Geocentric Solar Magnetospheric Research Articles

IMF Bx, By, and Bz dependence of energetic particle precipitation in the magnetosphere

The orientation of Interplanetary Magnetic Field (IMF B) components plays a major role in magnetospheric Energetic Particles Precipitation (EPP). This is because the IMF B components in the Geocentric Solar Magnetospheric (GSM) coordinate system control magnetic reconnection at the subsolar magnetopause. This study intends to unravel the dependence of the preceding conditions of the orientations of IMF B components that drove geomagnetic storms on the EPPs produced in the Van Allen radiation belt. We classified the geomagnetic storms based on their drivers and IMF Bz orientations prior to the storms’ onset. Energetic particle fluxes, IMF B, and solar wind data were used. The study showed that electrons are more reactive to solar wind speed than protons. Additionally, storms with southward IMF Bz orientation caused more EPP enhancement than with northward orientation. Our results also showed that protons are more influenced by magnetospheric compression than electrons at the northward orientation of IMF Bz. In fact, at this point, the proton’s motion violates adiabatic invariants. As particles interact with electric fields, a significant population of EPPs is produced by pitch angle diffusion. Generally, IMF Bx and IMF By are often anticorrelated. The preceding conditions of orientations of the IMF Bx indicated weaker dependence on auroral indices, and invariably on energetic particles precipitation than the IMF By.

Investigation of the relationship between geomagnetic activity and solar wind parameters based on a novel neural network (potential learning)

Predicting geomagnetic conditions based on in-situ solar wind observations allows us to evade disasters caused by large electromagnetic disturbances originating from the Sun to save lives and protect economic activity. In this study, we aimed to examine the relationship between the Kp index, representing global magnetospheric activity level, and solar wind conditions using an interpretable neural network known as potential learning (PL). Data analyses based on neural networks are often difficult to interpret; however, PL learns by focusing on the “potentiality of input neurons” and can identify which inputs are significantly utilized by the network. Using the full advantage of PL, we extracted the influential solar wind parameters that disturb the magnetosphere under southward Interplanetary magnetic field (IMF) conditions. The input parameters of PL were the three components of the IMF (Bx, By, Bz), solar wind flow speed (Vx), and proton number density (Np) in geocentric solar magnetospheric (GSM) coordinates obtained from the OMNI solar wind database between 1998 and 2019. Furthermore, we classified these input parameters into two groups (targets), depending on the Kp level: Kp = 6–9 (positive target) and Kp = 0 to 1 + (negative target). Negative target samples were randomly selected to ensure that numbers of positive and negative targets were equal. The PL results revealed that solar wind flow speed is an influential parameter for increasing Kp under southward IMF conditions, which was in good agreement with previous reports on the statistical relationship between the Kp index and solar wind velocity, and the Kp formulation based on the IMF and solar wind plasma parameters. Based on this new neural network, we aim to construct a more correct and parameter-dependent space weather forecasting model.Graphical

Invalid Polar Cap Indices: Erroneous Scaling Parameters

AbstractIn the publication Troshichev et al. (2006) (https://doi.org/10.1029/2005JA011402) on the polar cap (PC) indices, PCN and PCS, an error was made by using components of the interplanetary magnetic field (IMF) in their geocentric solar ecliptic (GSE) representation instead of the prescribed geocentric solar magnetospheric (GSM) representation for calculations of index scaling parameters. The mistake has caused a trail of incorrect relations and wrong conclusions extending since 2006 up to now (2020) which should be discontinued, for instance, by issuing a corrigendum note from the authors. The present contribution explains the error and discusses in an extended example its consequences for one of the publications that have referred to the invalid scaling parameter set. Further investigations reported here of the PC index versions recommended by the International Association for Geomagnetism and Aeronomy (IAGA) indicate occurrences of similar problems in the present derivation of index scaling parameters.

A Bayesian Inference‐Based Empirical Model for Scintillation Indices for High‐Latitude

AbstractSolar wind parameters, the solar radio flux index (F10.7), the Sun's declination and the SuperMAG Electrojet index are used to construct a Bayesian inference‐based empirical model for scintillation indices (S4 and σΦ) at high latitudes. For the present study, measurements from three Global Positioning System (GPS) L1 receivers located in the auroral zone, the cusp and in the polar cap are selected, respectively. The solar wind characteristics include the solar wind speed (VSW) and ram pressure (ρSW) as well as the Geocentric Solar Magnetospheric (GSM) By and the Bz components of the interplanetary magnetic field (IMF). Following a brief assessment on the independence of the variables (predictors), prior probabilities of occurrence in the case of a multinomial classification are constructed. Posterior‐probabilities are then deduced for any arbitrary set of predictors. We show that the model captures most variations seen in the measured indices whether they are associated or not with transient interplanetary events. Although the model tends to underestimate the actual phase index measurements, 95% of the validated events are predicted with an error less than 0.034 rad in σΦ. For the amplitude scintillation index, 5% of validated events have an error larger than 0.019.

Influence of MHD Turbulence on Ion Kappa Distributions in the Earth's Plasma Sheet as a Function of Plasma β Parameter

The possible influence of MHD turbulence on the energy distributions of ions in the Earth's plasma sheet was studied using data taken by the THEMIS satellites. Turbulence levels were traced using eddy diffusion coefficients (D), of which we measured one for each Geocentric Solar Magnetospheric (GSM) coordinates every 12 min. Ion fluxes between 1.75 and 210.5 keV during the same time windows that correspond to mainly suprathermal populations were fitted to Kappa distribution functions, which approximate a Maxwellian distribution when theκ-index (κ) is large. We found that the distribution of the eddy diffusion coefficients is bimodal, independently of both the eddy diffusion component and the plasma beta (β) parameter, which is defined as the ratio between plasma and magnetic pressures. The main peak corresponds to turbulent plasma flows withD> 103km2s−1. In such cases, the impact of turbulence on theκindex depends on the value ofβand also on the direction of the turbulent transport. For eddy diffusion perpendicular to the neutral sheet, the values ofκdecrease asDzzincreases forβ< 2; while for higher values ofβ,κincreases withDzz. For the other two directions, the values ofκdecrease asDincreases. This last tendency is stronger forβ~ 1 but almost null forβ~ 10. The secondary peak in the distribution ofDvalues might represent quasi-laminar flows forming part of very large vortices, correct detection and description of which is beyond the scope of this study.

Simulation of the Interplanetary Bz Using a Data-driven Heliospheric Solar Wind Model

Abstract Aimed to be ready for the transition from research to operation, we have developed a solar wind model by coupling a data-driven empirical coronal model with a magnetohydrodynamics heliospheric model. We performed a data-driven simulation of the solar wind for a two-year period during the declining and minimum phases of solar cycle 23. Comparisons with OMNI and Ulysses spacecraft data show that the model can reproduce the large-scale variations of the solar wind plasma parameters. The evolution of geocentric solar magnetospheric (GSM) B x and B z components are also reasonably duplicated by the model in terms of polarity and strength. Apparent signatures of the Russell–McPherron (R-M) effect are found from both observed data and simulated results, indicating that during the investigated interval the R-M effect is the dominant mechanism that controls the large-scale evolution of the north–south component of the interplanetary magnetic field in the GSM frame. The results demonstrate that the established model can provide valuable space weather information about the solar wind.

Measurements of the Vorticity in the Bursty Bulk Flows

AbstractCurlometer analysis from 4‐point Magnetospheric Multiscale spacecraft measurements is applied to obtain the vorticity (ω) in the bursty bulk flow (BBF) from tail reconnection. Increases of the vorticity is associated with enhancements of the high‐energy ion flux (above 10 keV) and the flow speed during the BBF. Finger‐like structure, consisting of the lower‐energy ions below 10 keV, occurs mainly during intervals of small vorticity. The vorticity during BBF consistently shows a dominant −Z Geocentric solar magnetospheric (GSM) component, indicating clockwise vortexes in the flows. Combined with bipolar signals of Vy‐geocentric solar magnetic, the Magnetospheric Multiscale observations provide evidence for flow eddies in the BBF. Power spectral analysis suggests existence of kinetic Alfvénic wave emissions, in addition to the flow eddy, in the high‐frequency regime in the BBF turbulence.

Ion Behaviors in the Reconnection Diffusion Region of a Corrugated Magnetotail Current Sheet

AbstractThe current sheet structure and ion behaviors in a magnetotail reconnection diffusion region are investigated. The multispacecraft analysis suggests a corrugated current sheet structure, interpreted as due to a flapping motion that propagates along geocentric solar magnetospheric along the +y direction in the Geocentric Solar Magnetospheric (GSM) coordinate. The electric field (E) and ion distributions have similarities with those in a planar current sheet. Energetic ions move along the current direction, suggesting the acceleration by the observed reconnection E during the meandering motion. Counterstreaming ions along the current sheet normal suggest the acceleration by the Hall E that is observed to be the dominant component. However, at certain locations, E and counterstreaming ions significantly deviate from the local normal direction, and more than one pair of counterstreaming populations exist, possibly because the corrugated current sheet enables ions entering the current sheet at different locations with different velocities to mix together.

Origin of Solar Rotational Periodicity and Harmonics Identified in the Interplanetary Magnetic Field B_{z} Component Near the Earth During Solar Cycles 23 and 24

Periodic variations in solar-wind and geomagnetic parameters have long been recognized. In this work, we examine the periodic properties in the interplanetary magnetic field (IMF) near the Earth using satellite observations from 1996 to 2017. We pay particular attention to short-term periodicities (solar rotational period and its harmonics) of IMF B_{z} by distinguishing between the geocentric solar ecliptic (GSE) and the geocentric solar magnetospheric (GSM) coordinates. We find that, for nearly all of the years in Solar Cycles 23 and 24, IMF B_{z} exhibits periodic changes with a period of the solar rotation or its harmonics or both. We emphasize that these changes are far more pronounced in the GSM coordinates than in the GSE coordinates and during the northern hemisphere spring and fall seasons than the other two seasons. We attribute this result to an exquisite harmony between the Russell–McPherron effect and a well-defined IMF polarity that shows either a two- or four-sector structure for a long period of time.

A Daily Determination of BZ Using the Russell‐McPherron Effect to Forecast Geomagnetic Activity

AbstractSince the middle of the last decade, UCSD has incorporated magnetic field data in its Institute for Space‐Earth Environmental Research interplanetary scintillation tomographic analysis. These data are extrapolated upward from the solar surface using the Current Sheet Source Surface model (Zhao & Hoeksema, 1995, https://doi.org/10.1029/94JA02266) to provide predictions of the interplanetary field in RTN coordinates. Over the years this technique has become ever more sophisticated, and allows different types of magnetogram data (SOLIS, Global Oscillation Network Group, etc.,) to be incorporated in the field extrapolations. At Earth, these fields can be displayed in a variety of ways, including Geocentric Solar Magnetospheric (GSM) Bx, By, and Bz coordinates. Displayed daily, the Current Sheet Source Surface model‐derived GSM Bz shows a significant positive correlation with the low‐resolution (few day variation) in situ measurements of the Bz field. The nano‐Tesla variations of Bz maximize in spring and fall as Russell and McPherron (1973, https://doi.org/10.1029/JA078i001p00092) have shown. More significantly, we find that the daily variations are correlated with geomagnetic Kp and Dst index variations, and that a decrease from positive to negative Bz has a high correlation with minor‐to‐moderate geomagnetic storm activity, as defined by NOAA Space Weather Prediction Center planetary Kp values. Here we provide an 11‐year study of the predicted Bz field, from the extrapolation of the Global Oscillation Network Group‐magnetograms. We provide a skill‐score analysis of the technique's geomagnetic storm prediction capability, which allows forecasts of moderate enhanced geomagnetic storm activity. UCSD and the Korean Space Weather Center currently operate a website that predicts this low‐resolution GSM Bz field component variation several days in advance.

Interplanetary Magnetic Field Bx Component Influence on Horizontal and Field‐Aligned Currents in the Ionosphere

AbstractStatistical analyses have shown that the sunward component of the interplanetary magnetic field, Bx (Geocentric Solar Magnetospheric), moderately but significantly affects the auroral intensity. These observations have been interpreted as signatures of a similar interplanetary magnetic field Bx control on Birkeland currents yet to be observed directly. Such a control, attributed to differences in magnetic tension on newly opened magnetic field lines, would lead to stronger region 1 (R1) Birkeland currents for Bx negative (positive) conditions in the Northern (Southern) Hemispheres than when Bx is positive (negative). In this paper we perform a detailed investigation of three different sets of magnetic field measurements, from the Challenging Minisatellite Payload and Swarm low Earth orbit satellites, from the Active Magnetosphere and Planetary Electrodynamics Response Experiment products derived from the Iridium satellite constellation, and from the SuperMAG ground magnetometer network, each analyzed using different techniques, to test these predictions. The results show that a change in sign of Bx changes the Birkeland currents by no more than ≈10%. The current patterns show little support for an interhemispheric asymmetry of the kind proposed to explain auroral observations. Instead, we propose an alternative interpretation, which is consistent with most of the auroral observations and with the current observations in the present paper, except for those based on Active Magnetosphere and Planetary Electrodynamics Response Experiment: The solar wind‐magnetosphere coupling is more efficient when the dipole tilt angle and Bx have the same sign than when they are different. We suggest that the higher coupling is because the dayside reconnection region is closer to the subsolar point when the dipole tilt angle and Bx have the same sign.

Cross-correlation and cross-wavelet analyses of the solar wind IMF Bz and auroral electrojet index AE coupling during HILDCAAs

Abstract. Solar-wind–geomagnetic activity coupling during high-intensity long-duration continuous AE (auroral electrojet) activities (HILDCAAs) is investigated in this work. The 1 min AE index and the interplanetary magnetic field (IMF) Bz component in the geocentric solar magnetospheric (GSM) coordinate system were used in this study. We have considered HILDCAA events occurring between 1995 and 2011. Cross-wavelet and cross-correlation analyses results show that the coupling between the solar wind and the magnetosphere during HILDCAAs occurs mainly in the period ≤ 8 h. These periods are similar to the periods observed in the interplanetary Alfvén waves embedded in the high-speed solar wind streams (HSSs). This result is consistent with the fact that most of the HILDCAA events under present study are related to HSSs. Furthermore, the classical correlation analysis indicates that the correlation between IMF Bz and AE may be classified as moderate (0.4–0.7) and that more than 80 % of the HILDCAAs exhibit a lag of 20–30 min between IMF Bz and AE. This result corroborates with Tsurutani et al. (1990) where the lag was found to be close to 20–25 min. These results enable us to conclude that the main mechanism for solar-wind–magnetosphere coupling during HILDCAAs is the magnetic reconnection between the fluctuating, negative component of IMF Bz and Earth's magnetopause fields at periods lower than 8 h and with a lag of about 20–30 min. Keywords. Magnetospheric physics (solar-wind–magnetosphere interactions)

Electron acceleration behind a wavy dipolarization front

In this paper, with the in-situ observations from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes we report a wavy dipolarization front (DF) event, where the DF has different magnetic structures and electron distributions at different $y$ positions in the Geocentric Solar Magnetospheric (GSM) coordinates. At $y \sim2.1R_{E}$ ( $R_{E}$ is the radius of Earth), the DF has a relatively simple structure, which is similar to that of a conventional DF. At $y \sim3.0R_{E}$ , the DF is revealed to have a multiple DF structure, where the plasma exhibits a vortex flow. Such a wavy DF could be the results of the interchange instability. The different structure of such a wavy DF at different sites has a great effect on electron acceleration. Fermi acceleration can occur at the site of the DF with a simple or multiple DF structure, while betatron acceleration as a local process has the contribution to energetic electrons only at the site of the DF with a simple structure.

Geomagnetic Effects of Corotating Interaction Regions

We present an analysis of the geoeffectiveness of corotating interaction regions (CIRs), employing the data recorded from 25 January to 5 May 2005 and throughout 2008. These two intervals in the declining phase of Solar Cycle 23 are characterised by a particularly low number of interplanetary coronal mass ejections (ICMEs). We study in detail how four geomagnetic-activity parameters (the Dst, Ap, and AE indices, as well as the Dst time derivative, $\mathrm{dDst}/\mathrm{d}t$ ) are related to three CIR-related solar wind parameters (flow speed, $V$ , magnetic field, $B$ , and the convective electric field based on the southward Geocentric solar magnetospheric (GSM) magnetic field component, $\mathit{VB}_{s}$ ) on a three-hour time resolution. In addition, we quantify statistical relationships between the mentioned geomagnetic indices. It is found that Dst is correlated best to $V$ , with a correlation coefficient of $\mathrm{cc}\approx0.6$ , whereas there is no correlation between $\mathrm{dDst}/\mathrm{d}t$ and $V$ . The Ap and AE indices attain peaks about half a day before the maximum of $V$ , with correlation coefficients ranging from $\mathrm{cc}\approx0.6$ to $\mathrm{cc}\approx0.7$ , depending on the sample used. The best correlations of Ap and AE are found with $\mathit{VB}_{s}$ with a delay of 3 h, being characterised by $\mathrm{cc}\gtrsim 0.6$ . The Dst derivative $\mathrm{dDst}/\mathrm{d}t$ is also correlated with $\mathit{VB}_{s}$ , but the correlation is significantly weaker $\mathrm{cc}\approx 0.4$ – 0.5, with a delay of 0 – 3 h, depending on the employed sample. Such low values of correlation coefficients indicate that there are other significant effects that influence the relationship between the considered parameters. The correlation of all studied geomagnetic parameters with $B$ are characterised by considerably lower correlation coefficients, ranging from $\mathrm{cc}=0.3$ in the case of $\mathrm{dDst}/\mathrm{d}t$ up to $\mathrm{cc}=0.56$ in the case of Ap. It is also shown that peak values of geomagnetic indices depend on the duration of the CIR-related structures. The Dst is closely correlated with Ap and AE ( $\mathrm{cc}=0.7$ ), Dst being delayed for about 3 h. On the other hand, $\mathrm{dDst}/\mathrm{d}t$ peaks simultaneously with Ap and AE, with correlation coefficients of 0.48 and 0.56, respectively. The highest correlation ( $\mathrm{cc}=0.81$ ) is found for the relationship between Ap and AE.

On the origins and timescales of geoeffective IMF

AbstractSouthward interplanetary magnetic field (IMF) in the geocentric solar magnetospheric (GSM) reference frame is the key element that controls the level of space weather disturbance in Earth's magnetosphere, ionosphere, and thermosphere. We discuss the relation of this geoeffective IMF component to the IMF in the geocentric solar ecliptic (GSE) frame, and using the almost continuous interplanetary data for 1996–2015 (inclusive), we show that large geomagnetic storms are always associated with strong southward, out‐of‐ecliptic field in the GSE frame: Dipole tilt effects, which cause the difference between the southward field in the GSM and GSE frames, generally make only a minor contribution to these strongest storms. The time‐of‐day/time‐of‐year response patterns of geomagnetic indices and the optimum solar wind coupling function are both influenced by the timescale of the index response. We also study the occurrence spectrum of large out‐of‐ecliptic field and show that for 1 h averages it is, surprisingly, almost identical in ICMEs (interplanetary coronal mass ejections), around CIRs/SIRs (corotating and stream interaction regions) and in the “quiet” solar wind (which is shown to be consistent with the effect of weak SIRs). However, differences emerge when the timescale over which the field remains southward is considered: for longer averaging timescales the spectrum is broader inside ICMEs, showing that these events generate longer intervals of strongly southward average IMF and consequently stronger geomagnetic storms. The behavior of out‐of‐ecliptic field with timescale is shown to be very similar to that of deviations from the predicted Parker spiral orientation, suggesting the two share common origins.

A statistical study on the shape and position of the magnetotail neutral sheet

Abstract. We study the average shape and position of the magnetotail neutral sheet based on magnetic field data obtained by Cluster, Geotail, TC-1, and THEMIS from the years 1995 to 2013. All data in the aberrated GSM (geocentric solar magnetospheric) coordinate system are normalized to the same solar wind pressure 2 nPa and downtail distance X ∼ −20RE. Our results show characteristics of the neutral sheet, as follows. (1) The neutral sheet assumes a greater degree of curve in the YZ cross section when the dipole tilt increases, the Earth dipole tilt angle affects the neutral sheet configuration not only in the YZ cross section but also in the XY cross section, and the neutral sheet assumes a more significant degree of tilt in the XY cross section when the dipole tilt increases. (2) Counterclockwise twisting of the neutral sheet with 3.10° is observed, looking along the downtail direction, for the positive interplanetary magnetic field (IMF) BY with a value of 3 to 8 nT, and clockwise twisting of the neutral sheet with 3.37° for the negative IMF BY with a value of −8 to −3 nT, and a northward IMF can result in a greater twisting of the near-tail neutral sheet than southward. The above results can be a reference to the neutral sheet model. Our large database also shows that the displaced ellipse model is effective to study the average shape of the neutral sheet with proper parameters when the dipole tilt angle is larger (less) than 10° (−10° ).

Draping of strongly flow-aligned interplanetary magnetic field about the magnetopause

Many dynamic processes of the magnetosphere are directly driven by the solar wind and the occurrence of magnetic merging at the magnetopause. The location of magnetopause magnetic merging, or reconnection, is now fairly well understood when the interplanetary magnetic field (IMF) contains large By and Bz components in relation to the Bx component (in Geocentric Solar Magnetospheric (GSM) coordinates). However, when the IMF contains a large X-component (i.e., is closely flow-aligned), it is not yet well understood how the shocked IMF drapes about the magnetopause, and how this affects the occurrence and location of magnetic merging. In this initial study, we examine from observations how a nearly flow-aligned IMF drapes about the magnetopause. The results of this study are expected to be useful for comparisons with both analytic and global numerical models of the magnetosheath magnetic field.

Database of ion temperature maps during geomagnetic storms.

Ion temperatures as a function of the x and y axes in the geocentric solar magnetospheric (GSM) coordinate system and time are available for 76 geomagnetic storms that occurred during the period July 2008 to December 2013 on CDAWeb. The method for mapping energetic neutral atom data from the Two Wide‐angle Imaging Spectrometers (TWINS) mission to the GSM equatorial plane and subsequent ion temperature calculation are described here. The ion temperatures are a measure of the average thermal energy of the bulk ion population in the 1–40 keV energy range. These temperatures are useful for studies of ion dynamics, for placing in situ measurements in a global context, and for establishing boundary conditions for models of the inner magnetosphere and the plasma sheet.

External field characterization using CHAMP satellite data for induction studies

Knowledge of external inducing source field morphology is essential for precise estimation of electromagnetic (EM) induction response. A better characterization of the external source field of magnetospheric origin can be achieved by decomposing it into outer and inner magnetospheric contributions, which are best represented in Geocentric Solar Magnetospheric (GSM) and Solar Magnetic (SM) reference frames, respectively. Thus we propose a spherical harmonic (SH) model to estimate the outer magnetospheric contribution, following the iterative reweighted least squares approach, using the vector magnetic data of the CHAMP satellite. The data covers almost a complete solar cycle from July 2001 to September 2010, spanning 54,474 orbits. The SH model, developed using orbit-averaged vector magnetic data, reveals the existence of a stable outer magnetospheric contribution of about 7.39 nT. This stable field was removed from the CHAMP data after transforming to SM frame. The residual field in the SM frame acts as a primary source for induction in the Earth. The analysis of this time-series using wavelet transformation showed a dominant 27-day periodicity of the geomagnetic field. Therefore, we calculated the inductive EM C-response function in a least squares sense considering the 27-day period variation as the inducing signal. From the estimated C-response, we have determined that the global depth to the perfect substitute conductor is about 1132 km and its conductivity is around 1.05 S/m.

The effects of seasonal and diurnal variations in the Earth's magnetic dipole orientation on solar wind–magnetosphere‐ionosphere coupling

The angle μ between the geomagnetic dipole axis and the geocentric solar magnetospheric (GSM) zaxis, sometimes called the “dipole tilt,” varies as a function of UT and season. Observations have shown that the cross‐polar cap potential tends to maximize near the equinoxes, when on averageμ= 0, with smaller values observed near the solstices. This is similar to the well‐known semiannual variation in geomagnetic activity. We use numerical model simulations to investigate the role of two possible mechanisms that may be responsible for the influence ofμon the magnetosphere‐ionosphere system: variations in the coupling efficiency between the solar wind and the magnetosphere and variations in the ionospheric conductance over the polar caps. Under southward interplanetary magnetic field (IMF) conditions, variations in ionospheric conductance at high magnetic latitudes are responsible for 10–30% of the variations in the cross‐polar cap potential associated withμ, but variations in solar wind–magnetosphere coupling are more important and responsible for 70–90%. Variations in viscous processes contribute slightly to this, but variations in the reconnection rate with μare the dominant cause. The variation in the reconnection rate is primarily the result of a variation in the length of the section of the separator line along which relatively strong reconnection occurs. Changes in solar wind–magnetosphere coupling also affect the field‐aligned currents, but these are influenced as well by variations in the conductance associated with variations inμ, more so than the cross‐polar cap potential. This may be the case for geomagnetic activity too.