We retrieved the 21-year EEJ strength data at 79°W using the geomagnetic horizontal components H recorded at Jicamarca (11.9°S, 76.8°W, 0.6°N mag. lat) and Piura (5.2°S, 80.6°W, 6.9°N mag. lat). To reveal ionospheric current disturbances, the average H at local midnight is used as a baseline to remove the main magnetic field of the geomagnetic field (Lühr, 2003). We then subtracted the baseline from the data to determine the EEJ strengths. These strengths were calculated as the difference between the H value recorded at the equatorial station and the off-equatorial station (Rastogi and Klobuchar, 1990; Anderson, 2002).

The OMNI solar wind data (velocity, density, dynamic pressure, and IMF B _z) at the Earth’s bow shock nose are collected from the Goddard Space Flight Center of NASA. To identify transient events, we selected the events of transient increase/decrease in solar wind velocity, density, dynamic pressure, and IMF B _z using the solar wind data spanning from 2001 to 2021. Subsequently, we compiled the corresponding EEJ data and conducted superposed epoch analyses to elucidate the corresponding EEJ reactions to the transient events. Our analysis was primarily focused on the dayside EEJ responses (i.e., the intervals between 6 and 18 LT at 79°W). This focus on daytime responses was chosen as nighttime EEJ responses to transient changes in solar wind tend to be less pronounced due to the lower ionospheric conductivity on the nightside. These criteria were meticulously designed to ensure an adequate number of valid samples while minimizing noise interference. Finally, we collected a total of 113 (40) events of solar wind velocity increase (decrease), a total of 47 (51) events of solar wind density increase (decrease), a total of 83 (58) events of solar wind dynamic pressure increase (decrease), a total of 31 (29) events of southward (northward) IMF B _z increase for statistics. In the subsequent equations, we use x ¯ to denote the average value of x , ∑ x to denote the total value of x , and ∆ x to denote the variation of x . The criteria are as follows.

The EEJ reacts rapidly and positively to transient changes in solar wind dynamic pressure and associated parameters. The solar wind dynamic pressure, denoted as P _dyn, is determined by both solar wind velocity V and density N and can be expressed as P dyn = ∑ m N V 2 , where m is the mass of a single particle (Dendy, 1995). To illustrate the EEJ responses due to variations in solar wind dynamic pressure and associated parameters, we chose three typical events of sharp solar wind velocity, density, and dynamic pressure enhancements as depicted in Figure 1. In the first event (column 1), a sudden increase solely in solar wind velocity occurred during an 11-min period on 06 May 2003, at 1328UT (0812LT), with a magnitude of 85 km/s, while other solar wind parameters including N, IMF B _y, and B _z were quite stable during this interval. Accordingly, the EEJ exhibited a sharp increase of ∼50 nT, peaked in ∼16 min, and returned to its original state at ∼1408UT (0852LT) (Figure 1A ₅). In this event, solar wind velocity displayed a sudden and strong increase along with a weak southward turning of IMF B _z with a magnitude of 5 nT. IMF B _z plays a dominant role in magnetosphere-ionosphere coupling (Bhaskar and Vichare, 2013). We cannot rule out the possibility that the variations in EEJ in this event may be influenced by IMF B _z. According to subsequent statistical analysis, the change in IMF B _z in this event is too small to be considered the primary cause of the sudden variation in EEJ, and the main reason for the EEJ’s sudden increase in this event is primarily due to the abrupt increase in solar wind velocity. The second column of Figure 1 shows an event in which only the solar wind density increased rapidly, with an amplitude of 10 particles/cm³ within 4 min at 1551UT (1035LT) on 02 August 2012. Correspondingly, the EEJ exhibited a simultaneous pulse-like enhancement of 67 nT as the solar wind density, peaked in 11 min, and returned to its original state in 37 min. The third column of Figure 1 demonstrates an event on 14 February 2011 characterized by sharp increases in solar wind density (black line), velocity (blue line), and dynamic pressure. The interplanetary shock with dynamic pressure enhancement by 5 nPa within 1 min at 1556UT (1040LT) and triggered a 56 nT growth pulse of EEJ in 20 min. In all three cases, the EEJ exhibited enhancements to the enhancements in solar wind parameters. The EEJ expression is EEJ = σ c E , where σ c is the cowling conductivity, and E is the dawn-to-dusk electric field in the ionosphere (Untiedt, 1967), EEJ depends on both cowling conductivity and the dawn-to-dusk electric field. The increased EEJ responses observed in all three events suggest the presence of eastward electric fields at low latitudes on the dayside.

As opposed to the enhancements in solar wind parameters, Figure 2 shows three events of sharply decreased solar wind velocity, density, and dynamic pressure as well as their effects on EEJ. In these cases, the EEJ responses were also positively correlated with the changes, which were decreases in the cases. These findings suggest the presence of westward electric fields on the dayside in response to decreases in solar wind velocity, density, and dynamic pressure. In summary, these observations indicate that the EEJ exhibits high responsiveness to sudden changes in the solar wind dynamic pressure and associated parameters.

Compared to individual case studies, conducting a statistical study on how the equatorial electric current responds to changes in solar wind parameters can eliminate randomness and reveal the underlying general principles of the phenomenon. Nilam et al. (2020) conducted a superposed epoch analysis of EEJ response to solar wind density in the India region, which resulted in a positive correlation between the EEJ and the variation in solar wind density. We also conducted a superposed epoch analysis to further investigate the EEJ responses to solar wind disturbances in South America and obtained the same result and extended it from the solar wind density to all the solar wind dynamic pressure associated parameters. Figures 3, 4 present the results of the analysis for transient increases and decreases in the solar wind parameters, respectively. In each figure, the individual events are shown as thin gray curves, while the thick red or black curves represent the average values of all events. The shaded areas in these panels represent the upper and lower quartiles. In Figure 3, the EEJ increased as the solar wind dynamic pressure parameters increased. For the 113 velocity increase events with a mean change of 85 km/s, the average increase in the EEJ is 8 nT. For the 47 density increase events with a mean change of 8 particles/cm³, the average increase in the EEJ is 6 nT, while for the 83 dynamic pressure increase events with a mean change of 3 nPa, the average increase in the EEJ is 6 nT. The statistical results showed that responses of the EEJ to the transient step-like increases in solar wind dynamic pressure parameters were also pulse-like in shape, with delays of ∼5 min and peaks at ∼11 min. To further illuminate our findings, Panel (a₅) of Figure 3 offers a bivariate histogram. This graphical representation illustrates the distribution of event samples across two-dimensional bins of EEJ strength and epoch time. The coloration of each bin serves as a visual cue, indicating the number of events contained within. Conversely, in Figure 4, the EEJ displayed rapid decreases when the solar wind dynamic pressure and associated parameters experienced a sudden drop. Overall, both cases and statistical results indicate that EEJ responses are positively correlated with changes in solar wind velocity, density, and dynamic pressure.

IMF B _z also has appreciable effects on the low-latitude ionospheric electrodynamics. Figures 5, 6 show that EEJ tends to increase/decrease when IMF B _z has a sharply southward/northward enhancement. In the first column of Figure 6, on 06 December 2014, IMF B _z underwent a sudden southward turning with a magnitude of 22 nT within 8 min, and EEJ exhibited a sudden increase of 69 nT within 14 min. In contrast, in the second column, on 23 May 2002, IMF B _z exhibited a northward turning with an amplitude of 32 nT within 12 min, leading to a synchronous decrease of 109 nT in EEJ within19 min. For the superposed epoch analysis of IMF B _z, there are totally 31 southward IMF B _z turning increase events and 29 northward IMF B _z turning increase events, with average amplitudes of 23 nT for both. The average responses are 57 nT and 32 nT, as shown in Figure 6. Compared with solar wind dynamic pressure parameters, IMF B _z has a greater influence on EEJ, due to the dominant role of magnetic reconnection in equatorial electric field fluctuations (Rastogi and Patel, 1975; Kelley et al., 1979; Abdu et al., 1995; Kikuchi et al., 2003; Basu et al., 2005; Tulasi Ram et al., 2012; 2016; Bhaskar and Vichare, 2013; Ohtani et al., 2013). On the other hand, we observed a significant asymmetry in the response of the EEJ to increase in southward IMF B _z and northward IMF B _z. The EEJ’s response to northward IMF B _z growth is nearly twice as strong as its response to southward IMF B _z growth.

The changes in EEJ are local time dependent (Bhaskar and Vichare, 2013). The factors influencing the variation of EEJ with local time are numerous. For example, the post-noon eastward ΔEEJ may be indirectly influenced by the upward electric field linked to the SAPS electric field within the SAPS channel, while the pre-noon ΔEEJ could be a direct result of the SAPS-associated polarization electric field penetration (Zhang et al., 2022). To further investigate the local time dependences in EEJ response to solar wind dynamic pressure parameters or IMF B _z changes, Figure 7 shows four distinct categories of increases: solar wind (a) velocity, (b) density, (c) dynamic pressure, (d) northward IMF B _z. Our methodology hinges on capturing the maximum change in EEJ within a 30-min window following the event onset, serving as a representation of the EEJ’s response intensity. Each point represents an event, with the coloration indicating the magnitude of the corresponding solar wind parameters. The horizontal axis represents the local time of the event onset, and the vertical axis represents the response intensity of EEJ in the event.

The EEJ responses to solar wind variations are most pronounced at local noon. Bhaskar and Vichare (2013) achieved a similar result before, they eliminated the local time dependence by eliminating the conductivity effect, which indicated the dependence is related to daytime conductivity. When solar radiation is enhanced, the ionization rate in the atmosphere increases, electron density and ionospheric conductivities increase. At noon, the conductivity is maximum and EEJ changes are most pronounced, while near sunrise and sunset, the conductivity is small and EEJ changes caused by the disturbance are small. We have not found any explicit relationship between the intensities of solar wind conditions and EEJ responses in our study.

EEJ undergoes an increase/decrease with a ∼5-min delay as solar wind velocity, density, and dynamic pressure increase/decrease. The impacts of these three parameters on EEJ are consistent. The EEJ responses to stepwise changes in the three parameters V, N, and P _dyn, are always impulsive with delays of ∼5 min, and they return to their original state within ∼20–40 min. To compare the strength of the EEJ responses to different solar wind dynamic pressure parameters, we analyzed the amplitude difference of EEJ responses. Over the span of 21 years of data, the average solar wind velocity measures ∼430 km/s, with an average velocity variation of ∼80 km/s (∼18%) during the selected events. This leads to an average EEJ response of ∼7 nT. The average solar wind density is ∼6 particles/cm³ and the density change during the chosen events amounts to ∼9 particles/cm³ (∼150%), resulting in an average EEJ response of ∼8 nT. The average dynamic pressure is estimated at ∼2 nPa, while the mean change in dynamic pressure during the chosen events is ∼4 nPa (∼200%), resulting in an average EEJ response of ∼10 nT.

The EEJ responds to changes in solar wind dynamic pressure possibly due to magnetopause deformation. Magnetopause is the boundary between solar wind pressure and the geomagnetic field pressure. When the solar wind dynamic pressure changes, the magnetopause contracts or expands accordingly. This process causes plasma flow towards the Earth’s magnetosphere through viscous interaction on the dayside, increasing plasma velocity near the magnetopause and magnetosphere flanks (Axford and Hines, 1961). As the plasma flows toward the interior of the magnetosphere, the velocity decreases, creating a flow shear near the magnetospheric flanks called “Transit Cell Convection” (Fujita et al., 2003). This shear enhances the region 1 field-aligned current (Kubota et al., 2015), leading to an imbalance between the region 1 field-aligned current and the region 2 field-aligned current. This imbalance can induce PPEF and increase EEJ. As the plasma flow gradually adapts to the new state, the response decays over ∼30–40 min (Huang et al., 2008). EEJ responses always have a 5-min delay because the solar wind data used is from the bow shock nose, which takes time to propagate into the magnetosphere and affects EEJ. Changes in conductivity may also affect EEJ responses. Li et al. (2021) observed that TEC fluctuations at low latitudes may be due to the impact of the interplanetary shock on the magnetopause, generating cavity mode oscillation in the cavity between the magnetosphere and plasmasphere, causing plasma oscillations (Wright, 1994). The resultant electron density variation due to the impact of interplanetary shock on the magnetopause can change the conductivity and thus EEJ.

The EEJ is most sensitive to changes in velocity and least sensitive to changes in dynamic pressure. Specifically, the different responses of EEJ to solar wind velocity, density, and dynamic pressure may be due to the individual effects of solar wind velocity and density on magnetospheric compression. Since P dyn = m N V 2 , the dynamic pressure is related to the square of the velocity and linearly related to the density. An increased solar wind electric field would lead to a higher magnetopause reconnection rate and smaller magnetopause standoff distance, due to the higher solar wind velocity (Lee and Lee, 2020). During the compression process, many empirical magnetopause models show that the relationship between solar wind dynamic pressure P _dyn and the magnetopause standoff distance R _sub is R sub ∼ P dyn − 1 / K (Beard, 1960; Shue et al., 1998; Lin et al., 2010; Liu et al., 2015), where K is a constant. However, Samsonov et al. (2020) showed a more accurate expression of R _sub using solar wind velocity and density: R s u b ∼ N − 1 / X V − 2 / Y , where X ≠ Y and both X and Y may depend on the sign of IMF B _z. Samsonov et al. (2020) indicated that the changes in R _sub for a velocity increase were greater than those for a density increase for the same P _dyn, indicating a greater magnetopause deformation. A greater compression of the magnetopause will cause stronger EEJ perturbations compared with the deformation caused by changes in solar wind density.

In addition, solar wind density can also affect EEJ through other mechanisms. Nilam and Tulasi Ram (2022) found that a decrease in solar wind density reduced EEJ, which was related to the location of magnetic reconnection. Magnetic reconnection plays a crucial role in controlling PPEF and magnetosphere-ionosphere coupling. Specifically, the reconnection rate at the magnetopause and the energy transfer to the magnetosphere are determined by the IMF B _z in the magnetosheath region (Kataoka et al., 2005). With a sudden decrease in solar wind density, the magnetopause where the dayside magnetic reconnection occurs is expanded, then the magnetosheath IMF B _z reduces and causes a reduction of the dayside reconnection rate at the magnetopause. Then the magnetic reconnection affects the EEJ by influencing the high-latitude convective electric field.

Our results revealed that EEJ tends to increase/decrease when there is sharp southward/northward enhancement of IMF B _z. Specifically, when there were transient southward increases in IMF B _z with an average amplitude of 23 nT, the EEJ experienced an average increase of 32 nT. Conversely, when there were transient northward increases in IMF B _z with an average amplitude of 23 nT, the EEJ experienced an average decrease of 57 nT. These results are consistent with previous findings that electric field perturbations are related to the north-south turning of IMF B _z (Huang et al., 2007), and show that the northward IMF B _z increases have stronger influences on EEJ than southward IMF B _z increases. When IMF B _z has a southward trend, the geomagnetic field and IMF are in opposite directions, then a magnetic reconnection occurs (Dungey, 1961). The solar wind drives large-scale magnetospheric convection through reconnection. The imbalance between region 1 field-aligned currents generated in the low-latitude boundary layer of the magnetosphere and the region 2 field-aligned currents generated in the inner boundary of the plasma sheet (Iijima and Potemra, 1978) leads to the PPEF from high latitude to low latitudes, which is eastward on the dayside and westward on the nightside (Richmond et al., 2003), leading to the enhancement of dayside EEJ. Therefore, EEJ is enhanced when IMF B _z has a southward turning. Conversely, if IMF B _z becomes more northward, the magnetic reconnection rate decreases, weakening the injection of field-aligned current into the ionosphere and decreasing EEJ. Bhaskar and Vichars (2013) found that the variations of ionospheric signatures were mainly controlled by the magnitude of southward IMF B _z, and the magnitude of northward IMF B _z did not influence the ionospheric characteristics. They also observed the asymmetry of the EEJ response to the southward and northward IMF B _z variation and estimated that for southward IMF B _z turnings, ∆ EEJ = 4.5 × ∆ E swy + 3.19 , for northward turnings, ∆ EEJ = 8.23 × ∆ E swy + 7.35 , where E _swy is a component of solar wind electric field along the y-axis (eastward) in GSM coordinate system. The response intensity of EEJ during IMF B _z northward is about twice as strong as during IMF B _z northward. This is consistent with our results. The asymmetry is mainly related to the stronger magnetospheric convection during the southward IMF B _z.

In addition to the variations in EEJ caused by solar wind changes investigated in this study, changes in the background ionosphere can also impact the low-latitude ionospheric current and electric field. For instance, during a solar eclipse, higher levels of solar obscuration result in lower electron density and stronger polarization electric field, leading to the appearance of gradient drift wave at higher altitudes with larger amplitude (Sekar et al., 2014). The Sun can not only influence the magnetosphere-ionosphere coupling in the form of solar wind but also create changes in the ionosphere through solar radiation. In this study, we primarily focus on the rapid response of EEJ to changes in solar wind conditions, with a time scale much shorter than the variations in the background ionosphere. When processing the data, we used the relative values of the geomagnetic horizontal components H, from which the background ionosphere has been subtracted, to calculate EEJ. This approach eliminated the influence of the background ionosphere.