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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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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 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]

Inside the magnetosheath, the IBEX‐Hi energetic neutral atom (ENA) imager measures a distinct background count rate that is more than 10 times the typical heliospheric ENA emissions observed when IBEX is outside the magnetosheath. The source of this enhancement is magnetosheath ions of solar wind (SW) origin that deflect around the Earth's magnetopause (MP), scatter and neutralize from the anti‐sunward part of the IBEX‐Hi sunshade, and continue into the instrument as neutral atoms, behaving indistinguishably from ENAs emitted from distant plasma sources. While this background pollutes observations of outer heliospheric ENAs, it provides a clear signature of IBEX crossings over the magnetospheric boundaries. In this study, we investigate IBEX encounters with the magnetosheath boundaries using ∼8 yr of orbital data, and we determine the MP and bow shock (BS) locations derived from this background signal. We find 280 BS crossings from XGSE ∼ 11 Re to XGSE ∼ −36 Re and 241 MP crossings from XGSE ∼ 6 Re to XGSE ∼ −48 Re. We compare IBEX BS and MP crossing locations to those from IMP‐8, Geotail, Cluster, Magion‐4, ISEE, and Magnetospheric Multiscale Mission, and we find that IBEX crossing locations overlap with the BS and MP locations inferred from these other data sets. In this paper, we demonstrate how IBEX can be used to identify magnetosheath crossings, and extend boundary observations well past the terminator, thus further constraining future models of magnetosheath boundaries. Furthermore, we use the IBEX data set to show observational evidence of near‐Earth magnetotail squeezing during periods of strong interplanetary magnetic field By. Plain Language Summary: The Interstellar Boundary Explorer (IBEX) spacecraft is designed to use remote‐sensing to examine the outer reaches of the solar system, yet it has been shown to have many magnetospheric applications as well. The primary interaction between the SW and the Earth's magnetosphere is the creation of a comet‐tail like structure known as the magnetosheath. The magnetosheath is highly dynamic, and its size and shape changes in response to changing SW conditions, with the exact details presently unknown. In this paper, we demonstrate how the IBEX's highly eccentric orbit allows for hundreds of encounters with both the inner and outer magnetosheath boundaries, and we use these crossings to create a new magnetosheath boundary data set. We then compare our new data set with data sets from previous magnetospheric missions. Finally, we use our new data set to provide observational evidence for a poorly understood phenomenon known as magnetotail squeezing. Key Points: We demonstrate a technique for producing both Bow Shock and Magnetopause data sets using the Interstellar Boundary ExplorerWe introduce new boundary crossings further tailward than previously used data sets into underexplored magnetosheath regionsWe provide observational evidence of magnetotail squeezing during periods when the interplanetary magnetic field has a strong y‐component [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]

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]

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]

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]

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]

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]

Ganymede is the only Solar System moon that generates a permanent magnetic field. Dynamics within the Ganymedean magnetosphere is thought to be driven by energy‐transfer interactions on its upstream magnetopause. Previously in Kaweeyanun et al. (2020), https://doi.org/10.1029/2019GL086228 we created a steady‐state analytical model of Ganymede's magnetopause and predicted global‐scale magnetic reconnection to occur frequently throughout the surface. This paper subsequently provides the first assessment of Kelvin‐Helmholtz (K‐H) instability growth on the magnetopause. Using the same analytical model, we find that linear K‐H waves are expected on both Ganymedean magnetopause flanks. Once formed, the waves propagate downstream at roughly half the speed of the external Jovian plasma flow. The Ganymedean K‐H instability growth is asymmetric between magnetopause flanks due to the finite Larmor radius effect arising from large gyroradii of Jovian plasma ions. A small but notable enhancement is expected on the sub‐Jovian flank according to the physical understanding of bulk plasma and local ion flows alongside comparisons to the well‐observed magnetopause of Mercury. Further evaluation shows that nonlinear K‐H vortices should be strongly suppressed by concurring global‐scale magnetic reconnection at Ganymede. Reconnection is therefore the dominant cross‐magnetopause energy‐transfer mechanism and driver of global‐scale plasma convection within Ganymede's magnetosphere. Plain Language Summary: Ganymede is the largest moon of Jupiter, and the only moon in the Solar System that can maintain a permanent magnetic field. Current research suggests Ganymede contains two internal magnetic field sources—a molten iron core and a subsurface ocean. The study of Ganymede's magnetic environment will be a primary objective for the JUpiter ICy moon Explorer (JUICE), the first moon‐orbiting satellite mission set to launch in 2022. Ganymede is surrounded by flows of plasma (energized gas) which are normally deflected away by the magnetic field along a boundary called the upstream magnetopause. However, the magnetic shield can be broken through interactions on the magnetopause such as Kelvin‐Helmholtz (K‐H) instability. Using a mathematical model established in Kaweeyanun et al. (2020), https://doi.org/10.1029/2019GL086228 we first determine that K‐H instability can grow as waves along Ganymede's magnetopause flank regions, and that the growth is enhanced on the magnetopause flank that is closest to Jupiter due to motions of local plasma. Finally, using Mercury as a comparison case, we argue that K‐H waves are unlikely to grow into turbulent vortices that can inject plasma across the magnetopause, as they will be torn apart by another magnetopause process known as magnetic reconnection. Key Points: We present the first assessment of Kelvin‐Helmholtz (K‐H) instability on Ganymede's upstream magnetopause using an analytical modelLinear K‐H waves can grow on both magnetopause flanks with small enhancement on the sub‐Jovian flank due to the finite Larmor radius effectNonlinear K‐H vortices should be suppressed by magnetic reconnection, so the latter likely dominates cross‐magnetopause plasma transport [ABSTRACT FROM AUTHOR]

The terrestrial magnetopause forms the boundary between the solar wind plasma with its embedded interplanetary magnetic field on one side, and the terrestrial magnetosphere, dominated by Earth's dipole field, on the other side. It is therefore a key region for the transfer of mass, momentum, and energy from the solar wind to the magnetosphere. The Cluster mission, comprising a constellation of four spacecraft flying in formation was launched more than 20 years ago to study boundaries in space. During its lifetime, Cluster has provided a wealth of new knowledge about the magnetopause. In this paper, we give an overview of Cluster‐based studies of this boundary, and highlight a selection of interesting results. Key Points: The magnetopause is a key region for transfer of mass, momentum and energy from the solar wind into the magnetosphereCluster has probed the terrestrial magnetopause for more than 20 yearsAn overview of key magnetopause results from the Cluster mission is given [ABSTRACT FROM AUTHOR]

Magnetopause flux ropes (FRs) play a crucial role in the transport of energy and plasma from the solar wind to the Earth's magnetosphere. Once formed by multiple X-line reconnection, the FRs move poleward, during which they can coalesce with each other through a re-reconnection process. The poleward moving FRs eventually coalesce into the cusp regions, through another re-reconnection process. In this paper, using three-dimensional (3-D) global hybrid simulations, we study the two re-reconnection processes of magnetopause FRs by examining the topological changes of magnetic field lines. These re-reconnection processes are essentially 3-D. Two FRs coalesce with each other and form two new FRs (instead of one new FR in the two-dimensional regime). When FRs move close to the cusp region, their field lines can reconnect with the cusp field lines so that the FRs can break into two shorter ones. The two re-reconnection processes increase the plasma energy and the magnetic flux connected to the Earth, which favors particle and energy transport toward the Earth's magnetosphere. [ABSTRACT FROM AUTHOR]

The magnetosheath and near‐Earth solar wind emit X‐rays due to charge‐exchange between the extended atmosphere and highly ionized particles in the solar wind. These emissions can be used to remotely sense the dynamic processes in this region. The Solar Wind Magnetosphere Ionosphere Link Explorer mission will carry out these measurements. In a previous paper, we looked at the effect of photon counting statistics on determining the location of the magnetopause and bow shock. In this paper we explore, through simulations, the more challenging question of orbital viewing geometry bias when the model and the emissions do not match each other exactly. Our simulations conclude that while care must be taken to avoid false minima in the fitting, there is very little to no orbital bias in extracting the position and large‐scale shape of the magnetopause and bow shocks from 2‐D X‐ray images from the future Solar Wind Magnetosphere Ionosphere Link Explorer mission. Key Points: A 3‐D model fit to 2‐D images can extract magnetopause and bow shock boundaries even when the model is not an exact fit to the observationsOrbital bias produced model/observation differences is within the science goal of the SMILE mission for large part of the orbitWhen using the simplex method, the initial guess must be chosen carefully to avoid false minima [ABSTRACT FROM AUTHOR]

During a dayside magnetopause crossing, the Magnetospheric Multiscale (MMS) mission observed a series of magnetic depletions in the total magnetic field, mainly due to the fluctuation of the component normal to the magnetopause. These fluctuations are adjacent to signatures suggesting that the MMS is at, or close to (

One of the main sources of magnetospheric particles is solar wind penetration through the magnetopause—a current sheet separating the cold, dense magnetosheath from the hot, rarified magnetospheric plasmas. The mechanism responsible for magnetosheath particle transport across the magnetopause has been better investigated for the near‐Earth dayside magnetopause (contrary to the nightside), where it was found that such a transport is controlled by the current sheet thickness, structure, and dynamics. Because plasma properties and magnetic field intensity in the magnetosheath significantly change as a function of radial distance from the Earth, the current sheet characteristics are different near Earth and at distant nightside magnetopause. Comparative investigations between near‐Earth and distant magnetopauses, however, require statistical observations at the two locations during the same time interval, that enable control for the same upstream solar wind driving conditions. In this paper, we perform such a study using the four Magnetosheric Multiscale (MMS) and two Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) probes to compare the characteristics of magnetic field and plasma populations during magnetopause crossings, which are separated by about 50 RE. We find that the current sheet profiles is similar at two locations. We also show that the magnetopause current sheet thickness scales with the local magnetosheath ion gyroradius. A weaker magnetic field on both sides of the current sheet is correlated with smaller current density at lunar distances. Key Points: Magnetopause current sheet thickness scales with the gyroradius of the magnetospheric ionsMagnetopause current density is smaller at lunar distances due to weaker magnetic fieldsThe magnetopause current sheet configuration is similar near Earth and at lunar distances [ABSTRACT FROM AUTHOR]

Plasma composition observations provide a useful mechanism for investigating plasma sources and subsequent evolution within planetary magnetospheres. While He++ is the second most abundant ion species in the solar wind, there are no known sources of He++ ions within the magnetosphere of Saturn, allowing He++ to serve as a tracer of solar wind ions within the Kronian magnetosphere. Meanwhile, water group ions (W+, consisting of O+, OH+, H2O+, and H3O+) and H2+, known to originate within the magnetosphere of Saturn, serve as a tracer of internally ionized plasma. In this paper, we investigate the relative abundances and properties of energetic (32–220 keV) ion species originating from sources within the magnetosphere and from the solar wind. Solar wind‐originating ions are observed to have significant relative abundance (up to ~0.05) in the midnight, dawn, and noon local time quadrants at high radial distances (~40, ~45, and ~20 RS, respectively). Several possible entry processes, such as reconnection and Kelvin‐Helmholtz instabilities, are outlined in this paper as well as a discussion of subsequent transport. Plain Language Summary: While the sources of plasma within the magnetosphere of Saturn have long focused on internal generation, this study seeks to estimate the amount of plasma entering into the magnetosphere from the solar wind. Using a mission averaged vantage point of the global relative abundances of 32 to 220 keV ions, the internal to external plasma is investigated in order to estimate the relative amount of solar wind‐originating ions within the magnetosphere of Saturn, as well as the regions of their importance. Solar wind‐originating ions are predominantly seen within the midnight and dawn quadrants of Saturn, while the dayside and inner magnetosphere is dominated by internally generated plasma. The energetic plasma is also seen to peak in density near a radial distance of 10 RS as a consequence of loss processes closer to the planet and radial diffusion further from Saturn. Key Points: Energetic (32‐220 keV) H+, H2+, He+, He++, and W+ densities are used to investigate internal versus external plasma sourcesEnergetic solar wind He++ observed in the magnetotail suggest access through Kelvin‐Helmholtz Instabilities and reconnection processesBoth internally and externally generated plasma species observe a peak in density at a radial distance of ~10 RS [ABSTRACT FROM AUTHOR]

The standard method for identifying magnetohydrodynamic rotational discontinuities in spacecraft data has been to examine how well the Walén relation is satisfied. In this paper, we apply two different versions of the Walén test to a database of nearly 1,000 dayside magnetopause crossings by the Magnetospheric Multi‐Scale spacecraft, with the objective of comparing their performance. The first approach is to evaluate the Walén relation as a jump condition, by determining the level of agreement between the change in plasma velocity across a discontinuity with the corresponding change in the Alfvén velocity. For this purpose, we use a recently developed quality index, Q, for which Q=±1 indicates perfect agreement. As was the case for a previously used quality index, ΔV∗, this new index employs data from two carefully chosen measurement times, located on opposite sides of the discontinuity. The second approach is to check the level of Alfvénicity of the flow for all measurements between those two points. Here, the quality index used is Wsl, the slope of the regression line in a scatter plot of plasma velocity components (after transformation into the deHoffmann‐Teller frame) versus the corresponding Alfvén velocity components, with Wsl=±1 indicating perfect agreement. For the two indices to give comparable numbers of rotational discontinuity candidates, a substantially higher threshold value is needed for |Q| than for |Wsl|. Even so, the events selected by the two methods are not identical. We also identify statistical relationships between Wsl and its associated correlation coefficient, Wcc, as well as between Wsl and Q and between Wsl and ΔV∗. Key Points: Statistical results from different versions of the Walén test are compared, using MMS data at the dayside magnetopauseThe quality measures should employ significantly different acceptance threshold values to identify rotational discontinuitiesRelationships between the various quality measures are explored and quantified [ABSTRACT FROM AUTHOR]

On 5 May 2017, MMS observed a crater‐type flux rope on the dawnside tailward magnetopause with fluctuations. The boundary‐normal analysis shows that the fluctuations can be attributed to nonlinear Kelvin‐Helmholtz (KH) waves. Reconnection signatures such as flow reversals and Joule dissipation were identified at the leading and trailing edges of the flux rope. In particular, strong northward electron jets observed at the trailing edge indicated midlatitude reconnection associated with the 3‐D structure of the KH vortex. The scale size of the flux rope, together with reconnection signatures, strongly supports the interpretation that the flux rope was generated locally by KH vortex‐induced reconnection. The center of the flux rope also displayed signatures of guide‐field reconnection (out‐of‐plane electron jets, parallel electron heating, and Joule dissipation). These signatures indicate that an interface between two interlinked flux tubes was undergoing interaction, causing a local magnetic depression, resulting in an M‐shaped crater flux rope, as supported by reconstruction. Plain Language Summary: Magnetic reconnection and Kelvin‐Helmholtz instability (KHI), two of the most fundamental physical processes occurring within the heliosphere and throughout the Universe, often occur simultaneously on the Earth's magnetopause. Previous studies indicate the importance of nonlinearly developed KH waves, which produce multiple kinetic layers facilitating reconnection both in and out of the velocity shear plane and resulting in the magnetic flux rope. However, these studies significantly lacked detailed in situ observations in quantity as well as appropriate 3‐D analyses of the structure of the KH vortex‐induced flux rope. In this paper, we use detailed observations by the MMS spacecraft to investigate both 2‐D and 3‐D structures of the flux rope developed along the KH waves. We found that two flux tubes interact through reconnection to form a single combined structure, which can explain the occurrence of M‐shaped crater flux rope. Key Points: MMS observed a magnetic flux rope formed on the boundary of a nonlinear Kelvin‐Helmholtz (KH) waveBoth in‐plane and midlatitude reconnection associated with the 3‐D structure of the KH vortex‐inducedflux rope were identifiedA current sheet at the flux rope center supported by reconstruction indicates two flux tubes interlinked to form a crater‐type flux rope [ABSTRACT FROM AUTHOR]

On the equatorward side of the dayside cusp there often appear diffuse auroras. In this paper, we report north‐south aligned discrete aurora events and show the spatial relationships between the auroras and the plasma flow. Several events of the north‐south aligned discrete auroral structures were identified using an all‐sky imager at Longyearbyen, Svalbard, during the recovery of a moderately disturbed period on 8 December 2013. During a brief interval of the moderately disturbed period, the cusp shifted to higher latitudes; as a result, the field‐aligned beam of the EISCAT Svalbard Radar (ESR) passed through the northern portion of the north‐south aligned auroral structures. Simultaneous observations from the all‐sky imager and ESR reveal that enhancements in ion temperature (caused by fast ion flow) occurred near the eastward and westward boundaries of the north‐south aligned auroral structures. However, within the region of the most enhanced aurora, the ion temperature enhancements were moderately suppressed. These features indicate that the ion flow slows down in the region of electron precipitation responsible for north‐south aligned auroral structures. We can quantitatively interpret the slowdown of the flow as the reduction of the electric field due to the polarization effect in the north‐south aligned region of the increased Pedersen conductivity. It thus appears that the magnetospheric source of the north‐south aligned discrete auroras is a limited area embedded in the region of plasma flow toward the dayside magnetopause. Key Points: First simultaneous observations of north‐south aligned discrete auroras equatorward of the cusp from an all‐sky imager and EISCAT are presentedA decrease in the elevated ion temperature is coincident with the north‐south aligned auroraThe plasma flow slows in the middle of the east‐west dimension of the north‐south aligned aurora [ABSTRACT FROM AUTHOR]

When spacecraft (s/c) missions probe plasma structures (PSs) the relative location of the s/c with respect to the PS is unknown. This information is, however, needed to measure the geometrical features of the PS (orientation and thickness) and to understand the physical processes underlying the PS dynamics. Methods to determine the s/c location exist, but they need strong assumptions to be satisfied (stationarity and special spatial dependencies). The number of cases for which these assumptions are likely to be valid for the entire PS seems to be limited, and even weak departures from these hypotheses may affect the results. For a quasi‐1‐D geometry in particular, the determination of the velocity component along the two quasi‐invariant directions is very inaccurate and the assumption of strict stationarity may lead these quantities to diverge. In this paper we present new methods to compute the s/c trajectory through a PS, without a priori assumption on its spatial geometry, and able to work even in the presence of weak nonstationarities. The methods are tested both on artificial and real data, the latter provided by the Magnetospheric Multiscale mission probing the Earth's magnetopause. The 1‐D and 2‐D trajectories of the Magnetospheric Multiscale are found that can be used as an initial step for future reconstruction studies. Advanced minimization procedures to optimize the results are discussed. Key Points: The spatiotemporal difference method is extended to determine nonmonodimensional spacecraft trajectoriesTwo new methods are developed for this determination: the single and the multivariate fit methods; they are compared to previous onesA gradient‐directed Monte Carlo approach is applied to optimize the results; the spacecraft path and magnetopause thickness are so computed [ABSTRACT FROM AUTHOR]

In this paper, we present observations made by the Magnetospheric Multiscale (MMS) mission of a small‐scale magnetic structure adjacent to a reconnecting current sheet at the dayside magnetopause. While MMS crosses the current sheet, it observes signatures of an electron diffusion region (EDR), including crescent shaped electron velocity distribution functions. Right after the EDR crossing, all spacecraft encounter an electron‐scale magnetic structure that can roughly be divided into two parts: a force‐free flux rope‐like part and a nonforce‐free magnetic enhancement. Within the leading force‐free part of the structure, the magnetic field component normal to the current sheet exhibits a bipolar signature, suggesting that the structure is flux rope‐like. In the trailing edge of the bipolar signature, three spacecraft observe a magnetic enhancement that has an amplitude almost twice the ambient magnetic field. The magnetic peak adjacent to an EDR can be associated with an electron vortex, where the perpendicular current is carried by E×B drifting electrons. The structure is advected tailward along the magnetopause while in the plasma frame, it propagates upstream suggesting that this reconnection event is highly three‐dimensional. Fluctuations in the plasma density and magnetic field and large peaks in the parallel electric field observed on the magnetospheric side of the EDR suggest that the current sheet is corrugated due to an electromagnetic drift instability. This or another instability of a thin reconnecting current sheet (e.g., tearing) is a likely cause of the formation of the observed magnetic structure. Key Points: MMS observed a complex electron‐scale magnetic structure adjacent to dayside electron diffusion regionMagnetic structure consists of a force‐free flux rope‐like part and a non‐force‐free magnetic enhancement supported by an electron vortexStructure propagates upstream in the plasma frame suggesting that it is formed due to an instability of a thin current sheet [ABSTRACT FROM AUTHOR]

Waves around the lower hybrid frequency are frequently observed at Earth's magnetopause and readily reach very large amplitudes. Determining the properties of lower hybrid waves is crucial because they are thought to contribute to electron and ion heating, cross‐field particle diffusion, anomalous resistivity, and energy transfer between electrons and ions. All these processes could play an important role in magnetic reconnection at the magnetopause and the evolution of the boundary layer. In this paper, the properties of lower hybrid waves at Earth's magnetopause are investigated using the Magnetospheric Multiscale mission. For the first time, the properties of the waves are investigated using fields and direct particle measurements. The highest‐resolution electron moments resolve the velocity and density fluctuations of lower hybrid waves, confirming that electrons remain approximately frozen in at lower hybrid wave frequencies. Using fields and particle moments, the dispersion relation is constructed and the wave‐normal angle is estimated to be close to 90° to the background magnetic field. The waves are shown to have a finite parallel wave vector, suggesting that they can interact with parallel propagating electrons. The observed wave properties are shown to agree with theoretical predictions, the previously used single‐spacecraft method, and four‐spacecraft timing analyses. These results show that single‐spacecraft methods can accurately determine lower hybrid wave properties. Key Points: The velocity and density fluctuations of magnetopause lower hybrid waves are resolved, showing that electrons are approximately frozen inLower hybrid wave dispersion relation and wave‐normal angle are computed from fields and particle measurementsSingle‐ and multi‐spacecraft methods yield consistent lower hybrid wave properties, confirming the accuracy of single‐spacecraft methods [ABSTRACT FROM AUTHOR]

The origin of interplanetary magnetic field that is frozen‐in to the solar wind plasma flow is clearly magnetic flux from the Sun's corona. However, the filamented structure of magnetic fields observed in the solar wind cannot be accounted for quite so simply. Given the 2 days or more for solar wind to travel from the Sun to 1 AU, some argue that many current sheets are present due to turbulence and other in‐transit effects in the dynamic plasma outflow. Alternatively, it is postulated that a "flux tube texture" of the solar wind exists as fossil structure of the corona. In this paper we examine the possible influence of magnetic reconnection occurring close to the Sun or in the solar wind on the character of current sheets observed by Magnetospheric Multiscale at 1 AU. Photospheric convection is used to perturb a magnetic carpet‐like configuration, which has well‐segmented open flux tubes defined by topological elements of the magnetic field. Flux tube boundaries in the model are defined by magnetic sepratrix surfaces which are a preferential location for strong currents and magnetic reconnection. Reconnection is associated with signatures in the magnetic field and plasma that may advect with the solar wind all the way to 1 AU. Aided by three‐dimensional coronal modeling and two‐dimensional simulation examples of reconnection layers, we examine properties of current sheets observed by Magnetospheric Multiscale and how these solar wind boundaries may relate to reconnection operating earlier in the solar wind or corona. Key Points: Magnetic flux tubes in the Sun's corona undergo reconnection at their boundariesMagnetic topological boundaries allow plasma in neighboring flux tubes to evolve independentlyAt least 50% of solar wind current sheets contain signatures of magnetic reconnection [ABSTRACT FROM AUTHOR]

Magnetic reconnection (MR) and the associated concurrently occurring waves have been extensively studied at large‐scale plasma boundaries, in quasi‐symmetric and asymmetric configurations in the terrestrial magnetotail and at the magnetopause. Recent high‐resolution observations by MMS (Magnetospheric Multiscale) spacecraft indicate that MR can occur also in the magnetosheath where the conditions are highly turbulent when the upstream shock geometry is quasi‐parallel. The strong turbulent motions make the boundary conditions for evolving MR complicated. In this paper it is demonstrated that the wave observations in localized regions of MR can serve as an additional diagnostic tool reinforcing our capacity for identifying MR events in turbulent plasmas. It is shown that in a close resemblance with MR at large‐scale boundaries, turbulent reconnection associated whistler waves occur at separatrix/outflow regions and at the outer boundary of the electron diffusion region, while lower hybrid drift waves are associated with density gradients during the crossing of the current sheet. The lower hybrid drift instability can make the density inhomogeneities rippled. The identification of MR associated waves in the magnetosheath represents also an important milestone for developing a better understanding of energy redistribution and dissipation in turbulent plasmas. Key Points: Magnetic reconnection associated instabilities and waves are investigated in the turbulent magnetosheathWhistler and lower hybrid drift waves occur in locations of turbulent reconnection with temperature anisotropy and density gradientsRippled density boundaries are observed which can be generated by lower hybrid drift instability or turbulent mixing [ABSTRACT FROM AUTHOR]

We present a continuing investigation of mass‐/charge‐dependent interactions between energetic ions (greater than tens of kiloelectron volts) and planetary magnetopauses and of the escape of the ions across the boundary. Previous studies at Earth using Magnetospheric Multiscale mission data are refined and advanced showing profound behavior differences between light (H, He) and singly charged heavy ions (O+). We highlight a distinctive feature of oxygen ions: an angular distribution bifurcation providing clear indication of entrainment along the magnetopause in Speiser‐like orbits during relatively stable magnetic conditions. This signature, interpreted using a simple kinetic model, suggests that these ions tend to be carried substantial distances along the boundary (even with boundary‐normal magnetic fields) in a fashion that impedes their full dayside escape. While large fluctuations and waves can likely sometimes disrupt the observed ordering, the following picture emerges. Energetic particles with gyroradii much smaller than the magnetopause thickness (e.g., electrons and absent boundary‐normal magnetic fields) and ions with gyroradii much larger than the thickness (e.g., O+) are impeded from fully escaping across the boundary. However, energetic ions with intermediate‐sized gyroradii commensurate with the thickness (e.g., H+, He++, and O6+) can be effectively scattered within the boundary causing them to escape much more readily, with and without boundary‐normal fields. This picture is supported by observations from the Juno spacecraft at the near‐dawn meridian side of Jupiter's magnetopause. There it is observed that energetic electrons and heavy ions are more strongly contained by the magnetopause than are the energetic protons and helium ions. Plain Language Summary: Strongly magnetized planets, like Earth, Jupiter, Saturn, Uranus, and Neptune, have space environments, called magnetospheres, that energize charge particles, like electrons and ions (charged atoms), to very high energies (thousands to millions of electron volts). Some of these energetic charge particles can leak across the "magnetopause" boundary that separates the magnetosphere from the interplanetary environment and become a part of that vast region of the sun‐generated solar wind. Charge particles in a magnetic field gyrate around in circular "gyro‐orbits." The heavier the charged particle, the larger is its gyro‐orbit. We find in this paper that the ability of the ions to leak across the magnetopause boundary depends on the sizes of the gyro‐orbits in a counterintuitive fashion. Traditionally, it has been thought that the larger the gyroradii, the easier it is for the particle to escape. Here we find that ions (and electrons) with the smaller gyroradii and ions with the larger gyroradii (heavy ions like those of oxygen) are both impeded in their abilities to leak across the magnetopause boundary. It is the ions with intermediate‐sized gyroradii (like protons), with gyroradii sizes close the thickness of the magnetopause that find it easiest to escape across the boundary. Key Points: Earth's >100‐keV‐injected magnetospheric O+ ions encountering the magnetopause tend to stay with it and are impeded from fully escapingEnergetic protons, with gyroradii similar to magnetopause thicknesses, are more likely to scatter within the boundary and escapeObservations at Jupiter's dawn magnetopause also show that large gyroradii ions appear less likely to escape than intermediate ones [ABSTRACT FROM AUTHOR]

The boundary separating solar wind plasma from ionospheric plasma is typically thought to be the magnetopause. A generalization of the magnetopause concept called the geopause was developed by Moore and Delcourt (1995, https://doi.org/10.1029/95RG00872). The geopause is a surface defined where solar wind quantities equal the ionospheric quantities. Geopause studies have helped characterize magnetospheric systems. However, comparative studies between the geopauses to the magnetopause have not been conducted. In this paper, we analyze the influence of inner boundary composition and interplanetary magnetic field (IMF) orientation on the steady state terrestrial geopauses and the magnetopause. This study simulates the Earth's magnetosphere by using the multifluid capabilities of the Block Adaptive Tree Solar wind Roe‐type Upwind Scheme magnetohydrodynamics model within the Space Weather Modeling Framework. The simulations show that the dayside magnetopause was not influenced by the presence of oxygen in the outflow for both IMF orientations and was larger than the other geopauses. In contrast, the nightside magnetopause was sensitive to the conditions in the outflow. The nightside magnetopause was smaller than the other geopauses with southward IMF. With northward IMF, the nightside magnetopause was the largest structure in comparison with the plasma‐based geopauses. Our results indicate that no single boundary surface dictates the transition from a solar wind dominated plasma to ionosphere dominated plasma. Key Points: Four definitions of the geopause are compared: number density, mass density, plasma pressure, and last closed field line (magnetopause)Multifluid magnetohydrodynamic modeling is used to calculate these geopauses for idealized north and south interplanetary magnetic fieldThe magnetopause is farthest out during north interplanetary field, but the plasma geopauses are farthest during south field [ABSTRACT FROM AUTHOR]

During geomagnetic storms, some fraction of the solar wind energy is coupled via reconnection at the dayside magnetopause, a process that requires a southward interplanetary magnetic field Bz. Through a complex sequence of events, some of this energy ultimately drives the generation of electromagnetic ion cyclotron (EMIC) waves, which can then scatter energetic electrons and ions from the radiation belts. In the event described in this paper, the interplanetary magnetic field remained northward throughout the event, a condition unfavorable for solar wind energy coupling through low‐latitude reconnection. While this resulted in SYM/H remaining positive throughout the event (so this may not be considered a storm, in spite of the very high solar wind densities), pressure fluctuations were directly transferred into and then propagated throughout the magnetosphere, generating EMIC waves on global scales. The generation mechanism presumably involved the development of temperature anisotropies via perpendicular pressure perturbations, as evidenced by strong correlations between the pressure variations and the intensifications of the waves globally. Electron precipitation was recorded by the Balloon Array for RBSP Relativistic Electron Losses balloons, although it did not have the same widespread signatures as the waves and, in fact, appears to have been quite patchy in character. Observations from Van Allen Probe A satellite (at postmidnight local time) showed clear butterfly distributions, and it may be possible that the EMIC waves contributed to the development of these distribution functions. Ion precipitation was also recorded by the Polar‐orbiting Operational Environmental Satellite satellites, though tended to be confined to the dawn‐dusk meridians. Key Points: Solar wind pressure fluctuations were directly transferred to the magnetosphere and then propagated throughout the magnetosphereGlobal pressure perturbations provided temperature anisotropies that drove EMIC waves throughout the magnetosphereElectron precipitation was recorded by the BARREL balloons, although it did not have the same widespread signatures as the wave signatures [ABSTRACT FROM AUTHOR]

Imaging magnetospheric satellite missions provide information, which is complementary to in situ observations. Imaging is often able to provide an instantaneous picture of large‐scale structures, whereas in situ measurements, even multipoint in situ measurements, can only provide an average view of large‐scale structure. But imaging also presents some challenges. When three‐dimensional structures need to be extracted from two‐dimensional images, it is necessary to either make suitable assumptions or record a large enough number of images from different viewing geometries to allow a reconstruction (e.g., tomography). Imaging data exist over a wide range of sources including visible light, ultraviolet light, extreme ultraviolet, energetic neutral atoms, and X‐rays, each informing different physical mechanisms. In this paper we consider the extraction of the geometry of the magnetopause and the bow shock from single X‐ray images expected from the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) mission. We examine the effect of photon‐counting noise in determining the boundary geometries. We also consider the effect of different viewing geometries in the form of orbital vantage point and target look direction. Finally, we consider the effect of background noise. We find that our approach is relatively robust to viewing geometry effects and works at low count rates. Key Points: We demonstrate reconstructing the three‐dimensional shape of the magnetopause and bow shock from single two‐dimensional imagesIn our simulations the reconstruction is not strongly dependent on viewing geometryThe reconstruction algorithm is successful for low count rates, less than one per pixel [ABSTRACT FROM AUTHOR]

This paper describes properties and behavior of magnetic reconnection and flux transfer events (FTEs) on the dayside magnetopause using the global hybrid‐Vlasov code Vlasiator. We investigate two simulation runs with and without a sunward (positive) Bx component of the interplanetary magnetic field (IMF) when the IMF is southward. The runs are two‐dimensional in real space in the noon‐midnight meridional (polar) plane and three‐dimensional in velocity space. Solar wind input parameters are identical in the two simulations with the exception that the IMF is purely southward in one but tilted 45° toward the Sun in the other. In the purely southward case (i.e., without Bx) the magnitude of the magnetosheath magnetic field component tangential to the magnetopause is larger than in the run with a sunward tilt. This is because the shock normal is perpendicular to the IMF at the equatorial plane, whereas in the other run the shock configuration is oblique and a smaller fraction of the total IMF strength is compressed at the shock crossing. Hence, the measured average and maximum reconnection rate are larger in the purely southward run. The run with tilted IMF also exhibits a north‐south asymmetry in the tangential magnetic field caused by the different angle between the IMF and the bow shock normal north and south of the equator. Greater north‐south asymmetries are seen in the FTE occurrence rate, size, and velocity as well; FTEs moving toward the Southern Hemisphere are larger in size and observed less frequently than FTEs in the Northern Hemisphere. Key Points: Sunward IMF tilt results in a smaller tangential field at the magnetopause, slowing dayside reconnectionSmaller tangential field results from a decrease in the IMF component that is shocked up at the bow shockA positive IMF Bx component introduces north‐south asymmetries, where fewer (but larger) FTEs appear on the Southern Hemisphere [ABSTRACT FROM AUTHOR]

The terrestrial magnetopause is the boundary that shields the Earth's magnetosphere on one side from the shocked solar wind and its embedded interplanetary magnetic field on the other side. In this paper, we show observations from two of the Time History of Events and Macroscales Interactions during Substorms (THEMIS) satellites, comparing dayside magnetopause crossings with flank crossings near the terminator. Macroscopic properties such as current sheet thickness, motion, and current density are examined for a large number of magnetopause crossings. The results show that the flank magnetopause is typically thicker than the dayside magnetopause and has a lower current density. Consistent with earlier results from Cluster observations, we also find a persistent dawn‐dusk asymmetry with a thicker and more dynamic magnetopause at dawn than at dusk. Key Points: Observations from THEMIS are used to characterize the magnetopauseThe flank magnetopause is thicker than the dayside magnetopauseA dawn‐dusk asymmetries exists in many magnetopause parameters [ABSTRACT FROM AUTHOR]

In this paper, we study electron radial diffusion coefficients derived from Pc4‐Pc5 ultralow frequency (ULF) wave power during the intense geomagnetic storm on 17–18 March 2015. During this storm the population of highly relativistic electrons was depleted within 2 hr of the storm commencement. This radial diffusion, depending upon the availability of source populations, can cause outward radial diffusion of particles and their loss to the magnetosheath, or inward transport and acceleration. Analysis of electromagnetic field measurements from Geostationary Operational Environment Satellite (GOES), Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite, and ground‐based magnetometers shows that the main phase storm‐specific radial diffusion coefficients do not correspond to statistical estimates. Specifically, during the main phase, the electric diffusion (DLLE) is reduced, and the magnetic diffusion (DLLB) is increased, compared to empirical models based on Kp. Contrary to prior results, the main phase magnetic radial diffusion cannot be neglected. The largest discrepancies, and periods of dominance of DLLB over DLLE, occur during intervals of strongly southward IMF. However, during storm recovery, both magnetic and electric diffusion rates are consistent with empirical estimates. We further verify observationally, for the first time, an energy coherence for both DLLB and DLLE where diffusion coefficients do not depend on energy. We show that, at least for this storm, properly characterizing main phase radial diffusion, potentially associated with enhanced ULF wave magnetopause shadowing losses, cannot be done with standard empirical models. Modifications, associated especially with southward IMF, which enhance the effects of DLLB and introduce larger main phase outward transport losses, are needed. Key Points: Event‐specific electric and magnetic diffusion both show coherent energy dependence during the intense March 2015 magnetic stormMain phase event‐specific diffusion coefficients are poorly represented by empirical Kp‐dependent solar cycle averaged statistical modelsIntervals of southward IMF during the main phase drive strong compressional perturbations significantly enhancing magnetic diffusion [ABSTRACT FROM AUTHOR]

Kinetic Alfvén wave (KAW) and whistler wave play a key role in the framework of turbulence and reconnection. There are lot of observations of these wave in the magnetopause region. The present paper deals with the nonlinear evolution of KAW and a weak whistler wave through the preexisting magnetic reconnection site. For this study the dynamical evolution equations are derived by taking into account the ponderomotive force driven density modification and magnetic field fluctuations due to shear field modeled by the Harris sheet. Furthermore, these equations have been solved numerically as well as semianalytically. For numerical integrations we have used the pseudospectral method and finite difference method and for semianalytically Runge Kutta method. Simulated results have shown the evolution of coherent structures or current sheets, which are capable to energy transfer efficiently. These structures have scale size around ion gyroradius as well as electron inertial length as calculated analytically. At a later time the chaotic structures arise in this reconnection site. This gives the signature of turbulence generation. Therefore, the magnetic power spectrum with scaling is also presented in this manuscript and their relevance with the observed spectrum (calculated from the Cluster data (Chaston, 2008, https://doi.org/10.1029/2008GL033601)) is also pointed out. Key Points: Role of KAW and Whistler waves in the reconnection site is very importantA model is developed to study the effects of background Harris field and KAW nonlinearity in evolution of KAW and weak whistler wave in magnetic reconnection siteLocalized structures have scale sizes varying from ion gyroradious to electron skin depth [ABSTRACT FROM AUTHOR]

This paper investigates the excitation of waveguide modes in a nonuniform dipole equilibrium and, further, their coupling to field line resonances (FLRs). Waveguide modes are fast compressional ultralow frequency (ULF) waves, whose structure depends upon the magnetospheric equilibrium and the solar wind driving conditions. Using magnetohydrodynamic simulations, we consider how the structure of the excited waveguide mode is affected by various forms of magnetopause driving. We find that the waveguide supports a set of normal modes that are determined by the equilibrium. However, the particular normal modes that are excited are determined by the structure of the magnetopause driver. A full understanding of the spatial structure of the normal modes is required in order to predict where coupling to FLRs will occur. We show that symmetric pressure driving about the noon meridian can excite normal modes which remain around to drive resonances for longer than antisymmetric driving. Further, the critical quantity in terms of efficient coupling is the magnetic pressure gradient aligned with the resonance. Plain Language Summary: Earth's magnetic field is constantly being disturbed by the outflow from the Sun (the solar wind). Such disturbances generate periodic oscillations of the Earth's magnetic field, which can be measured by instruments on the ground and in space. These oscillations (waves) have been shown to have an impact on the aurora and the motion of energetic particles within the radiation belts (regions of trapped energetic particles around the Earth). In this study, we use a computational model of the Earth's magnetic field to investigate how these waves propagate. We consider various ways of driving the system, together with different density distributions and measure the resulting wave response. We identify that the system supports what are called "fast normal modes," which are the natural waves of the system. These can be thought of in the same way as blowing across an open bottle, which will produce a specific frequency of sound. We discuss how the structure of these modes affects how energy propagates and accumulates throughout the Earth's magnetic environment. Key Points: Waveguide normal mode structure is determined by the equilibriumStructure of waveguide normal modes is key in determining coupling efficiency and resonance locationMagnetopause driver determines which normal modes are excited and hence the FLR locations [ABSTRACT FROM AUTHOR]

In this paper we report on a sequence of large‐amplitude Alfvénic fluctuations terminating in a field and flow discontinuity and their effects on electromagnetic fields and plasmas in the near‐magnetopause magnetosheath. An arc‐polarized structure in the magnetic field was observed by the Time History of Events and Macroscale Interactions during Substorms‐C in the solar wind, indicative of nonlinear Alfvén waves. It ends with a combined tangential discontinuity/vortex sheet, which is strongly inclined to the ecliptic plane and at which there is a sharp rise in the density and a drop in temperature. Several effects resulting from this structure were observed by the Magnetospheric Multiscale spacecraft in the magnetosheath close to the subsolar point (11:30 magnetic local time) and somewhat south of the geomagnetic equator (−33° magnetic latitude): (i) kinetic Alfvén waves; (ii) a peaking of the electric and magnetic field strengths where E·J becomes strong and negative (−1 nW/m3) just prior to an abrupt dropout of the fields; (iii) evolution in the pitch angle distribution of energetic (a few tens of kilo‐electron‐volts) ions (H+, Hen+, and On+) and electrons inside a high‐density region, which we attribute to gyrosounding of the tangential discontinuity/vortex sheet structure passing by the spacecraft; (iv) field‐aligned acceleration of ions and electrons that could be associated with localized magnetosheath reconnection inside the high‐density region; and (v) variable and strong flow changes, which we argue to be unrelated to reconnection at partial magnetopause crossings and likely result from deflections of magnetosheath flow by a locally deformed, oscillating magnetopause. Key Points: We examine effects at MMS in inner magnetosheath of an arc‐polarized structure and a combination of a tangential discontinuity and vortex sheetOne effect is that kinetic Alfvén waves are observed by the MMS spacecraftAccelerated plasma flows are seen which are likely due to magnetopause motions resulting from tangential stresses [ABSTRACT FROM AUTHOR]