which could help improve the stability of results. To further confirm the stability and generalizability of results, we conduct analyses on separate datasets, from either CHAMP (2000−2010) or Swarm (2014−2020) satellites. Our primary findings exhibit considerable consistency (figures not shown). These observations potentially indicate the reliability of the current statistical analyses. Future study may involve using additional observational data to validate the conclusions drawn in this work.

Figure  1.  Counts of auroral crossings events, recorded by CHAMP and Swarm A, as a function of IMF B_x cone angle when E_m is less than 3 mV/m. The data are divided into two panels: the left panel presents Northern Hemisphere event counts; the right panel presents Southern Hemisphere counts. The bars are color-coded to distinguish among different local time sectors.

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3.   Statistical Results

PEJs typically exhibit an eastward (positive value) flow in the dusk sector, and a westward (negative value) flow in the dawn and midnight sectors; PEJ flow direction at noon can vary between eastward or westward, based on the IMF B_y polarity. Our primary focus in this study lies on assessing PEJ strength. Therefore, we use the absolute value of PEJ density in all local time sectors, treating both eastward and westward PEJ as positive values.

Figure 2 displays how absolute PEJ strength varies as a function of IMF B_x cone angle and magnetic local time. We calculate the mean absolute PEJ value within each local time, under all IMF B_x cone angle conditions, and then subtract these local time mean values from the recorded PEJ data in order to better represent PEJ variation as a function of IMF B_x cone angle, i.e., IMF B_x sign. The left panel of Figure 2 presents Northern Hemisphere results; the right panel, Southern Hemisphere results. Figures 2a−d are for positive PEJ around noon; Figures 2e−h are for negative PEJ around noon. When the IMF cone angle is greater than 90°, it represents IMF B_x < 0; when it is less than 90°, IMF B_x > 0. The closer the angle of IMF B_x is to 0° or 180°, the more influential the B_x component becomes. Conversely, when the angle is closer to 90°, the dominance shifts towards IMF B_y or B_z.

Figure  2.  IMF B_x cone angle versus magnetic local time variations of absolute PEJ in the Northern (left column) and Southern (right column) Hemispheres. Panels (a)−(d) are for positive PEJ around noon; (e)−(h) are for for negative PEJ around noon. The mean value has been subtracted in each local time. Panels (a), (b), (e), and (f) are June solstice; (c), (d), (g), and (h) are December solstice.

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Obvious local time, seasonal, and hemispheric differences can be found when considering PEJ dependence on the cone angle. In local summer (Figures 2a, 2d, 2e, 2h), the dependence exhibits more significance on the dayside than on the nightside. Conversely, in local winter (Figures 2b, 2c, 2f, 2g), the nightime variation becomes more pronounced than in the daytime; moreover, the cone angle dependence between the daytime and nighttime is different. For example, in the summer NH (Figure 2e), the daytime westward PEJ shows its maximum value around 60°−90° (i.e. IMF B_x > 0). In the winter NH (Figures 2c, 2g), the nighttime westward PEJ shows its maximum value around 120°−180° (i.e. IMF B_x < 0). The B_x sign dependence of eastward and westward PEJ around noontime is nearly opposite. The noontime eastward PEJ (Figure 2a) in the NH shows its maximum value at IMF B_x < 0, while the noontime westward PEJ (Figure 2e) shows its maximum value at IMF B_x > 0. The variation of PEJ with IMF B_x is less prominent at dawn and dusk when compared to the noon and midnight sectors. The response of noontime and midnight PEJ to the IMF B_x sign in the Southern Hemisphere is reversed when compared to that in the Northern Hemisphere.

One can notice that the peak value of PEJ around noon (Figures 2a, 2d, 2e, 2f, 2h) aligns around cone angle 90°, while the minimum value aligns near 0° or 180°, indicating that the influences of IMF B_y and B_z surpass that of the IMF B_x component. However, during nighttime (Figures 2b, 2c, 2d, 2f, 2g, 2h), the maximum PEJ shift towards 0° or 180°, indicating that the effect of IMF B_x on the nighttime PEJ might surpass that of IMF B_y or B_z.

Figure 3 shows the difference in the MLat (ΔMLat) of peak PEJ as a function of IMF B_x cone angle and local time in the same format as Figure 2. The mean absolute MLat of PEJ within each local time under all IMF B_x cone angle conditions is determined and is then subtracted in each local time in order to better represent peak PEJ variation in latitude with IMF B_x cone angle. One can notice that the most prominent response occurs around noon and midnight, with the dawn and dusk less affected. Changes in the summer hemisphere are notably greater compared to those in the winter hemisphere. PEJ locations show local time and hemispheric asymmetry in their dependence on the IMF B_x orientation. During the Northern Hemisphere’s summer, the noontime eastward PEJ (Figure 3a) is positioned at a higher latitude when IMF B_x < 0 (cone angle >90°), while the noontime westward PEJ (Figure 3e) and nighttime westward PEJ (Figures 3a, 3c, 3e, 3g) are situated at higher latitudes when IMF B_x > 0 (cone angle <90°). In contrast, during the Southern Hemisphere's summer, the eastward PEJ at noon (Figure 3d) is located at a higher latitude when IMF B_x > 0, whereas the westward PEJ at noon (Figure 3h) and night (Figures 3b, 3d, 3f, 3h) is positioned at a higher latitude when IMF B_x < 0.

Figure  3.  The same format as Figure 2, but for the MLat of peak PEJ. IMF B_x cone angle versus magnetic local time variations of the MLat of peak PEJ in the Northern (left column) and Southern Hemispheres (right column). Panels (a)−(d) are for positive PEJ around noon, and (e)−(h) for negative PEJ around noon. The mean value has been subtracted in each local time. Panels (a), (b), (e), and (f) are in June solstice and (c), (d), (g), and (h) are in December solstice.

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In order to separate the impact of IMF B_x from the other IMF components, we have subtracted the result of dominant IMF B_x > 0 (i.e. cone angle ≤30°) from that of IMF B_x < 0 (i.e. cone angle ≥150°) to obtain ΔPEJ and ΔMLat, i.e.

ΔPEJ = |PEJ| cone angle ≤30° – |PEJ| cone angle ≥150°,

and

ΔMLat = |MLat| cone angle ≤30° – |MLat| cone angle ≥150°.

The closer the cone angle is to 0° or 180°, the more influential the B_x component becomes. Such explicit impact of the orientation of IMF B_x on the strength and position of PEJ is illustrated in Figure 4. A positive value means the current is stronger and its location is at a higher latitude for IMF B_x > 0 than for IMF B_x < 0. The asterisk denotes the summer solstice; the circle denotes the winter solstice. The blue dashed line indicates the Northern Hemisphere; the red dashed line indicates the Southern Hemisphere.

Figure  4.  Variations of absolute peak PEJ and MLat between IMF cone angle ≤30° and cone angle ≥150° as a function of MLT. The asterisk denotes June solstice, and the circle represents December solstice. Blue lines indicate Northern Hemisphere data; red lines, Southern Hemisphere.

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For the eastward PEJ in the dusk sector (15–20 MLT), in both hemispheres, the ΔPEJ of IMF B_x > 0 is a little weaker than that of IMF B_x < 0, a difference more evident in NH summer. For IMF B_x > 0, the latitude of the PEJ is located a little more poleward than that of IMF B_x < 0, except for that in the winter SH. In the 21–02 MLT sector, the westward current is obviously weaker for IMF B_x > 0 than for IMF B_x < 0 in local NH winter, while the PEJ is obviously stronger for IMF B_x > 0 than for IMF B_x < 0 during the local SH winter. A weaker IMF B_x effect can be observed in the local summer hemisphere. The NH PEJ for IMF B_x > 0 is located at a higher latitude than for IMF B_x < 0, while the SH PEJ shows an opposite trend. The IMF B_x dependence of the westward electrojet in the dawn sector (03–08 MLT) is less obvious than in the nighttime (21–02 MLT). In the summer NH, the PEJ is stronger for IMF B_x > 0, while in the winter NH, the PEJ of IMF B_x < 0 is stronger. The SH response is weaker than that in the NH. The PEJ for IMF B_x > 0 is located at higher latitudes, compared to that for IMF B_x < 0, except for the SH summer. Around noontime, the responses of the eastward and westward PEJ to IMF B_x are more significant in local summer. For IMF B_x > 0 (IMF B_x < 0), the eastward (westward) PEJ in the NH is weaker and located at lower latitudes, whereas the opposite effect is true in the SH, with the latitudinal differences being greater in local summer than in local winter. This confirms the effect of ionospheric conductance on the MLat of PEJ, i.e., the auroral region retreats equatorward under dark conditions (Wang H et al., 2005).

We further calculate the relative change of PEJ compared to the background value, which is defined as ΔPEJ/M_PEJ. The average peak PEJ values for IMF B_x cone angles ≤30° and ≥150° is defined as

M_PEJ = 0.5 × (|PEJ|cone angle ≤30° + |PEJ|cone angle ≥150°).

Notably, the changes in PEJ density during winter night and summer day can reach nearly 6%−22% relative to background values, differences that can be regarded significant.

4.   Discussion

In this study, we investigate differences in local time and hemispheric impact of IMF B_x on peak PEJ current density and MLat by examining more than 16 years of observations from CHAMP (2000–2010) and Swarm (2014–2020) satellites. We find that IMF B_x has obvious impacts on the latitude and peak strength of PEJs; prominent local time, seasonal, and hemispheric differences in both are well correlated with variations in IMF B_x.

This study examines periods dominated by IMF B_x. When the IMF cone angle is ≤30° or ≥150°, IMF is primarily aligned sunward or anti-sunward, and the IMF B_y and B_z are relatively weaker compared to IMF B_x. As E_m is primarily associated with the amplitudes of IMF B_y and B_z, it remains weak during times when IMF B_x prevails.

Figure 5 illustrates the variation of IMF B_x, B_y, B_z, and E_m between periods of IMF cone angle ≤30° (IMF B_x > 0) and IMF cone angle ≥150° (IMF B_x < 0) in different local time sectors. It reveals a difference of about −1.2 nT in IMF B_y, about 0.25 nT in IMF B_z, about 0.05 mV/m in E_m. However, these fluctuations appear insufficient to explain the 6−20% variations observed in PEJ strength during midnight and noon periods. Previous researchers have studied the impact of IMF B_x on the AL index, and have typically restricted IMF |B_y| to less than 1 nT (Laundal et al., 2018b) or less than 2 nT (Holappa et al., 2023) in order to eliminate the influence of IMF B_y. In our study, the variation in IMF B_y falls within these specified ranges. However, IMF B_x variance exceeds 4 nT, suggesting that the differences in local time, season, and hemisphere in PEJ strength and latitude, as revealed in Figure 4, stem primarily from the influence of IMF B_x.

Figure  5.  Differences of IMF B_x, B_y, B_z, and E_m between IMF cone angle ≤30° and cone angle ≥150° as a function of MLT. The asterisk denotes June solstice, and the circle represents December solstice. Blue lines represent Northern Hemisphere data; red lines, Hemisphere.

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Overall, the explicit effect of IMF B_x is more pronounced around noontime and midnight, compared to dawn and dusk sectors. This might be attributed to the alignment of IMF B_x along the noon-meridional direction. Notably, the IMF B_x effect on the westward PEJ near midnight (21–02 MLT) is more significant in winter than in summer. During NH winter, when IMF B_x < 0, the westward PEJ is stronger than when IMF B_x > 0; conversely, in SH winter, the westward electrojet is stronger for IMF B_x > 0 compared to IMF B_x < 0. This result is consistent with previous studies (Kubyshkina et al., 2023), indicating that in NH winter, IMF B_x > 0 suppresses the AL index while B_x < 0 increases AL under similar driving conditions. Kubyshkina et al. (2023) explained that IMF B_x could impact the nightside magnetospheric configuration, causing a shift in the current sheet either northward or southward for IMF B_x < 0 or B_x > 0. In cases of a large negative dipole tilt (i.e. NH winter) with plasma sheet positioned below the equatorial plane, IMF B_x < 0 might realign the current sheet back towards the equatorial plane, restoring symmetry and resulting in increased aurora activity. A similar scenario arises in SH winter with a large positive dipole tilt for IMF B_x > 0.

Another explanation for the winter preference of IMF B_x effect on the nighttime PEJ is related to the nighttime aurora precipitation. In local winter, ionospheric conductance is induced mainly by particle precipitation, whereas in local summer, solar illuminated ionospheric conductance dominates. Particle precipitation shows explicit IMF B_x dependence. Shue et al. (2002) reported that the NH nighttime auroral power was higher for IMF B_x < 0 than for IMF B_x > 0 under similar IMF B_y conditions when IMF is southward. The opposite trend was observed in the SH. This IMF B_x dependence of auroral intensity was further confirmed by Reistad et al. (2014). This might explain why the IMF B_x effect on PEJs is relatively more pronounced in local winter compared to local summer.

Unlike the midnight sector, the IMF B_x effect on the noontime PEJ is more significant in the summer hemisphere with higher conductance. During periods dominated by IMF B_x < 0 (B_x > 0), the reconnection site tends to manifest around the northern (southern) magnetotail lobe, facilitating antiparallel reconnection between the earthward (sunward) IMF B_x and the northward geomagnetic field. In summer, the magnetotail shifts closer to the Sun, creating a geomagnetic geometry more conducive to lobe reconnection. Lobe reconnection seems to explain why the noontime PEJ in the NH is situated at a higher latitude when IMF B_x < 0, as compared to when IMF B_x > 0. Conversely, in the SH, the reversed IMF B_x sign effect occurs: IMF B_x > 0 promotes the southern lobe reconnection, resulting in a larger PEJ located at a higher latitude in the SH.

In the Northern Hemisphere, the eastward current is stronger when IMF B_x < 0 than when IMF B_x > 0, while the westward PEJ shows a reversed variation with the IMF B_x sign. Conversely, the SH PEJ shows the opposite IMF B_x sign dependence. One plausible explanation might be that in the NH, IMF B_x < 0 results in a larger eastward PEJ, whereas in the SH, it is IMF B_x > 0 that results in enhanced eastward PEJ. This also explains why the westward PEJ gets weakened in the NH during IMF B_x < 0 compared to IMF B_x > 0, as the negative B_x strengthens the eastward PEJ. Therefore, the pre-existing westward PEJ gets reduced and the center of peak westward PEJ shifts toward lower latitudes.

When IMF B_x < 0, the eastward current intensifies on the dayside of NH, which might be attributed to the magnetic tension force acting on open field lines during IMF B_x periods. As proposed by Cowley (1981) and Reistad et al. (2014), negative (positive) IMF B_x results in a larger magnetic tension force on the northern (southern) open magnetic field line. This amplifies the magnetopause current perpendicular to the magnetic field, flowing into the noon-midnight meridional plane when observed from the duskside. This magnetopause current, known as the generator of FACs, strengthens and flows into the ionosphere in the pre-noon sector but flows out of the ionosphere in the post-noon sector. Consequently, an enhanced ionospheric eastward current is expected to connect FACs around noontime. Although the direction of PEJs has been regarded to be controlled by IMF B_y (e.g., Friis-Christensen and Wilhjelm, 1975), this work suggests that the direction of the IMF B_x component, too, impacts the flow direction of the noontime PEJ, with a more significant impact in the summer hemisphere.

5.   Summary

In the present work, we have studied the important impact of IMF B_x on PEJ peak current density and location. Our study is based on magnetic field observations gathered from CHAMP (2000–2010) and Swarm (2014–2020) satellites. The effect of IMF B_x on PEJ strength and location displays obvious local time, seasonal, and hemispherical differences, suggesting distinct dominant physical processes in different local time sectors.

Generally, IMF tend to amplify the westward PEJ in the midnight sectors when IMF B_x < 0 in the Northern Hemisphere (NH), and when IMF B_x > 0 in the Southern Hemisphere (SH). This effect is relatively stronger in the local winter hemisphere. Meanwhile, in the noon regions, IMF B_x intensifies the eastward current, consequently reducing the westward current, when it is <0 in the NH and when it is >0 in the SH; this pattern is stronger in the local summer. At dusk and dawn, the effect of IMF B_x on the PEJs is significantly weaker than in other local time sectors.

In addition, latitudinal positions of the NH and SH PEJs are unconjugated, but each does correlate with the orientations of its IMF B_x. For both the eastward and westward PEJs around noon and midnight, the dependence on IMF B_x polarity of hemispheric latitude displacement of PEJ is entirely opposite. When IMF B_x < 0 (IMF B_x > 0) in the NH (SH), the eastward current around noon shifts towards higher latitudes, while the westward current around midnight moves towards lower latitudes.