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Using data from the 2D IMAGE network and magnetic stations located in Russia, Pc5 geomagnetic pulsations with a frequency of ~2.8 mHz, which occurred in the afternoon sector against the background of the magnetic storm of August 27, 2014, preceded by a 5-day period with low magnetic activity, are studied in detail. In two time intervals, at the beginning of the storm and during the period of maximum magnetic activity, instantaneous 2D distributions of Pc5 magnetic field amplitudes on Earth's surface are plotted. It has been found that the ionospheric sources of Pc5 (vortex Hall currents) have an elliptical shape with a larger axis in the south–north direction. At the beginning of the magnetic storm, a single burst of Pc5 pulsations was detected, the center of the source of which was located at the geomagnetic latitude ~67.5° (L ~ 6.8 RE) and moved westward with a velocity of ~0.7 km/s. The estimated size of this ionospheric source is ~150 km in the west–east direction and ~330 km in the south–north direction. During the maximum of the magnetic storm, Pc5 pulsations are produced by two ionospheric sources following each other. These ionospheric sources have a more elongated elliptical shape with axes of ~250 km in the west–east direction and ~670 km in the south–north direction. The centers of these sources were shifted by 4° to a more southern geomagnetic latitude ~63.5° (L ~ 5 RE) and moved westward with a velocity of ~1.7 km/s. Estimates of the size of the two-time magnetic field tube in which the resonance MHD waves have been generated and its velocity in the equatorial plane of the magnetosphere are presented. [ABSTRACT FROM AUTHOR]

The injection of energy into the Earth's magnetosphere during geomagnetic storms and substorms has a direct impact on the ionosphere in the auroral and sub-auroral regions. This influence can be observed through phenomena such as the expansion of the auroral oval, and fluctuations in plasma density and the intensity of the field-aligned current (FAC) system. Changes in the geomagnetic environment significantly affect the main ionospheric trough (MIT) and field-aligned currents (FACs). In this study, we analysed two geomagnetic storms of different origins—October 2015 and September 2017 that occurred during solar cycle 24. Both storms were characterised by two minima of the Dst index and classified as two-step storms. In this work, we investigate the evolution of FACs density, the displacement of MIT and FACs during the development of geomagnetic storms, and we quest for the relationship between field-aligned currents and the main ionospheric trough. We noticed a connection between the positions of the main ionospheric trough minima and the Dst index, found cases especially during the main and recovery phase of the storms when FACs increase after leaving the ionospheric trough, and observed the predominance of downward currents in the area of the MIT which suggest their influence on the formation of this region of depleted plasma density. [ABSTRACT FROM AUTHOR]

The ionospheric conductance is the major quantity that determines the interaction of the magnetosphere with the ionosphere, where the magnetosphere is the large region of space affected by Earth's geomagnetic field, and the ionosphere is its electrically conducting inner boundary, lying right on the edge of the atmosphere. Storms and substorms in space dissipate their energy through ionospheric currents, which heat the atmosphere and disrupt satellite orbits. The ionospheric conductance has, heretofore, been estimated using the staticized physics known as electrostatic theory, which finds that it can be computed by integrating the zero-frequency conductivity along the lines of Earth's geomagnetic field. In this work we test this supposition by deriving an electromagnetic solution for collisional plasma, and applying it to obtain a first-ever fully-electromagnetic calculation of ionospheric conductance. We compare the results to the field line integrated conductivity, and find significant differences on all scales investigated. We find short-wavelength, mode-mixing, and wave-admittance effects that were completely unexpected. When this theoretical result is matched with recent observational findings for the scale of the magnetosphere-ionosphere coupling-interaction, there results a situation where small- to intermediate-scale effects really may contribute to global modeling of the Sun-Earth system. [ABSTRACT FROM AUTHOR]

Abstract—The concept of Alfvén waves plays a key role in the theory of ultralow-frequency (ULF) electromagnetic oscillations of extraterrestrial origin. This article is dedicated to the 80th anniversary of the discovery of Alfvén waves. It focuses on the ionospheric Alfvén resonator (IAR). IAR excites ultralow-frequency oscillations in the Pc1 range (0.2–5 Hz). When computing the oscillation spectrum within the standard model, it is assumed that the IAR is an autonomous dynamical system. In contrast, in this paper, IAR is treated as a specific subsystem of the general system of Alfvén oscillations of geomagnetic field lines. In other words, we proceed from the idea that IAR, in general, is not an autonomous oscillatory system. The problem about IAR spectrum is discussed in the context of the general problem on the spectrum of magnetohydrodynamic oscillations of the Earth's magnetosphere. The corresponding Sturm–Liouville problem is formulated. Analytical solutions of the problem are considered in the Wentzel–Kramers–Brillouin approximation. It is pointed out that the problem of IAR spectrum has to be solved by numerical methods due to the rather complicated distribution of the Alfvén velocity along the geomagnetic field lines. [ABSTRACT FROM AUTHOR]

On 24 April 2023, a Coronal Mass Ejection event caused the solar wind to become sub‐Alfvénic, leading to the development of an Alfvén Wing configuration in the Earth's magnetosphere. Alfvén Wings have previously been observed as cavities of low flow around moons in Jupiter's and Saturn's magnetospheres, but the observing spacecraft did not have the ability to directly measure the Alfvén Wings' current structures. Through in situ measurements made by the Magnetospheric Multiscale spacecraft, the 24 April event provides us with the first direct measurements of current structures during an Alfvén Wing configuration. These structures are observed to be significantly more anti‐field‐aligned and electron‐driven than the typical diamagnetic magnetopause current, indicating the disruption caused to the magnetosphere current system by the Alfvén Wing formation. The magnetopause current is then observed to recover more of its typical, perpendicular structure during the magnetosphere's recovery from the Alfvén Wing formation. Plain Language Summary: The solar wind applies pressure on the Earth's magnetic field, distorting it from a dipole into its compressed dayside and stretched tail configuration. However, this typical structure can be disrupted by eruptive solar events such as Coronal Mass Ejections (CMEs), which may cause the solar wind's pressure to drop low enough that it is no longer able to push the magnetosphere back to form a single unified tail. When this occurs, the tail splits into two separate structures, called Alfvén Wings. While this configuration is rare at Earth, it is common from interactions of the outer planets' magnetosphere's with their moons, where Alfvén Wing configurations have been studied and modeled. However, because the observing spacecraft lacked the necessary instrumentation, we have not yet directly observed the Alfvén Wing current structures. On 24 April 2023, a CME event led to the creation of an Alfvén Wing formation in the Earth's magnetosphere. We observed this event using the Magnetospheric Multiscale spacecraft, which enabled us to make the first direct observations of Alfvén Wing current structures. These currents were found to be mainly parallel to the local magnetic field, in contrast to typical magnetopause currents. Key Points: On 24 April 2023, the Magnetospheric Multiscale (MMS) spacecraft observed an Alfvén Wing formation along the dawn‐flank of Earth's magnetosphereMMS's observations represent the first in situ measurements of Alfvén Wing current structuresThe current structures are found to be primarily anti‐field‐aligned, electron‐driven, and filamentary [ABSTRACT FROM AUTHOR]

Using Time History of Events and Macroscale Interactions during Substorms (THEMIS) data, we studied the stepwise development in high‐latitude geomagnetic perturbations and Pi1 and Pi2 pulsations during substorm onsets and their association with stepwise auroral onset arc development by analyzing four substorm events. We found that the geomagnetic perturbations and pulsations which are magnetic signatures of the substorm on the ground show stepwise changes and excitation similar to the development of the auroral onset arc which is the visual manifestation of the substorm. We observed minor to small changes in magnetic perturbations and excitation of Pi2 pulsations before initial brightening (IB), and the subsequent excitation of Pi1 and the second Pi2 at or around the further enhancement of onset arc (FE). Then, a steep fall in the magnetic northward component, and the largest‐amplitude and highest‐frequency Pi1 and Pi2 pulsations appeared at or after poleward expansion (PE). The appearance of FE in all four events and its association with magnetic perturbations and pulsations suggest that FE is an important step in addition to IB and PE. The detailed analysis of the FE step using ground‐ and space‐based data may provide information on the substorm triggering mechanism, the sequence of mechanisms behind the substorm, as well as the mechanisms responsible for the excitation of Pi1 and Pi2 pulsations. Plain Language Summary: The substorm is a very complex phenomenon and the triggering mechanisms behind it and their sequence of processes in the magnetosphere are still not fully understood. Geomagnetic pulsations and perturbations are considered as excellent indicators for onset timing, and Pi pulsation's arrival time on the ground may also be used to identify the location of substorm onset in the magnetotail. We studied the stepwise development in geomagnetic perturbations and Pi pulsations at substorm onsets and their association with the development of auroral onset arc which is a four‐step phenomenon. We observed that the geomagnetic perturbations and pulsations which are the magnetic signature of the substorm on the ground show stepwise changes and excitation similar to the development of the auroral onset arc. Additionally, we suggest that further enhancement of auroral onset arc (FE), which did not get much attention previously, is another important and separate step besides initial brightening and poleward expansion. As the actual mechanism behind FE is still unknown, we anticipate that a detailed analysis of the FE step using ground‐ and space‐based data may provide crucial information on the mechanism behind FE as well as the sequence of mechanisms behind the substorm. Key Points: Pi pulsations and magnetic perturbations are excited in steps during substorm onsetA relation exists between further enhancement of auroral onset arc and large‐amplitude, high‐frequency Pi pulsationsWe suggest that further enhancement of the auroral onset arc is an important step in auroral onset arc development [ABSTRACT FROM AUTHOR]

The 1859 Carrington event is the most intense geomagnetic storm in recorded history, and the literature provides numerous explanations for what drove the negative H perturbation on the Earth. There is debate on what dominated the event. Our analysis shows a combination of causes of similar orders of magnitude. Previous analyses generally rely upon the observed H perturbation at Colaba, India; historic newspaper reports; and empirical models. We expand the analysis using two Space Weather Modeling Framework simulations to examine what drove the event. We compute contributions from currents and geospace regions to the northward B field on Earth's surface, BN. We examine magnetospheric currents parallel and perpendicular to the local B field, ionospheric currents, and gap region field–aligned currents (FACs). We also evaluate contributions from the magnetosheath, near–Earth, and neutral sheet regions. A combination of currents and geospace regions significantly contribute to BN on the Earth's surface, changing as the storm evolves. At storm onset, magnetospheric currents and gap–region FACs dominate in the equatorial region. At auroral latitudes, gap–region FACs and ionospheric currents are the largest contributors. At storm peak, azimuthal magnetospheric currents and gap–region FACs dominate at equatorial latitudes. Gap–region FACs and ionospheric currents dominate in the auroral zone, down to mid‐latitudes. Both the magnetosheath and FACs contribute at storm peak, but are less significant than that from the near–Earth ring current. During recovery, the near–Earth ring current is the largest contributor at equatorial latitudes. Ionospheric currents and gap–region FACs dominate in the auroral zone. Key Points: Prior Carrington event analyses attributed the ΔH perturbation observed on Earth to various causes, with debate on which dominatedUsing MHD simulations, we compute how currents in various systems and geospace regions contribute to ΔHRather than a dominant contributor, we find that multiple current systems and regions contribute with the same order of magnitude [ABSTRACT FROM AUTHOR]

Magnetosheath high‐speed jets (HSJs), localized impulses of dynamic pressure, are attracting growing attention due to their geoeffectiveness. However, how HSJs modulate chorus waves in the magnetosphere still remains unclear. Utilizing combined observations of the Time History of Events and Macroscale Interactions during Substorms satellites A and E, we report, for the first time, the prompt disappearance of the magnetospheric chorus waves caused by a HSJ. Such wave disappearance is directly due to the flux drop of energetic electrons (∼10–100 keV), leading to the cessation of wave generation, which is supported by the linear theoretical analysis. We propose that the flux drop results from the local indentation of magnetopause after the HSJ impact, where two new smaller magnetic mirrors are formed off the equator and part of electrons are then expelled by the mirror force. The HSJs should be an important factor in modulating chorus waves because of their high occurrence rate. Plain Language Summary: The solar wind with enhanced dynamic pressure can globally compress the dayside magnetosphere, causing the excitation and intensification of chorus waves. Besides the enhancement of solar wind RAM pressure, there also exists the ion‐scale dynamic pressure impulse in the magnetosheath, known as a high‐speed jet (HSJ), even during quiet solar wind conditions. Previous studies have shown that the HSJs have a high occurrence rate and they are geoeffective (causing disturbances in the Earth's magnetosphere). However, it is still unknown whether HSJs can modulate chorus waves in the magnetosphere. In this study, based on joint observations of THEMIS‐A and E probes, we report the prompt disappearance of chorus waves in the magnetosphere caused by the HSJs. We propose that a HSJ causes the local indentation of the dayside geomagnetic fields at the magnetic equator, and creates two small magnetic mirrors. Consequently, the poleward mirror force will expel some of the electrons from the equator, and lead to a significant decrease of the electron flux. Finally, the generation of chorus waves will stop due to the lack of energy sources. This study reveals that HSJs are a new and important factor in modulating chorus waves in the magnetosphere. Key Points: With joint observations of THMIS‐A and THMIS‐E, we first report the prompt disappearance of chorus waves due to the high‐speed jets (HSJs)HSJs cause the local indentation of the magnetopause, creating two new magnetic mirrors, and electrons are expelled by the mirror forceThe electron flux drop leads to the cessation of the chorus wave generation, that is, the disappearance of waves [ABSTRACT FROM AUTHOR]

This paper presented an analysis of geomagnetic disturbance observed on the ground during geomagnetic storms with different intensities in 2015 using the meridian chain data at geomagnetic mid and low latitudes. Ground observation records superimpose varying types of space-current system and noise interference. Geomagnetic disturbance with variation of discontinuity and irregularities are difficult to identify and distinguish. We proposed a variational mode decomposition (VMD) algorithm for reconstructing geomagnetic horizontal (H ) disturbance signals. We decomposed the geomagnetic signals into geomagnetic disturbance signals, diurnal variation signals, and noise disturbance signals using the VMD algorithm. Intrinsic mode functions (IMFs) were selected to form the reconstructed signal, which represented a geomagnetic disturbance during a geomagnetic storm. We investigated the decreased amplitude of H component obtained from the reconstructed signals during main phase of geomagnetic storms with different geomagnetic storms intensities and seasons at mid and low latitudes. The maximum values of gradient variation of H component disturbance with geomagnetic latitude cosine are near magnetic latitude 30°N during geomagnetic storms with different intensities and seasons. Ionopheric structural changes in the low-to-mid latitude transition zone maybe the primary cause. The result provides a reference for the complex coupling relationship between the ionosphere and magnetosphere during geomagnetic storms. [ABSTRACT FROM AUTHOR]

Ionospheric outflow supplies nearly all of the heavy ions observed within the magnetosphere, as well as a significant fraction of the proton density. While much is known about upflow and outflow energization processes, the full global pattern of outflow and its evolution is only known statistically or through numerical modeling. Because of the dominant role of heavy ions in several key physical processes, this unknown nature of the full outflow pattern leads to significant uncertainty in understanding geospace dynamics, especially surrounding storm intervals. That is, global models risk not accurately reproducing the main features of intense space storms because the amount of ionospheric outflow is poorly specified and thus magnetospheric composition and mass loading could be ill‐defined. This study defines a potential mission to observe ionospheric outflow from several platforms, allowing for a reasonable and sufficient reconstruction of the full outflow pattern on an orbital cadence. An observing system simulation experiment is conducted, revealing that four well‐placed satellites are sufficient for reasonably accurate outflow reconstructions. The science scope of this mission could include the following: reveal the global structure of ionospheric outflow; relate outflow patterns to geomagnetic activity level; and determine the spatial and temporal nature of outflow composition. The science objectives could be focused to be achieved with minimal instrumentation (only a low‐energy ion spectrometer to obtain outflow reconstructions) or with a larger scientific scope by including contextual instrumentation. Plain Language Summary: Earth's upper atmosphere above 500 km altitude constantly loses charged particles to outer space in a process called ionospheric outflow. This outflow is important for the dynamics of the near‐Earth space environment ("space weather") yet is poorly understood on a global scale. A mission is needed to observe the global patterns of ionospheric outflow and its relation to space weather driving conditions. The science objectives of such a mission could include not only the reconstruction of global outflow patterns but also the relation of these patterns to geomagnetic activity and the spatial and temporal nature of outflow composition. A study is presented to show that four well‐placed spacecraft would be sufficient for reasonable outflow reconstructions. Key Points: A simulation study is conducted to determine the number of spacecraft needed for accurate reconstruction of 2D ionospheric outflow patternsDetermining the global pattern of ionospheric outflow is needed to understand the geospace system, especially during geomagnetic stormsA potential ionospheric outflow mission concept is defined that could address this unresolved key issue of space physics and space weather [ABSTRACT FROM AUTHOR]

During geomagnetic storms, the increase in energy input into the ionosphere in the form of Poynting flux and electron precipitation leads to an enhanced ionospheric outflow that results in an increase of the O+ content in the magnetosphere. Using different missions and instrumentation, two main ionospheric sources have been identified for the oxygen ions reaching the inner magnetosphere during geomagnetic storms: the dayside cusp, and the night side auroral region. Evidence of both pathways have been presented in the literature. However, the relative contribution of each of these pathways to the enhancement of O+ observed in near‐Earth plasma sheet, as well as the dynamics involved during the development of geomagnetic storms remains an open question. Here, we present the first statistical study to date to address this question, in the form of a superposed epoch analysis of O+ and H+ moments obtained by the Magnetospheric Multiscale (MMS) mission throughout the main phase of 90 geomagnetic storms with a minimum SYM‐H of at least −50 nT. The results show a clear increase in the oxygen density in the near‐Earth plasma sheet, with values further from Earth remaining low. Temperature values for both species show an increase with the progress of the storms. These results combined suggest that, during the main phase of geomagnetic storms, most of the oxygen ions observed in the near‐Earth plasma sheet are traveling directly from the nightside auroral region. Key Points: Superposed epoch analysis shows dynamic evolution of O+ ions during geomagnetic stormsSignificant enhancement of the O+‐to‐H+ number density ratio observed at radial distances less than 12 REScalar temperature for both species show the arrival of heated ions far from Earth throughout the analyzed period [ABSTRACT FROM AUTHOR]

• neutron monitor measurements provide information on statement of geomagnetic field. • strong cosmic ray anisotropy observed inside magnetic cloud. • atmospheric temperature variations are reconstructed using muon telescope data. We investigated the conditions of the heliosphere, magnetosphere, and atmosphere from cosmic ray (CR) observations during the 2018 Aug 25–26 strong geomagnetic storm. The analysis involved the global survey (GS) and the spectrographic global survey (SGS) methods created and developed at the Yu.G. Shafer Institute of Cosmophysical Research and Aeronomy of the Siberian Branch of the Russian Academy of Sciences (ShICRA SB RAS) and at the Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Sciences (ISTP SB RAS). Also, in our analysis, we used the data on direct measurements of the interplanetary medium parameters from the known OMNI database, CR measurements at the GOES geostationary satellites, from the global network of neutron monitors and muon detectors, from the Sayan spectrograph, and from the A.I. Kuzmin Yakutsk spectrograph of CR. When analyzing this event, the SGS enabled to obtain the data on the orientation of the mean interplanetary magnetic field, on the geomagnetic cutoff rigidity and its variations during the geomagnetic storm. Also, this method allowed us to estimate the bulk temperature over the point of recording CR, the ring current and the magnetopause current, as well as their contributions to the Dst-index, and also to establish differential rigidity spectra of CR at different stages of the magnetic storm evolution. Through the GS method, we determined the value and the directions for the first two spherical harmonics of CR distribution, and the direction to their anisotropy source. The results obtained through two different global survey methods are shown to be agree and mutually supplement each other. Using the Yakutsk spectrograph records enabled to determine the index for the power energy spectrum of variations in CR intensity during the investigated event. [ABSTRACT FROM AUTHOR]

The boundary between the interplanetary magnetic field and the terrestrial magnetic field is the magnetopause. This magnetopause is influenced by dynamic changes in the solar wind, that is different solar wind conditions lead to a change in the shape and location of the magnetopause. The interaction between the solar wind and the magnetosphere can be studied from in-situ spacecraft observations. Many studies focus on the equatorial plane, as this is where recent spacecraft constellations such as THEMIS or MMS operate. However, to fully capture the interaction, it is important to study the high latitude regions as well. The Cluster spacecraft allow us to collect a dataset of high-latitude magnetopause crossings and study magnetopause motion in this region, as well as deviations from established magnetopause models. We use multi-spacecraft analysis tools to investigate the direction of magnetopause motion in the high latitudes and compare the occurrence of crossings at different locations with the result in the equatorial plane. We find that the high-latitude magnetopause motion is generally consistent with previously reported values and seems to be more often associated with a closed magnetopause boundary. We show that on average the magnetopause moves faster inwards than outwards. Furthermore, the occurrence of magnetopause positions beyond those predicted by the Shue et al. (1998) model at high latitudes is found to be caused by the similar solar wind parameters as in the equatorial plane. Finally, we highlight the importance of the dipole tilt angle at high latitudes. Our results may be useful for the interpretation of plasma measurements from the upcoming SMILE mission (Branduardi-Raymont et al., 2018), as this spacecraft will also fly frequently through the high-latitude magnetopause. [ABSTRACT FROM AUTHOR]

We discuss the reasons for the extreme compression of the magnetosphere during the storm on February 27, 2023, when the magnetopause crossed the geostationary orbit. At the same time, aurora was observed at middle latitudes. The global parameters of magnetospheric current systems were calculated from data on the parameters of the interplanetary medium and geomagnetic indices characterizing the evolution of the ring current and the westward auroral electrojet, using a paraboloid model of the magnetosphere. We have calculated the contributions of various current systems to the observed value of the index. The contribution of the tail current sheet is comparable with the contribution of the ring current for this storm. The calculated modelled field is compared with the data of the GOES-16, 18 magnetometers; the results are in good agreement with observations. [ABSTRACT FROM AUTHOR]

The interaction of a solar wind discontinuity with the backstreaming particles of the Earth's ion foreshock can generate hot, tenuous plasma transients such as foreshock bubbles (FB) and hot flow anomalies (HFA). These transients are known to have strong effects on the magnetosphere, distorting the magnetopause (MP), either locally during HFAs or globally during FBs. However, previous studies on the global impact of FBs have not been able to determine whether the response stems directly from the transverse scale size of the phenomenon or its fast motion over the magnetosphere. Here we present the observation of an FB and its impact on the magnetosphere from different spacecraft scattered over the dayside magnetosphere. We are able to constrain the size of the transverse scale of an FB from direct observations to be about 10 RE. We go on to discuss how the magnetosphere responds to this transient, which seems to have a similar scale across the dayside. Plain Language Summary: The solar wind is a fast plasma flow of charged particles originating from the Sun. Earth's magnetic field diverts this flow around the planet forming the magnetosphere. The bow shock forms upstream of Earth to decelerate the solar wind and deflect the flow around Earth's magnetic field. A fraction of the solar wind particles are reflected back into the solar wind stream, forming the foreshock. Discontinuities in the solar wind can concentrate these backstreaming particles forming a bubble of hot and tenuous plasma called a foreshock bubble. These transients structures move with the solar wind, arriving at and modifying the bow shock and leading to an expansion of the magnetosphere due to lower pressure within the transients' core. Such a response was reported before in different studies which conclude that foreshock bubbles have a global impact on the magnetosphere. In our study we report on another foreshock bubble observed by a multi‐spacecraft constellation. The observations allowed us to constrain the size of the foreshock bubble. We observe that the large scale response of the magnetosphere seems to reflect with the size of the FB. Key Points: Multi satellite observation of a large foreshock bubble (FB) hitting Earth's magnetosphereThe transverse (y − z) scale size of the FB can be constrained from observations to be at least 8–10 RE fitting with simulationsThe magnetospheric response to the FB seems to be visible across almost 10 RE of the dayside, matching the FB size [ABSTRACT FROM AUTHOR]

Understanding the motion of charged particles in the electromagnetic field in the inner magnetosphere is essential for space science and space weather. Charge accumulation can occur due to the dipole magnetic and convective electric fields in this region. However, until the recent Magnetospheric Multiscale (MMS) mission, there were few means to detect charge distribution in situ. We report unambiguous in situ observation of the spatial distribution of the excess charge density in the inner magnetosphere by the MMS mission. We find that a positive (negative) charge accumulates in the dusk (dawn) side inner magnetospheres, which is contrary to the long assumed overall quasi-neutrality of space plasma. These observations and results offer insight into magnetosphere–ionosphere coupling. The magnetospheric multiscale mission (MMS) consists of four identical spacecraft used to collect data on the interaction between the Sun and Earth's magnetic fields and study how the various interactions govern the dynamics of phenomena such as plasmas and particle movement. Here, the authors analyse MMS data on charge distribution, showing that there is a separation of charge on the dawn and dusk sectors of the inner magnetosphere. [ABSTRACT FROM AUTHOR]

We examined rapid variations in the electron zebra stripe patterns, specifically at L = 1.5, over a three‐month duration, using twin Van Allen Probes within Earth's inner magnetosphere. During geomagnetically quiet intervals, these stripes exhibit a peak‐to‐valley ratio (Δj) ∼1.25 in detrended electron fluxes. However, during geomagnetic storms, they became highly prominent, with Δj > 2.5. The correlation between Δj and net field‐aligned currents (FACs) is observed to be high (0.70). Global magnetohydrodynamic (MHD) simulation results indicate that the westward electric field at midnight at low latitudes in the deep inner magnetosphere correlates well with net FACs. An increase in net FACs could amplify the dawn‐to‐dusk electric field in the deep inner magnetosphere, thereby causing the inward transport of electrons. Given that FACs are linked to the interaction between solar wind and the magnetosphere, our findings emphasize the importance of solar wind‐magnetosphere coupling in the deeper regions of the inner magnetosphere. Plain Language Summary: The intensity of hundreds of keV electron fluxes displays a distinctive pattern in the energy versus L‐value spectrogram, characterized by periodic valleys and peaks, commonly referred to as zebra stripes. These patterns have been observed in the magnetospheres of multiple planets, including Earth, Jupiter, and Saturn. Our study reveals that during geomagnetically quiet intervals, Earth's inner magnetospheric zebra stripes exhibit well‐defined banded features. However, these bands become highly pronounced during geomagnetic storms. The peak‐to‐valley ratio (Δj) of detrended electron fluxes shows a correlation with net field‐aligned currents (FACs), and these FACs, in turn, align with the westward component of the electric field at midnight. Consequently, FACs play a significant role in controlling electron flux dynamics deep within the inner magnetosphere. This research illuminates the solar wind‐magnetosphere‐ionosphere couplings. Key Points: Electron zebra stripes are a persistent feature in Earth's inner magnetosphere, although they become intensified during geomagnetic stormsPeak‐valley ratio (Δj) in detrended electron flux within the zebra stripes enhances by ≥1 factor at L = 1.5 during three geomagnetic stormsΔj is well correlated with the net field‐aligned current (FAC) in polar region, suggesting the dominant role of convection driven by FAC [ABSTRACT FROM AUTHOR]

Flux transfer events (FTEs) are transient magnetic flux ropes at Earth's dayside magnetopause formed due to magnetic reconnection. As they move across the magnetopause surface, they can generate disturbances in the ultralow frequency (ULF) range, which then propagate into the magnetosphere. This study provides evidence of ULF waves in the Pc2 wave frequency range (>0.1 Hz) caused by FTEs during dayside reconnection using a global 3D hybrid‐Vlasov simulation (Vlasiator). These waves resulted from FTE formation and propagation at the magnetopause are particularly associated with large, rapidly moving FTEs. The wave power is stronger in the morning than afternoon, showing local time asymmetry. In the pre and postnoon equatorial regions, significant poloidal and toroidal components are present alongside the compressional component. The noon sector, with fewer FTEs, has lower wave power and limited magnetospheric propagation. Plain Language Summary: The Earth's magnetosphere is a dynamic region shaped by the interplay between the solar wind and Earth's magnetic field. This interaction occurs at the boundary of the magnetosphere (magnetopause) through a process known as magnetic reconnection, giving rise to Flux Transfer Events (FTEs), which are magnetic structures that carry flux and energy into the magnetosphere. These FTEs form either in sudden bursts, patchy patterns or in a continuous, and relatively stable way making the magnetopause surface dynamic. As the FTEs move along the boundary of the magnetosphere, they create compressed regions and lead to wave generation that can extend into the magnetosphere. The study uses an advanced 3D hybrid‐Vlasov simulation model to analyze waves originated from FTE formation and propagation at the magnetopause. We find that rapidly moving and large FTEs have a significant impact on the magnetopause, leading to the generation of ULF waves with frequency above 0.1 Hz. This shows first direct evidence supporting previous theoretical speculations regarding the ability of FTEs to generate waves near the magnetopause. Key Points: Dayside Pc2 waves (>0.1 Hz) have been detected in a 3D hybrid‐Vlasov simulationThese waves exhibit lower intensity within the magnetosphere at noon, compared to the prenoon and postnoon sectorsPc2 waves observed in the simulation are associated with largest and fast moving flux transfer events initiated by subsolar reconnection [ABSTRACT FROM AUTHOR]

This review presents the progress made in the last decade in the field of large-scale electromagnetic (EM) induction with natural sources, which fluctuate at periods from seconds to years and originate in oceans, ionosphere and magnetosphere. These mechanisms produce field variations that can be used to image subsurface electrical structure of Earth and planets across scales and depths from the shallow crust to the lower mantle. In the last decade, we have seen a substantial progress made in different areas related to methods, observations and 3-D numerical modelling of EM phenomena at crustal and mantle scales. Specifically, new methods for handling complex ionospheric and magnetospheric sources were proposed, accompanied by more efficient forward and inverse modelling tools that allowed us to combine several broadband sources and constrain electrical conductivity on multiple scales simultaneously. Magnetic signals due to oceanic tides were established as a new source to probe conductivity of the sub-oceanic upper mantle. Further, the launch of ESA Swarm satellites in 2013 and their successful ongoing operation have marked a new era in the field of large-scale EM induction, unlocking a set of new opportunities, but also posing new challenges. These developments were backed by new lab measurements of electrical conductivity for mantle minerals at temperatures and pressures that are getting closer to the relevant pressure and temperature conditions in the mantle, alleviating the need for inaccurate extrapolations. The latter enabled more plausible quantitative estimates of water content, melt fractions and temperature in the mantle. In parallel, crust and mantle conductivity models along with developed modelling techniques have become an integral part of geomagnetic field and geomagnetically induced currents (GICs) modelling workflows, establishing new inter-disciplinary knowledge domains. [ABSTRACT FROM AUTHOR]

In 1961, Dungey suggested that magnetic reconnection occurs due to the solar-terrestrial interaction. The interplanetary magnetic field (IMF) is thought to merge with Earth's geomagnetic field (GMF). After the reconnection process the newly formed magnetic flux tube, consisting of both the IMF and GMF, moves anti-sunward. Poleward-moving auroral forms (PMAFs) are believed to be the ionospheric signatures of this process, which transfers magnetic flux from the dayside to the nightside. This paper looks at the connection between the solar wind speed and the motion of the PMAF as it moves from the auroral oval, anti-sunward, into the polar cap. PMAFs are identified using both the meridian scanning photometer (MSP) and colored all-sky camera (ASC). Once the PMAFs are identified, the PMAF-SLOPE, v[sub α] (units of degrees per time) and the angle (αPMAF) the PMAF makes with the horizontal (Time axis), in the MSP plot are calculated. These values (vα and αPMAF) are individually plotted against the vx-component of the solar wind speed and the flow speed (total solar wind speed). The plots generate linear a relationship between PMAF-SLOPEs, vα, [or PMAF angles (αPMAF)], and the vx-component of the solar wind speed (or the flow speed). A total of 57 PMAF events from 8 different days were associated with solar wind speeds (vx-component) ranging from 344 to 679 km/s. The first linear plot, between the PMAF-SLOPE and solar wind speed (vx-component), shows a high correlation: rvα = 0.944. A second linear plot, between αPMAF and the solar wind speed (vx-component) shows a very high correlation: rα[sub PMAF] =0.973. The conclusions obtained from this statistical study are: 1) both the PMAF-SLOPE vα and αPMAF are highly correlated to the vx-component of the solar wind, increasing when vx increases and vice versa, 2) PMAFs must be connected to both the IMF and GMF and are dragged anti-sunward, mostly by the vx-component of the solar wind, and 3) PMAFs are indeed the ionospheric footprints of a newly formed magnetic flux tube, due to dayside magnetic reconnection, being transferred from the dayside to nightside. [ABSTRACT FROM AUTHOR]

Field Line Resonances (FLRs) are a critical component in Earth's magnetospheric dynamics, associated with the transfer of energy between Ultra Low Frequency waves and local plasma populations. In this study we investigate how the polarisation of FLRs are impacted by cold plasma density distributions during geomagnetic storms. We present an analysis of Van Allen Probe A observations, where the spacecraft traversed a storm time plasmaspheric plume. We show that the polarisation of the FLR is significantly altered at the sharp azimuthal density gradient of the plume boundary, where the polarisation is intermediate with significant poloidal and toroidal components. These signatures are consistent with magnetohydrodynamic modeling results, providing the first observational evidence of a 3D FLR associated with a plume in Earth's magnetosphere. These results demonstrate the importance of cold plasma in controlling wave dynamics in the magnetosphere, and have important implications for wave‐particle interactions at a range of energies. Plain Language Summary: Earth's space environment is home to electrons and ions across a wide range of energies, trapped in the region by our global geomagnetic field. Energy can be transferred to and from the trapped particles through oscillations in the magnetic field, and these processes are responsible for the extreme energization of trapped electrons to hazardous levels for local spacecraft. In this paper we explore a type of magnetic field oscillation termed Field Line Resonances (FLRs): standing waves on a field line analogous to the oscillatory motion of guitar strings. We use spacecraft observations to show that the direction of the field line oscillations changes significantly depending on the density of the background plasma. The results confirm previous modeling work, and are the first observational evidence of 3D FLRs at a plume. The findings have important consequences for how FLRs transfer energy between the electrons and ions. Key Points: We present the first observational evidence of a 3D Field Line Resonance at the sharp density gradient of a plume edgeThe observed polarisation change confirms magnetohydrodynamic modeling results and predictions made by Elsden and Wright (2022)The presence of 3D Field Line Resonances during storm times has impacts for how Ultra Low Frequency waves couple and interact with local plasma [ABSTRACT FROM AUTHOR]

This paper is devoted to the 80th anniversary of the discovery of Alfven waves, which play an important role in physics, radiophysics, astrophysics, and Earth physics. The emphasis is on the ponderomotive redistribution of plasma in the Earth's magnetosphere under the action of Alfven and ion-cyclotron waves. At relatively small distances from the Earth, the ponderomotive force is buoyant, i.e., is directed upwards, regardless of whether an Alfven wave propagates towards the Earth or away from it. In the near-equatorial zone of the central regions of magnetosphere, waves in the Pc 1 range push the plasma to the minimum of geomagnetic field, so that a maximum of plasma density arises on the equator at sufficiently high wave intensity. A bifurcation occurs at the magnetosphere's periphery, and the maximum is split into two maxima, the distance between which increases while moving away from the Earth. The polar wind, acceleration of heavy ions, and fictitious nonlinearity of the surface impedance of the Earth's crust are also briefly discussed. [ABSTRACT FROM AUTHOR]

The magnetosphere is the outermost part of the Earth, formed by the interaction of the Earth's dipole magnetic field and the solar wind. Solar wind conditions depend on solar activity, which can affect space weather. One of the solar activities that significantly impact space weather is Coronal Mass Ejection (CME). The magnetosphere is observed using satellites in space-based and using a magnetometer on earth-based. However, these observations are limited to a specific location and time. In this work, we are interested in employing magnetohydrodynamics (MHD) to investigate the interaction of solar wind in the magnetosphere. The MHD has four equations: transfer of mass, momentum, magnetic, and thermal energy, which explain the four main parameters of the solar wind: density, velocity, magnetic field, and pressure, respectively. From these four parameters, the response of Earth's magnetosphere can be identified. Here, we used both analytical and numerical calculation via the SHASTA-FCT. The results show that the positive interplanetary magnetic field merges to Earth's magnetic field. However, the negative interplanetary magnetic field does not merge with Earth's magnetic field. We also observed that the higher solar wind speed results in the shorter simulation time. The bow shock as a result of the interaction of the solar wind and the Earth's magnetic field is formed at a distance of ∼ 50 , 000 km , and the magnetopause as a result of the equilibrium of the solar wind pressure and the pressure of the Earth's magnetic field has a thickness of ∼ 5000 km . In addition, we also discuss Alfvén velocity represents the motion of the magnetic field. [ABSTRACT FROM AUTHOR]

The Earth's magnetosphere supports a variety of Magnetohydrodynamic (MHD) normal modes with Ultra Low Frequencies (ULF) including standing Alfvén waves and cavity/waveguide modes. Their amplitudes and frequencies depend in part on the properties of the magnetosphere (size of cavity, wave speed distribution). In this work, we use ∼13 years of Time History of Events and Macroscale Interactions during Substorms satellite magnetic field observations, combined with linearized MHD numerical simulations, to examine the properties of MHD normal modes in the region L > 5 and for frequencies <80 mHz. We identify persistent normal mode structure in observed dawn sector power spectra with frequency‐dependent wave power peaks like those obtained from simulation ensemble averages, where the simulations assume different radial Alfvén speed profiles and magnetopause locations. We further show with both observations and simulations how frequency‐dependent wave power peaks at L > 5 depend on both the magnetopause location and the location of peaks in the radial Alfvén speed profile. Finally, we discuss how these results might be used to better model radiation belt electron dynamics related to ULF waves. Plain Language Summary: The solar wind constantly disturbs plasma in the near‐Earth space environment on a broad range of frequencies. However, plasma waves in the Earth's magnetosphere, a region of space where the Earth's magnetic field plays a dominant role in shaping plasma dynamics, often exhibit standing wave structure with a narrow range of frequencies. In other words, the magnetosphere selects standing waves with discrete frequencies from drivers with a broadband frequency spectrum. These standing waves have properties that depend on the size of the magnetosphere and plasma wave speeds. In this study, we use a database of magnetic field measurements from the Time History of Events and Macroscale Interactions during Substorms satellites along with numerical simulations to isolate natural frequencies from noisy and variable driving conditions and extract standing wave spatial structure. We show how the standing wave properties change as the outer boundary of the magnetosphere and internal wave speeds change. We finally discuss how the properties of these standing waves might be used to improve space weather models. Key Points: Magnetohydrodynamic normal modes are identified using ∼13 years of observations and comparisons with numerical simulationsRadial Alfvén speed profile peaks outside 8 Earth radii significantly alter frequencies and spatial structure of normal modesFrequencies and nodal structure of cavity/waveguide modes vary with magnetopause location, with power peaks well inside the magnetopause [ABSTRACT FROM AUTHOR]

We present a new model of the near‐Earth magnetospheric field produced by electric currents in the inner magnetosphere and the associated induced magnetic field. The model is designed to track hourly variations of these fields and accounts for their local time asymmetries. It is built by applying spherical harmonic analysis to vector measurements from the ground observatory network at low and mid‐latitudes. The primary and induced fields are separated with an approach in the time domain that uses a a priori radially‐symmetric electric conductivity model of the Earth. The model coefficients are computed at one‐hour time steps between 1997 and 2022. This model is shown to be consistent to within a few nT with previously developed indices which track the magnetospheric ring current. It is also validated against data from the Swarm, CHAMP and Øersted satellites. The fit to satellite data is comparable to that of the CHAOS‐7.15 model for geomagnetically quiet times, and improved by up to 20% on some components for geomagnetically moderate and active times. We attribute these differences mostly to a better representation of local time asymmetries, both on average and during individual geomagnetic storms. This model can be used in various applications, such as investigating the properties of the magnetospheric field and its sources and separating the magnetospheric field from the fields of other sources in geomagnetic field modeling. Plain Language Summary: Geomagnetic field modeling aims at building data‐based mathematical representations, or models, of the various contributions to the total Earth's magnetic field measured at or near the Earth's surface. One of such contributions is the magnetic field generated by electric currents in the inner magnetosphere, including the so‐called ring current. In this paper, we present a new model of the near‐Earth magnetic field generated in the inner magnetosphere based on data collected in ground magnetic observatories. The model covers the 1997–2022 time span, includes improved representations of the local time asymmetries of the field and of the effect of electrical induction in the Earth's mantle, and is validated against data collected in low Earth orbits by the Swarm, CHAMP and Oersted satellites. We find that the model provides an improved representation of the magnetic field generated in the inner magnetosphere during periods of moderate and high geomagnetic activity, including magnetic storms. Key Points: A new ground‐data based model of hourly variations of the primary inner near‐Earth magnetospheric and associated induced field is presentedComparison with satellite data shows up to 20% performance improvement compared to the CHAOS‐7 model for moderate and active timesThe model accounts for variations of the field with local time and can be used to study geomagnetic storms [ABSTRACT FROM AUTHOR]

The Earth's magnetotail at lunar distances (R ≈ 60 RE) serves as a unique laboratory to study plasma dynamics in a weak, highly fluctuating magnetic field with a strong magnetic field gradient. In particular, studies of the of quiet‐time plasma in the lunar‐distant magnetotail can inform us about plasma entry to the magnetosphere and sources of magnetospheric plasma populations. We use the data collected by the two lunar‐orbiting Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft during its 2013–2019 magnetotail traversals to infer the average thermodynamic and spectral properties of plasma populations in the quiet‐time (i.e., low geomagnetic activity and absence of fast plasma flows) magnetotail at lunar distances. We found that plasma temperature and density in the quiet‐time magnetotail at R ≈ 60 RE are organized by the magnetic field. Three distinct regions with plasma β ≫ 1, β ∼ 1, and β ≪ 1, the central plasma sheet (CPS), outer plasma sheet (OPS), and lobes are sampled. We found that temperatures and energy spectra of ion populations in CPS, OPS, and lobes regimes are different: the hotter CPS temperatures scale with the kinetic energy of solar wind protons; cold OPS/lobe ions are, likely, of ionospheric origin. The ion and electron particle spectra in CPS, OPS, and lobes are nonthermal and reasonably well fitted by the Kappa function, with κ exponent varying between 2.5 and 3.5. Plain Language Summary: Earth's only natural satellite, the Moon orbits around the Earth at a distance of about 239,000 miles (385,000 km). The two lunar‐orbiting Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft orbit around the Moon in stable equatorial, high‐eccentricity orbits, of 100‐km × 19,000‐km altitude. The Moon and the ARTEMIS satellites spend about 25 days in the solar wind and 3–4 days inside the Earth's magnetosphere in the magnetotail. Using the comprehensive instrumentation of the ARTEMIS satellites, we study properties of the plasma and the magnetic field in the Earth's magnetotail at lunar distances. Our statistical results are valuable for the NASA Lunar Gateway HERMES mission preparation. Key Points: We examine the properties of plasma populations in the magnetotail at R ≈ 60 RE under quiet conditionsNon‐Maxwellian ion and electron populations dominate at βi ≤ 1 and βi > 1 regimesAverage ion and electron equatorial temperatures scale with solar wind kinetic energy [ABSTRACT FROM AUTHOR]

Universal Time (UT) variations in many magnetospheric state indicators and indices have recently been reviewed by Lockwood and Milan (2023, https://doi.org/10.3389/fspas.2023.1139295). Key effects are introduced into magnetospheric dynamics by the eccentric nature of Earth's magnetic field, features that cannot be reproduced by a geocentric field model. This paper studies the UT variation in the occurrence of substorm onsets and uses a simple Monte‐Carlo model to show how it can arise for an eccentric field model from the effect of the diurnal motions of Earth's poles on the part of the geomagnetic tail where substorms are initiated. These motions are in any reference frame that has an X axis that points from the center of the Earth to the center of the Sun and are caused by Earth's rotation. The premise behind the model is shown to be valid using a super‐posed epoch study of the conditions leading up to onset. These studies also show the surprising degree of preconditioning ahead of the growth phase that is required, on average, for onset to occur. A key factor is the extent to which pole motions caused by Earth's rotation influence the near‐Earth tail at the relevant X coordinate. Numerical simulations by a global MHD model of the magnetosphere reveal the effect required to generate the observed UT variations and with right order of amplitude, albeit too small by a factor of about one third. Reasons why this discrepancy may have arisen for the simulations used are discussed. Plain Language Summary: Earth's magnetic field is eccentric in that the main magnetic (dipole) axis does not pass through the center of the Earth. This introduces a wobble into many aspect of near‐Earth space (the "magnetosphere") as Earth rotates. Many consequences of this have been noted in previous papers. This paper investigates the effect of the eccentricity on the phenomenon of magnetospheric substorms. It is shown that the explosive releases of energy stored in the tail of the magnetosphere are more likely to start ("onset") at some Universal Times (and therefore geographic longitudes) than others and an explanation of why is provided. Key Points: Universal Time (UT) effects in the magnetosphere are caused by the eccentric nature of Earth's intrinsic magnetic fieldThere is a UT dependence of the open flux (and hence also the integrated magnetopause reconnection voltage) needed to trigger substorm onsetGrowth phases that lead to substorm onset show considerable preconditioning by prior reconnection [ABSTRACT FROM AUTHOR]

Although many substorm‐related observations have been made, we still have limited insight into propagation of the plasma and field perturbations in Pi2 frequencies (∼7–25 mHz) in association with substorm aurora, particularly from the auroral source region in the inner magnetosphere to the ground. In this study, we present conjugate observations of a substorm brightening aurora using an all‐sky camera and an inner‐magnetospheric satellite Arase at L ∼ 5. A camera at Gakona (62.39°N, 214.78°E), Alaska, observed a substorm auroral brightening on 28 December 2018, and the footprint of the satellite was located just equatorward of the aurora. Around the timing of the auroral brightening, the satellite observed a series of quasi‐periodic variations in the electric and magnetic fields and in the energy flux of electrons and ions. We demonstrate that the diamagnetic variations of thermal pressure and medium‐energy ion energy flux in the inner magnetosphere show approximately one‐to‐one correspondence with the oscillations in luminosity of the substorm brightening aurora and high‐latitudinal Pi2 pulsations on the ground. We also found their anti‐correlation with low‐energy electrons. Cavity‐type Pi2 pulsations were observed at mid‐ and low‐latitudinal stations. Based on these observations, we suggest that a wave phenomenon in the substorm auroral source region, like ballooning type instability, play an important role in the development of substorm and related auroral brightening and high‐latitude Pi2, and that the variation of the auroral luminosity was directly driven by keV electrons which were modulated by Alfven waves in the inner magnetosphere. Plain Language Summary: One of the most frequent disturbances in the Earth's magnetosphere is called substorm. Through Earth's magnetic field line, the development of a magnetospheric substorm is projected onto Earth's high‐latitude ionosphere as auroral evolution. During a substorm, the energy in the tail of the magnetosphere is released to the earth, and quite a part of this energy is transmitted as different types of wave phenomena. These wave oscillations can be observed by magnetometers on the ground and satellites in the magnetosphere. It is still uncertain what mechanisms trigger these substorm‐related waves, how these waves correlate with other substorm phenomena, and how they transmit and dissipate, especially for the wave phenomena at ∼4–7 Earth radii away from the Earth. This paper presents a case study of such substorm event, where we show the variation of magnetic field and auroral luminosity on the ground, and the electromagnetic wave that is called as Pi2 pulsations and oscillation of plasma flux in the inner magnetosphere. We demonstrate that there are one‐to‐one correspondences in these oscillations. We suggest the possibility that a ballooning plasma instability triggers the observed oscillations in particle flux, as well as the electromagnetic wave that modulates the auroral luminosity. Key Points: Observations of plasma and field characteristics near the source region of a substorm auroral brightening at L ∼ 5Pi2, auroral luminosity variation and fluctuations of in‐situ electromagnetic field and particle flux were observed during substorm onsetCorrespondence between these fluctuations suggest waves in the magnetosphere like ballooning instability control auroral intensity [ABSTRACT FROM AUTHOR]

Global ultra‐low frequency (ULF) oscillations are believed to play a significant role in the mass, energy, and momentum transport within the Earth's magnetosphere. In this letter, we observe a ∼1.2 mHz radial standing wave in the dusk‐sector magnetosphere accompanied by the field line resonance (FLR) on 16 July 2017. The frequency estimation from the simple box model also confirms the radial standing wave. The essential characteristics of FLR are concurrently identified at the dusk‐sector magnetosphere and the conjugated ground location. Further, the radial standing wave dissipates energy into upper atmosphere to enhance the local aurora by coupling itself to the FLR. The magnetospheric dominant 1.2/1.1 mHz ULF waves plausibly correspond well with the discrete ∼1 mHz magnetosheath ion dynamic pressure/velocity oscillation, suggesting this radial standing wave and FLR in the flank magnetosphere may be triggered by the solar‐wind and/or magnetosheath dynamic pressure/velocity fluctuations. Plain Language Summary: Just like thumping the strings, the Earth's magnetic field line can also be disturbed by the external impulses or internal instability, generating the rich ultra‐low frequency (ULF) oscillations with period of 1–1,000 s. They are believed to play a significant role in the mass, energy, and momentum transport within the Earth's magnetosphere. But the question of how an external driving mechanism can produce field line disturbances deep inside the magnetosphere is a topic of much debate. One classical global mode resonance model suggests that the magnetospheric space sometimes acts as a huge closed or semi‐closed cavity, where the ULF waves form the radial standing wave structure. So far, the observational evidence of radial standing waves with period of above 8 min accompanied by the field line disturbance in the flank magnetosphere is sparse. In this letter, we report a radial standing wave with period of ∼14 min within dusk‐sector magnetosphere accompanied by the field line disturbance phenomenon by multiple‐satellite observations. The radial standing wave energy sinks down to upper atmosphere to light the local aurora by coupling itself to the field line resonance. We conclude the radial standing wave may be caused by the solar‐wind and/or magnetosheath dynamic pressure/velocity fluctuations. Key Points: A radial standing Pc5‐6 ultra‐low frequency wave is identified within the dusk‐sector magnetosphereRadial standing wave dissipates energy into upper atmosphere by coupling itself to the field line resonanceSolar‐wind and/or magnetosheath dynamic pressure/velocity fluctuations possibly trigger this radial standing wave [ABSTRACT FROM AUTHOR]

Magnetic storms and substorms cause global disturbances in the Earth's magnetosphere. Plasma clouds injected from the magnetotail during storm or substorm drift around the Earth and generate ultra‐low frequency (ULF) waves via various mechanisms. At the same time, the inner part of the magnetosphere called plasmasphere is filled with cold particles and its characteristics are sensitive to the geomagnetic activity level. Previous theoretical and some observational studies suggested plasmasphere and its boundary, plasmapause, are special regions for ULF waves to interact with charged particles. We present a statistical analysis of ULF waves during different geomagnetic conditions. We utilized Arase satellite magnetic field and electron density measurements from March 2017 to December 2020 to investigate spatial distribution of ULF waves and its dependence on the plasmapause location. A 1–2 RE gap between the plasmapause and a region of high transverse waves occurrence rate was found. This gap keeps during quiet geomagnetic conditions when plasmasphere expands, and we concluded that the plasmapause controls the ULF wave distribution in the magnetosphere. ULF wave occurrence rate significantly decreases at quiet time, but dayside and dawnside maxima still occur for poloidal and compressional, and toroidal waves, respectively. Thus, we can distinguish internally and externally excited waves. Average wave frequency distribution revealed field‐line resonance character of toroidal waves as frequency increases toward the Earth. Poloidal and compressional waves distributions clearly distinguish low frequency externally excited waves and high frequency storm‐time pulsations. Plain Language Summary: Plasma environment and magnetic field inside the magnetosphere are very dynamic and strongly depend on geomagnetic activity level. Magnetic storms and substorms affect all the processes and systems, including the plasmasphere formation and ultra‐low frequency (ULF) wave generation. The plasmasphere is a dense inner part of the magnetosphere filled with cold protons and electrons, while ULF waves are responsible for energy transfer on a large distance and intensively interact with charged particles. We used 46 months statistics of Arase satellite measurements and found a clear dependence between ULF waves observations and the plasmasphere boundary location. ULF waves are mostly observed in 1–2 Earth radii from the plasmasphere boundary even when it moves outward during long period of quiet geomagnetic conditions. Although ULF waves rare occur at quiet geomagnetic conditions, they still appear at the dayside and dawnside magnetosphere. We think, these waves are excited by external sources from the solar wind or magnetosphere boundary—magnetopause. This conclusion is supported by wave frequency analysis showing a clear difference between lower frequency dayside waves and higher frequency nightside waves, which are associated with external and internal sources, respectively. Thus, we found distinct signatures of plasmasphere control of ULF wave spatial distribution. Key Points: Poloidal and toroidal waves are detected mostly outside the plasmasphere with 1–2 Earth radii from itPlasmasphere expansion and shrinkage impact the ultra‐low frequency wave distribution at the dusk and night sectors of the magnetosphereExternally excited dayside waves usually have lower frequencies than nightside waves excited by internal instabilities [ABSTRACT FROM AUTHOR]

This paper studies extremely low frequency (ELF) whistler‐mode waves' behavior within small‐scale magnetic field irregularities in the Earth's magnetosphere, known as magnetic ducts. Based on the magnetic fields' magnitude inside and outside these ducts, they are categorized as high‐magnetic ducts and low‐magnetic ducts (LBD). Using the whistler‐mode dispersion relation analysis, our primary focus is to show that LBDs are prone to leak electromagnetic energy outside the duct. We further investigate the hypothesis that whistlers can propagate within LBDs without any signal loss when the width of the duct corresponds to an integer multiple of the perpendicular wavelengths of the waves inside it. This condition offers a straightforward and effective method for identifying non‐leaking eigenmodes of LBDs. Our analysis of this non‐leaking condition reveals that every LBD possesses a finite number of non‐leaking eigenmodes directly proportional to the duct's width and the magnitude of the ambient magnetic field within it. The analytical results are then validated using two‐dimensional, time‐dependent simulations of the electron‐Magnetohydrodynamics model. Also, we model the non‐leaking propagation of an ELF whistler‐mode wave observed inside the LBD by the NASA Magnetospheric Multiscale mission satellites. Key Points: Magnetospheric Multiscale mission satellites observe whistler‐mode waves inside the low‐magnetic ducts (LBD)We provide an analytical explanation for the occurrence of leaking ducts and show that the LBD are prone to leakSimulations confirm our analytical prediction of the most trapped extremely low frequency wave in the LBD, provided by least degree of leakage [ABSTRACT FROM AUTHOR]

Since the discovery of the large‐scale field‐aligned currents it is widely acknowledged that gaps exist between the Region 1 (R1) and Region 2 (R2) currents in which the current values are relatively small as compared to neighboring regions. Assuming that the field‐aligned currents are generated by plasma pressure gradients, we analyzed data collected by the THEMIS satellites between 2007 and 2011 to identify regions with very low plasma pressure gradients (pressure plateaus), which could be responsible for the appearance of these gaps. It was found that the pressure profiles with low radial gradients are typically located between 8 and 10 Radii around the Earth. Projections of pressure plateau regions onto ionospheric altitudes, for both individual events and on a statistical basis, coincide with the locations of gaps between Iijima and Potemra field‐aligned currents. The role played by identified pressure plateaus in shaping the pattern of large‐scale field‐aligned currents is discussed. Plain Language Summary: Field‐aligned currents that flow between the magnetosphere and ionosphere along magnetic field lines play a crucial role in magnetosphere‐ionosphere interactions. Despite being discovered at the start of the space era, their origins are still subject to debate. In our current study, we have established that the observed gaps between the upward and downward currents correspond to the plateau regions of constant plasma pressure, as obtained using data from the THEMIS mission. This finding provides strong evidence in favor of the generation of field‐aligned currents by plasma pressure gradients in the transition region between the dipole and tail‐like geomagnetic field. This is a critical piece of information for understanding the global dynamics of Earth's magnetosphere. Key Points: During quiet geomagnetic conditions, the regions of plasma pressure plateau are observed in the inner magnetosphereThese regions are located at geocentric distances of 8–10 RE forming a ring surrounding the EarthThe localization of the plasma pressure plateau region and gaps between the Region 1 and 2 currents should be located at the same distance [ABSTRACT FROM AUTHOR]

Auroral kilometric radiation (AKR) is a terrestrial radio emission, excited by the same accelerated electrons which excite auroral emissions. Although it is well correlated with auroral and geomagnetic activity, the coupling timescales between AKR and different magnetospheric/ionospheric regions are yet to be determined. Estimation of these coupling timescales is non-trivial as a result of complex, non-linear processes which rarely occur in isolation. In this study, the mutual information between AKR intensity and different geomagnetic indices is used to assess the correlation between variables. Indices are shifted to different temporal lags relative to AKR intensity, and the lag at which the variables have the greatest shared information is found. This lag is interpreted as the coupling timescale. The AKR source region receives the effects of a shared driver before the auroral ionosphere. Conversely, the polar ionosphere reacts to a shared driver before the AKR source region. Bow shock IMF BZ is excited about an hour before AKR enhancements. [ABSTRACT FROM AUTHOR]

In this study, we have investigated the effect of supersubstorm of September 7 and 8, occurred during magnetic storm of September 7–8, 2017 on horizontal component of geomagnetic field (H) upon the high latitude (ABK, FCC, MGD and UPS), middle latitude (ABG, BOU, DUR and PHU) and low latitude (API, GUA, KOU, and MBO) stations. From these different stations, complexities in the characteristics of H component were identified, showing diverse effects during the supersubstorm. To characterize the common features associated with various intrinsic processes for transfer of energy during the event, the continuous wavelet technique (CWT) and cross correlation analysis are used. The CWT identified a significant energy pattern in the signal of high spectral variability from time scale 1 to 2 minutes during 24 and 37 hours. The cross correlation analysis suggests that the perturbation in H component at all latitude strongly correlates with FAC, PCV, TE, AP, JH and RC respectively. This study helps to understand energy dynamics, potential and current within the magnetosphere, in the form of FAC, PCV, RC, JH, AP and TE during SSSs. [ABSTRACT FROM AUTHOR]

The off‐equatorial minimum‐B‐pocket (OEMBP) at high latitudes in the dayside outer magnetosphere is expected as another source region of the electrostatic electron cyclotron harmonic (ECH) waves, in addition to the magnetic equator. However, its crucial role in the excitation of the dayside ECH waves has never been directly investigated due to its spatiotemporal‐varying and difficulty to be located. Based on the observations from Magnetospheric Multiscale satellites, we statistically analyze the properties of dayside ECH waves by organizing them according to the latitude distance from the OEMBP located by the spatiotemporal‐varying geomagnetic field model. Such mapping confirms larger occurrence rates and electric field amplitudes of dayside ECH waves in the vicinity of the OEMBP, which gradually fall down nearby with the latitude distance increasing. Our results provide statistical evidence for the OEMBP as another source region of ECH waves besides the magnetic equator. Plain Language Summary: The off‐equatorial minimum‐B‐pocket is the local minimum of magnetic field strength at high latitude in the dayside outer magnetosphere, which is expected as another source region of the electrostatic electron cyclotron harmonic (ECH) waves besides the magnetic equator. However, its importance for the dayside ECH waves at high latitudes has never been directly investigated since the off‐equatorial minimum‐B‐pocket is spatiotemporal‐varying and is difficult to be located. We use the solar wind‐dependent geomagnetic field model to locate the off‐equatorial minimum‐B‐pocket along the field line threading the observational point, and subsequently introduce the latitude distance to quantify the distance from the off‐equatorial minimum‐B‐pocket. Our results prove that dayside ECH waves in the outer magnetosphere are more frequent and stronger in the vicinity of the off‐equatorial minimum‐B‐pocket, demonstrating that it is indeed a source region of ECH waves. Key Points: Dayside electron cyclotron harmonic (ECH) waves on bifurcated magnetic field lines are statistically analyzed according to their latitude distance from minimum‐B‐pocketThese dayside ECH waves have larger occurrence rates and amplitude in the vicinity of the off‐equatorial minimum‐B‐pocketThe occurrence and amplitude of these dayside ECH waves are asymmetric relative to the off‐equatorial minimum‐B‐pocket [ABSTRACT FROM AUTHOR]

In this study employing data from the Swarm satellites at an altitude of ≈500 km, we present evidence of field‐line resonances (FLRs) in the Pc2‐3 (0.02–0.2 Hz) frequency band in the undisturbed post‐sunset region of the equatorial ionosphere. We identify 26 events extending to magnetic latitudes as low as 2°, which was previously thought to be too low for FLRs to exist due to ionospheric damping. Identification of FLRs is supported by a narrow‐band fundamental and first harmonics, an oscillating field‐aligned Poynting vector, a relative phase shift between E and δB of ≈90°, and in one case observation of one wave in conjugate hemispheres by two satellites separated by 4 min. We show that, contrary to the non‐uniform trend of ground‐based observations, the fundamental frequencies of these occurrences decline monotonically with increasing field‐line length. Their frequencies differ from those of ground‐based magnetometer observations reported in the literature. Plain Language Summary: Ultra‐low‐frequency (ULF) waves play a crucial role in the coupling of the magnetosphere‐ionosphere system. These waves have been reported abundantly in the high latitude ionosphere, where they are driven directly by processes in the magnetosphere and solar wind. ULF waves have also been reported at the equatorial ionosphere, but less frequently. In some cases, low‐latitude waves are thought to be driven by high‐latitude waves ducting through the ionosphere or by compressional waves, which can propagate across geomagnetic field lines from high altitudes in the equatorial region. These waves can then excite interhemispheric field‐line resonances (FLRs), in which Earth's magnetic field lines vibrate like a guitar string, with longer field lines oscillating at lower frequencies. In this work, we use the Swarm satellites to study low‐latitude FLRs at an altitude of ≈500 km in the quiet ionosphere at geomagnetic latitudes below 3°. We demonstrate the existence of FLRs with periods between 5 and 50 s which increase with magnetic latitude, as expected when field line lengths increase. Key Points: Field line resonances (FLRs) are identified at the lowest magnetic latitude ever reportedThe fundamental frequency of our events decreases with increasing magnetic latitude in a uniform trendFLRs detected at conjugate latitudes by two spacecraft at the equatorial ionosphere [ABSTRACT FROM AUTHOR]

Understanding global space weather effects is of great importance to the international scientific community, but more localized space weather predictions are important on a national level. In this study, data from a ground magnetometer at Valentia Observatory is used to characterize space weather effects on the island of Ireland. The horizontal component of magnetometer observations and its time derivative are considered, and extreme values of these are identified. These extremes are fit to a generalized extreme value distribution, and from this model return values (the expected magnitude of an observation within a given time window) are predicted. The causes of extreme values are investigated both in a case study, and also statistically by looking at contributions from geomagnetic storms, substorms, and sudden commencements. This work characterizes the extreme part of the distribution of space weather effects on Ireland (and at similar latitudes), and hence examines those space weather observations which are likely to have the greatest impact on susceptible technologies. Plain Language Summary: Space weather describes the effects of the interaction between the Sun's solar wind and the Earth's magnetic field, and can have dramatic impacts on human technology. Space weather can be characterized both globally, and on a local level. In this study, space weather over the island of Ireland is characterized using an Irish ground based magnetometer at Valentia Observatory. Magnetometers record a measure of the magnetic field, and these changes will be large when space weather effects are enhanced. Extreme observations from this magnetometer are detected, and these are used to predict future extreme observations. Additionally, the contributions of different types of space weather phenomena to extreme observations are determined. Understanding extreme observations on a national level will help to enhance space weather resilience which is increasingly important as society becomes more and more dependent on technology. Key Points: More extreme Valentia observations are observed in the premidnight sectorReturn values of extreme Valentia observations at given return periods are predictedGeomagnetic storms contribute more extremes than substorms or sudden commencements [ABSTRACT FROM AUTHOR]

By using the magnetic field data from Van Allen Probes, we analyzed the distribution characteristics of the electromagnetic environment in the inner magnetosphere on different Dst* index and magnetic local time. Our results show that for the response of different current systems, the dawn‐dusk and noon‐midnight asymmetry distribution of the residual magnetic field δB increases with Dst* index. When Dst* < −60 nT, a "banana"‐shaped geomagnetic field negative disturbance peak region appears in the sector from midnight to dusk. Then, we obtained the azimuthal current density and found the asymmetric internal eastward and external westward ring current. Through the vector analysis of three‐dimensional current density, the current density vector distribution in the magnetic equatorial plane is completely displayed for the first time, which directly proves the existence of banana current near r = 3.0–4.0 RE during strong geomagnetic storms. Plain Language Summary: By using the magnetic field data from Van Allen Probes, we find that the electromagnetic environment of the Earth's inner magnetosphere exhibits different distributions on different geomagnetic disturbance strengths. During the occurrence of strong magnetic disturbances, the residual magnetic field δB will exhibit distinct dawn‐dusk and noon‐midnight asymmetric distributions. And a "banana"‐shaped peak region of negative geomagnetic field perturbation will appear in the midnight to dusk sector, while both the eastward and westward ring currents exhibit asymmetry. The presence of the banana current near r = 3.0–4.0 RE during strong geomagnetic storms is then directly demonstrated by vector analysis of the 3D current density. Key Points: The negative perturbation response of space currents to the geomagnetic field in the inner magnetosphere is analyzedBoth the eastward ring current and westward ring current is asymmetricThe vector distribution diagram of banana current density is obtained [ABSTRACT FROM AUTHOR]

During the Earth's magnetic reversal, the dipole component of the magnetic field weakens, and the non‐dipole component becomes dominant, resulting in a far more complex magnetospheric topology than that of a dipole. In this study, we used a particle tracing technique to investigate the motion of ions within an irregular magnetosphere during the Matuyama‐Brunhes magnetic polarity reversal. Compared to the scenario in which the geomagnetic field is dominated by a dipole component, earthward‐moving particles can be hardly "trapped" in the inner magnetosphere when the geomagnetic field experiences the polarity reversal, and particles can directly precipitate into the Earth's atmosphere on a global scale. It suggests that under an irregular magnetospheric configuration, the traditional trapped region of particles (e.g., radiation belt or ring current) no longer exists. Plain Language Summary: During the Earth's paleomagnetic reversal, the strength of the Earth's magnetic field was approximately 10% of its current value, accompanied by a weakening of the dipole component and an enhancement of the non‐dipole component. Under the superposition of this non‐axisymmetric multipole magnetic field, the magnetospheric structures may undergo significant changes. How charged particles travel in such an irregular magnetosphere and how different their trajectories are compared to the well‐known textbook scenario are still unknown. In this study, by utilizing global MHD simulations and test‐particle tracing techniques, we trace the charged particle's trajectory within the magnetosphere during the Matuyama‐Brunhes magnetic polarity reversal. We found that under an irregular magnetosphere in the middle stage of the geomagnetic reversal, particles in the inner magnetosphere cannot be constantly trapped around the Earth. The traditional trapped region (e.g., radiation belt or ring current) no longer exists. Particles moving Earthward do not gain the same acceleration effect as those under the present‐day magnetic field topology, resulting in a globally distributed pattern of particle precipitation. These differences can affect the global energy deposition and particle distribution in near‐Earth space. Key Points: We simulated charged particles' trajectories in the magnetosphere during the Matuyama‐Brunhes reversalParticles cannot be "trapped" in an irregular magnetosphereThe irregular geomagnetic field results in global particle precipitation [ABSTRACT FROM AUTHOR]