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The solar wind is a continuous outflow of charged particles from the Sun's atmosphere into the solar system. At Earth, the solar wind's outward pressure is balanced by the Earth's magnetic field in a boundary layer known as the magnetopause. Plasma density and temperature differences across the boundary layer generate the Chapman‐Ferraro current which supports the magnetopause. Along the dayside magnetopause, magnetic reconnection can occur in electron diffusion regions (EDRs) embedded into the larger ion diffusion regions (IDRs). These diffusion regions form when opposing magnetic field lines in the solar wind and Earth's magnetic field merge, releasing magnetic energy into the surrounding plasma. While previous studies have given us a general understanding of the structure of the diffusion regions, we still do not have a good grasp of how they are statistically differentiated from the non‐diffusion region magnetopause. By investigating 251 magnetopause crossings from NASA's Magnetospheric Multiscale (MMS) Mission, we demonstrate that EDR magnetopause crossings show current densities an order of magnitude higher than regular magnetopause crossings—crossings that either passed through the reconnection exhausts or through the non‐reconnecting magnetopause, providing a baseline for the magnetopause current sheet under a wide range of driving conditions. Significant current signatures parallel to the local magnetic field in EDR crossings are also identified, which is in contrast to the dominantly perpendicular current found in the regular magnetopause. Additionally, we show that the ion velocity along the magnetopause is highly correlated with a crossing's location, indicating the presence of magnetosheath flows inside the magnetopause. Plain Language Summary: The magnetopause is a dynamic boundary layer created through the interaction of the solar wind with Earth's magnetic field. This boundary is supported by a current sheet and acts as the "entry gate" of the solar wind's energy into the magnetosphere through a process called magnetic reconnection where energy previously stored in the magnetic field is released into the surrounding magnetopause plasma. The reconnection process is initiated in localized diffusion regions, which form in the magnetopause's current sheet during specific solar wind conditions. In this paper, we clarify what makes the diffusion regions stand out from the background magnetopause current sheet by utilizing data from NASA's Magnetospheric Multiscale mission. Our analysis reveals that the diffusion regions have stronger currents than the background magnetopause and that a significant portion of this current becomes parallel to the local magnetic field. Key Points: Electron diffusion region (EDR) crossings show current densities an order of magnitude higher than regular magnetopause crossingsEDR crossings contain significant current components parallel to the local magnetic fieldIon velocity along the magnetopause is highly correlated with an event's location—indicating the presence of magnetosheath flows [ABSTRACT FROM AUTHOR]

A quantitative model of the magnetosheath (MS) magnetic structure is developed, using a multi‐year set of Geotail, Themis, Cluster, and MMS magnetometer and plasma instrument data. The MS database is created using an identification algorithm, based on observed magnetic field magnitudes and proton densities, normalized by their concurrent interplanetary values, followed by additional filtering with the help of standard bow shock (BS) and magnetopause (MP) models. The model architecture is based on the toroidal/poloidal formalism and a coordinate system that naturally accounts for the tailward flaring of both boundaries. The magnetic field expansions include 960 free coefficients, derived by fitting the model to a grand data set, split into independent training and validation subsets with 1,291,380 and 411,933 1‐min records, respectively. The model faithfully reproduces basic types of the interplanetary magnetic field (IMF) wrapping around the MP. Regular IMF sectors result in strongly dawn‐dusk asymmetric draping, with much larger magnitudes at the quasi‐perpendicular dusk side of BS, and weaker at the quasi‐parallel dawn side, where the MS field lines are bent and dragged tailward. Except in the case of the flow‐aligned IMF orientation, the subsolar field steadily grows toward the MP, and the effect is clearly IMF Bz‐dependent: the field and its gradient are larger (smaller) for northward (southward) IMF Bz, implying a pile‐up of the magnetic flux in the first case and stronger reconnection in the second. Model distributions of the MS field magnitude reveal local depressions, associated with polar cusps near the high‐latitude limits of data coverage. Plain Language Summary: The terrestrial magnetosheath is a relatively wide transition region, separating our planet's magnetosphere from the undisturbed flow of magnetized solar wind. Due to the sudden compression of the incoming plasma flow at the bow shock and high conductivity of the magnetosheath medium, the relatively weak magnetic field of interplanetary origin undergoes abrupt compression and drapes around the magnetosphere boundary, the magnetopause. Huge amounts of archived spacecraft data accumulated in the world data centers during past decades of space flight made it possible to develop quantitative models of the magnetosheath magnetic field, based on direct in situ observations. This paper presents first results of such a modeling study, providing a closed analytical representation of the magnetosheath magnetic field, driven by input from upstream monitors of the interplanetary medium. Key Points: A closed empirical model of the magnetosheath magnetic field is developed, based on satellite data and a flexible mathematical architectureDriven by interplanetary parameters and dipole tilt, the model faithfully reproduces basic features of the IMF draping around the magnetopauseNorthward (southward) IMF results in higher (lower) subsolar fields, indicating magnetic flux pile‐up (stronger reconnection), respectively [ABSTRACT FROM AUTHOR]

Reconnection at Earth's magnetopause drives magnetospheric convection and provides mass and energy input into the magnetosphere/ionosphere system thereby driving the coupling between solar wind and terrestrial magnetosphere. Despite its importance, the factors governing the location of dayside magnetopause reconnection are not well understood. Though a few models can predict X‐line locations reasonably well, the underlying physics is still unresolved. In this study we present results from a comparative analysis of 274 magnetic reconnection events as observed by the Magnetospheric Multiscale (MMS) mission to determine what quantities affect the accuracy of such models and are most strongly associated with the occurrence of dayside magnetopause reconnection. We also attempt to determine under what upstream solar wind conditions each global X‐line model becomes least reliable. Plain Language Summary: Magnetic reconnection is an energy transferring process that happens at the Earth's magnetopause, the boundary between Earth's local plasma environment and the Sun's interplanetary environment. Reconnection helps move energy and particles into the region of space surrounding the earth called the magnetosphere. Dayside reconnection is important because it is a defining element to the amount of energy deposited into our magnetosphere. Despite its importance, scientists still do not completely understand how it works and where it happens. This study looks at many observations of magnetic reconnection to try and understand its precise location and figure out under which conditions it is more likely to occur. This paper compares in situ data with different models for predicting reconnection locations. Key Points: Compared four different reconnection line modelsOn average, the Maximum Bisection Field Model gives the best prediction of reconnection line locationThe Maximum Exhaust Velocity model gives the worst prediction of the reconnection line location [ABSTRACT FROM AUTHOR]

During its polar orbits around Jupiter, Juno often crosses the boundaries of the Jovian magnetosphere (namely the magnetopause and bow shock). From the boundary locations, the upstream solar wind dynamic pressure can be inferred, which in turn illustrates the state of compression or relaxation of the system. The aim of this study is to examine Jovian radio emissions during magnetospheric compressions, in order to determine the relationship between the solar wind and Jovian radio emissions. In this paper, we give a complete list of bow shock and magnetopause crossings (from June 2016 to August 2022), and the associated solar wind dynamic pressure and standoff distances inferred from Joy et al. (2002, https://doi.org/10.1029/2001JA009146). We then select two sets of magnetopause crossings with moderate to strong compression of the magnetosphere for two case studies of the response of the Jovian radio emissions. We confirm that magnetospheric compressions lead to the activation of new radio sources. Newly activated broadband kilometric emissions are observed almost simultaneously with compression of the magnetosphere, with sources covering a large range of longitudes. Decametric emission sources are seen to be activated more than one rotation later only at specific longitudes and dusk local times. Finally, the activation of narrowband kilometric radiation is not observed until the magnetosphere is in its expansion phase. Key Points: This paper provides a list of the Jovian magnetosphere boundary crossings by the Juno spacecraft from June 2016 to August 2022Jovian magnetospheric compressions lead to increased bKOM radio emissions (immediately) and DAM on the dusk sector (more than one rotation later)nKOM radio emission appears later during relaxation phase of the compression [ABSTRACT FROM AUTHOR]

Mercury possesses a miniature but dynamic magnetosphere driven primarily by the solar wind through magnetic reconnection. A prominent feature of the dayside magnetopause reconnection that has been frequently observed is flux transfer events (FTEs), which are thought to be an important player in driving the global convection at Mercury. Using the BATSRUS Hall magnetohydrodynamics model with coupled planetary interior, we have conducted a series of global simulations to investigate the generation and characteristics of FTEs under different solar wind Alfvénic Mach numbers (MA) and interplanetary magnetic field (IMF) orientations. An automated algorithm was also developed to consistently identify FTEs and extract their key properties from the simulations. In all simulations driven by steady upstream conditions, FTEs are formed quasi‐periodically with recurrence time ranging from 2 to 9 s, and their characteristics vary in time as they evolve and interact with the surrounding plasma and magnetic field. Our statistical analysis of the simulated FTEs reveals that the key properties of FTEs, including spatial size, traveling speed and core field strength, all exhibit notable dependence on the solar wind MA and IMF orientation, and the trends identified from the simulations are generally consistent with previous MErcury Surface Space ENvironment, GEochemistry, and Ranging observations. It is also found that FTEs formed in the simulations contribute about 3%–13% of the total open flux created at the dayside magnetopause that participates in the global circulation, suggesting that FTEs indeed play an important role in driving the Dungey cycle at Mercury. Key Points: 3D global Hall magnetohydrodynamics simulations and an automated identification algorithm are developed to study flux transfer event formation and associated dynamics at MercuryProperties of simulated FTEs agree well with MErcury Surface Space ENvironment, GEochemistry, and Ranging (MESSENGER) observations and exhibit clear dependence on solar wind MA and interplanetary magnetic field orientationFTEs make a significant contribution to the open flux generation in Mercury's magnetosphere, consistent with previous MESSENGER findings [ABSTRACT FROM AUTHOR]

The solar wind ion transport across the magnetopause is one of the main sources of plasma for the Earth's magnetotail. Such a transport is supported by various dynamic processes at the flank magnetopause, where wave‐particle interactions play a crucial role in ion flow thermalization and diffusion across magnetic field surfaces of the magnetopause tangential discontinuity. In this paper we numerically model such ion thermalization by the most intense electromagnetic waves observed in the magnetosheath, kinetic Alfven waves. We aim to develop an approach for long‐term simulations of ion scattering by waves and ion dynamics around realistic magnetopause magnetic fields. This approach is based on a combination of test particle simulations and stochastic differential equations modeling ion diffusion in velocity space. We demonstrate that for realistic magnetopause configuration and wave characteristics, the magnetosheath ion flow can be substantially thermalized around the magnetopause. This result explains observations of ion energy conservation across the flank magnetopause: kinetic and thermal energies of flowing magnetosheath ions approximately equal to the thermal energy of stagnant magnetospheric ions. Key Points: We model the ion flow thermalization by kinetic Alfven waves around the flank magnetopauseThe model uses stochastic differential equations for particle trajectories to include effects of wave‐particle interactionsScattering and thermalization may explain ion energy conservation across the magnetopause [ABSTRACT FROM AUTHOR]

Tracking the location and motion of the bow shock and magnetopause can reveal many aspects concerning the nature of the solar wind‐magnetosphere interaction. In the absence of missions that image the magnetopause, we propose to detect and track the magnetopause and bow shock motion using multipoint in situ observations of the magnetosheath. Making theoretical assumptions corroborated by numerical simulations, Sibeck et al. (2022, https://doi.org/10.1029/2022JA030704) proposed a method to predict the subsolar magnetopause and the bow shock from the linear variation in magnetosheath plasma velocity expected from global magnetohydrodynamic simulations along the Sun‐Earth line. This paper presents a case study that employs the velocity gradient method using multi‐point THEMIS plasma observations from 21 to 22 June 2007. Using the five THEMIS spacecraft we demonstrate that the radial gradient in plasma velocity remains nearly linear across the subsolar magnetosheath, except when the magnetosheath lies behind the quasi‐parallel bow shock and conditions are very turbulent. The bow shock and magnetopause move outward in response to a gradual decrease in solar wind dynamic pressure. We show that results from the velocity gradient method agree well with predictions of global magnetohydrodynamic and empirical models for this time interval. The velocity gradient method can therefore be used to remotely determine the location of the magnetopause and bow shock, solely based on two‐spacecraft measurements at the subsolar magnetosheath when not downstream of the quasi‐parallel shock. Key Points: Plasma velocities diminish linearly from ∼0.25 VSW inside the quasi‐perpendicular subsolar bow shock to 0 outside the subsolar magnetopauseSimultaneous two‐point measurements of stagnation streamline velocity gradients define the positions of the subsolar magnetopause and bow shockTurbulent magnetosheath flows preclude the use of the method behind the quasi‐parallel bow shock [ABSTRACT FROM AUTHOR]

In Trung et al. (2019b, https://doi.org/10.1029/2019ja026636), we compared the geopause, a surface where the ionospheric plasma and solar wind plasma has equal parameters (by mass density, number density, or pressure), to the magnetopause to show that there was a transition in physics governing the motion of the plasmas between the different boundaries. In this study, we use multifluid magnetohydrodynamics simulation data to examine the relation of the source terms in the momentum equation and to the geopauses. We find that the geopauses do represent a transition in the physics governing the motion of the plasmas. While friction does not play a dominant role in the motion of the heliospheric and ionospheric plasmas, we find that friction shows indirectly that the ionospheric plasma can travel faster than the heliospheric plasma. Further satellite data is needed to verify the possibility of ionospheric plasma reaching speeds exceeding the solar wind. Key Points: We used numerical simulations to compare the structure of source terms in the multifluid momentum equation to the geopauseGeopauses represent a transition in how source terms govern the motion of individual plasma componentsFriction shows that ionospheric plasma can travel faster than the solar wind [ABSTRACT FROM AUTHOR]

Cold ions from the plasmasphere can reach the Earth's magnetopause through the plasma wind and plume, affecting the reconnection at the magnetopause, such as changing the reconnection rate and influencing the energy budget. Recent observations found that cold ions were substantially heated by magnetopause reconnection. Although a few mechanisms have been proposed to account for the cold ion acceleration at the magnetopause, the dominant mechanism remains unclear. In this paper, we perform 2.5-dimensional fully kinetic simulations to study cold ion heating in asymmetric reconnection at Earth's magnetopause. We find that cold ions absorb about 10%-25% of the total released magnetic energy, which is predominantly converted into the thermal energy of cold ions. The proportion of the energy gained by cold ions increases with the increment of the initial density ratio of the cold ions to hot ions in the magnetosphere. Cold ions are primarily accelerated by the Hall electric field at the magnetospheric separatrix region, while the out-of-plane electric field does negative work. The increase in thermal energy of cold ions is mainly caused by stochastic heating, that is, the viscous heating associated with the non-gyrotropic pressure tensor, -..., contributes the most. The work done by pressure is most significant around the magnetosheath separatrix region. These results can significantly deepen our understanding of the cold ion dynamics at Earth's magnetopause. [ABSTRACT FROM AUTHOR]

When a solar wind discontinuity interacts with foreshock ions, foreshock transients such as hot flow anomalies and foreshock bubbles can form. These create significant dynamic pressure perturbations disturbing the bow shock, magnetopause, and magnetosphere-ionosphere system. However, presently these phenomena are not predictable. In the accompanying paper, we derived analytical equations of foreshock ion partial gyration around a discontinuity and the resultant current density. In this study, we utilize the derived current density strength to model the energy conversion from the foreshock ions, which drives the outward motion or expansion of the solar wind plasma away from the discontinuity. We show that the model expansion speeds match those from local hybrid simulations for varying foreshock ion parameters. Using MMS, we conduct a statistical study showing that the model expansion speeds are moderately correlated with the magnetic field strength variations and the dynamic pressure decreases around discontinuities with correlation coefficients larger than 0.5. We use conjunctions between ARTEMIS and MMS to show that the model expansion speeds are typically large for those already-formed foreshock transients. Our results show that our model can be reasonably successful in predicting significant dynamic pressure disturbances caused by foreshock ion-discontinuity interactions. We discuss ways to improve the model in the future. [ABSTRACT FROM AUTHOR]

Surface waves on Earth's magnetopause have a controlling effect upon global magnetospheric dynamics. Since spacecraft provide sparse in situ observation points, remote sensing these modes using ground‐based instruments in the polar regions is desirable. However, many open conceptual questions on the expected signatures remain. Therefore, we provide predictions of key qualitative features expected in auroral, ionospheric, and ground magnetic observations through both magnetohydrodynamic theory and a global coupled magnetosphere‐ionosphere simulation of a magnetopause surface eigenmode. These show monochromatic oscillatory field‐aligned currents (FACs), due to both the surface mode and its non‐resonant Alfvén coupling, are present throughout the magnetosphere. The currents peak in amplitude at the equatorward edge of the magnetopause boundary layer, not the open‐closed boundary as previously thought. They also exhibit slow poleward phase motion rather than being purely evanescent. We suggest the upward FAC perturbations may result in periodic auroral brightenings. In the ionosphere, convection vortices circulate the poleward moving FAC structures. Finally, surface mode signals are predicted in the ground magnetic field, with ionospheric Hall currents rotating perturbations by approximately (but not exactly) 90° compared to the magnetosphere. Thus typical dayside magnetopause surface modes should be strongest in the East‐West ground magnetic field component. Overall, all ground‐based signatures of the magnetopause surface mode are predicted to have the same frequency across L‐shells, amplitudes that maximize near the magnetopause's equatorward edge, and larger latitudinal scales than for field line resonance. Implications in terms of ionospheric Joule heating and geomagnetically induced currents are discussed. Plain Language Summary: Waves on the boundary of the magnetosphere, the magnetic shield established by the interplay of the solar wind with Earth's magnetic field, play a controlling role on energy flow into our space environment. While these waves can be observed as they pass over satellites in orbit, due to the small number of suitable satellites available it would be helpful to be able to detect these waves from the surface of the Earth with instruments that measure the northern/southern lights, motion of the top of our atmosphere, or magnetic field on the ground. However, we do not currently understand what the signs of these waves should look like in such instruments. In this paper we develop theory and use computer simulations of these boundary waves to predict key features one might expect to measure from the ground. Based on these predictions, we also discuss how the waves might contribute to the hazards of space weather. Key Points: Theory and global simulations of magnetopause surface waves' effects on the aurorae, ionosphere, and ground magnetic field are investigatedWe predict poleward‐moving periodic aurora, convection vortices, and ground pulsations, with larger latitudinal scales than Alfvén modesAmplitudes of all signals peak near the projection of the inner/equatorward edge of the magnetopause rather than the open–closed boundary [ABSTRACT FROM AUTHOR]

We present in‐depth analysis of three southward‐moving meso‐scale (ion‐to magnetohydrodynamic‐scale) flux transfer events (FTEs) and subsequent crossing of a reconnecting magnetopause current sheet (MPCS), which were observed on 8 December 2015 by the Magnetospheric Multiscale spacecraft in the subsolar region under southward and duskward magnetosheath magnetic field conditions. We aim to understand the generation mechanism of ion‐scale magnetic flux ropes (ISFRs) and to reveal causal relationship among magnetic field structures, electromagnetic energy conversion, and kinetic processes in magnetic reconnection layers. Results from magnetic field reconstruction methods are consistent with a flux rope with a length of about one ion inertial length growing from an electron‐scale current sheet (ECS) in the MPCS, supporting the idea that ISFRs can be generated through secondary reconnection in an ECS. Grad‐Shafranov reconstruction applied to the three FTEs shows that the FTEs had axial orientations similar to that of the ISFR. This suggests that these FTEs also formed through the same secondary reconnection process, rather than multiple X‐line reconnection at spatially separated locations. Four‐spacecraft observations of electron pitch‐angle distributions and energy conversion rate j·E′=j·E+ve×B $\mathbf{j}\cdot {\mathbf{E}}^{\prime }=\mathbf{j}\cdot \left(\mathbf{E}+{\mathbf{v}}_{\mathrm{e}}\times \mathbf{B}\right)$ suggest that the ISFR had three‐dimensional magnetic topology and secondary reconnection was patchy or bursty. Previously reported positive and negative values of j·E′ $\mathbf{j}\cdot {\mathbf{E}}^{\prime }$, with magnitudes much larger than expected for typical MP reconnection, were seen in both magnetosheath and magnetospheric separatrix regions of the ISFR. Many of them coexisted with bi‐directional electron beams and intense electric field fluctuations around the electron gyrofrequency, consistent with their origin in separatrix activities. Plain Language Summary: Magnetic reconnection is a physical process that converts magnetic energy into plasma energy by changing the connectivity of magnetic field lines from one region to another. Magnetic reconnection at the outer boundary of planetary magnetospheres, known as the magnetopause (MP), is key to the entry of solar wind plasma and energy into the magnetospheres that forms the basis for space weather phenomena in the magnetospheres. MP reconnection often occurs in a transient or patchy manner, forming magnetic flux ropes (FRs) with helical field lines of various sizes. They may become an important pathway for fast coupling between the solar wind and magnetosphere. However, the generation mechanism of a subclass of FRs, relatively small "ion‐scale" FRs, is poorly understood. Computer simulations show that they are formed in thin and elongated current sheets of single active reconnection site, but this scenario has not been confirmed by observations. Our observations based on NASA's Magnetospheric Multiscale mission show that ion‐scale FR can form in a thin current sheet of single ongoing reconnection site at Earth's MP. The observed FR showed signatures of complex field line connectivity and localized conversion from electromagnetic to electron energy and vice versa, indicating complex MP dynamics. Key Points: Ion‐scale magnetic flux rope (ISFR) can be generated from reconnecting electron‐scale current sheet at the subsolar magnetopause (MP)Preceding mesoscale flux ropes had axial directions akin to that of the ISFR in the MP, suggesting the same generation mechanismThe ISFR had complex magnetic topology with three‐dimensional effects and involved patchy, intense energy conversion in separatrix regions [ABSTRACT FROM AUTHOR]

The launch of the Twin Rockets to Investigate Cusp Electrodynamics‐2 (TRICE‐2) took place on the morning of 08 December 2018. The two rockets of this campaign each sampled the low‐altitude Northern Hemisphere cusp region at approximately the apex of each rocket's trajectory (1,042 and 757 km for the High flyer and Low flyer, respectively). Ion and electron electrostatic analyzers (ESAs) on board each rocket measured in situ particle populations throughout the flights. Energy‐flux spectrograms of ions and electrons from each ESA clearly show the passage of each rocket through the cusp region. This work examines the locations of these entrance/exit points in relation to cusp models as provided in and compiled from the published literature, along with a discussion of model variables that have been optimized to best fit the cusp region boundary sampling locations by TRICE‐2. The results of this study set the stage for understanding the upcoming NASA Small Explorer Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites orbital plane intersections with (i.e., "trajectory cuts" through) the cusp region. Key Points: The low‐altitude cusp region was directly sampled by instruments onboard the two Twin Rockets to Investigate Cusp Electrodynamics‐2 rocketsCusp models have been compiled from published studies, with free parameters representing the low‐latitude boundary layer and mantleCusp model free parameters have been optimized to best match the observed cusp region location [ABSTRACT FROM AUTHOR]

Lower hybrid waves (LHWs) are frequently observed associated with magnetic reconnection. A general picture of the statistical properties of LHWs in magnetopause reconnection, however, has not yet been established. Here we report such a statistical study of LHWs using magnetospheric multiscale mission data in the magnetopause reconnection exhaust region. LHWs are identified in reconnection outflow regions over a broad range of magnetic local time. Most of the LHWs are near strong density gradients located on the low‐density magnetospheric side of the reconnection layer. The wave amplitude is typically several mV/m but can occasionally reach a few tens of mV/m. Compared with intervals that do not exhibit LHWs, intervals with LHWs show a ∼50% increase in parallel electron temperature, providing statistical evidence that LHWs contribute to parallel electron heating in reconnection exhausts and the dayside low‐latitude boundary layer. Plain Language Summary: In this study, we provide a statistical study of the wave strength, the location, and the potential effect of lower hybrid waves (LHWs) on particles in reconnection exhaust regions. We find that LHWs are mainly located near strong density gradients on the low‐density magnetospheric side of the reconnection layer. The wave strength and distribution do not depend on the magnetic shear condition. Te,‖ and Te,‖/Te,⊥ show significant increases associated with LHWs, suggesting that LHWs contribute to electron heating in reconnection exhaust regions and the low‐latitude boundary layer. Key Points: Lower hybrid waves (LHWs) in reconnection outflow regions are found from 06 MLT to 18 MLT near the dayside magnetopauseMost of the LHWs are near strong density gradients located on the magnetospheric side of the reconnection layerSignificant increases in Te,‖ and Te,‖/Te,⊥ are observed during intervals of LHWs [ABSTRACT FROM AUTHOR]

A cavity mode wave, referring to a trapped or radially standing fast mode wave between different magnetospheric boundaries, has been developed in theory and reported in observation studies. In this study, we present an interplanetary shock (IPS)‐induced cavity mode wave event observed outside the plasmasphere on 31 August 2017 with multispacecraft measurements. The phase delay of 90° between the azimuthal electric field and compressional magnetic field indicates that the fast‐mode wave triggered by the IPS is a standing wave, presumably radially trapped in the cavity between the magnetopause and plasmapause. Taking advantage of the location of Van Allen Probe B spacecraft right outside the plasmapause and the Arctic and Antarctic Research Institute ground‐based high‐latitude array mapped in the noon sector, it is suggested that the observed compressional wave associates to cavity mode with its inner boundary at the plasmapause and its outer boundary at the magnetopause. The peak frequency of the wavelet spectrum of the compressional magnetic field increases from 10.5 to 12.5 mHz, which is consistent with the theoretically calculated cavity eigenfrequencies before and after the IPS. We also provide the first evidence that the peak frequency of the cavity mode increases due to the inward motion of the magnetopause during IPS compression. Key Points: We report a cavity mode ULF wave event between the plasmapause and magnetopause induced by an interplanetary shockNarrow‐band compressional waves are observed at noon sector outside the plasmasphere by spacecraft and ground‐based measurementThe variation in the peak frequency is related to the compressional motion of the magnetopause [ABSTRACT FROM AUTHOR]

This paper presents the magnetospheric multiscale (MMS) observations of three flux ropes (FRs) sequentially in a reconnection exhaust at Earth's magnetopause. Three active X‐lines, which are responsible for the formation of these FRs, were also detected by MMS, providing strong evidence that the FRs were generated by multiple X‐line reconnection. We find that the core field inside the FRs considerably affects the Hall current in the adjacent X‐lines. The intense core field of the FRs deflected the electron outflow jet, forming the electron‐scale Hall magnetic field on the magnetosheath side. Additionally, an active secondary reconnection was observed at the center of one FR. This secondary reconnection split the FR into two smaller FRs and produced sharp density gradients and intense fluctuations of electric field and current within the FR. These observations advance the understanding of the formation and evolution of FRs during multiple X‐line reconnection at magnetopause. Plain Language Summary: Flux transfer event (FTE) is generally explained as the flux rope (FR) generated by multiple X‐line reconnection at the dayside magnetopause. Although the theory and simulations have shown that the multiple active X‐lines and FRs can be produced simultaneously by the tearing instability in the magnetopause current sheet, there is no in‐situ evidence to directly support this scenario yet. Using high‐resolution measurements from the magnetospheric multiscale mission, we present a case study that all active X‐lines expected at the edges of three successive FRs were observed. These observations confirm that reconnection could simultaneously occur at multiple sites to produce the FRs at Earth's magnetopause. Interestingly, the electron‐scale Hall magnetic field was observed at the magnetosheath side in this multiple X‐line reconnection. This is due to the electron outflow jet deflected into the magnetosheath side by the strong core field of FR. Additionally, an ongoing secondary reconnection was observed at the center of one FR. Intense waves and current perturbations were observed in a smaller FR which was produced by this secondary reconnection. Key Points: Ongoing multiple X‐line reconnection generates a series of flux ropes at the magnetopauseElectron‐scale Hall magnetic fields are formed due to the high deflection of electron jets caused by the core field of the flux ropeSecondary reconnection within the flux rope split the flux rope into smaller flux ropes and produces intense electromagnetic fluctuations [ABSTRACT FROM AUTHOR]

We revisited a typical interplanetary (IP) shock event on 7 November 2004, with high time resolution (1 s) total electron content (TEC) observations. The IP shock impinged on the dayside magnetopause at ∼18:27 UT, and launched a pulse into the magnetosphere. Using the multi‐spacecraft timing method, the pulse was estimated to propagate at an angular velocity of ∼0.1 MLT/s. While the in situ spacecraft observations only provided a first‐order estimation of the propagation speed, a more precise monitoring of the propagating pulse was achieved by the densely distributed network of the Global Navigation Satellite System (GNSS). Following the shock arrival, GNSS receivers throughout the equatorial region recorded the associated TEC impulses. The signatures were sequentially observed from the sub‐solar to the nightside via both flanks, which in turn demonstrated the concrete propagation pattern of the pulse. In addition to a consistent angular velocity, a ∼40 s difference was systematically found between the TEC measurements and the in situ spacecraft observations, revealing the timescale of the shock‐induced magnetospheric‐ionospheric coupling. Key Points: Global Positioning System total electron content (TEC) impulses associated with the interplanetary shock impingement were systematically observed in the equatorial regionThe TEC observations showed an azimuthal propagation velocity of ∼0.1 MLT/s and a time delay of ∼40 s between the spacecraft observationsThe TEC technique achieved a monitoring of the shock‐induced pulse and revealed the timescale for the pulse to couple to the ionosphere [ABSTRACT FROM AUTHOR]

Magnetic reconnection at the magnetopause has long been studied with multi‐spacecraft observations. In this work, data from the five satellite THEMIS mission during the years of 2008–2010 are used to generate statistics regarding the spatial extent of magnetopause reconnection. The presence of a reconnecting magnetopause is determined with the Walén relation as two satellites cross the magnetopause simultaneously. In some cases both satellites measure reconnection whereas in others one satellite measures reconnection and the other does not. This study finds that two spacecraft are more likely to observe a contiguous reconnection region the closer they are spatially, and that reconnection is not always extended around the entire magnetopause. Plasma β gradient drifts are investigated as a cause of local reconnection suppression. Spacecraft position along the magnetopause flanks is also investigated as a possible spatial limitation to reconnection due to changes in shear flow or boundary thickness. Plain Language Summary: Magnetic reconnection, a process by which magnetic fields break and reform, is observed with the THEMIS satellites at Earth's magnetopause. Using two satellites crossing the magnetopause near simultaneously, the spatial extent of reconnection can be sampled. This study found 208 of these such events from 2008 to 2010, and find that simultaneous reconnection is more likely to be measured the closer satellites spatially are. We additionally conclude that reconnection is not always extended around the magnetopause because events are found in which only one satellite observes reconnection. We explore the possible causes of reconnection confinement with analysis of the magnetic field shear angle and plasma beta, as well as spacecraft position along the magnetopause flanks. Key Points: THEMIS conjunction observations at the magnetopause are used to determine the spatial extent of magnetopause reconnectionTwo spacecraft are more likely to both observe reconnection jets the closer they are spatiallyDiamagnetic suppression is a reason why reconnection at the magnetopause is not always spatially extended [ABSTRACT FROM AUTHOR]

The magnetopause standoff distance characterizes global magnetospheric compression and deformation in response to changes in the solar wind dynamic pressure and interplanetary magnetic field orientation. We cannot derive this parameter from in situ spacecraft measurements. However, time series of the magnetopause standoff distance can be obtained in the near future using observations by soft X‐ray imagers. In two companion papers, we describe methods of finding the standoff distance from X‐ray images. In Part 1, we present the results of MHD simulations which we use for the calculation of the X‐ray emissivity in the magnetosheath and cusps. Some MHD models predict relatively high density in the magnetosphere, larger than observed in the data. Correcting this, we develop magnetospheric masking methods to separate the magnetosphere from the magnetosheath and cusps. We simulate the X‐ray emissivity in the magnetosheath for different solar wind conditions and dipole tilts. Plain Language Summary: The highly dynamic solar wind continuously bombards the Earth's magnetosphere, which changes shape in response. The magnetopause is the outer boundary of the magnetosphere, which is known to move. We are limited in our knowledge of the overall shape of the magnetopause, as current in situ measurements can only tell us about any change in the magnetopause at one specific location. Spacecraft carrying soft X‐ray imagers, however, will soon revolutionize our understanding by monitoring large areas of the magnetopause as the solar wind varies. In this first of a series of two papers, we simulate X‐ray emissions in the vicinity of the Earth using two magnetohydrodynamic models, and for two case studies with vastly different incoming solar wind conditions. In a subsequent paper, we examine methods how to extract the magnetopause shape from the simulated X‐ray images. Key Points: The SMILE mission will carry soft X‐ray imager onboardMagnetospheric masking methods separate the magnetosphere from the magnetosheath and cuspsWe make magnetospheric masks using threshold conditions and flowlines [ABSTRACT FROM AUTHOR]

The Earth's magnetosheath and cusps are the sources of soft X‐rays. In the accompanying paper (Part 1) and this paper, we discuss the methods of finding the magnetopause position by analyzing the X‐ray images. We use the software developed for the Soft X‐ray Imager (SXI) on board the forthcoming Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) mission. We show how to find the maximum SXI count rate in noisy count maps. We verify the assumption that the maximum of the X‐ray emissivity integrated along the Line‐of‐Sight (Ix) is tangent to the magnetopause. We consider two cases using two MHD models and apply different methods of magnetospheric masking. Overall, the magnetopause is located close to the maximum Ix gradient or between the maximum Ix gradient and the maximum Ix depending on the method used. But since the angular distance between the maximum Ix gradient and the maximum Ix is relatively small (about 3°), the maximum Ix might be used as an indicator of the outer boundary of a wide magnetopause layer usually obtained in MHD simulations. Plain Language Summary: This is the second of two papers presenting the techniques to estimate the Earth's magnetopause location (the outer boundary of the magnetosphere) under the impact of the highly dynamic solar wind. Our knowledge of the overall shape of the magnetopause will be vastly improved when we start using X‐ray imagers to monitor large areas around this boundary as the solar wind varies. In this second paper of the series, we make use of the X‐ray emissions in the vicinity of the Earth simulated in the first paper for two case studies with vastly different incoming solar wind conditions. Here we examine different methods of how to extract the magnetopause shape and position from X‐ray maps of the type that will be returned by the X‐ray imager due to flying on the SMILE mission. Key Points: Different methods of finding the location and shape of the magnetopause from X‐ray images give consistent resultsMaximum integrated emissivity indicates the outer boundary of the magnetopauseThis work is made in preparation for the forthcoming Solar wind Magnetosphere Ionosphere Link Explorer mission [ABSTRACT FROM AUTHOR]

Pc5 ultralow frequency waves are important for transferring energy between the magnetosphere and ionosphere. While many observations have been performed on Pc5 waves properties, it has been difficult to determine the source region, signal propagation path, and the two‐dimensional structure of Pc5 waves beyond coverage by a small number of satellites. Pc5 waves often show a dawn‐dusk asymmetry, but the cause of the asymmetry is under debate. To address these issues, we used conjunction events between the THEMIS satellites and all‐sky imagers and analyzed two Pc5 wave events that were stronger on the dawnside. For both events, the Pc5 waves propagated from dawnside magnetopause toward the nightside magnetosphere. The Pc5 waves were also associated with dawnside magnetopause surface waves, which were probably induced by the Kelvin‐Helmholtz instability. The ionospheric equivalent currents identified multiple vortices on the dawnside associated with quasi‐periodic auroral arcs and much weaker perturbations on the duskside. Global auroral imaging also presented a similar dawn‐dusk asymmetry with multiple arcs on the dawnside, while only one or two major arcs existed on the duskside. Pc5 waves in the magnetosphere had an anti‐phase relation between the total magnetic field and thermal pressure, with a slower propagation velocity compared with magnetohydrodynamic waves. The Poynting flux was anti‐sunward with an oscillating field‐aligned component. These properties suggest that Pc5 waves were slow or drift mirror mode waves coupled with standing Alfven waves. The ground‐based and multi‐satellite observations provide crucial information for determining the Pc5 waves properties, possible source region, and signal propagation path. Plain Language Summary: Pc5 ultralow frequency waves (1.66–6.66 mHz) are large‐scale waves in space, and Pc5 waves are capable of interacting with particles and modulating higher frequency waves, resulting in energy transfer in the magnetosphere‐ionosphere system. However, it is difficult to find the exact signal propagation path and identify the source due to the limited sparse satellite observations in space. We investigated two Pc5 wave events by using the conjunctions of ground magnetometers, all‐sky imagers, and Time History of Events and Macroscale Interaction During Substorms (THEMIS) satellites. We found that the aurora shows periodic intensification and the ionospheric currents had multiple vortices with dawn‐dusk asymmetry. The Pc5 waves in the magnetosphere are identified as standing waves that were coupled with surface waves at the magnetopause traveling anti‐sunward. We suggest that magnetopause surface waves were induced by Kelvin‐Helmholtz instability and drive the Pc5 waves. The dawn‐dusk asymmetry of Kelvin‐Helmholtz instability in the magnetosheath may explain the current asymmetric pattern in the ionosphere. Key Points: Two Pc5 wave events we analyzed were highly dawn‐dusk asymmetric, with stronger magnetic field perturbations on the dawnside than dusksideSlow or drift mirror mode waves coupled with standing Alfven waves in the magnetosphere were driven by the magnetopause surface wavesSurface waves by Kelvin‐Helmholtz instability along the magnetopause is suggested to be the source of the dawnside Pc5 waves [ABSTRACT FROM AUTHOR]

Flux Transfer Events (FTEs) are transient magnetic flux ropes typically found at the Earth's magnetopause on the dayside. While it is known that FTEs are generated by magnetic reconnection, it remains unclear how the details of magnetic reconnection controls their properties. A recent study showed that the helicity sign of FTEs positively correlates with the east‐west (By) component of the Interplanetary Magnetic Field (IMF). With data from the Cluster and Magnetospheric Multiscale missions, we performed a statistical study of 166 quasi force‐free FTEs. We focus on their helicity sign and possible association with upstream solar wind conditions and local magnetic reconnection properties. Using both in situ data and magnetic shear modeling, we find that FTEs whose helicity sign corresponds to the IMF By are associated with moderate magnetic shears while those that do not correspond to the IMF By are associated with higher magnetic shears. While uncertainty in IMF propagation to the magnetopause may lead to randomness in the determination of the flux rope core field and helicity, we rather propose that for small IMF By, which corresponds to high shear and low guide field, the Hall pattern of magnetic reconnection determines the FTE core field and helicity sign. In that context we explain how the temporal sequence of multiple X‐line formation and the reconnection rate are important in determining the flux rope helicity sign. This work highlights a fundamental connection between kinetic processes at work in magnetic reconnection and the macroscale structure of FTEs. Plain Language Summary: In the vicinity of the Earth's magnetosphere outer boundary, the magnetopause, twisted magnetic field structures known as "Flux Transfer Events" (FTEs) are often detected by spacecraft in‐situ. They temporarily connect the solar wind to the Earth's ionosphere, allowing the transfer of solar wind flux into the magnetosphere. It is known that FTEs are produced as a consequence of magnetic reconnection, a process that rearranges the topology of sheared magnetic fields, between the shocked solar wind and the geomagnetic field. However, our understanding of how the microphysics of magnetic reconnection can lead to the macroscopic structures of FTEs is still limited. We revisit the in‐situ observations of FTEs made by the Cluster and Magnetospheric Multiscale missions. We focus on the twist feature of FTEs as characterized by their helicity and investigate its relationship to solar wind conditions and possible link to magnetic reconnection properties. By investigating local magnetic shear conditions around FTE locations, we found that the FTE helicity is determined by a kinetic feature of magnetic reconnection known as the "Hall magnetic field". Our study highlights a close connection between a kinetic process of magnetic reconnection and the global structure of FTEs, constituting a cross‐scale coupling effect in solar‐terrestrial interaction. Key Points: We study the helicity sign of Flux Transfer Events and investigate upstream solar wind conditions and local magnetic shear around themThe helicity sign is found to be unassociated to the Interplanetary Magnetic Field (By) component when the local magnetic shear is highThe FTEs' helicity sign in such cases may relate to the Hall field of magnetic reconnection in the absence of a guide field [ABSTRACT FROM AUTHOR]

The magnetopause is treated as a boundary where the pressure of the incoming solar wind is balanced by the pressure of the geomagnetic field and the plasma pressure inside the magnetopause is often neglected. However, published studies of pressure balance at the magnetopause reveal an excess of the magnetosheath pressure. Moreover, our survey of about 50,000 THEMIS magnetopause crossings shows that about 1% of them exhibits even larger magnetic field in the magnetosheath than in the magnetosphere. A subsequent analysis of crossings observed in the subsolar region under a southward interplanetary magnetic field shows the pressure of the dense cold ion population as an important component of the total magnetospheric pressure. This component is too cold to be registered by standard ion analyzers but its density can reach 300 cm−3. The second effect connected with a presence of cold plasma population is a reduction of the reconnection rate that slows down the transport of the magnetic flux down the tail and leads to magnetic pile‐up in the subsolar magnetosheath. Plain Language Summary: The magnetopause is usually treated as a boundary where the pressure of the incoming solar wind is balanced by the pressure of the geomagnetic field and the plasma pressure inside the magnetopause is often neglected. However, we have found a large number of subsolar magnetopause crossings exhibiting larger magnetic field in the magnetosheath than in the adjacent magnetospheric region. Taking into account the contribution of the magnetosheath plasma, the pressure balance at the magnetopause seems to be violated. Statistical analysis revealed that these events are observed predominantly during geomagnetic storms when the cold plasma from the plasmasphere comes toward the magnetopause. This plasma is too cold to be registered by the ion spectrometers but its density is sufficient to keep the pressure balance across the magnetopause as we demonstrate in two case studies. Our analysis explains contradiction that can be found in several already published papers pointing out a lack of the magnetospheric pressure at the magnetopause. Key Points: About 1% of subsolar magnetopause crossing exhibits larger magnetic field in the magnetosheath than in the magnetosphereSuch magnetopause configuration is typical for periods of geomagnetic stormsThe cold dense plasma of plasmaspheric origin was identified as a component keeping the pressure balance across the magnetopause [ABSTRACT FROM AUTHOR]

This commentary paper presents summaries of my research work on magnetospheric reconnection and on geomagnetic storms, to which I have devoted most of my professional career in space physics. Thus, the paper summarizes: (a) my early studies on the magnetospheric reconnection-electric field, involving observations and a model of the related polar cap-electric field during events with observed interplanetary driving parameters and (b) studies on several fundamental aspects associated with the origin of geomagnetic storms, especially leading to intense and extreme levels of activity, mostly associated with the arrival of geoeffective Coronal Mass Ejections (CMEs). The latter also involves studies on recurrent geomagnetic activity caused by interplanetary high-speed streams, among which “High-Intensity, Long-Duration and Continuous Auroral Activity (HILDCAAs for short)” events were shown to represent a special class. In the final Section, I have added some comments on recent issues dealing with fundamental concepts on reconnection at the Earth's magnetopause. [ABSTRACT FROM AUTHOR]

Global magnetohydrodynamic models predict that plasma velocities vary almost linearly from 0 km s−1 at the nose of the stationary magnetopause on the stagnation streamline to a limiting value of 0.25 VSW for high solar wind Alfvénic Mach numbers at the nose of the bow shock, where VSW is the solar wind velocity. This paper presents a proof‐of‐concept study showing how two‐point measurements of the plasma velocity in the subsolar magnetosheath can be used to determine gradients in the plasma velocity and consequently the steady‐state or slowly moving locations of both the nose of the magnetopause and the nose of the bow shock. The results may be of use to those binning magnetosheath observations to develop empirical models, those testing models for magnetopause erosion or deposition in response to magnetic reconnection, and those determining the stand‐off distance of the bow shock as a function of solar wind conditions. Plain Language Summary: We employ dual‐spacecraft observations to determine the radial gradient in plasma velocity through the subsolar magnetosheath. Relying upon results from global magnetohydrodynamic simulations indicating that this gradient is linear, we use it to determine the locations of the bow shock and magnetopause positions. Key Points: Plasma velocities vary almost linearly along the stagnation streamline from the nose of the magnetopause to the nose of the bow shockMagnetohydrodynamic theory and models predict magnetosheath plasma velocities at the noses of the bow shock and magnetopauseRadial gradients in the plasma velocity define the locations of the stationary or slow‐moving subsolar magnetopause and bow shock [ABSTRACT FROM AUTHOR]

Global simulations predict that the low‐latitude mantle may be an important pathway for the solar wind entry into the tail magnetosphere close to the current sheet when interplanetary magnetic field (IMF) By dominates over IMF Bz. To evaluate this entry mechanism in the near‐Earth tail (X ∼ −10–−20 RE), we investigate the predictions from 3D global hybrid simulations as well as in situ observations by magnetospheric multiscale (MMS) spacecraft. The simulations predict that the low‐latitude mantle plasma can appear in the near‐Earth tail lobe extending inward approximately 5 RE from the flank magnetopause. The low‐latitude mantle plasma appears in the dawnside northern lobe and duskside southern lobe during positive IMF By, while the opposite asymmetry is seen during negative IMF By. After a change in the IMF By direction arriving at the bow shock nose, it takes another ∼15–30 min for the asymmetry to completely reverse to the opposite sense in the near‐Earth tail. We present six MMS events in the tail lobe showing that the existence and absence of the low‐latitude mantle plasma is consistent with the predicted asymmetries. Statistical analysis of 5 years of MMS observations shows that the dependencies of the magnitudes of the lobe densities and tailward field‐aligned flow speeds on the IMF By directions are consistent with the predicted contributions from the low‐latitude mantle plasma in the expected lobe regions. Key Points: Global hybrid simulations predict that the low‐latitude mantle plasma can form in the near‐Earth lobe controlled by interplanetary magnetic field (IMF) ByLow‐latitude mantle is in dawnside northern lobe and duskside southern lobe for positive IMF By. The asymmetry reverses for negative IMF ByStatistical analysis of magnetospheric multiscale data supports the simulation predicted dependence of near‐Earth low‐latitude mantle on the IMF By direction [ABSTRACT FROM AUTHOR]

Whistler waves are often observed in magnetopause reconnection associated with electron beams. We analyze seven MMS crossings surrounding the electron diffusion region (EDR) to study the role of electron beams in whistler excitation. Waves have two major types: (a) Narrow‐band waves with high ellipticities and (b) broad‐band waves that are more electrostatic with significant variations in ellipticities and wave normal angles. While both types of waves are associated with electron beams, the key difference is the anisotropy of the background population, with perpendicular and parallel anisotropies, respectively. The linear instability analysis suggests that the first type of wave is mainly due to the background anisotropy, with the beam contributing additional cyclotron resonance to enhance the wave growth. The second type of broadband waves are excited via Landau resonance, and as seen in one event, the beam anisotropy induces an additional cyclotron mode. The results are supported by particle‐in‐cell simulations. We infer that the first type occurs downstream of the central EDR, where background electrons experience Betatron acceleration to form the perpendicular anisotropy; the second type occurs in the central EDR of guide field reconnection. A parametric study is conducted with linear instability analysis. A beam anisotropy alone of above ∼3 likely excites the cyclotron mode waves. Large beam drifts cause Doppler shifts and may lead to left‐hand polarizations in the ion frame. Future studies are needed to determine whether the observation covers a broader parameter regime and to understand the competition between whistler and other instabilities. Key Points: In EDRs observed by MMS, electron distributions of background plus beams excite whistler by beam drift and anisotropy of both populationsDifferent types of distributions and waves are inferred to depend on the distance from the X‐lineA parametric study with the linear instability analysis is used to discuss the competition between different whistler modes [ABSTRACT FROM AUTHOR]

In this paper, we analyze the fine structure of rotational discontinuities (RDs) in the magnetopause of an open magnetosphere using four‐point magnetic field measurements recorded by the Magnetospheric MultiScale mission. An RD is commonly observed within the magnetopause when reconnection occurs in the magnetopause. Furthermore, an RD usually occurs closer to the magnetosheath side of the magnetopause. The magnetic structure of RD is usually much thinner than 110 km. The magnetic field rotation maxima and magnetic strength minima are within the RD. The radius of curvature of the magnetic field lines reaches a minimum in the RD (∼83–1,491 km). In addition, the radius of curvature of the magnetic field lines of the RD is usually much larger than the thickness of the magnetic structure of the RD, indicating that the magnetic field lines almost lie in the RD. Generally, a field‐aligned current dominates in the RD. Plain Language Summary: The magnetopause is the sharp outer boundary of the magnetosphere. It is a key region in space, where the mass, momentum, and energy of the solar wind is transferred into the magnetosphere. In a magnetohydrodynamics framework, the magnetopause usually takes the form of either a tangential discontinuity or a rotational discontinuity (RD). If magnetopause reconnection occurs, then it is expected that there will be an RD‐type magnetopause. However, the fine magnetic structure and current distribution of RDs remain unclear. In this study, by using four‐point magnetic field measurements from the Magnetospheric MultiScale Mission, we studied the inner magnetic structures of RDs which are closely related to reconnection processes. Investigating RD structures can enhance our understanding of the interaction between the solar wind and the magnetosphere. Key Points: The fine magnetic structure of rotational discontinuities (RDs) at the dayside magnetopause is revealedThe magnetic structure of a RD is very thin, and it is usually at the magnetosheath side of the magnetopauseThe RD corresponds to the minimum of the magnetic field strength and strong rotation in the magnetic field [ABSTRACT FROM AUTHOR]

Following the St. Patrick's Day (17 March) geomagnetic storm of 2013, the interplanetary magnetic field had near‐zero clock angle for almost two days. Throughout this period multiple cusp‐aligned auroral arcs formed in the polar regions; we present observations of, and provide a new explanation for, this poorly understood phenomenon. The arcs were observed by auroral imagers onboard satellites of the Defense Meteorological Satellite Program. Ionospheric flow measurements and observations of energetic particles from the same satellites show that the arcs were produced by inverted‐V precipitation associated with upward field‐aligned currents (FACs) at shears in the convection pattern. The large‐scale convection pattern revealed by the Super Dual Auroral Radar Network and the corresponding FAC pattern observed by the Active Magnetosphere and Planetary Electrodynamics Response Experiment suggest that dual‐lobe reconnection was ongoing to produce significant closure of the magnetosphere. However, we propose that once the magnetosphere became nearly closed complicated lobe reconnection geometries arose that produced interleaving of regions of open and closed magnetic flux and spatial and temporal structure in the convection pattern that evolved on timescales shorter than the orbital period of the DMSP spacecraft. This new model naturally explains many features of cusp‐aligned arcs, including why they focus in from the nightside toward the cusp region. Plain Language Summary: The geomagnetic storm that occurred on St. Patrick's Day in 2013 was followed by a period of almost two days during which the magnetic field embedded within the solar wind was pointing purely northwards, a rare occurrence. Auroral observations reveal that a series of auroral arcs formed near the geomagnetic poles which were aligned along the sunwards direction. Measurements of flows within the polar ionosphere reveal that these arcs were associated with shears in the convection pattern and electrical currents linking the magnetosphere and ionosphere. We propose a mechanism by which these flows could be produced, invoking complicated patterns of magnetic reconnection occurring at high latitudes on the dayside magnetopause. These observations shed light on the poorly understood solar wind‐magnetosphere‐ionosphere coupling processes that occur when the interplanetary magnetic field is directed northwards. Key Points: Cusp‐aligned arcs were observed for 2 days of near‐zero clock angle interplanetary magnetic field following the St. Patrick's Day storm of 2013Arcs were produced by upwards field‐aligned currents associated with vorticity within a highly structured ionospheric convection patternWe propose that the magnetosphere was nearly closed, but complicated lobe reconnection geometries interleaved open and closed flux [ABSTRACT FROM AUTHOR]

Observations at the reconnection site from the Magnetospheric Multiscale mission are used to examine the occurrence of magnetic reconnection along the magnetopause in relation to both the upstream interplanetary magnetic field (IMF) orientation and local magnetosheath variations. While there is a statistically larger number of reconnection intervals observed downstream of the quasi‐parallel bow shock than is accounted for based on the histogram of overall IMF orientations, it is surmised that increased fluctuations downstream of the quasi‐parallel bow shock cause greater large‐scale motion of the magnetopause surface; but do not preferentially enhance the occurrence of steady magnetic reconnection. An examination of the locations of observed reconnection sites relative to model predictions also suggest that enhanced magnetosheath fluctuations do not result in steady reconnection occurring at random locations at the magnetopause. Key Points: Magnetospheric Multiscale (MMS) encounters the magnetopause reconnection site over a wide range of external conditionsThe occurrence of reconnection region encounters is consistent with that of large‐scale magnetopause motion, and depends on interplanetary magnetic field orientationDistributions of MMS locations with respect to model reconnection line predictions are independent of region downstream of the bow shock [ABSTRACT FROM AUTHOR]

Magnetospheric Multiscale (MMS) observations during an extended crossing of the Earth's dayside magnetopause show evidence of multiple reconnections at the boundary. This crossing occurred when the Interplanetary Magnetic Field (IMF) was southward and had a significant BY component. Approximately 2 hr after this crossing, the Twin Rocket Investigation of Cusp Electrodynamics‐2 rockets were launched into the northern hemisphere cusp and observed overlapping cusp ion injections. These overlapping injections are also evidence of multiple reconnections at the magnetopause. Observations more than 2 hr apart do not constitute conjunction between the spacecraft and the rockets. However, the IMF conditions during the magnetopause crossing and the cusp traversal were very similar. Therefore, had the magnetopause crossings and cusp traversals occurred at the same time, the observations would have been similar. Thus, these two events illustrate the link between multiple reconnections at the magnetopause and overlapping cusp ion injections. Plain Language Summary: Magnetic reconnection is a fundamental process in plasmas that interconnects magnetic field lines on either side of a current layer and converts magnetic energy into particle energy. It is the primary process that interconnects the magnetic field of the Sun with the magnetic field of the Earth at the current layer interface called the magnetopause. Ions and electrons from the solar wind follow these interconnected field lines into the Earth's magnetospheric cusps. Magnetic reconnection at the magnetopause occurs along long lines, called X‐lines. This paper discusses observations at the magnetopause and in the cusp that indicate that there are actually multiple X‐lines at the magnetopause and these multiple X‐lines produce a specific signature in the cusp called overlapping ion injections. Key Points: Magnetospheric multiscale observations often show evidence of multiple reconnection at the magnetopauseTwin Rocket Investigation of Cusp Electrodynamics‐2 observations of overlapping cusp ion injections are also evidence of multiple reconnections at the magnetopauseThere is a strong link between multiple reconnections at the magnetopause and overlapping cusp ion dispersions [ABSTRACT FROM AUTHOR]

During the 9 March 2018 event with two consecutive interplanetary shocks compressing the dayside magnetosphere, the azimuthal mode structure and frequency spectrum of ultra low frequency magnetic pulsations are resolved using a cross‐spectral analysis based on high‐fidelity multi‐probe Magnetospheric Multiscale Mission (MMS) magnetometer data. The results based on the MMS 4 and MMS 3 pair of measurements show that shock arrival leads to low mode (|m|≤3 $\vert m\vert \le 3$) magnetic fluctuations in the Pc4‐5 regimes, and smaller spatial scale fluctuations implied by the dominant high mode numbers are observed after both shock signatures hit and passed the magnetosphere. Detailed evolution of the mode structure is also shown for the first shock to reveal the development of high mode structure from a bump‐on‐tail distribution at m≈20 $m\approx 20$ to a dominant peak at m≈50 $m\approx 50$ in about 10 min. In addition, an interesting change of sign in m $m$ from negative to positive is observed as MMS crosses ∼11 MLT pre‐noon, which is consistent with the picture of wave generation by dayside magnetopause compression and then anti‐sunward propagation. For both shocks, the contribution of higher frequency waves (Pc‐4 range compared with Pc‐5) to the total wave power is found to be negligible before and after the shock impact, but it becomes more significant during the shock impact. Key Points: The azimuthal mode structure and frequency spectrum of Ultra Low Frequency (ULF) magnetic pulsations during two consecutive interplanetary shocks are analyzedThe azimuthal mode structure and frequency spectrum of Ultra Low Frequency (ULF) magnetic pulsations during two consecutive interplanetary shocks are analyzedContribution of higher frequency ULF waves to total wave power is more significant during the shock impact than before and after the shocks [ABSTRACT FROM AUTHOR]

In response to a solar wind dynamic pressure enhancement, the compression of the magnetosphere generates strong ionospheric signatures and a sharp variation in the ground magnetic field, termed sudden commencement (SC). While such compressions have also been associated with a contraction of the ionospheric polar cap due to the triggering of reconnection in the magnetotail, the effect of any changes in dayside reconnection is less clear and is a key component in fully understanding the system response. In this study, we explore the time‐dependent nature of dayside coupling during SC by performing global simulations using the Gorgon magnetohydrodynamic code and impact the magnetosphere with a series of interplanetary shocks with different parameters. We identify the location and evolution of the reconnection region in each case as the shock propagates through the magnetosphere, finding strong enhancement in the dayside reconnection rate and prompt expansion of the dayside polar cap prior to the eventual triggering of tail reconnection. This effect pervades for a variety of interplanetary magnetic field orientations, and the reconnection rate is most enhanced for events with higher dynamic pressure. We explain this by repeating the simulations with a large explicit resistivity, showing that compression of the magnetosheath plasma near the propagating shock front allows for reconnection of much greater intensity and at different locations on the dayside magnetopause than during typical solar wind conditions. The results indicate that the dynamic behavior of dayside coupling may render steady models of reconnection inaccurate during the onset of a severe space weather event. Plain Language Summary: The Earth's magnetic field is often impacted by strong impulses of magnetized plasma ejected from the Sun. These compress the boundaries of the region shielded by the magnetic field and generate sudden signatures in the upper atmosphere and on the ground which can have societal effects. During the compression, some mass and energy is able to penetrate this shield more effectively than usual through a process called magnetic reconnection. We perform computer simulations of several such compressions to explore the effect on reconnection in detail, quantifying its enhancement and the dependence on various parameters. We find that reconnection is intensified immediately at the point of compression and undergoes a highly time‐dependent behavior for several minutes after impact. This demonstrates that basic models of reconnection that do not account for this time‐dependence may be inaccurate at the onset of severe space weather events. Key Points: The dayside reconnection line responds dynamically to the compression and distortion of the magnetopause by an interplanetary shockThe reconnection rate is enhanced near the shock as it propagates and appears to be modulated by large‐scale magnetopause motionsThe complex time‐dependence of dayside coupling during sudden commencement deviates significantly from steady models of reconnection [ABSTRACT FROM AUTHOR]

We present an initial assessment of using tomography on single‐spacecraft images to reconstruct 3D X‐ray emissions from the Earth's magnetosheath. 3D structures in the Earth's magnetosphere have been studied using superposed epoch techniques with single‐point single‐spacecraft observations. They have yielded great insights, but some studies are observation starved, particularly for infrequent solar wind conditions. Global imaging data have provided more insight about these structures, but are 2D projections of 3D structures. We explore the use of tomographic reconstruction techniques to understand what can be extracted from global images from a single spacecraft. The Solar wind Magnetosphere Ionosphere Link Explorer mission, due to launch in 2024 on a 3‐year mission, will carry a soft X‐ray imager which will capture emissions from portions of the magnetosheath and upstream solar wind. We already demonstrated that the 3D shape of the magnetopause and the bow shock can be extracted from such images with suitable assumptions. The next step is to examine whether full 3D reconstructions of the emissions are possible. We explore the limited range of viewing angles, which affect the accuracy of the reconstructions and introduce artifacts in some cases, and the low count‐rates in the images which introduce noise in the reconstructions which must be filtered out. Despite these limitations we show that it is possible to reconstruct some aspects of the magnetosheath global morphology using single‐spacecraft soft X‐ray imaging. Key Points: Tomographic reconstruction of the magnetosheath with Solar wind Magnetosphere Ionosphere Link Explorer is possibleHigh noise/low‐count images can be used with regularization constraintsSuperposed epoch over a year works better than a single orbit [ABSTRACT FROM AUTHOR]

We report results of our multi‐spacecraft analysis of a solar wind reconnecting current sheet (RCS) and its solar wind magnetic hole (SWMH) observed on November 20, 2018. In the solar wind, the normal vector to the current sheet plane makes an angle of 32° with the Sun‐Earth line. A combination of tilted current sheet plane and foreshock effects cause an asymmetric interaction with the bow shock, in which the structure arrives at the quasi‐perpendicular side of the bow shock before the quasi‐parallel side. The magnetic field strength inside the magnetic hole decreases by ∼69 percent in the solar wind, with a similar depression rate observed inside the magnetosheath due to this structure. The solar wind flow slowdown and deflection during the bow shock crossing significantly disrupt the reconnection exhausts within the RCS. The interaction of the RCS and SWMH with the bow shock creates enhanced fluxes of accelerated electrons and ions. Plasma flow deflection in the magnetosheath also increases with the passage of the RCS. The ion density and temperature both increase within the current sheet to form a roughly pressure balanced structure. Field rotation and change in the dynamic pressure during this event modify the reconnection zones at the magnetopause and cause asymmetric inward motions in portions of the bow shock and the magnetopause boundaries (i.e., deformation). Unlike localized magnetosheath jets, an RCS and its associated SWMH in the solar wind have a global impact on the bow shock and the magnetopause. Plain Language Summary: Space Weather is the study of effects of solar inputs on the space environment surrounding Earth. A source of solar input is through the solar wind, a stream of charged particles from the Sun carrying the interplanetary magnetic field. In this study, we analyze effects of a particular type of solar wind anomaly on Earth. The structure is initially observed by solar wind monitors far upstream of Earth, and later appears in the data of several near Earth spacecraft. We show that the structure can pass through the outer most boundary around Earth, the bow shock, and propagate closer to Earth. This study has significance in shaping our understanding of space weather as it describes near‐Earth effects of a commonly observed solar wind phenomenon. Key Points: Asymmetric interaction is caused by the current sheet plane orientation in the solar wind and foreshock effectsFlux of accelerated electrons and ions increase in the magnetosheath due to current sheet and magnetic hole interactions at the bow shockThe structure causes deformation of the bow shock and magnetopause boundaries and modulates the reconnection process at the magnetopause [ABSTRACT FROM AUTHOR]

The Earth's magnetosheath and cusps emit soft X‐rays due to the charge exchange between highly charged solar wind ions and exospheric hydrogen atoms. The Lunar Environment Heliospheric X‐ray Imager and Solar wind Magnetosphere Ionosphere Link Explorer missions are scheduled to image the Earth's dayside magnetosphere system in soft X‐rays to investigate global‐scale magnetopause reconnection modes under varying solar wind conditions. The exospheric neutral hydrogen density distribution, especially the value of this density at the subsolar magnetopause is of particular interest for understanding X‐ray emissions near this boundary. This paper estimates the exospheric density during solar minimum using the X‐ray Multimirror Mission (XMM) astrophysics observatory. We selected an event on 12 November 2008 from the XMM data archive, which detects soft X‐rays of magnetosheath origin while solar wind and interplanetary magnetic field conditions are relatively constant. During the event the location of the magnetopause was measured in situ by the THEMIS mission, thus the location of the solar wind ions responsible for the magnetosheath emission is well constrained by observation. We estimated the exospheric density using the Open Geospace Global Circulation Model (OpenGGCM) and a spherically symmetric exosphere model. The ratio of the magnetosheath plasma flux between the OpenGGCM model and the THEMIS, was nearly 1, which means the magnetohydrodynamic model reasonably reproduces the magnetosheath plasma conditions. The OpenGGCM magnetosheath parameters were used to deconvolve soft X‐rays of exospheric origin from the XMM signal. The lower‐limit of the exospheric density of this solar minimum event is 36.8 ± 11.7 cm−3 at 10 RE subsolar location. Key Points: From 11 years of X‐ray Multimirror Mission‐Newton X‐ray observations, we identified 193 intervals that can be used to estimate the exospheric neutral densityWe derived a solar minimum exospheric neutral density from the 12 November 2008 event using a global MagnetoHydroDynamics model and a simplified exosphere modelThe density estimate is 36.8 cm−3 near the subsolar magnetopause and is likely a lower‐limit value [ABSTRACT FROM AUTHOR]

We present a statistical study on the properties of equatorial electron pitch angle distributions (PADs) in the Earth's outer radiation belt, for the first time based on particle measurements from the entire Van Allen Probes mission. A detailed selection criteria is used to identify intervals when flux measurements at energies from 0.2 to 3.4 MeV are available across a wide range of pitch angles close to the geomagnetic equatorial plane. To better characterize the shape of each pitch angle distribution, the flux data is fitted to functions of the equatorial pitch angle based on Legendre polynomials. Using this technique, we show that the shape of the PADs strongly depends on the particle's energy, location and geomagnetic activity. These results are used to identify the dominant physical processes responsible for creating PADs with different shapes. The results presented here mainly focus on the occurrence statistics and properties of butterfly PADs. Significantly, we find clear evidence that butterfly PADs have a peak flux that preferentially occurs at two distinct and discrete equatorial pitch angles, either 35° ± 5° or 65° ± 5°. Energy, L‐shell and magnetic local time variations indicate that the flux peaks at equatorial pitch angles ∼35° appear consistent with magnetopause shadowing, whilst those peaking at ∼65° may be associated with wave‐particle interactions. We also show that the flux at low and high pitch angles, for both butterfly and nonbutterfly PADs, remains remarkably well correlated even as the flux intensity varies by four orders of magnitude. Plain Language Summary: We present a statistical study on the properties and occurrence of equatorial electron pitch angle distributions (PADs) in the Earth's outer radiation belt based on particle measurements from the entire Van Allen Probes mission. The flux data is fitted to functions of the equatorial pitch angle. Using the PAD fits, the characteristics of PADs that have a local flux minimum near equatorial pitch angles of 90°, known as butterfly PADs, are determined. We find clear evidence that these butterfly PADs have a peak flux that preferentially occurs at two distinct and discrete equatorial pitch angles, either 35° ± 5° or 65° ± 5°. Energy, L‐shell and magnetic local time variations indicate that the butterfly PADs with flux peaks at equatorial pitch angles of ∼35° appear consistent with outer radiation belt electron loss through the magnetopause, whilst those peaking at ∼65° may be associated with wave‐particle interactions. We also show that the flux at low and high pitch angles, for both butterfly and nonbutterfly PADs, remains remarkably well correlated even as the flux intensity varies by four orders of magnitude. Key Points: Equatorial butterfly pitch angle distributions have preferred flux peaks at ∼35° or ∼65°, each occurring across different L and MLT rangesThe depth of butterfly distribution dips tends to get smaller as the flux increases, inconsistent with growing off‐equatorial peaksFrom L*∼3 to ∼5, the flux at 90° is highly correlated with the flux at lower pitch angles, even as the flux varies by four orders of magnitude [ABSTRACT FROM AUTHOR]

The Magnetospheric Multiscale (MMS) mission has presented a new opportunity to study the fine scale structures and phenomena of the Earth's magnetosphere, including cross scale processes associated with the Kelvin–Helmholtz Instability (KHI), but such studies of the KHI and its secondary processes will require a database of MMS encounters with Kelvin–Helmholtz (KH) waves. Here, we present an overview of 45 MMS observations of the KHI from September 2015 to March 2020. Growth rates and unstable solid angles for each of the 45 events were calculated using a new technique to automatically detect plasma regions on either side of the magnetopause boundary. There was no apparent correlation between solar wind conditions during the KHI and its growth rate and unstable solid angle, which is not surprising as KH waves were observed downstream of their source region. We note all KHI were observed for solar wind flow speeds between 295 and 610 km/s, possibly due to a filtering effect of the instability onset criteria and plasma compressibility. Two‐dimensional Magnetohydrodynamic (2D MHD) simulations were compared with two of the observed MMS events. Comparison of the observations with the 2D MHD simulations indicates that the new region sorting method is reliable and robust. The ability to automatically detect separate plasma regions on either side of a moving boundary and determine the KHI growth rate may prove useful for future work identifying and studying secondary processes associated with the KHI. Key Points: A survey of the Magnetospheric Multiscale (MMS) data from September 2015 to March 2020 identified 45 Kelvin–Helmholtz wave eventsEvents are observed for the full range of solar wind conditions. Growth rates are independent of solar wind conditionsA new method is developed for the automatic detection of magnetosheath and magnetospheric regions within the KHI [ABSTRACT FROM AUTHOR]

Geomagnetic pole reversals occur frequently throughout geologic history, although one has not yet occurred in recorded time. Magnetohydrodynamic models of Earth's core have revealed that during a reversal, the magnetic dipole moment disappears, leaving higher‐order moments. Previous research examined quadrupole magnetic field topologies and quantitatively specified the magnetic equators of those topologies but did not fully examine charged particle drift motion and stability in the inner magnetosphere. Earth's closed magnetosphere is primarily dominated by two electric fields, the corotational and convection generated electric fields. E⃗×B⃗ $\vec{E}\times \vec{B}$ drifts from these fields ultimately drives the behavior of the cold plasma of the plasmasphere. In a quadrupole‐dominated magnetic field, the plasma motion generated by the E⃗×B⃗ $\vec{E}\times \vec{B}$ drifts would be dramatically different from the classical dipole field plasma convection. Three quadrupole topologies were evaluated, and the E⃗×B⃗ $\vec{E}\times \vec{B}$ drift was analyzed along the magnetic equators of these topologies to characterize and quantify the resultant plasma motion and evaluate the behavior, structure and stability of the plasmasphere. We also tested for plasmaspause and magnetopause boundary sensitivity to magnetic field strength. The direction of the convection flow is hemispherically dependent for the η = 0 and 0.5 quadrupole topologies, that is, the plasma in the Northern Hemisphere convects tailward, and the Southern Hemisphere convects sunward. The η = 1 topology demonstrates evidence of strong plasmasphere erosion due to the intersection of the magnetic equators, and the magnetopause and plasmapause of the η = 1 topology are particularly sensitive to reductions in magnetic field strength. Plain Language Summary: Earth's magnetic field protects the planet from high‐energy particles from the Sun. Little is known about what happens to Earth's magnetic field during a geomagnetic pole reversal, yet pole reversals occur regularly on geological timescales. Previous studies suggested that during pole reversals, Earth's magnetic field becomes more complex, taking on the structure of magnetic quadrupoles, and the overall magnetic field strength decreases. We studied the effects a pole reversal would have on Earth's electric fields, and how that would affect the cold, atmosphere‐sourced plasma in the near‐Earth space environment known as the plasmasphere. We find that the stability of the plasmasphere is highly dependent on the shape of the magnetic field, and that magnetic field shapes lacking a rotational symmetry around Earth's rotational axis lead to stronger erosion of Earth's plasmasphere, and leaves Earth's atmosphere more vulnerable to changes in magnetic field strength. Key Points: Pole reversals dramatically affect electric fields and plasmasphere stability due to changes in the topology of Earth's magnetic fieldQuadrupole magnetic field topologies generate sunward convection flows in one hemisphere, and tailward convection flows in the other hemisphere. Quadrupole topologies that exhibit rotational modulation with respect to the solar wind do not support steady‐state convection and result in stronger plasmasphere erosionThe plasmapause and magnetopause boundary positions are sensitive to changes in magnetic field strength and are highly dependent on magnetic field topology [ABSTRACT FROM AUTHOR]

Magnetic reconnection is universal in various plasma environments like the solar atmosphere, planetary magnetosphere, and laboratory to trigger explosive phenomena. Its nature and magnetic structure in two dimensions (2D) have been abundantly analyzed. The three‐dimensional (3D) characteristics of reconnection are different from 2D, and the mysteries of its 3D geometry are under exploration. A critical character in 3D is the presence of magnetic nulls, which is essential when the magnetic field topology is changing. The four identical spacecraft from the Cluster mission enable several methods for analyzing and visualizing the 3D nature of the magnetic structure. In this paper, we focus on reviewing the magnetic topology of the reconnection geometry containing 3D magnetic null points. Applying the fitting‐reconstruction method to Cluster data, magnetic null pairs or multiple magnetic nulls linked by separators or helically wrapped spines are found in the reconnection diffusion region and turbulent outflow region. The reconstructed magnetic structures show that the 2D features are parts of local approximations of the 3D geometries. The 2D regimes of antiparallel and component reconnection can simultaneously occur in 3D separator reconnection. The flux ropes are formed along the spine line or in a region enclosed by wrapped fan surfaces in the presence of spiral null points. Key Points: Cluster and magnetospheric multiscale observations and data‐based reconstruction of 3D reconnection geometry with magnetic nulls are reviewedNull pairs can form configurations with both X‐type and O‐type 2D projectionsThe null points serve as skeletons of the 3D magnetic reconnection region [ABSTRACT FROM AUTHOR]

Loss mechanisms act independently or in unison to drive rapid loss of electrons in the radiation belts. Electrons may be lost by precipitation into the Earth's atmosphere, or through the magnetopause into interplanetary space—a process known as magnetopause shadowing. While magnetopause shadowing is known to produce dropouts in electron flux, it is unclear if shadowing continues to remove particles in tandem with electron acceleration processes, limiting the overall flux increase. We investigated the contribution of shadowing to overall radiation belt fluxes throughout a geomagnetic storm starting on the 7 September 2017. We use new, multimission phase space density calculations to decipher electron dynamics during each storm phase and identify features of magnetopause shadowing during both the net‐loss and the net‐acceleration storm phases on sub‐hour time scales. We also highlight two distinct types of shadowing; "direct," where electrons are lost as their orbit intersects the magnetopause, and "indirect," where electrons are lost through ULF wave driven radial transport toward the magnetopause boundary. Plain Language Summary: Charged particles with extremely high energies are trapped by Earth's geomagnetic field. These particles form rings around Earth called the Van Allen radiation belts, which vary in intensity. This radiation poses a risk to satellites orbiting Earth, so it is important to understand how changes in geomagnetic conditions produce variations in the radiation belt intensity. In this work, we take measurements of electron radiation from many satellites to observe electron "dropouts," where nearly the entire radiation belt is lost in a matter of hours. We found that it is necessary to use multimission measurements to make observations of dropouts because a dropout may occur quicker than a single satellite can traverse the radiation belt. In early September 2017, we observed that movements of the geomagnetic outer boundary, the magnetopause, were responsible for removing electrons, combined with diffusive processes. This agreed with the predictions by previous studies. We further observed that the magnetopause continued to remove electrons from the belt while electrons were simultaneously accelerated by fluctuations in the geomagnetic field. This is significant because electrons may be removed from the belt soon after they were created, limiting the overall growth of the radiation belt while the magnetopause was compressed. Key Points: Multimission phase space density observations are necessary to resolve relativistic electron dynamics during September 2017 stormRelativistic electron losses to the magnetopause were identified, which led to further diffusion of electrons toward the magnetopauseElectron loss to the magnetopause was observed simultaneous to prompt local energization in the heart of the radiation belt [ABSTRACT FROM AUTHOR]

In a companion statistical study, we showed that the expression of the magnetopause surface as a power law of an elliptic function of the zenith angle holds at lunar distances, that the flaring of the magnetopause surface is influenced by the Interplanetary Magnetic Field (IMF) By component and that the IMF Bx component had no influence on the stand-off distance. As a follow-up to these statistical results, this paper presents a new empirical analytical asymmetric and non-indented model of the magnetopause location and shape. This model is obtained from fitting of 15,349 magnetopause crossings using 17 different spacecraft and is parametrized by the upstream solar wind dynamic and magnetic pressures, the IMF clock angle and the Earth dipole tilt angle. The constructed model provides a more accurate prediction of the magnetopause surface location than current Magnetopause surface models, especially on the night side of the magnetosphere. [ABSTRACT FROM AUTHOR]

Observations from the magnetospheric multiscale mission in or near electron diffusion regions (EDRs) at the Earth's magnetopause are used to determine the orientation of reconnection X‐lines. The results highlight cross‐scale coupling of magnetic reconnection and the differences between component and anti‐parallel reconnection. These observations are consistent with a model that has a continuous, component reconnection X‐line that extends many Earth Radii (RE) across the dayside magnetopause when the interplanetary magnetic field (IMF) is southward and |BY| ∼ |BZ|. Encounters anywhere along the X‐line have similar cross‐section structure in the direction normal to the magnetopause, indicating that component reconnection is quasi‐two‐dimensional, at some scale larger than the electron scale. EDR encounters far from this primary component X‐line may be associated with transient or spatially limited reconnection structures. On the magnetopause flanks on either side of the component X‐line, there are regions where anti‐parallel reconnection occurs. These regions dominate the entire dayside magnetopause when the IMF is southward and |BY| << |BZ|. In parts of these regions, observations in or near EDRs are consistent with reconnection occurring at a series of short, anti‐parallel reconnection X‐lines. These X‐lines may be quasi‐stationary or propagating along an anti‐parallel ridge at the magnetopause. In other parts of these regions, X‐lines may be much longer. Plain Language Summary: Magnetic reconnection is the process that couples the solar wind and the Earth's magnetosphere. This process produces long reconnection lines that stretch across the Earth's magnetopause. This paper describes the orientation of these reconnection lines. Key Points: Magnetospheric multiscale observations are used to determine the orientation of reconnection X‐lines at the Earth's magnetopauseFor southward interplanetary magnetic field and BY ∼ BZ, there is a long, continuous, component reconnection X‐line extending many RE across the dayside magnetopauseIn high shear regions, there may be many, short, anti‐parallel reconnection X‐lines or a long X‐line along a high magnetic shear ridge [ABSTRACT FROM AUTHOR]

Using a three‐dimensional global hybrid simulation, we investigate the formation and evolution of ion‐scale magnetic reconnection inside an interplanetary rotational discontinuity (RD) owing to its interaction with the quasi‐perpendicular (Q‐⊥ $\perp $) bow shock and the magnetosphere. The interplanetary magnetic field (IMF) is initially predominantly northward, while it changes to purely southward across the RD. A significantly thinned RD current layer with a width ∼ ion skin depth and normal magnetic field Bn≈0 ${\mathrm{B}}_{n}\approx 0$ is formed by a shock compression process as the RD interacts with the Q‐⊥ $\perp $ shock. Magnetic reconnection thereupon takes place inside the thin RD current layer, where Hall magnetic and electric fields, reconnection electric field, and high‐speed ion outflow jets are identified. Simultaneously, flux ropes form with an extension of a few ion inertial lengths. As the RD is transmitted into the magnetosheath, multiple reconnection sites lead to the formation of longer flux ropes. Moreover, magnetosheath reconnection is also found at new reconnection sites inside the RD. No reconnection is found in the Q‐⊥ $\perp $ shock alone outside the RD. The flux ropes propagate poleward and tailward in the magnetosheath. In addition, magnetopause reconnection takes place under the southward IMF on the sunward side of the RD after the RD passes through the magnetopause. Key Points: Reconnection current sheets form at the Q‐⊥per bow shock by the compression of an interplanetary RD as it interacts with the shockThe evidence of reconnection includes the formation of flux ropes, high‐speed outflows, and Hall magnetic and electric fieldsMore reconnection sites inside the RD current layer are found in the magnetosheath downstream of the Q‐⊥per shock [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive study of the intense current structures (ICSs) at the dayside magnetopause, by using the high‐resolution data from the Magnetospheric Multiscale (MMS) mission. About 3,600 ICSs with a current density exceeding 1.2 μA/m2 have been detected within the magnetopause boundary layer (MBL). We find that most ICSs crossed the spacecraft in less than 1 s, corresponding to an average thickness of less than one ion inertial length (di). The number of ICSs decreases with the thickness increasing from the electron‐scale to the ion‐scale. Boundary layers closer to Earth have more ICS than boundary layers further away from Earth, probably caused by the large solar wind dynamic pressure. The occurrence rate of the ICS is higher in the dusk sector near the meridian. For most ICSs, the current is carried by electrons. The perpendicular current (90° to the magnetic field) is larger than the parallel current (0° or 180° to the magnetic field) for more ICSs. The energy conversion between the magnetic field and plasma as measured by J·E $\boldsymbol{J}{\dot{}}\boldsymbol{E}$ is primarily through the perpendicular current and electric field, while the energy dissipation J·E′=J·(E+Ve×B) $\boldsymbol{J}{\dot{}}{\boldsymbol{E}}^{\prime }=\boldsymbol{J}{\dot{}}(\boldsymbol{E}+{\boldsymbol{V}}_{\boldsymbol{e}}\times \boldsymbol{B})$ is mainly dominated by the parallel component. ICSs provide much stronger energy conversion and dissipation compared to the ambient plasma in the MBL. This study improves our understanding of the characteristics of the ICS and its role in solar wind‐magnetosphere coupling. Plain Language Summary: Current sheet is the hotbed for various instabilities and an important site for energy exchange between electromagnetic fields and plasmas. Current structures with electric current density exceeding 1 μA/m2 have frequently observed by the Magnetospheric Multiscale mission at the dayside magnetopause boundary layer. Important features of these intense current structures, such as the occurrence rate, current carrier, contribution to energy conversion, are not clear. This paper presents a comprehensive analysis of more than three thousand intense current structures at the magnetopause. We show that most of these current structures are extremely short in time domain and have thicknesses less than the ion inertial length. Moreover, they play important roles in energy dissipation within the boundary layer. These results are of great help in understanding the characteristics of the intense current structures and their role in the solar wind‐magnetosphere coupling. Key Points: Most intense current structures crossed the spacecraft in less than 1 s, and their thickness are mostly less than one ion inertial lengthThe intense current structure has a higher occurrence rate closer to Earth and in the dusk sector near the meridianThe intense current structure is important for energy dissipation in the magnetopause boundary layer [ABSTRACT FROM AUTHOR]

In this paper we study electrostatic waves with time‐dependent frequency features in the terrestrial foreshock. These short (0.1–0.3 s) duration waves are characterized by a significant frequency drift where the peak wave power shifts from a few hundred Hz to 2–4,000 Hz in a few hundred milliseconds. Based on the electric field data from the Magnetospheric Multiscale Mission (MMS) we have identified 46 of these wave packets. Using four spacecraft timing approach we find that these waves have a propagation direction pointing upstream. However, their plasma frame velocity is less than the solar wind speed, therefore they are eventually convected downstream toward the bow shock. We use the double‐probes of MMS and present an interferometric analysis, which allows us to obtain the dispersion relation of these waves and directly compare them to theoretical ones. We show that the measured dispersion relations are in good agreement with Doppler shifted ion acoustic waves and discuss potential mechanisms related to impulsive reflected ions that may allow the growth of these waves and cause time‐dependent frequency features. Key Points: We analyze electrostatic waves with time‐dependent frequency features in the foreshock using the double‐probes of Magnetospheric Multiscale MissionThe measured wavenumber‐frequency plots are in excellent agreement with ion acoustic wave dispersion relationsWe discuss potential mechanisms, which may lead to the time‐dependent frequency features of the waves [ABSTRACT FROM AUTHOR]

We investigate the properties of the ion sky maps produced by the Dual Ion Spectrometers (DIS) from the Fast Plasma Investigation (FPI). We have trained a convolutional neural network classifier to predict four regions crossed by the Magnetospheric Multiscale Mission (MMS) on the dayside magnetosphere: solar wind, ion foreshock, magnetosheath, and magnetopause using solely DIS spectrograms. The accuracy of the classifier is >98%. We use the classifier to detect mixed plasma regions, in particular to find the bow shock regions. A similar approach can be used to identify the magnetopause crossings and reveal regions prone to magnetic reconnection. Data processing through the trained classifier is fast and efficient and thus can be used for classification for the whole MMS database. Plain Language Summary: Magnetospheric Multiscale Mission (MMS) has been traversing the Earth's magnetosphere to help scientists understand how the tremendous amounts of energy are released through the phenomenon known as magnetic reconnection. The spacecraft can transfer to the Earth only 4% of its measurements due to link limitations. The success of the mission relies on the selection of the most relevant measurement intervals to be sent down to the science operation center. We have trained a small deep convolutional neural network which identifies the kind of plasma the spacecraft is traversing at each measurement interval with an excellent accuracy >98%. We have used our model to identify some of the most interesting regions, bow shocks. It took only a day for the model to process all observations collected by the MMS within 3 years. The model can save a substantial amount of time for the scientists in the loop whose role is to locate such regions manually. The proposed model is suitable for the hierarchy of models being built to fully automate the on‐ground data processing. Moreover, it is small enough to be embedded in the on‐board software of future missions. Key Points: We develop a technique for automated classification of plasma regions traversed by the Magnetospheric Multiscale Mission spacecraftOur model distinguishes solar wind, megnetosheath, magnetosphere, and ion foreshock with 98% accuracyIt can be used to detect mixed plasma regions, bow shock and magnetopause crossings [ABSTRACT FROM AUTHOR]

A variety of plasma waves have been observed in the separatrix region during magnetic reconnection, and their roles on reconnection and exciting mechanisms are still controversial. In this paper, we report the whistler wave with a very narrow frequency band just above half of the electron cyclotron frequency in the separatrix region. This whistler wave is accompanied with the electron cyclotron harmonic (ECH) waves. The ECH waves have frequencies between the first and fifth harmonic of electron cyclotron frequency fce. The wave normal angle of the whistler waves is ∼20°–∼40°, meaning the quasi‐parallel propagation, while the ECH waves propagate perpendicular to the background magnetic field with θ∼87°±2°. In the separatrix region, the electrons have a loss‐cone distribution while these two kinds of waves are observed. Based on the linear theory, such an electron distribution is unstable for both whistler waves and ECH waves. It indicates that these two kinds of waves are generated by the electron loss‐cone instability. Key Points: A very narrow band whistler wave and electron cyclotron harmonic waves are simultaneously observed in the magnetosphere separatrix regionThe electron loss‐cone distributions are continuously detected while the two kinds of waves are observedThe two kinds of waves are probably excited by the electron loss‐cone instability [ABSTRACT FROM AUTHOR]

Solar wind parameters play a dominant role in reconnection rate, which controls the solar wind‐magnetosphere coupling efficiency at Earth's magnetopause. Besides, low‐energy ions from the ionosphere, frequently detected on the magnetospheric side of the magnetopause, also affect magnetic reconnection. However, the specific role of low‐energy ions in reconnection is still an open question under active discussion. In the present work, we report in situ observations of a multiscale, multi‐type magnetopause reconnection in the presence of low‐energy ions using NASA's Magnetospheric Multiscale data on September 11, 2015. This study divides ions into cold (10–500 eV) and hot (500–30,000 eV) populations. The observations can be interpreted as a secondary reconnection dominated by electrons and cold ions (mainly in XYGSE plane) located at the edge of an ion‐scale reconnection (mainly in XZGSE plane). This analysis demonstrates a dominant role of cold ions in the secondary reconnection without hot ions' response. Cold ions and electrons are accelerated and heated by the secondary process. The case study provides observational evidence for the simultaneous operation of antiparallel and component reconnection. Our results imply that the pre‐accelerated and heated cold ions and electrons in the secondary reconnection may participate in the primary ion‐scale reconnection affecting the solar wind‐magnetopause coupling and the complicated magnetic field topology could affect the reconnection rate. Key Points: We report Magnetospheric Multiscale observations of a multiscale, multi‐type magnetopause reconnection in the presence of low‐energy ionsAn electron‐cold ion reconnection, dominated by low‐energy ions and electrons, is located at the edge of an ion‐scale reconnectionThe multiscale reconnection provides observational evidence for the simultaneous operation of antiparallel and component reconnection [ABSTRACT FROM AUTHOR]

Mercury's magnetosphere is a unique and dynamic system, primarily due to the proximity of the planet to the Sun and its small size. Interactions between solar wind and embedded interplanetary magnetic field (IMF) and the dayside Hermean magnetosphere drive an electric current on the system's magnetopause boundary. So far, electromagnetic induction due to magnetopause motion in response to changing external pressure has been used to constrain Mercury's iron core size. Here we assess the impact a changing IMF direction has on the Hermean magnetopause currents, and the resulting inducing magnetic field. Observations made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during dayside magnetopause boundary crossings in the first "hot season," are used to demonstrate the importance of the IMF direction to Mercury's magnetopause currents. Our 16 boundary crossings show that introduction of external IMFs change the magnetopause current direction by 10° to 100°, compared to the case where only the internal planetary field is considered. Analytical modeling was used to fill in the bigger picture and suggests for an east‐west reversal of the IMF, typical of the heliospheric current sheet sweeping over Mercury's magnetosphere, the inducing field at Mercury's surface caused by the resulting magnetopause current dynamics is on the order of 30% of the global planetary field. These results suggest that IMF variability alone has an appreciable effect on Mercury's magnetopause current and generates a significant inducing magnetic field around the planet. The arrival of the BepiColombo mission will allow this response to be further explored as a method of probing Mercury's interior. Plain Language Summary: Mercury has a large iron core, in which a magnetic field is produced. Determining the precise size of this core and the composition of Mercury's interior are key to developing our understanding of the terrestrial planet's formation and evolution and, therefore, the mechanisms involved in the formation of our solar system. Changes to the sheet of electric current that shields Mercury from the stream of charged particles ejected from the Sun can be used as a natural metal detector to reveal properties of Mercury's subsurface. Through observational data and modeling of the interactions between Mercury's magnetic field and the external magnetic field generated by the Sun, we assess a way in which this electric current sheet can be altered. Our results showed that variability in the orientation of the external magnetic field from the Sun has an appreciable impact on Mercury's shielding electric current sheet. The arrival of the BepiColombo mission in 2025 will allow this response to be further explored as a method of probing Mercury's interior. Key Points: We assess the impact of a changing interplanetary magnetic field (IMF) direction on Mercury's magnetopause current and resulting inducing fieldWe show that variability of the IMF direction alone has an appreciable effect on Mercury's magnetopause currentsThis response, and BepiColombo's arrival, will allow inducing fields to be further explored as a method of probing Mercury's interior [ABSTRACT FROM AUTHOR]