Solar Wind Dynamic Pressure Pulses Research Articles

An Extreme Auroral Electrojet Spike During 2023 April 24th Storm

AbstractAbrupt variations of auroral electrojets can induce geomagnetically induced currents, and the ability to model and forecast them is a pressing goal of space weather research. We report an auroral electrojet spike event that is extreme in magnitude, explosive in nature, and global in spatial extent that occurred on 24 April 2023. The event serves as a fundamental test of our understanding of the response of the geospace system to solar wind dynamics. Our results illustrate new and important characteristics that are drastically different from existing knowledge. Most important findings include (a) the event was only of ∼5‐min duration and was limited to a narrow (2°–3°) band of diffuse aurora; (b) the longitudinal span covered the entire nightside sector, possibly extending to the dayside; (c) the trigger seems to be a transient solar wind dynamic pressure pulse. In comparison, substorms usually last 1–2 hr and span almost the entire latitudinal width of the auroral oval. Magnetic perturbation events (MPEs) span hundreds km in radius. Both substorms and MPEs are mainly driven by disturbances in the magnetotail. A possible explanation is that the pressure pulse compresses the magnetosphere and enhances diffuse precipitation of electrons and protons from the inner plasma sheet, which elevates the ionospheric conductivity and intensifies the auroral electrojet. Therefore, the event exhibits a potentially new type of geomagnetic disturbance and highlights a solar wind driver that is enormously influential in driving extreme space weather events.

Influence of an Upstream Dynamic Pressure Pulse on Venus–Solar Wind Interaction by Multispecies Magnetohydrodynamic Simulation

As an unmagnetized planet, Venus does not have a global dipole magnetic field like Earth. The solar wind interacts directly with the Venusian ionosphere. In this paper, the interaction between solar wind and the Venusian upper atmosphere is investigated by conducting a three-dimensional multispecies magnetohydrodynamic (MHD) model, in which the Venusian ionosphere is developed self-consistently by considering chemical reactions among four major ion species (H+, O2 +, O+, and CO2 +). Based on this developed MHD model, we mainly study the effects of the impulsive solar wind dynamic pressure on the interaction process. Simulation results reveal that the locations of the bow shock and magnetic pileup boundary show an abrupt compression, then expansion, and finally shrinkage to reach a new steady state with the influence of the impulsive dynamic pressure. The recovery time of the large-scale magnetic field is a few minutes. In addition, the peak escape fluxes are enhanced by about 1 order of magnitude due to the increased solar wind dynamic pressure pulse.

Equatorward Moving Auroral Arcs Associated with Impulse-Excited Field Line Resonance

The theory of equatorward moving east-west elongated auroral arcs associated with field line resonance (FLR) has been proposed for decades. However, confirming this theory requires in-situ observations of FLR within the magnetosphere and simultaneous all-sky imager observations of equatorward moving auroral arcs near satellite footpoints. In this study, we present the first observations of multiple equatorward moving auroral arcs related to impulse-excited FLR, using datasets from the WIND, Geotail satellites, and an all-sky imager at China’s Zhongshan Station (ZHS) in Antarctica. In the presented event, the ultra-low-frequency waves associated with solar wind dynamic pressure pulse was mainly toroidal mode, which is consistent with the theory that the toroidal mode waves usually related with external source. The all-sky imager located in Zhongshan station recorded several equatorward moving auroral arcs, followed by reverse propagating ones. The latitudinal width of the equatorward moving auroral arcs was on the order of 25 km and had an average equatorward propagation of ~0.37 km/s, which is very similar to the value from previous work. To better illustrate the observed evolution of auroral arcs related with the FLRs we proposed a simple model to evaluate the FACs induced by the FLRs in different latitudes. The latitudinal distribution evolution of FACs agrees well with the ground-based optical observations.

Prompt responses of magnetospheric whistler-mode waves to solar wind dynamic pressure pulses

Whistler-mode waves play a critical role in shaping the Earth’s radiation belts, and their spatiotemporal distribution is vital for forecasting and modeling geospace weather. Previous works have extensively investigated the influences of geomagnetic activities, such as storms and substorms, on the modification of whistler-mode waves, but the direct impacts of solar wind disturbances have received relatively less attention. Recently, increasing research has highlighted the prompt impacts of solar wind dynamic pressure pulses on magnetospheric whistler-mode waves. This paper reviews the current progress in this field, specifically the prompt responses of chorus waves and plasmaspheric hiss to the solar wind dynamic pressure pulses. It will summarize the underlying mechanisms and pose some outstanding questions.

PC index as a ground-based indicator of the solar wind energy incoming into the magnetosphere: (1) relation of PC index to the solar wind electric field EKL

The polar cap magnetic activity PC index was approved by IAGA as a proxy for energy that enters into the magnetosphere during solar wind-magnetosphere coupling (IAGA Resolutions, 2013; IAGA Resolution, 2021). The paper summarizes experimental results attesting the validity of this PC index essence. The following issues are examined: the PC index derivation method, making allowance for regular and irregular variations of ionospheric conductivity; relationships between the PC index and solar wind electric field (EKL) and factors controlling response of PC index to the EKL field alterations; relation of PC index to the magnetospheric field-aligned currents (FAC) and to solar wind dynamic pressure pulses (Pdyn). The PC index alterations in course of 23/24 solar activity cycles have been analyzed in relation to various solar wind parameters and the conclusion was made that linear link between the PC index and solar wind electric field EKL remained valid irrespective of solar activity cycle. New ideas concerning (1) the nature of occasional differences between the PC indices in summer and winter polar caps and (2) two simultaneously acting mechanisms of the solar wind influence on the magnetosphere (Dungey and Tverskoy concepts) are discussed, as a result of the PC index application. It is emphasized that the ground-based PC index ensures a permanent on-line information on geoefficiency of the solar wind impact on the magnetosphere and, correspondingly, on the varying geophysical situation.

Immediate Impact of Solar Wind Dynamic Pressure Pulses on Whistler‐Mode Chorus Waves in the Inner Magnetosphere

AbstractSolar wind‐magnetosphere coupling is a key link of the space weather chain. There have been increasing reports of solar wind dynamic pressure pulses influencing the whistler‐mode chorus wave growth in the inner magnetosphere, but the response conditions and mechanisms of chorus growth remain under debate. Here we present a statistical study of the immediate impact of solar wind dynamic pressure pulses on inner magnetospheric chorus waves using Van Allen Probes data from 2012 to 2019. A stronger pulse is found to have a greater likelihood to change the chorus amplitude particularly on the dayside. Positive pulses can significantly enhance chorus amplitudes, while negative ones result in a weakening in chorus amplitudes. As supported by direct observations, these pulses alter the linear growth of waves by modifying energetic electron distributions; in contrast, geomagnetic field modeling indicates no significant changes in the geomagnetic field inhomogeneity controlling the nonlinear growth threshold of waves.

The Relationship Between Solar Wind Dynamic Pressure Pulses and Solar Wind Turbulence

Solar wind dynamic pressure pulses (DPPs) are small-scale plasma structures with abrupt and large-amplitude plasma dynamic pressure changes on timescales of seconds to several minutes. Overwhelming majority of DPP events (around 79.13%) reside in large-scale solar wind transients, i.e., coronal mass ejections, stream interaction regions, and complex ejecta. In this study, the intermittency, which is a typical feature of solar wind turbulence, is determined and compared during the time intervals in the undisturbed solar wind and in large-scale solar wind transients with clustered DPP events, respectively, as well as in the undisturbed solar wind without DPPs. The probability distribution functions (PDFs) of the fluctuations of proton density increments normalized to the standard deviation at different time lags in the three types of distinct regions are calculated. The PDFs in the undisturbed solar wind without DPPs are near-Gaussian distributions. However, the PDFs in the solar wind with clustered DPPs are obviously non-Gaussian distributions, and the intermittency is much stronger in the large-scale solar wind transients than that in the undisturbed solar wind. The major components of the DPPs are tangential discontinuities (TDs) and rotational discontinuities (RDs), which are suggested to be formed by compressive magnetohydrodynamic (MHD) turbulence. There are far more TD-type DPPs than RD-type DPPs both in the undisturbed solar wind and large-scale solar wind transients. The results imply that the formation of solar wind DPPs could be associated with solar wind turbulence, and much stronger intermittency may be responsible for the high occurrence rate of DPPs in the large-scale solar wind transients.

Global Magnetosphere Response to Solar Wind Dynamic Pressure Pulses During Northward IMF Using the Heliophysics System Observatory

AbstractWe analyzed the magnetospheric global response to dynamic pressure pulses (DPPs) using the Heliophysics System Observatory (HSO) and ground magnetometers. During northward Interplanetary Magnetic Field (IMF) Bz conditions, the magnetosphere acts as a closed “cavity” and reacts to solar wind DPPs more simply than during southward IMF. In this study we use solar wind data collected by ACE and WIND together with magnetic field observations of Geotail, Cluster, Time History of Events and Macroscale Interactions during Substorms (THEMIS), Magnetospheric Multiscale Mission (MMS), Van Allen Probes, GOES missions, and ground magnetometer arrays to observe the magnetosphere (dayside, nightside, inner magnetosphere, magnetotail, magnetosheath, etc.) and ionosphere response simultaneously in several local time sectors and regions. A total of 37 events were selected during the period between February 2007 to December 2017. We examine the global response of each event and identify systematic behavior of the magnetosphere due to DPPs' compression, such as MHD wave propagation, sudden impulses, and Ultra Low Frequency waves (ULF) in the Pc5 range. Our results confirm statistical studies with a more limited coverage that have been performed at different sectors and/or regions of the magnetosphere. We present observations of the different signatures generated in different regions that propagate through the magnetosphere. The signature of the tailward traveling DPP is observed to move at the same solar wind speed, and in superposition of other known magnetospheric perturbations. It is observed that the DPP also generates or increases the amplitude of Pc4‐5 waves observed in the inner magnetosphere, while similar waves are observed on the ground.

Occurrence of EMIC Waves in the Magnetosphere According to Their Distance to the Magnetopause

AbstractWave growth of electromagnetic ion cyclotron (EMIC) emissions observed in the outer magnetosphere is mainly controlled by compression events resulting from solar wind dynamic pressure pulses. During such events wave growth is expected to be maximum close to the magnetopause. In previous studies, distribution of EMIC waves was analyzed according to their distance from the Earth, which is inadequate for studying the magnetopause region. We map a data set of EMIC waves observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft according to their distance from case‐by‐case modeled magnetopause. EMIC occurrence rate is found to be maximum within two Earth radii from the magnetopause and then it linearly decreases with an increasing distance, especially close to the local noon. Asymmetries between the morning and evening magnetic sectors are explained by asymmetries in the upstream conditions and by the presence of another EMIC population of a different origin.

On the loss mechanisms of radiation belt electron dropouts during the 12 September 2014 geomagnetic storm

Radiation belt electron dropouts indicate electron flux decay to the background level during geomagnetic storms, which is commonly attributed to the effects of wave-induced pitch angle scattering and magnetopause shadowing. To investigate the loss mechanisms of radiation belt electron dropouts triggered by a solar wind dynamic pressure pulse event on 12 September 2014, we comprehensively analyzed the particle and wave measurements from Van Allen Probes. The dropout event was divided into three periods: before the storm, the initial phase of the storm, and the main phase of the storm. The electron pitch angle distributions (PADs) and electron flux dropouts during the initial and main phases of this storm were investigated, and the evolution of the radial profile of electron phase space density (PSD) and the (<italic>μ</italic>, <italic>K</italic>) dependence of electron PSD dropouts (where <italic>μ</italic>, <italic>K</italic>, and <italic>L</italic>* are the three adiabatic invariants) were analyzed. The energy-independent decay of electrons at <italic>L</italic> &gt; 4.5 was accompanied by butterfly PADs, suggesting that the magnetopause shadowing process may be the major loss mechanism during the initial phase of the storm at <italic>L</italic> &gt; 4.5. The features of electron dropouts and 90°-peaked PADs were observed only for &gt;1 MeV electrons at <italic>L</italic> &lt; 4, indicating that the wave-induced scattering effect may dominate the electron loss processes at the lower <italic>L</italic>-shell during the main phase of the storm. Evaluations of the (<italic>μ</italic>, <italic>K</italic>) dependence of electron PSD drops and calculations of the minimum electron resonant energies of H⁺-band electromagnetic ion cyclotron (EMIC) waves support the scenario that the observed PSD drop peaks around <italic>L</italic>* = 3.9 may be caused mainly by the scattering of EMIC waves, whereas the drop peaks around <italic>L</italic>* = 4.6 may result from a combination of EMIC wave scattering and outward radial diffusion.

Three Different Episodes of Prompt Equatorial Electric Field Perturbations Under Steady Southward IMF Bz During St. Patrick's Day Storm

AbstractThree different episodes of prompt penetration electric field (PPEF) disturbances are observed during the main phase of the St. Patrick's Day storm on 17 March 2015 under steady southward interplanetary magnetic field (IMF) Bz conditions unlike the conventional PPEF associated with southward or northward turnings of IMF Bz. These PPEF events took place during the period when strong disturbance dynamo fields are prevailing in the background. The first event is triggered by a solar wind dynamic pressure pulse that caused a sharp eastward PPEF and strong enhancement of equatorial electrojet current in Brazilian dayside. The second event caused another short but strong westward PPEF on dayside due to the reversal of IMF By from duskward to dawnward under steady IMF Bz. The third event caused a longer eastward PPEF in association with a solar wind dynamic pressure pulse followed by the onset of a substorm, which has led to strong enhancement of equatorial electrojet, quick rejuvenation and symmetric redistribution of equatorial ionization anomaly in the Brazilian sector. The signatures of the PPEF with opposite polarity and smaller magnitudes are also observed in the Asian sector on the nightside. The possible mechanisms for the observed PPEF events under steady IMF Bz are discussed in terms of changes in the high‐latitude field‐aligned currents and reconfiguration of high‐latitude convection fields using Active Magnetosphere and Planetary Electrodynamics Response Experiment and Super Dual Auroral Radar Network high‐frequency radar observations.

Unusual geomagnetic response to the solar wind dynamic pressure pulse during storm-time

Unusual geomagnetic response to the solar wind dynamic pressure pulse during storm-time

Dynamic responses of the Earth's radiation belts during periods of solar wind dynamic pressure pulse based on normalized superposed epoch analysis

AbstractUsing the electron flux measurements obtained from five satellites (GOES 15 and POES 15, 16, 18, and 19), we investigate the flux variations of radiation belt electrons during forty solar wind dynamic pressure pulses identified between September 2012 and December 2014. By utilizing the mean duration of the pressure pulses as the epoch timeline and stretching or compressing the time phases of individual events to normalize the duration by means of linear interpolation, we have performed normalized superposed epoch analysis to evaluate the dynamic responses of radiation belt energetic electrons corresponding to various groups of solar wind and magnetospheric conditions in association with solar wind dynamic pressure pulses. Our results indicate that by adopting the timeline normalization we can reproduce the typical response of the electron radiation belts to pressure pulses. Radiation belt electron fluxes exhibit large depletions right after the Pdyn peak during the periods of northward interplanetary magnetic field (IMF) Bz and are more likely to occur during the Pdyn pulse under southward IMF Bz conditions. For the pulse events with large negative values of (Dst)min, radiation belt electrons respond in a manner similar to those with southward IMF Bz, and the corresponding postpulse recovery can extend to L ~ 3 and exceed the prepulse flux levels. Triggered by the solar wind pressure enhancements, deeper earthward magnetopause erosion provides favorable conditions for the prompt electron flux dropouts that extend down to L ~ 5, and the pressure pulses with longer duration tend to produce quicker and stronger electron flux decay. In addition, the events with high electron fluxes before the Pdyn pulse tend to experience more severe electron flux dropouts during the course of the pulse, while the largest rate of electron flux increase before and after the pulse occurs under the preconditioned low electron fluxes. These new results help us understand how electron fluxes respond to solar wind dynamic pressure pulses and how these responses depend on the solar wind and geomagnetic conditions and on the preconditions in the electron radiation belts.

Multi-satellite simultaneous observations of magnetopause and atmospheric losses of radiation belt electrons during an intense solar wind dynamic pressure pulse

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; Radiation belt electron flux dropouts are a kind of drastic variation in the Earth's magnetosphere, understanding of which is of both scientific and societal importance. Using electron flux data from a group of 14 satellites, we report multi-satellite simultaneous observations of magnetopause and atmospheric losses of radiation belt electrons during an event of intense solar wind dynamic pressure pulse. When the pulse occurred, magnetopause and atmospheric loss could take effect concurrently contributing to the electron flux dropout. Losses through the magnetopause were observed to be efficient and significant at &lt;i&gt;L&lt;/i&gt; ≳ 5, owing to the magnetopause intrusion into &lt;i&gt;L&lt;/i&gt; ∼ 6 and outward radial diffusion associated with sharp negative gradient in electron phase space density. Losses to the atmosphere were directly identified from the precipitating electron flux observations, for which pitch angle scattering by plasma waves could be mainly responsible. While the convection and substorm injections strongly enhanced the energetic electron fluxes up to hundreds of keV, they could delay other than avoid the occurrence of electron flux dropout at these energies. It is demonstrated that the pulse-time radiation belt electron flux dropout depends strongly on the specific interplanetary and magnetospheric conditions and that losses through the magnetopause and to the atmosphere and enhancements of substorm injection play an essential role in combination, which should be incorporated as a whole into future simulations for comprehending the nature of radiation belt electron flux dropouts.&lt;/p&gt;

Magnetospheric vortices and their global effect after a solar wind dynamic pressure decrease

AbstractUsing multipoint data from three Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites, we report a magnetospheric flow vortex driven by a negative solar wind dynamic pressure pulse. The observed vortex rotated in a direction opposite to that associated with positive solar wind dynamic pressure pulses. The vortex was moving tailward, as confirmed by a global magnetohydrodynamics (MHD) simulation. In addition, the equivalent ionospheric currents (EICs) deduced from ground magnetometer station data reveal that a current vortex in the ionosphere near the foot point of the satellites has a rotation sense consistent with that observed in the magnetosphere. The field‐aligned current (FAC) density estimated from three THEMIS satellites is about 0.15 nA/m2, and the total FAC of the vortex is about 1.5–3 × 105 A, on the order of the total FAC in a pseudobreakup, but less than the total FAC in most moderate substorms, 106 A.

STRONG SOLAR WIND DYNAMIC PRESSURE PULSES: INTERPLANETARY SOURCES AND THEIR IMPACTS ON GEOSYNCHRONOUS MAGNETIC FIELDS

In this investigation, we first present a statistical result of the interplanetary sources of very strong solar wind dynamic pressure pulses (DPPs) detected by WIND during solar cycle 23. It is found that the vast majority of strong DPPs reside within solar wind disturbances. Although the variabilities of geosynchronous magnetic fields (GMFs) due to the impact of positive DPPs have been well established, there appears to be no systematic investigations on the response of GMFs to negative DPPs. Here, we study both the decompression effects of very strong negative DPPs and the compression from strong positive DPPs on GMFs at different magnetic local time sectors. In response to the decompression of strong negative DPPs, GMFs on the dayside near dawn and near dusk on the nightside, are generally depressed. But near the midnight region, the responses of GMF are very diverse, being either positive or negative. For part of the events when GOES is located at the midnight sector, the GMF is found to abnormally increase as the result of magnetospheric decompression caused by negative DPPs. It is known that under certain conditions magnetic depression of nightside GMFs can be caused by the impact of positive DPPs. Here, we find that a stronger pressure enhancement may have a higher probability of producing the exceptional depression of GMF at the midnight region. Statistically, both the decompression effect of strong negative DPPs and the compression effect of strong positive DPPs depend on the magnetic local time, which are stronger at the noon sector.

A STATISTICAL SURVEY OF DYNAMIC PRESSURE PULSES IN THE SOLAR WIND BASED ONWINDOBSERVATIONS

Solar wind dynamic pressure pulse (DPP) structures, across which the dynamic pressure changes abruptly over timescales from a few seconds to several minutes, are often observed in the near-Earth space environment. The space weather effects of DPPs on the magnetosphere-ionosphere coupling system have been widely investigated in the last two decades. In this study, we perform a statistical survey on the properties of DPPs near 1 AU based on nearly 20 years of observations from the WIND spacecraft. It is found that only a tiny fraction of DPPs (around 4.2%) can be regarded as interplanetary shocks. For most DPPs, the total pressure (the sum of the thermal pressure and magnetic pressure) remains in equilibrium, but there also exists a small fraction of DPPs that are not pressure-balanced. The overwhelming majority of DPPs are associated with solar wind disturbances, including coronal mass ejection-related flows, corotating interaction regions, as well as complex ejecta. The annual variations of the averaged occurrence rate of DPPs are roughly in phase with the solar activity during solar cycle 23, and during the rising phase of solar cycle 24.

GPS phase scintillation at high latitudes during geomagnetic storms of 7–17 March 2012 – Part 1: The North American sector

Abstract. The interval of geomagnetic storms of 7–17 March 2012 was selected at the Climate and Weather of the Sun-Earth System (CAWSES) II Workshop for group study of space weather effects during the ascending phase of solar cycle 24 (Tsurutani et al., 2014). The high-latitude ionospheric response to a series of storms is studied using arrays of GPS receivers, HF radars, ionosondes, riometers, magnetometers, and auroral imagers focusing on GPS phase scintillation. Four geomagnetic storms showed varied responses to solar wind conditions characterized by the interplanetary magnetic field (IMF) and solar wind dynamic pressure. As a function of magnetic latitude and magnetic local time, regions of enhanced scintillation are identified in the context of coupling processes between the solar wind and the magnetosphere–ionosphere system. Large southward IMF and high solar wind dynamic pressure resulted in the strongest scintillation in the nightside auroral oval. Scintillation occurrence was correlated with ground magnetic field perturbations and riometer absorption enhancements, and collocated with mapped auroral emission. During periods of southward IMF, scintillation was also collocated with ionospheric convection in the expanded dawn and dusk cells, with the antisunward convection in the polar cap and with a tongue of ionization fractured into patches. In contrast, large northward IMF combined with a strong solar wind dynamic pressure pulse was followed by scintillation caused by transpolar arcs in the polar cap.

Properties of Solar Wind Dynamic Pressure Pulses at 1 AU during the Deep Minimum between Solar Cycles 23 and 24

Observations during the deep solar minimum between Solar Cycles 23 and 24 offer an opportunity for characterizing the nature of solar wind dynamic pressure pulses (DPPs) under extreme solar activity. In this study, we identify 226 DPPs from July 2008 to June 2009 using an automatic detection algorithm based on high-resolution plasma data from the Wind spacecraft to investigate the features of DPPs during the deep solar minimum. For comparison, the similarities and differences of the statistical characteristics of the DPPs during the deep solar minimum and during the previous solar minimum are also examined. It is found that the number and the occurrence rate of DPPs during the deep solar minimum are only about one-third of those during the previous minimum, which may be attributed to lower solar wind dynamic pressure and weaker dynamic pressure fluctuations. From a statistical perspective, however, no obvious difference is apparent between the other basic DPP properties in the two solar minima, such as the absolute and relative amplitude of the dynamic pressure changes and the durations of the transition regions of DPPs. Other basic properties of the DPPs during the deep solar minimum are as follows: 1) the distribution of the absolute value of the dynamic pressure amplitude change peaks at 1.0 – 1.5 nPa, 2) the most probable relative pressure changes are 0.2 – 0.8, 3) DPP durations are broad-peaked between 150 s and 210 s with a mean of about 171 s, 4) 76.7 % of the DPPs can be considered as pressure balance structures, 5) dynamic pressure changes across DPPs are dominated by density changes, 6) specially, during the deep solar minimum, a considerable portion of DPPs, 86.7 %, are associated with large-scale solar wind transients such as interplanetary coronal mass ejections (ICMEs) and stream interaction regions (SIRs).

Polar cap response to the solar wind density jump under constant southward IMF

Sharp changes of the solar wind parameters determining the dynamic pressure jump lead to strong magnetosphere-ionosphere disturbances. Here the effect on the Earth’s ionospheric high latitudes of the solar wind dynamic pressure pulse caused only by the increase of the interplanetary plasma density under southward constant IMF is considered. We investigate reaction of the cross-polar cap potential on the increase of AL index and/or jump of the solar wind density. It is found that for the case of 10 January 1997 the main contribution to the polar cap potential drop increase gave the growth of AL index relative to the input of the solar wind density jump. We also study the influence of the solar wind density increase on the crosspolar cap potential for the quiet magnetospheric conditions. It occurred that the polar cap potential difference decreases with the great increase of the interplanetary plasma density. For the disturbed magnetosphere the main role in the polar cap potential drop increase plays increase of AL. Thus, we found the change of the cross-polar cap potential due to the AL index variations and/or the solar wind density drop even in a case when the interplanetary electric field is constant.