Field-aligned Current System Research Articles

The Structure of Field‐Aligned Current Systems as Inferred From the Multiscale Minimum Variance Analysis

AbstractAuroral field‐aligned currents (FACs) have an intrinsic complexity caused by the superposition of contributions from a broad spectrum of scales and diversity of locations. The complex FAC systems are investigated by using the multiscale minimum variance analysis. This technique provides a scale based decomposition of the FAC systems by identifying the constituting FAC elements as well as their structure. At the basis, the analysis exploits the scale dependence of the eigenvalues of the magnetic field variance matrix. The scale decomposition along the transversal (latitudinal) direction results from the scale derivative of the maximum eigenvalue. The complementary information from the scale derivative of the minimum eigenvalue helps to infer the full structure of each FAC element by providing estimates of the FAC length (longitudinal) scale. The scale derivative of minimum and maximum eigenvalues are used to identify FAC signatures associated to different types of aurora (e.g., highly extended, finite arcs, gradient regions) as well as to characterize the influence of the crossing location with respect to the FAC structures (e.g., near edge crossings). The multiscale analysis is applied to simulated FACs and to spacecraft observations made by the Swarm mission. The use with real world data illustrates the power of this analysis, whose full benefits for magnetosphere‐ionosphere coupling investigations are yet to be explored.

Different Origin Field‐Aligned Currents and Their Relationship With Auroral Intensity

AbstractThe ionospheric field‐aligned current (FAC) consists of three components related to ionospheric vortices and gradients of Pedersen and Hall conductances, with the first term being of magnetospheric origin and the latter two being of ionospheric origin (Eq. 1). We calculate each of them between 2010 and 2016 using line‐of‐sight velocity data from the Super Dual Auroral Radar Network radar and auroral electron precipitation data from the Defense Meteorological Satellite Program. Then, we investigate the contribution of each FAC component, as well as the relationship between each FAC component and auroral activity. Our result shows that ionospheric‐origin FACs contribute up to 54% (dawn) and 42% (dusk) within the auroral oval. Both magnetospheric‐origin FACs and ionospheric‐origin FACs initially increase and then stabilize as the SuperMAG electrojet (SME) index increases. After stabilization, these two currents have almost equally important roles, with an average contribution rate of 56% and 44%, respectively. Additionally, the ionospheric‐origin FACs are found to have a higher dependency on auroral intensity. We also find that magnetospheric‐origin FACs exhibit distribution patterns similar to the R1/R2 FAC systems because their directions are determined by vorticities. When the SME index exceeds 200 nT, the contribution from the Pedersen conductance gradient consistently exhibits downward currents near 70° magnetic latitude in the dusk and predawn sectors due to the combined effects of velocities and Pedersen conductance gradients. However, the distribution pattern from the Hall conductance gradient is not so clear due to the uncertainty in the angle between velocity and Hall conductance gradient.

Dayside Magnetic Depression Following Interplanetary Shock Arrivals During the February 1958 and July 1959 Superstorms

AbstractThe present study investigates mid‐ and low‐latitude ground magnetic disturbances observed following the arrival of three interplanetary (IP) shocks during the super‐geomagnetic storms of February 1958 and July 1959. One may expect that after IP shocks, the H (northward) magnetic component increases globally but especially on the dayside. However, in each event, the H component was depressed sharply for 1–2 hr in the dawn‐to‐noon sector, whereas it increased in other local time (LT) sectors. Observed magnetic deflections suggest that there existed field‐aligned currents (FACs) flowing into and out of the auroral zone around the western and eastern edges of the LT sector of the dayside H depression. These features strongly suggests that the observed H depression was a remote effect of a R1‐sense FAC system. It was previously reported that similar ground magnetic disturbances were observed after the SSC of the 2003 Halloween storm, which reveals striking similarities to the well‐known H depression observed at Colaba during the 1859 Carrington storm. It is therefore suggested that the external driving behind IP shocks, especially those associated with major storms, is most optimum for the sharp reduction of the dayside H component through the formation and intensification of the dayside FAC system. Associated magnetic disturbances are considered to be larger in magnitude with increasing magnetic latitude, and oriented azimuthally as well as meridionally. Such magnetic disturbances in dayside midlatitudes may not be discussed very often as a target of space weather, but their potential impacts on ground infrastructures probably require closer attention.

Estimation of Ionospheric Field‐Aligned Currents Using SuperDARN Radar and DMSP Observations

AbstractStudies commonly assumed that variations in ionospheric conductance were insignificant and proposed that vorticities can be a reliable proxy or diagnostic for ionospheric field‐aligned currents (FACs). We propose a complete method for measuring FACs using data from the Super Dual Auroral Radar Network radar and the Defense Meteorological Satellite Program. In our method, the FACs are determined by three terms. The first term is referred to as magnetospheric‐origin FACs, while the second and third terms are known as ionospheric‐origin FACs. This method incorporates height‐integrated conductances based on observational data, thereby addressing the limitation of assuming uniform conductances. Different from previous works, we can calculate FACs at a low altitude of 250 km and obtain high‐resolution measurements within observable areas. Another advantage of this method lies in its ability to directly calculate and analyze the impact of ionospheric vorticity and conductance on FACs. We apply this method to obtain FACs in the Northern Hemisphere from 2010 to 2016 and analyze the distributions of height‐integrated conductances and total FACs. Our analysis reveals that the average FACs clearly exhibit the large‐scale R1 and R2 FAC systems. We conduct statistical analysis on magnetospheric‐origin FACs and ionospheric‐origin FACs. Our findings show that within the auroral oval, ionospheric‐origin FACs reach a comparable level to magnetospheric‐origin FACs. However, ionospheric‐origin FACs are significantly minor and almost negligible in other regions. This implies that height‐integrated conductance gradients and vorticities play equally significant roles within the auroral oval, whereas vorticities dominate in other regions.

Effects of the evolving early Moon and Earth magnetospheres

Recently it has been identified that our Moon had an extensive magnetosphere for several hundred million years soon after it was formed when the Moon was within 20 Earth Radii (RE) from the Earth. Some aspects of the interaction between the early Earth-Moon magnetospheres are investigated by mapping the interconnected field lines between the Earth and the Moon and investigating how the early lunar magnetosphere affects the magnetospheric dynamics within the coupled magnetospheres over time. So long as the magnetosphere of the Moon remains strong as it moves away from the Earth in the antialigned dipole configuration, the extent of the Earth’s open field lines decreases. As a result, at times it significantly changes the structure of the field-aligned current system, pushing the polar cusp significantly northward, and forcing magnetotail reconnection sites into the deeper tail region. In addition, the combined magnetospheres of the Earth and the Moon greatly extend the number of closed field lines enabling a much larger plasmasphere to exist and connecting the lunar polar cap with closed field lines to the Earth. That configuration supports the transfer of plasma between the Earth and the Moon potentially creating a time capsule of the evolution of volatiles with depth. This paper only touches on the evolution of the early Earth and Moon magnetospheres, which has been a largely neglected space physics problem and has great potential for complex follow-on studies using more advanced tools and due to the expected new lunar data coming in the next decade through the Artemis Program.

SwarmFACE: A Python package for field-aligned currents exploration with Swarm

The SwarmFACE package utilizes magnetic field measurements by the Swarm satellites to study systems of field-aligned currents (FACs). Improvements of well-established techniques as well as novel single- and multi-satellite methods or satellite configurations are implemented to extend the characterization of FAC systems beyond the Swarm official Level-2 FAC product. Specifically, the included single-satellite algorithm allows to consider the FAC sheet inclination with respect to the satellite orbit and can work with low- or high-resolution data. For dual-satellite FAC estimation the package provides three algorithms, based on the least-squares, on the singular value decomposition, and on the Cartesian boundary-integral methods. These algorithms offer advantages over the corresponding Level-2 algorithm by providing more stable solutions for ‘extreme’ configurations, e.g. close to the orbital cross-point, and by allowing for a more general geometry of the spacecraft configuration. In addition, the singular value decomposition algorithm adapts itself to the spacecraft configuration, allowing for continuous, dual-satellite based FAC solutions over the entire polar region. Similarly, when Swarm forms a close configuration, the package offers the possibility to estimate the FAC density with a three-satellite method, obtaining additional information, associated to a different (larger) scale. All these algorithms are incorporating a robust framework for FAC error assessment. The SwarmFACE package further provides useful utilities to automatically estimate the auroral oval location or the intervals when Swarm forms a close configuration above the auroral oval. In addition, for each auroral oval crossing, a series of FAC quality indicators, related to the FAC methods’ underlying assumptions, can be estimated, like the current sheet inclination and planarity or the degree of current sheet stationarity.

Signatures of wedgelets over Fennoscandia during the St Patrick’s Day Storm 2015

During the long main phase of the St Patrick’s Day storm on March 17, 2015, we found three separate enhancements of the westward electrojet. These enhancements are observed in the ionospheric equivalent currents computed using geomagnetic data over Fennoscandia. Using data from the IMAGE magnetometer network, we identified localised field-aligned current (FAC) systems superimposed on the pre-existing ionospheric current system. We suggest that these localised current systems are wedgelets and that they can potentially contribute to a larger-scale structure of a substorm current wedge (SCW). Each wedgelet is associated with a negative BXspike. Each spike is recorded at a higher latitude than the former one and all three are very localised over Fennoscandia. The first spike occurred at 17:34 UT and was observed at Lycksele, Rørvik and Nurmijärvi, the second spike was recorded at 17:41 UT and located at Lycksele and Rørvik, whereas the last spike occurred at 17:47 UT and was observed at Kevo and Abisko. Simultaneous optical auroral data and electron injections at the geosynchronous orbit indicate that one or more substorms took place in the polar ionosphere at the time of the wedgelets. This study demonstrates the occurrence of small and short-lived structures such as wedgelets at different locations over a short time scale, 15 min in this case.

Main features of magnetospheric dynamics in the conditions of pressure balance

We analyzed the applicability of a hydrodynamic approach to the analysis of large-scale magnetospheric dynamics in the conditions of quiet geomagnetic activity. The main parameter, determining the applicability of the ideal MHD, is the ratio between the Alfvén and plasma velocities. We obtained the 2D distribution of this parameter using data of the THEMIS mission at geocentric distances <20 RE, and showed that not only the ring current region can be described suggesting full pressure balance. The external part of the ring current, which is localized in the surrounding the Earth plasma ring and earlier considered as a plasma sheet continuation, can be regarded as a stress balance region as well. Azimuthal plasma pressure gradients in this region create a system of large-scale field-aligned currents. Their closing in the ionosphere generates a large-scale convection in the ionosphere and magnetosphere. The mechanism of the large-scale convection control due to “penetration” of large-scale solar wind magnetic field is discussed. We discuss the role of turbulent transport in the analysis of magnetospheric regions, in which high level of turbulence is observed simultaneously with large-scale stress balance.

Laboratory Modeling of a System of Field-Aligned Currents Generated by Flows of Inner Magnetospheric Laser Plasma

Laboratory Modeling of a System of Field-Aligned Currents Generated by Flows of Inner Magnetospheric Laser Plasma

Reconstructing Mercury's magnetic field in magnetosphere using radial basis functions

Based on the Magnetometer observations of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit around Mercury, we construct a global magnetic field model of Mercury's magnetosphere using radial basis functions (RBFs). This model is parameterized by magnetic disturbance level and the fields are a sum of internal and external sources in the magnetosphere, including magnetopause and magnetospheric currents. The RBF-based model does not impose preconceived assumptions on the fields and constructs the system of tail currents and field-aligned currents (FACs) simultaneously. Compared with the previous models, the residuals between the observed and modeled magnetic field are significantly reduced at different magnetic activity levels and this indicates that our model is reliable and robust. Some features in Mercury's magnetosphere are also constructed based on the RBF model. The magnetic field is compressed in the boundary layer inside the dayside magnetosphere and stretched at the nightside due to the strong dawn-dusk cross-tail currents beyond 1.5 RM(RM is Mercury's radius). The X-line location at the nightside is reduced from 4.6 RM in the magnetic quiet period to 2.9 RM in the magnetic active period and fits the magnetic disturbance indices in linear form. The FACs are downward (upward) in the dawn (dusk) sector and are increased with magnetic disturbance levels. In summary, the RBF-based model could reveal all principal features in the magnetosphere and their changes associated with magnetic disturbance levels.

Laboratory modeling of a field-aligned currents system generated by a flow of inner magnetospheric plasma

For the first time in laboratory conditions, an experiment to simulate a system of field-aligned currents arising on planets such as Hot Jupiters in the presence of dense inner-magnetospheric plasma was carried out. The magnitude and transit time of field-aligned currents were measured as a function of the magnetic field using flat electrodes. The geometry of the expansion of plasma streams was pictured by gated camera. Also, in a first approximation, the efficiency of energy transfer from plasma flows to field-aligned currents was calculated. The results obtained create a basis for future laboratory experiments on this topic and improve existing numerical models.

An Examination of Magnetosphere‐Ionosphere Influences During a SAPS Event

AbstractThe sub‐auroral polarization stream (SAPS) is a region of westward high velocity plasma convection equatorward of the auroral oval that plays an important role in mid‐latitude space weather dynamics. In this study, we present observations of SAPS flows extending across the North American sector observed during the recovery phase of a minor geomagnetic storm. A resurgence in substorm activity drove a new set of field‐aligned currents (FACs) into the ionosphere, initiating the SAPS. An upward FAC system is the most prominent feature spreading across most SAPS local times, except near dusk, where a downward current system is pronounced. The location of SAPS flows remained relatively constant, firmly inside the trough, independent of the variability in the location and intensity of the FACs. The SAPS flows were sustained even after the FACs weakened and retreated polewards with a decline in geomagnetic activity. The observations indicate that the mid‐latitude trough plays a crucial role in determining the location of the SAPS and that SAPS flows can be sustained even after the magnetospheric driver has weakened.

A Rotating Azimuthally Distributed Auroral Current System on Saturn Revealed by the Cassini Spacecraft

Abstract Stunning aurorae are mainly produced when accelerated electrons travel along magnetic field lines to collide with the atmosphere. The motion of electrons often corresponds to the evolution of a magnetic field-aligned current system. In the terrestrial magnetosphere, the current system is formed at the night-side sector, and thus produces an auroral bulge at night. Due to the different energy sources between Saturn and the Earth, it is expected that their auroral current systems are fundamentally different, although the specific auroral driver at Saturn is poorly understood. Using simultaneous measurements of the aurora, particles, magnetic fields, and energetic neutral atoms, we reveal that a chain of paired currents, each of which includes a downward and an upward current branch, is formed in Saturn's magnetosphere, which generates separated auroral patches. These findings inform similar auroral current structures between the Earth and Saturn, while the difference is that Saturn's unique mass and energy sources lead to a rotational characteristic.

AMPERE polar cap boundaries

Abstract. The high-latitude atmosphere is a dynamic region with processes that respond to forcing from the Sun, magnetosphere, neutral atmosphere, and ionosphere. Historically, the dominance of magnetosphere–ionosphere interactions has motivated upper atmospheric studies to use magnetic coordinates when examining magnetosphere–ionosphere–thermosphere coupling processes. However, there are significant differences between the dominant interactions within the polar cap, auroral oval, and equatorward of the auroral oval. Organising data relative to these boundaries has been shown to improve climatological and statistical studies, but the process of doing so is complicated by the shifting nature of the auroral oval and the difficulty in measuring its poleward and equatorward boundaries. This study presents a new set of open–closed magnetic field line boundaries (OCBs) obtained from Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) magnetic perturbation data. AMPERE observations of field-aligned currents (FACs) are used to determine the location of the boundary between the Region 1 (R1) and Region 2 (R2) FAC systems. This current boundary is thought to typically lie a few degrees equatorward of the OCB, making it a good candidate for obtaining OCB locations. The AMPERE R1–R2 boundaries are compared to the Defense Meteorological Satellite Program Special Sensor J (DMSP SSJ) electron energy flux boundaries to test this hypothesis and determine the best estimate of the systematic offset between the R1–R2 boundary and the OCB as a function of magnetic local time. These calibrated boundaries, as well as OCBs obtained from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) observations, are validated using simultaneous observations of the convection reversal boundary measured by DMSP. The validation shows that the OCBs from IMAGE and AMPERE may be used together in statistical studies, providing the basis of a long-term data set that can be used to separate observations originating inside and outside of the polar cap.

The increase in the curvature radius of geomagnetic field lines preceding a classical dipolarization

Abstract. Based on assumptions that substorm field line dipolarization at geosynchronous altitudes is associated with the arrival of high-velocity magnetotail flow bursts referred to as bursty bulk flows, the following sequence of field line dipolarization is proposed: (1) slow magnetoacoustic wave excited through ballooning instability by enhanced inflows in pre-onset intervals towards the equatorial plane; (2) in the equatorial plane, slow magnetoacoustic wave stretching of the flux tube in dawn–dusk directions resulting in spreading plasmas in dawn–dusk directions and reduction in the radial pressure gradient in the flux tube. As a consequence of these processes, the flux tube assumes a new equilibrium geometry in which the curvature radius of new field lines increased in the meridian plane, suggesting an onset of field line dipolarization. The dipolarization processes associated with changing the curvature radius preceded classical dipolarization caused by a reduction of cross-tail currents and pileup of the magnetic fields. Increasing the curvature radius induced a convection surge in the equatorial plane as well as inductive westward electric fields of the order of millivolts per meter (mV m−1). Electric fields transmitted to the ionosphere produce an electromotive force in the E layer for generating a field-aligned current system of Bostrom type. This is also equivalent to the creation of an incomplete Cowling channel in the ionospheric E layer by the convection surge.

Magnetospheric Dynamo Driving Large‐scale Birkeland Currents

AbstractBased upon recent global magnetohydrodynamic (MHD) simulations, we provide a theoretical foundation for magnetospheric dynamos driving large‐scale field‐aligned current (FAC) systems. In the framework of MHD, the dynamo is a process in which plasma thermal energy is converted to electromagnetic energy. Theoretical analysis using ideal MHD equations indicates the existence of two types of dynamos that allow this energy conversion. One is the expanding slow dynamo associated with flux tube expansion, and the other is the contracting slow dynamo associated with flux tube contraction. The expanding slow is a stationary dynamo without significant magnetic field curvature, with the field intensity gradient playing a central role. In contrast, the contracting slow is a dynamo with finite time variation and finite magnetic field curvature. A supplementary MHD simulation in this study, together with those in the past studies, indicates the existence of a general relation between the main voltage generator (i.e., dynamo) and the main FAC generator (i.e., divergence of FACs) that appears commonly for large‐scale FAC systems.. That is, the dynamo overlaps the FAC generator in a specific spatial configuration and that at the transition between compressible high beta plasma and incompressible low beta plasma. This overlap represents a coupling of slow mode and Alfvén mode disturbances. We performed linear analysis of that special configuration using ideal MHD equations and derived the general expression relating the voltage generator and the FAC generator.

Impact of geomagnetic variation over sub-auroral ionospheric region during high solar activity year 2014

The present work is an attempt to evaluate the impact of changing space weather condition over sub-auroral ionosphere during high solar activity year 2014. In view of this, the GPS based TEC along with Ionosonde data over Indian permanent scientific base “Maitri”, Antarctica (70°46′00″S, 11°43′56″E) has been utilized. The results suggested that the nature of ionospheric responses to the geomagnetic disturbances not only depended upon the status of high latitudinal electro-dynamic processes but also influenced by the seasonal variations. The results revel both negative and positive type of ionospheric response in a single year but during different seasons. The study suggested that the combination of equator-ward plasma transportation along with ionospheric compositional changes causes a negative ionospheric impact especially during summer and equinox seasons. However, the combination of pole-ward contraction of the oval region along with particle precipitation may lead to exhibit positive ionospheric response during the winter season. The plasma transportation direction has been validated with the help of convection boundary (HM boundary) deduced with the help of SuperDARN observations. The ground based ionosonde observations clearly provided the evidence of deep penetration of high energetic particles up to the E-layer heights which results a sudden and strong appearance of E-layer. The strengthening of E-layer is responsible for modification of auroral electrojet and field-aligned current system. Also, the sudden appearance of E-layer along with a decrease in F-layer electron density suggested the dominance of NO+ over O+ in a considered region under geomagnetic disturbed condition.

Features of the inter-hemispheric field-aligned current system over Malaysia ionosphere

Magnetic records of the declination (D) component for the solar quiet year 2011–2013 obtained from Magnetic Data Acquisition System (MAGDAS) at Langkawi (Geog. Lon. 99.68∘ E, Geog. Lat. 6.30∘N), Malaysia were utilized in this study. The minutes averages were used to delineate the diurnal (Sq(D)) variation. The monthly mean (MSq(D)) and their seasonal variabilities (SVq(D)) were also analysed. The Sq(D) and their MSq(D) exhibit smooth regular occurring pattern in the month of April–September and became highly perturbed in October–March across the years. The highest positive (∼3.5 arc-min) and the negative (∼−3.0 arc-min) values were observed in August 2011 during the dawn and noon sectors. These maxima shifted to July and September in 2012 with peaks ∼3.2 and −3.0 arc-min. In 2013, the positive maximum (∼3.0 arc-min) and its negative (∼−2.5 arc-min) were again seen in August. This implies that the dawn and noon sectors of August 2011 and 2013 are strongly influenced by IHFACs and this effect shifted to July and September in 2012. IHFACs through the years flow from the winter to summer hemisphere during the noon and dusk sectors and flow in opposite direction during the dawn sector. The day-to-day magnitudes of Sq(D) and MSq(D) seems to suggest the inter-hemispheric imbalance of the ionospheric Sq current earlier established by Van Sabben as the cause of IHFACs is not strongly affected by the changes in annual solar variation. Dusk-side IHFACs were observed to be northbound in all the seasons with the exception of June solstice. The direction of IHFACs does not change except in April and November. The current intensity is not large in solstices except in August 2011 and 2013 but it shifted to July in 2012. The result further showed that the magnitude of the duskside IHFACS is determined to some extent by the strength of the noontime IHFACs. IHFACs were generally observed to be greater during the daytime than night-time hours.

Investigating the Development of Localized Neutral Density Increases During the 24 August 2005 Geomagnetic Storm

AbstractWe investigate localized thermospheric neutral density increases (or density spikes) developed during the 24 August 2005 geomagnetic storm and utilize a multi‐instrument database for monitoring the underlying thermospheric, ionospheric, magnetospheric, and solar wind conditions. We study five scenarios occurring under eastward/westward BY dominations and demonstrating the development of density spikes during various events. These scenarios show (1) a northern density spike associated with enhanced antisunward polar flows driven by dayside merging along old‐open field lines during the initial phase, (2) some southern density spikes associated with enhanced sunward auroral flows caused by auroral brightening events during the storm initial phase and main phase, (3) two pairs of southern dayside‐nightside density spike configuration associated with enhanced antisunward polar flows caused by dayside merging and nightside magnetotail reconnections along old‐open field lines during the underlying substorm activity, and (4) a series of enhanced polar sunward flows developed in the polar cap driven by different processes during the recovery phase. We provide a detailed analysis of these events including the specification of underlying field aligned current systems, flow channel types, and auroral forms. Observational results demonstrate the various density spikes and their associated auroral forms and/or flow channels and thus provide a better understanding of their development during this intense geomagnetic storm.

Swarm Observation of Field‐Aligned Currents Associated With Multiple Auroral Arc Systems

AbstractAuroral arcs occur in regions of upward field‐aligned currents (FACs); however, the relation is not one to one, since kinetic energy of the current‐carrying electrons is also important in the production of auroral luminosity. Multiple auroral arc systems provide an opportunity to study the relation between FACs and auroral brightness in detail. In this study, we have identified two types of FAC configurations in multiple parallel arc systems using ground‐based optical data from the Time History of Events and Macroscale Interactions during Substorms all‐sky imagers, magnetometers and electric field instruments on board the Swarm satellites. In “unipolar FAC” events, each arc is an intensification within a broad, unipolar current sheet and downward return currents occur outside of this broad sheet. In “multipolar FAC” events, multiple arc systems represent a collection of multiple up/down current pairs. By collecting 17 events with unipolar FAC and 12 events with multipolar FACs, we find that (1) unipolar FAC events occur most frequently between 20 and 21 magnetic local time and multipolar FAC events tend to occur around local midnight and within 1 h after substorm onset. (2) Arcs in unipolar FAC systems have a typical width of 10–20 km and a spacing of 25–50 km. Arcs in multipolar FAC systems are wider and more separated. (3) Upward currents with more arcs embedded have larger intensities and widths. (4) Electric fields are strong and highly structured on the edges of multiple arc system with unipolar FAC. The fact that arcs with unipolar FAC are much more highly structured than the associated currents suggests that arc multiplicity is indicative not of a structured generator deep in the magnetosphere, but rather of the magnetosphere‐ionosphere coupling process.