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We use the three-dimensional (3-D) global hybrid code ANGIE3D to simulate the interaction of four solar wind tangential discontinuities (TDs) observed by ARTEMIS P1 from 0740 UT to 0800 UT on 28 December 2019 with the bow shock, magnetosheath, and magnetosphere. We demonstrate how the four discontinuities produce foreshock transients, a magnetosheath cavity-like structure, and a brief magnetopause crossing observed by THEMIS and MMS spacecraft from 0800 UT to 0830 UT. THEMIS D observed entries into foreshock transients exhibiting low density, low magnetic field strength, and high temperature cores bounded by compressional regions with high densities and high magnetic field strengths. The MMS spacecraft observed cavities with strongly depressed magnetic field strengths and highly deflected velocity in the magnetosheath downstream from the foreshock. Dawnside THEMIS A magnetosheath observations indicate a brief magnetosphere entry exhibiting enhanced magnetic field strength, low density, and decreased and deflected velocity (sunward flow). The solar wind inputs into the 3-D hybrid simulations resemble those seen by ARTEMIS. We simulate the interaction of four oblique TDs with properties similar to those in the observation. We place virtual spacecraft at the locations where observations were made. The hybrid simulations predict similar characteristics of the foreshock transients, a magnetosheath cavity, and a magnetopause crossing with characteristics similar to those observed by the multi-spacecraft observations. The detailed and successful comparison of the interaction involving multiple TDs will be presented. [ABSTRACT FROM AUTHOR]

As the shocked solar wind flows past the flank magnetopause, surface waves can form and roll up into flow vortices. This is known as the Kelvin–Helmholtz Instability, which is thought to be an important mechanism whereby energy and momentum are transferred from the solar wind into the magnetosphere. One mechanism whereby this can occur is magnetic reconnection in the equatorial plane on compressed current sheets between vortices. In 2015, the NASA Magnetospheric Multiscale (MMS) mission observed this form of in‐plane reconnection during a KHI event in the post‐noon sector of the dayside magnetopause. We use data from MMS to investigate 12 KHI events at different positions along the magnetospheric flanks. For each event, we identify periodic compressed current sheets and perform Walén tests for each to identify reconnection jets. We then investigate the fraction of current sheets that exhibit reconnection signatures, and refer to it as the "event ratio." Results show that the event ratio decreases for events further down the magnetospheric flanks. Additionally, we investigate solar wind parameters during each event and find that the event ratio increases with an increasing northward component of the interplanetary magnetic field. Key Points: 12 Kelvin‐Helmholtz events observed by MMS are investigatedWe determine the fraction of current sheets where magnetic reconnection occursThe fraction of currents sheets with reconnection signatures correlates with northward interplanetary magnetic field strength [ABSTRACT FROM AUTHOR]

In Earth's magnetotail, we report three types of current sheets: Quiet current sheets supported by antiparallel lobe fields, "electron‐only" reconnecting current sheets, and traditionally reconnecting current sheets. We survey Earth's magnetotail for each current sheet type and perform event studies on one of each. One quiet current sheet, three electron‐only reconnecting current sheets, and one traditionally reconnecting current sheet are placed in order to showcase the possible time‐evolution of reconnection onset. Over time, electron‐only reconnecting current sheets increase in thickness, electron and ion outflow exhaust, perpendicular ion temperature, parallel electron temperature, and Hall electric field. These features support a transition phase from a quiet current sheet to traditional reconnection. Statistics of each current sheet type show that, during electron‐only reconnection, electrons are heated and evacuate the reconnection region while ions retain the properties of a quiet current sheet. In addition, statistics of solar wind properties indicate that electron‐only and traditional reconnection do not have unique solar wind drivers. Guide field statistics of electron‐only reconnection imply that aspects of the driving mechanism of electron‐only reconnection remain unclear. Plain Language Summary: The Magnetosphere Multiscale mission is a four satellite mission that is well prepared to study field and plasma properties of magnetic reconnection. Electron‐only reconnection is a newly discovered, transient form of magnetic reconnection that does not couple with ions. In this article, we present satellite observations of quiet magnetotail current sheets, traditional magnetotail reconnection, and "electron‐only" magnetotail reconnection events. We compare individual events of each current sheet type and the statistical solar wind and magnetotail features of each current sheet type to show that "electron‐only" reconnection can transition to traditional magnetic reconnection. Key Points: Magnetosphere Multiscale observed 47 quiet current sheets, 19 electron‐only reconnection events, and 155 traditional reconnection events in the magnetotailOne quiet current sheet, three electron‐only events, and one traditional electron diffusion region show evolution from quiet current sheet to traditional reconnectionElectrons are heated during electron‐only reconnection, while ions are not [ABSTRACT FROM AUTHOR]

We report a unique conjugate observation of fast flows and associated current sheet disturbances in the near‐Earth magnetotail by MMS (Magnetospheric Multiscale) and Cluster preceding a positive bay onset of a small substorm at ∼14:10 UT, September 8, 2018. MMS and Cluster were located both at X ∼ −14 RE. A dipolarization front (DF) of a localized fast flow was detected by Cluster and MMS, separated in the dawn‐dusk direction by ∼4 RE, almost simultaneously. Adiabatic electron acceleration signatures revealed from the comparison of the energy spectra confirm that both spacecraft encounter the same DF. We analyzed the change in the current sheet structure based on multi‐scale multi‐point data analysis. The current sheet thickened during the passage of DF, yet, temporally thinned subsequently associated with another flow enhancement centered more on the dawnward side of the initial flow. MMS and Cluster observed intense perpendicular and parallel current in the off‐equatorial region mainly during this interval of the current sheet thinning. Maximum field‐aligned currents both at MMS and Cluster are directed tailward. Detailed analysis of MMS data showed that the intense field‐aligned currents consisted of multiple small‐scale intense current layers accompanied by enhanced Hall‐currents in the dawn‐dusk flow‐shear region. We suggest that the current sheet thinning is related to the flow bouncing process and/or to the expansion/activation of reconnection. Based on these mesoscale and small‐scale multipoint observations, 3D evolution of the flow and current‐sheet disturbances was inferred preceding the development of a substorm current wedge. Key Points: Evolution of localized fast flows and dipolarization front is obtained from multi‐scale multi‐point observations in near‐Earth magnetotailCurrent sheet thinning accompanied by intense field‐aligned currents is detected following the passage of the dipolarization frontFrom signatures of adiabatic electron acceleration it is confirmed that the same flow front was detected by the multi‐point measurements [ABSTRACT FROM AUTHOR]

On December 08, 2018 the Twin Rocket Investigation of Cusp Electrodynamics 2 (TRICE 2) mission was successfully launched. The mission consisted of two sounding rockets, each carrying a payload capable of measuring electron and ion distributions, electric and magnetic fields, and plasma waves occurring in the northern magnetospheric cusp. This study highlights the ion and wave observations obtained by TRICE 2 in the cusp and observations from the magnetospheric multiscale (MMS) spacecraft at the low‐latitude magnetopause two hours prior to the TRICE 2 traversal of the cusp. Within the cusp, typical ion cusp features were observed, that is, energy‐latitude dispersion of injected magnetosheath plasma. However, a lower energy population was also measured near the equatorward edge of the cusp on open field lines. Pitch‐angle distributions of the low‐energy ions suggest that this population was magnetospheric in origin, and not from ionospheric upflows. Data from MMS show that counterstreaming ions were present in the outer magnetosphere and low‐latitude boundary layer at similar energies to those observed by TRICE 2 in the cusp. Correlations between the low‐energy ions within the cusp and broadband extremely low frequency waves suggest that the low‐energy magnetospheric ions that filled the flux tube may have undergone wave‐particle interactions. These interactions may cause pitch‐angle scattering of low‐energy magnetospheric ions closer to the loss cone, thereby allowing them to precipitate into the cusp and be measured by the TRICE 2 sounding rockets. Key Points: Low‐energy ions measured near the equatorward edge of the cusp by TRICE 2 are locally mirroring and propagating toward the ionosphereA similar counter‐streaming ion population is observed at the magnetopause by MMS two hours prior to the TRICE 2 cusp traversalIon and wave data in the cusp suggest wave‐particle interactions are pitch angle scattering low‐energy magnetospheric ions [ABSTRACT FROM AUTHOR]

We present multi‐point observations of foreshock bubbles (FBs) for comparison with the predictions of hybrid simulations. The four Magnetospheric Multiscale (MMS) spacecraft observed a series of discontinuities in the region upstream from the bow shock on December 18, 2017. Two solar wind discontinuities were associated with fully developed FBs with core regions exhibiting greatly decelerated and deflected antisunward flows, significant increases in temperature, and depressed plasma densities and magnetic field strengths. A single shock lies upstream (sunward) from each of the two bubbles. The connection to the foreshock before and/or after the discontinuities can play an important role in the foreshock transient formation. We present the predictions of a 2.5‐D global hybrid simulation for the presence of the FBs for two different interplanetary magnetic field (IMF) cone angles (66° and 25°). The topology of the simulated FB is nearly hemispherical for small angles, but becomes linear for large angles. We show that the characteristics of the two simulated FBs for the two different discontinuity normals and IMF cone angles are in good agreement with the observational results. Key Points: The Magnetospheric Multiscale (MMS) spacecraft observe two fully developed foreshock bubbles (FBs) whose properties are compared with the predictions of the hybrid code modelA global hybrid simulation predicts the characteristics of two FBs in the region upstream from the bow shockThe simulation results match MMS observations of the two FBs, but uncertainties associated with solar wind conditions need to be considered [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 (

High resolution observations of dense plasma impulsive structures moving through an ambient background magnetospheric flows were captured by the Magnetospheric Multiscale mission. The observations show particle heating and acceleration, shock‐like wave formation, and whistler wave excitation inside the interface between the dense impulsive plasma structures and the ambient plasma. A multiscale hybrid kinetic simulation provides an explanation of the observed wave‐particle interactions with the assumption that the dense plasma structures may be represented by plasma clouds which are formed at the magnetopause layer due to reconnection processes. Plain Language Summary: Dense, impulsive plasma structures moving through a background plasma were captured by the NASA Magnetospheric Multiscale mission. The observations show that the dense structures can generate strong perturbations in the electromagnetic field and shock‐like waves. Interactions between these electromagnetic waves and the particles results in particle acceleration. 3‐D hybrid kinetic modeling (particle description for ions and fluid description for electrons) was used to investigate the plasma physics of the observed structures. It was assumed that the plasma clouds were formed by magnetic field reconnection inside the magnetopause, which is the interface between the solar wind particles and the cold low‐density magnetospheric plasma. The work helps us understand the plasma environment at the interface between the Earth and solar wind, near planetary moons, within astrophysical explosions, and possibly at the interface between the solar wind and local interstellar medium. Key Points: MMS has captured high‐resolution observations of dense plasma structures moving through sub‐Alfvenic and supersonic ambient background magnetospheric flowsStrong acceleration of the ambient ions are observed at the interface between the plasma structure and the background plasmaThe observations, combined with hybrid kinetic modeling, provide for the first time insights into the physics of wave‐particle interactions triggered by dense impulsive structures moving through background plasmas [ABSTRACT FROM AUTHOR]

We present observations of mirror mode structures in the dawn-side magnetosphere from the Magnetospheric Multiscale Mission. The observations were made during a period of relatively stable northward interplanetary magnetic field conditions. We observed magnetic troughs only with reductions of up to 90% in the magnitude of the local magnetic field. The structures were closely aligned with the local magnetic field and had sizes perpendicular to the magnetic field of the order of 1Re and ion densities up to twice that of the surrounding regions. We used ion, magnetic field, and electric field data to examine the plasma conditions inside and outside of these structures to investigate the stability and evolution of the structures and estimate the magnitude of changes in the ion distributions. Comparing the ion bulk properties to theoretical predictions, we concluded that variations in the parameters with trough depth were due to the spacecraft traversing them at different locations along their length. Our calculations of heating and cooling of the ions at different pitch angles is consistent with theory. We conclude that these troughs were likely long-lived, bistable structures that originated in the magnetosheath near the nose of the magnetopause and were captured into the magnetosphere by dual-lobe reconnection. [ABSTRACT FROM AUTHOR]

Two‐dimensional Hall reconnection model can explain the fast release of magnetic energy and most of its predictions have been demonstrated by in situ satellite observations. However, the three‐dimensional effect to reconnection remains poorly known. Recent numerical simulations have shown that three‐dimensional evolution is more complex than two‐dimensional in that reconnection occurs in multiple sites that are not necessarily in the primary neutral current sheet. Here, we present the first observational evidence for localized secondary reconnection at the separatrix surface of a magnetic flux rope (MFR). This secondary reconnection occurs between the axial magnetic field of the MFR, which points out‐of‐plane of the Earth's magnetopause reconnection, and the magnetospheric field. This three‐dimensional electron‐scale reconnection in the exhaust facilitates the cross‐scale energy conversion from the macro‐scale down to electron‐scale. Plain Language Summary: Magnetic reconnection releases large amounts of magnetic energy in astrophysical, space and laboratory plasmas. For the decades, the reconnection has been investigated largely based on two‐dimensional models while the three‐dimensional effect to reconnection remains poorly known. It has been shown by numerical simulation that three‐dimensional evolution, because of the freedom in the out‐of‐plane direction, is turbulent and distinct from the two‐dimension laminar reconnection in that reconnection occurs in multiple sites that are not necessarily in the primary neutral current sheet. The turbulent reconnection is characterized by the formation and interaction of multiple flux ropes, or chaotic magnetic field lines in the outflow region. Although small‐scale magnetic flux ropes and their interactions have been observed, one important missing piece is whether reconnection occurs at the region away from the neutral current sheet. Here we present the first evidence for localized secondary reconnection at the separatrix surface of the primary reconnection. This secondary reconnection occurs between the axial magnetic field of the flux rope, which points out‐of‐plane of the magnetopause reconnection, and the magnetospheric field. This three‐dimensional patchy reconnection in the exhaust facilitates the cross‐scale energy conversion from the macro‐scale down to electron‐scale and leads to a turbulent evolution of reconnection. Key Points: Three‐dimensional secondary electron‐scale reconnection was observed at the separatrix of a flux rope in the magnetosphereNew light on the destiny of flux rope from a three‐dimensional perspective: dissipation through the reconnection of the core fieldThe cross‐scale energy conversion is through the formation and dissipation of flux ropes [ABSTRACT FROM AUTHOR]

Characteristics of the reconnection region are examined based on Magnetospheric Multiscale (MMS) observation of a plasma sheet boundary layer crossing on July 12, 2018 when both high-energy (>few keV) electrons as well as ions flowing parallel to the magnetic field showed distinct energy dispersion. Remote sensing techniques are applied to estimate location of reconnection region and injection time of these particles. The reconnection region is estimated to be located at X = (-23 ± 1.9) RE from electrons, comparable to that from ions, X = (-24.5 ± 0.7) RE by taking into account the time of flight effect as well as the effect of separatrix motion relative to the plasma. The electron injection times precede that of ions by ~6 s. We found that the time dispersed energetic electron beams away from X-line are accompanied by short-lived high-frequency parallel-electric field disturbances around plasma frequency. These wave packets are the first observable remote feature of reconnection. Using multi-point measurements of high energy ions, electrons, magnetic field, and parallel-electric field disturbances, reconnection electric field is estimated as 1.6-2.5 mV/m initially and decrease to ~0.8 mV/m within ~20 s. This study shows that the remote sensing scheme is an alternative method to infer the temporal/spatial changes of the magnetotail reconnection. Plain Language Summary Magnetic reconnection is a process in which magnetic field energy is converted to particle energy in association with a change of the topology of the magnetic field in a small region smaller than the plasma particles can complete its gyration. Although the reconnection region is small, the effect of the outflowing particles from reconnection can be detected remotely. We study the remote effects of magnetic reconnection based on an observation by the four Magnetospheric Multiscale (MMS) spacecraft on July 12, 2018. Based on the energy and species dependent features and difference among the spacecraft, we deduced the characteristics of the remote reconnection such as the reconnection rate and its evolution. [ABSTRACT FROM AUTHOR]

Observations from the Magnetospheric Multiscale (MMS) mission are used to quantify the maximum effect of magnetospheric H+ and He+ on dayside magnetopause reconnection. A data base of current-sheet crossings from the first 2 years of the MMS mission is used to identify magnetopause crossings with the highest He+ concentrations. While all of these magnetopause crossings exhibit evidence of plasmaspheric plume material, only half of the crossings are directly associated with plasmaspheric plumes. The He+ density varies dramatically within the magnetosphere adjacent to the magnetopause, with density variations of an order of magnitude on timescales as short as 10 s, the time resolution of the composition instrument on MMS. Plasma wave observations are used to determine the total electron density, and composition measurements are used to determine the mass density in the magnetosheath and magnetosphere. These mass densities are then used with the magnetic field observations to determine the theoretical reduction in the reconnection rate at the magnetopause. The presence of high-density plasmaspheric plume material at the magnetopause causes transient reductions in the reconnection rate of up to -40%. Plain Language Summary As the solar wind propagates from the Sun to the Earth, it encounters two boundaries that limit its access to the near-Earth environment. The first is a bow shock that heats, slows, and deflects the solar wind. The second is the Earth's magnetopause, where the shocked solar wind is deflected around the Earth's magnetic field region called the magnetosphere. Magnetic reconnection at the magnetopause creates an interconnection between the magnetic field of the shocked solar wind and the Earth's magnetic field. The rate at which these magnetic fields interconnect, the reconnection rate, depends on the amount of plasma (ions and electrons) on either side of the magnetopause. This research uses observations from the Magnetospheric Multiscale mission to look at this rate for times when there is substantial plasma on the magnetospheric side of the magnetopause. These instances are rare; however, when they do occur, they reduce the reconnection rate by a substantial amount (up to 40%). [ABSTRACT FROM AUTHOR]

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]

Magnetospheric multiscale (MMS) encountered the primary low‐latitude magnetopause reconnection site when the interspacecraft separation exceeded the upstream ion inertial length. Classical signatures of the ion diffusion region (IDR), including a subion‐Alfvénic demagnetized ion exhaust, a superion‐Alfvénic magnetized electron exhaust, and Hall electromagnetic fields, are identified. The opening angle between the magnetopause and magnetospheric separatrix is 30° ± 5°. The exhaust preferentially expands sunward, displacing the magnetosheath. Intense pileup of reconnected magnetic flux occurs between the magnetosheath separatrix and the magnetopause in a narrow channel intermediate between the ion and electron scales. The strength of the pileup (normalized values of 0.3 – 0.5) is consistent with the large angle at which the magnetopause is inclined relative to the overall reconnection coordinates. MMS‐4, which was two ion inertial lengths closer to the X line than the other three spacecraft, observed intense electron‐dominated currents and kinetic‐to‐electromagnetic‐field energy conversion within the pileup. MMS‐1, MMS‐2, and MMS‐3 did not observe the intense currents nor the particle‐to‐field energy conversion but did observe the pileup, indicating that the edge of the generation region was contained within the tetrahedron. Comparisons with particle‐in‐cell simulations reveal that the electron currents and large inclination angle of the magnetopause are interconnected features of the asymmetric Hall effect. Between the separatrix and the magnetopause, high‐density inflowing magnetosheath electrons brake and turn into the outflow direction, imparting energy to the normal magnetic field and generating the pileup. The findings indicate that electron dynamics are likely an important influence on the magnetic field structure within the ion diffusion region. Plain Language Summary: The Earth's and Sun's magnetic fields meet and can interconnect at the outermost boundary of the Earth's magnetosphere, the magnetopause. Reconnection of the two magnetic fields requires that the motions of the ions and electrons become decoupled from the motion of the field itself. Owing to their greater inertia, the ions becomes decoupled from the magnetic field within a much larger volume of space compared to the electrons. The demagnetization of ions and electrons during magnetic field reconnection are difficult to study simultaneously with in situ data, as both the larger ion and smaller electron scale sizes need to be simultaneously resolved. In this study, we report an observation of magnetopause reconnection with the magnetospheric multiscale (MMS) mission. MMS has instruments capable of resolving the electron scales, and we analyze an event for which the spacecraft were separated by a large enough distance to resolve the ion scales. We find that the electron dynamics are important for influencing the structure of the magnetic fields in the region where ion motions are decoupled from the field. Key Points: With ion‐scale interspacecraft separations magnetospheric multiscale crossed an ion diffusion region with embedded electron currentsIntense pileup of reconnected magnetic flux is observed within the ion diffusion region where the exhaust opens at a wide angleKinetic simulations indicate that the intense flux pileup may be a regular feature related to the asymmetric Hall electron currents [ABSTRACT FROM AUTHOR]

The Kelvin‐Helmholtz Instability (KHI) is thought to be an important driver of mass and momentum transfer from the solar wind to the Earth's magnetosphere. We present observations from NASA's Magnetospheric Multiscale mission of ion acoustic‐like waves associated with turbulence during the KHI interval on 8 September 2015. These parallel electrostatic waves are nonlinear, can have amplitudes in excess of 100 mV/m, and may be associated with a field‐aligned potential drop. We perform a survey of all vortices in the 8 September 2015 event, investigating the occurrence of electrostatic waves below the ion plasma frequency. We find that they tend to occur in the turbulent region in the middle of the KHI vortices, independent of the presence of compressed or intermittent currents. This suggests that the waves occur in the presence of plasma mixing in the vortex region and that they may play a part in the overall turbulent dissipation process. Key Points: Ion‐acoustic‐like waves occur within the Kelvin‐Helmholtz instability at Earth's magnetosphereThese waves appear in the vortex regions of the Kelvin‐Helmholtz instability and are not related to electric currentsThese waves are potentially due to mixing of plasma populations and may impact the turbulent dissipation process [ABSTRACT FROM AUTHOR]

We report the observation of solar wind‐magnetosphere‐ionosphere interactions using a series of flux transfer events (FTEs) observed by Magnetospheric MultiScale (MMS) mission located near the dayside magnetopause on 18 December 2017. The FTEs were observed to propagate duskward and either southward or slightly northward, as predicted under duskward and southward interplanetary magnetic field (IMF). The Cooling model also predicted a significant dawnward propagation of northward‐moving FTEs. Near the MMS footprint, a series of poleward‐moving auroral forms (PMAFs) occurred almost simultaneously with those FTEs. They propagated poleward and westward, consistent with the modeled FTE propagation. The intervals between FTEs, relatively consistent with those between PMAFs, strongly suggest a one‐to‐one correspondence between the dayside transients and ionospheric responses. The FTEs embedded in continuous reconnection observed by MMS and corresponding PMAFs individually occurred during persistent auroral activity recorded by an all‐sky imager strongly indicate that those FTEs/PMAFs resulted from the temporal modulation of the reconnection rate during continuous reconnection. With the decay of the PMAFs associated with the FTEs, patch‐like plasma density enhancements were detected to form and propagate poleward and then dawnward. Propagation to the dawn was also suggested by the Super Dual Auroral Radar Network (SuperDARN) convection and Global Positioning System (GPS) total electron content data. We relate the temporal variation of the driving solar‐wind and magnetospheric mechanism to that of the high‐latitude and polar ionospheric responses and estimate the response time. Plain Language Summary: The solar wind‐magnetosphere coupling often occurs in a nonsteady manner. Such disturbances modify the magnetosphere‐ionosphere system. One of the most common/important processes of such nonsteady phenomena is time‐dependent dayside reconnection, as observationally evidenced by flux transfer events (FTEs), which have been, in turn, represented by ionospheric poleward‐moving auroral forms (PMAFs). Decaying PMAFs have, then, been followed by the occurrence of polar cap patches, which are regions of plasma density enhancements observed in the polar cap. This study indicates a sequence of dynamic processes from the driving solar wind, nonsteady reconnection/FTEs, PMAFs, and polar cap patches. Although a portion of this link has been studied and reported, the complete sequence of these connections and full picture of the solar wind‐magnetopause‐ionosphere coupling have rarely been reported and examined. The series of FTEs embedded in continuous reconnection and corresponding PMAFs individually occurred during persistent auroral activity strongly indicate that the FTEs and PMAFs were in response to the temporal modulation of the reconnection rate during continuous reconnection, rather than the repeated, complete turn‐on and turnoff of dayside reconnection. Our study advances the knowledge of Magnetosphere‐Ionosphere‐Coupling near the cusp region and in the open field‐line region that has been less developed than on closed field lines. Key Points: Multiple flux transfer events (FTEs) were observed to form on the dayside magnetopause under southward and duskward IMFGround‐based observations indicate the poleward‐moving plasma streams or auroral forms (PMAFs) associated with the FTEsDevelopment and transpolar motion of polar cap patches following the PMAFs complete the link of solar wind‐magnetosphere‐ionosphere coupling [ABSTRACT FROM AUTHOR]

In the magnetosheath, intense whistler mode waves, called "Lion roars," are often detected in troughs of magnetic field intensity in mirror mode structures. Using data obtained by the four Magnetospheric Multiscale (MMS) spacecraft, we show that reversals of gradient of magnetic field intensity along the magnetic field correspond to reversals of the field‐aligned component of Poynting flux of whistler mode waves in the troughs. Such a characteristic is consistent with the idea that the whistler mode waves are effectively generated near the local minima of magnetic field intensity because of the smallest cyclotron resonance velocity and propagate toward regions of larger magnetic field intensity along the magnetic field lines on both sides. We use the reversal of the Poynting flux as an indicator of wave source regions. In these regions, we find that pancake or an outer edge of butterfly electron distributions above ~100 eV are good candidates for wave generation. Unclear correlations of phase difference and amplitude variations of whistler mode waves in cases of ~40 km spacecraft separation indicate that a simple plane wave approximation with a constant amplitude is not valid at this spatial scale that is much smaller than the ion gyroradius. The whistler mode waves consist of small coherent wave packets from multiple sources with spatial scales smaller than tens of electron gyroradii transverse to the background magnetic field in a mirror mode structure. Key Points: Whistler mode waves are generated near local minima of magnetic field intensity and propagate along field lines on both sidesPancake or an outer edge of butterfly electron distributions above ~100 eV are good candidates for wave generationThe waves consist of many small coherent wave packets of tens of electron gyroradii perpendicular to magnetic fields [ABSTRACT FROM AUTHOR]

The four Magnetospheric Multiscale (MMS) spacecraft observed energetic (E>50 keV) ion bursts exhibiting an inverse dispersion in the magnetosheath on 28 December 2015. We consider the possibility that these ions originate from the magnetosphere. The ion composition ratios, flux levels, and the spectral slopes of the energetic ion energy spectra observed in the foreshock and in the magnetosheath resemble those in the outer magnetosphere but differ significantly from those seen further upstream from the bow shock at ACE. The particle gyrocenters lie earthward from the spacecraft, indicating that the maximum ion fluxes come from close to the magnetosphere. We provide important evidence that argues against an explanation of the particle source in terms of hot flow anomaly acceleration. A three‐dimensional global hybrid simulation shows that escaping magnetospheric ions can be scattered and transported across the magnetosheath. Ground magnetometer observations suggest that a solar wind pressure increase accelerates escaping magnetospheric ions via betatron acceleration, resulting in an inverse energy dispersion in the magnetosheath. However, there are no pressure changes detected on the MMS and ACE spacecraft and the ground magnetic field strength does not appear to be large enough to be consistent with the large magnetospheric compression needed to account for a betatron acceleration. Therefore, we suggest that the inverse energy dispersion event can be explained by a magnetic field rotation that connects MMS to the subsolar magnetosphere, enabling high‐energy particles from deep within the inner magnetosphere gain access to the magnetopause and magnetosheath. Key Points: The four MMS spacecraft observed an energetic ion burst exhibiting an inverse dispersion in the magnetosheathThe energetic ion energy spectra observed in the foreshock and in the magnetosheath resemble those in the outer magnetosphereA 3‐D global hybrid simulation shows that escaping magnetospheric ions can be scattered and transported across the magnetosheath [ABSTRACT FROM AUTHOR]

The Kelvin‐Helmholtz instability (KHI) at Earth's magnetopause and associated turbulence are suggested to play a role in the transport of mass and momentum from the solar wind into Earth's magnetosphere. We investigate electromagnetic turbulence observed in Kelvin‐Helmholtz vortices encountered at the dusk flank magnetopause by the Magnetospheric Multiscale (MMS) spacecraft under northward interplanetary magnetic field (IMF) conditions in order to reveal its generation process, mode properties, and role. A comparison with another MMS event at the dayside magnetopause with reconnection but no KHI signatures under a similar IMF condition indicates that while high‐latitude magnetopause reconnection excites a modest level of turbulence in the dayside low‐latitude boundary layer, the KHI further amplifies the turbulence, leading to magnetic energy spectra with a power law index −5/3 at magnetohydrodynamic scales even in its early nonlinear phase. The mode of the electromagnetic turbulence is analyzed with a single‐spacecraft method based on Ampère's law, developed by Bellan (2016, https://doi.org/10.1002/2016JA022827), for estimating wave vectors as a function of spacecraft frame frequency. The results suggest that the turbulence does not consist of propagating normal‐mode waves but is due to interlaced magnetic flux tubes advected by plasma flows in the vortices. The turbulence at sub‐ion scales in the early nonlinear phase of the KHI may not be the cause of the plasma transport across the magnetopause but rather a consequence of three‐dimensional vortex‐induced reconnection, the process that can cause an efficient transport by producing tangled reconnected field lines. Plain Language Summary: Turbulence is ubiquitous in nature and plays an important role in material mixing and energy transport. Turbulence in space plasmas is characterized by fluctuations of flow velocity and/or electromagnetic fields over a broad frequency range and/or length scales and is believed to be the key to efficient plasma transport and heating. However, its generation mechanism is not fully understood because turbulence in space is often fully developed or already relaxed when observed. By analyzing high‐resolution plasma and electromagnetic field data taken by the Magnetospheric Multiscale spacecraft, we study the generation process of electromagnetic turbulence at the outer boundary of Earth's magnetosphere, called the magnetopause, where either a flow shear‐driven Kelvin‐Helmholtz instability or magnetic reconnection or both could drive turbulence. It is shown that while dayside reconnection generates a modest level of turbulence at the magnetopause near noon, the flow shear instability further amplifies the turbulence at the flank magnetopause. Our analysis also suggests that the turbulence may not be the primary cause of plasma transport from solar wind into the magnetosphere but rather a consequence of the flow shear‐induced reconnection that is likely the primary cause of plasma transport at the dayside flank under northward solar wind magnetic field conditions. Key Points: The Kelvin‐Helmholtz instability (KHI) amplifies electromagnetic fluctuations in the magnetopause boundary layerThe turbulent fluctuations in the vortices may not be due to propagating waves but to magnetic structures, that is, interlaced flux tubesThe observed turbulent power law spectra at sub‐ion scales are consistent with those in kinetic simulations of KHI‐driven reconnection [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]

When the interplanetary magnetic field (IMF) is southward and has a substantial Y component, reconnection at the magnetopause occurs at low latitudes. Under these conditions, the maximum magnetic shear model for the reconnection X‐line at the magnetopause predicts a continuous X‐line stretching from the dawn to dusk terminators. During the solstices, the X‐line is not at the subsolar point and may be located in a region where the magnetosheath bulk flow is super‐Alfvenic. For a fixed IMF direction, the maximum shear model also predicts a stationary X‐line. In response to IMF clock angle changes on the timescale of minutes, the X‐line moves on the same timescale. The stationarity of the reconnection X‐line is testable observationally under certain, restrictive conditions. This stationarity is tested using observations from the Magnetospheric Multiscale mission. For two events, the distance from the Magnetospheric Multiscale spacecraft to the X‐line is constant over several minutes (within relatively large error bars) and the X‐line is also near the location predicted by the maximum magnetic shear model. Thus, the reconnection X‐line at the magnetopause appears to be quasi‐stationary for constant IMF clock angle. These observations also place constraints on the formation and motion of multiple X‐lines at the magnetopause. Key Points: The maximum magnetic shear model predicts a stationary reconnection X‐line in a region where the magnetosheath flow is likely to be super‐AlfvenicObservations of transmitted and return beams in the low latitude boundary layer for some restrictive conditions show that there is a quasi‐stationary reconnection X‐line near the predicted locationAlternatively, there could be multiple X‐lines formed at the magnetopause and these observations place constraints on the formation frequency and motion of multiple X‐lines [ABSTRACT FROM AUTHOR]

One of the major unresolved questions regarding the magnetic reconnection phenomenon is how plasma waves impact the process. In 2015, the National Aeronautics and Space Administration launched the four‐satellite Magnetospheric Multiscale Mission to study magnetic reconnection, especially on the electron scale. Since launch, it has identified several wave modes below the electron plasma frequency that occur near the dayside reconnection X‐line. These include large‐amplitude parallel electrostatic waves, whistler mode waves, lower hybrid waves, and turbulence associated with a corrugated current structure. We survey 23 electron diffusion region events observed by Magnetospheric Multiscale Mission at the dayside magnetopause to help understand how these wave modes impact the reconnection process. Common wave modes are identified, as well as their typical location within the reconnection layer (e.g., electron diffusion region, ion diffusion region, separatrix, and inflow and outflow jets). We find that, with a few exceptions, electromagnetic whistlers are most commonly confined to the separatrices of the exhaust boundary. Lower hybrid waves are found on the magnetosphere side of the current layer and do not make it to the X‐line. The wave modes that typically occur closest to the dissipation region are the corrugated current structures and large‐amplitude parallel electrostatic waves. Key Points: We survey waves below the electron plasma frequency near 23 dayside magnetopause electron diffusion region eventsThe main waves investigated are whistlers, parallel electrostatic waves, current corrugation, and the lower hybrid drift instabilityWe find waves most typically occur on the magnetospheric side, with current corrugation most likely to reach the diffusion region [ABSTRACT FROM AUTHOR]

Data are analyzed from a Magnetospheric Multiscale encounter with a dayside magnetopause reconnection region on 29 December 2016. The uniqueness of the event stems from the small (~7 km) average spacecraft separation and the sequential sampling of an electron diffusion region with electron crescent distributions. We quantitatively investigate the earthward acceleration of magnetosheath electrons through the in‐plane null by the polarization electric field EN that points radially outward from the magnetopause. The results compare favorably with previous plasma simulations with one important difference that the reconnection electric field (EM) extends throughout the region of strong EN so that both fields energize electrons in the same region. This acceleration is quantified here for the first time. As the spacecraft penetrate deeper into the region of enhanced EN, the magnetic reflection of lower‐energy electrons produces a thinner crescent. Plain Language Summary: Reconnection causes the conversion of magnetic energy to particle energy and the breaking and reconnection of solar wind and geomagnetic field lines. In asymmetric reconnection at the dayside magnetopause with different plasma densities and magnetic field strengths on the Earth and Sun sides of an X‐line, this energy conversion is displaced to the earthward side of the current layer. An electric field pointing outward normal to the dayside magnetopause drives reconnection by accelerating solar wind electrons across the reconnection current sheet. These electrons take up dawnward‐directed meandering orbits, and they are ultimately the carriers of the reconnection current. This paper reports the first direct measurement of this acceleration. Key Points: Parallel and perpendicular energy conversion and electron acceleration are studied with data from four spacecraft with average separations of 7 kmThe magnetopause normal electric field (EN) accelerates magnetosheath electrons across the reconnection current sheet to form crescent distributionsMagnetic reflection causes the crescent energy peaks to become narrower with increasing depth into the diffusion region [ABSTRACT FROM AUTHOR]

Data from the Magnetospheric Multiscale (MMS) mission, in particular, the spacecraft potential measured with and without the ion beams of the active spacecraft potential control (ASPOC) instruments, plasma electron moments, and the electric field, have been employed for an improved determination of plasma density based on spacecraft potential. The known technique to derive plasma density from spacecraft potential sees the spacecraft behaving as a plasma probe which adopts a potential at which the ambient plasma current and one of photoelectrons produced at the surface and leaving into space are in equilibrium. Thus, the potential is a function of the plasma current, and plasma density can be determined using measurements or assumptions on plasma temperature. This method is especially useful during periods when the plasma instruments are not in operation or when spacecraft potential data have significantly higher time resolution than particle detectors. However, the applicable current–voltage characteristic of the spacecraft has to be known with high accuracy, particularly when the potential is actively controlled and shows only minor residual variations. This paper demonstrates recent refinements of the density determination coming from: 1) the reduction of artifacts in the potential data due to the geometry of the spinning spacecraft and due to effects of the ambient electric field on the potential measurements and 2) a calibration of the plasma current to the spacecraft surfaces which is only possible by comparison with the variable currents from the ion beams of ASPOC. The results are discussed, and plasma densities determined by this method are shown in comparison with measurements by the Fast Plasma Instrument (FPI) for some intervals of the MMS mission. [ABSTRACT FROM AUTHOR]

While vorticity defined as the curl of the velocity has been broadly used in fluid and plasma physics, this quantity has been underutilized in space physics due to low time resolution observations. We report Magnetospheric Multiscale (MMS) observations of enhanced electron vorticity in the vicinity of the electron diffusion region of magnetic reconnection. On 11 July 2017 MMS traversed the magnetotail current sheet, observing tailward‐to‐earthward outflow reversal, current‐carrying electron jets in the direction along the electron meandering motion or out‐of‐plane direction, agyrotropic electron distribution functions, and dissipative signatures. At the edge of the electron jets, the electron vorticity increased with magnitudes greater than the electron gyrofrequency. The out‐of‐plane velocity shear along distance from the current sheet leads to the enhanced vorticity. This, in turn, contributes to the magnetic field perturbations observed by MMS. These observations indicate that electron vorticity can act as a proxy for delineating the electron diffusion region of magnetic reconnection. Plain Language Summary: Magnetic reconnection, causing explosive magnetic energy conversion into particle energy, is one of the most fundamental physical processes occurring both within the heliosphere and throughout the universe. The multiscale kinetic structures associated with reconnection have long been a focus in space plasma physics. We investigated how electron vorticity, a physical quantity widely used in fluid physics, but underutilized in the plasma, in particular, reconnection physics, enables us to delineate multiscale kinetic boundaries of reconnection sites using the unprecedented time resolution data from National Aeronautics and Space Administration's Magnetospheric Multiscale spacecraft. The magnitude of electron vorticity to be compared with the electron gyrofrequency provides a frame‐independent indicator of the electron diffusion region, therefore, greatly advancing our ability to delineate the multiscale reconnection boundaries. This study, directly relevant to plasma/reconnection physics, will improve our understanding of fundamental physics with far‐reaching implications in astrophysics. Key Points: We report MMS observation of enhanced electron vorticity in the reconnection siteThe magnitude of electron vorticity greater than the electron gyrofrequency indicates the electron diffusion region of magnetic reconnectionThe enhanced electron vorticity contributes to magnetic field perturbations observed by MMS [ABSTRACT FROM AUTHOR]

This study analyzes 25 ion‐scale flux ropes in the Magnetospheric Multiscale (MMS) observations to determine their structures. The high temporal and spatial resolution MMS measurements enable the application of multispacecraft techniques to ion‐scale flux ropes. Flux ropes are identified as quasi‐one‐dimensional (quasi‐1‐D) when they retain the features of reconnecting current sheets; that is, the magnetic field gradient is predominantly northward or southward, and quasi‐2‐D when they exhibit circular cross sections; that is, the magnetic field gradients in the plane transverse to the flux rope axis are comparable. The analysis shows that the quasi‐2‐D events have larger core fields and smaller pressure variations than the quasi‐1‐D events. These two types of flux ropes could be the result of different processes, including magnetic reconnection with different dawn‐dusk magnetic field components, temporal transformation of flattened structure to circular, or interactions with external environments. Plain Language Summary: Magnetic flux ropes are fundamental magnetic structures in space plasma physics and are commonly seen in the universe, such as, astrophysical jets, coronal mass ejections, and planetary magnetospheres. Flux ropes are important in mass and energy transport across plasma and magnetic boundaries, and they are found in a wide range of spatial sizes, from several tens of kilometers, that is, ion‐scale flux ropes, to tens of millions of kilometers, that is, coronal mass ejections, in the solar system. The ion‐scale flux ropes can be formed during magnetic reconnection and are hypothesized to energize electrons and influence the reconnection rate. Previous examinations of the structure of ion‐scale flux ropes were greatly limited by measurement resolution. The unprecedented Magnetospheric Multiscale (MMS) mission high temporal and spatial resolution measurements provide a unique opportunity to investigate flux rope structures. By employing multispacecraft techniques, this study has provided new insights into the magnetic field variations and dimensionality of ion‐scale flux ropes in the Earth's magnetotail. The results are consistent with the evolution of ion‐scale flux ropes from initially flattened current sheet‐like flux ropes near the time of formation into lower energy state with circular cross section predicted by theory and termed as the "Taylor" state. Key Points: Ion‐scale flux ropes are observed to have either flattened or circular cross sections using MDD and GS reconstructionAnalysis of 25 flux ropes show that circular cross‐section flux ropes have stronger core field and smaller thermal pressures than flattened flux ropesThe two types of flux ropes may be the results of reconnection, temporal evolution, or interactions with external environment [ABSTRACT FROM AUTHOR]

The electron rolling pin distribution, showing electron pitch angles primarily at 0°, 90°, and 180°, has recently been observed behind dipolarization fronts (DFs) and explained using an analytical model. However, the energy range of such distribution has been unknown so far, owing to the low‐resolution data in previous spacecraft missions. Here using the high‐resolution measurements of Magnetospheric Multiscale, we reveal the energy range of electron rolling pin distribution behind DFs for the first time. We find that such distribution appears only above 1.7 keV, falling well into the suprathermal energy range. Below 1.7 keV, electrons exhibit a Maxwell distribution, while above 1.7 keV, they exhibit a power law distribution. In addition, such distribution appears primarily in the growing phase of the flow and disappears quickly in the decaying phase. During the formation of the rolling pin distribution, electrons are gyrotropic. These findings have greatly improved our knowledge of electron dynamics around DFs. Plain Language Summary: Examining the electron pitch angle distribution can help us understand the electron dynamics behind dipolarization fronts (DFs). Up to now, the energy range of electron rolling pin distribution, which shows electron pitch angles primarily at 0°, 90°, and 180°, has been unknown, owing to the low‐resolution data in previous spacecraft missions. Here using the high‐resolution Magnetospheric Multiscale measurements, we reveal the energy range of electron rolling pin distribution behind DFs for the first time. Our findings can significantly improve the knowledge of electron dynamics around DFs. Key Points: Rolling pin distribution appears only above 1.7 keV, falling well into suprathermal energy rangeBelow 1.7 keV, electrons have Maxwell distribution; above 1.7 keV, they have power law distributionDuring the formation of the rolling pin distribution, electrons are gyrotropic [ABSTRACT FROM AUTHOR]

Using data from the MMS mission and the First‐Order Taylor Expansion (FOTE) method, here we reveal electron distribution functions around a reconnection X‐line at the Earth's magnetopause. We find cigar distribution of electrons in both the magnetosphere‐side and magnetosheath‐side inflow regions, isotropic distribution of electrons at the separatrix, and loss of high‐energy electrons in the antiparallel direction in the magnetosheath‐side inflow region. We interpret the formation of cigar distribution in the inflow regions using the Fermi mechanism—as suggested in previous simulations, the loss of high‐energy electrons in the magnetosheath side using the parallel electric fields—which evacuate electrons to escape the diffusion region along the antiparallel direction, and the isotropic distribution at the separatrix using the pitch angle scattering by whistler waves—which exist frequently at the separatrix. We also find that the electron distribution functions can change rapidly (within 60 ms) from isotropic to cigar as the spacecraft moves slightly away from the separatrix. Plain Language Summary: Magnetic reconnection is a key process responsible for many explosive phenomena in nature such as solar flares and magnetospheric substorms. Up to now, the electron behaviors (particularly electron distribution functions) around the reconnection X‐line have not been well revealed, because the X‐line topology is unavailable in previous study. Using data from the MMS mission and the newly developed FOTE method, here we investigate electron distribution functions around a reconnection X‐line and explain the formation mechanisms of these distributions successfully. Key Points: For the first time, we reveal electron distributions around a reconnection X‐line using the FOTE method, rather than cartoons or simulationsWe find different types of electron distributions around the X‐line, and explain the formation mechanisms of these distributions successfullyWe find a rapid change of electron distribution within 60 ms. Such rapid change can only be revealed by FOTE but not other methods [ABSTRACT FROM AUTHOR]

One of the most important properties of Earth's magnetotail thin current sheet (TCS) is that its current is predominantly contributed by magnetized electrons. The Hall electric field, normal to the TCS and generated by charge separation, is critical to the generation of this electron current as well as a dawn‐dusk asymmetry of the magnetotail, such as duskside preference of magnetic reconnection and related structures and phenomena. However, systematic investigation of the Hall electric field has so far been lacking. Utilizing observations of TCS by Magnetospheric Multiscale and Time History of Events and Macroscale Interactions during Substorm spacecraft, we study the properties of this field. Our results, from various, complementary methods, show that the Hall electric field Ez (in the geocentric solar magnetospheric, coordinate system) or En (normal to the TCS plane) can be clearly observed to point toward the center of the current sheet. The typical magnitude of this electric field is several tenths of 1 mV/m. Statistics of Magnetospheric Multiscale magnetotail TCS crossings show that the Hall electric field is stronger on the duskside, indicating a stronger Hall effect there, which confirms predictions from global‐scale hybrid and particle‐in‐cell simulations. Key Points: We utilize MMS and THEMIS observations of the Hall electric field in Earth's magnetotail TCSThe Hall electric field is directed toward the center of the TCS, with an average magnitude of several tenths of 1 mV/mStatistics show that the Hall electric field is stronger on the duskside, consistent with predictions from simulations [ABSTRACT FROM AUTHOR]

The Fast Plasma Investigation (Pollock et al., , https://doi.org/10.1007/s11214-016-0245-4; FPI) onboard the Magnetospheric Multiscale mission (Burch, Moore, et al., , https://doi.org/10.1007/s11214-015-0164-9; MMS) uses a discrete wavelet transform and bit plane encoder (DWT/BPE; Winterrowd et al., , https://doi.org/10.1109/AERO.2010.5446664) for data compression. This is the first plasma spectrometer suite to use this method of compression and thus serves as a benchmark for future plasma spectrometers. Flight data from MMS confirm viability of this compression algorithm in large areas of the magnetosphere. Because much of the FPI data were compressed losslessly on orbit, this flight data can be used as seed data to investigate performance of the DWT/BPE‐based compression at increased rates of compression. In this study, data from several representative regions of the magnetosphere have been compressed to increasingly small sizes and the resulting error was analyzed. Wavelet‐based compression is shown to be effective in all regions of the magnetosphere and solar wind for plasma count data, with performance varying with local environment. Specifically, plasma distributions that are characterized by low temperature and/or low density are compressed better leading to excellent performance in plasma regions such as solar wind and the magnetosheath. In general, ion data are compressed better than electron data, primarily due to a higher drift velocity of ions relative to their thermal speed and lower count rates. DWT/BPE compression can therefore be recommended for future instruments measuring count data in Earth's magnetosphere and in solar wind. Plain Language Summary: Space science missions are using increasingly advanced instrumentation and are taking larger and larger amounts of data. As such, better forms of data compression are required. This work explores using a wavelet‐based data compression scheme to compress plasma count data. The results indicate that wavelet compression can store plasma count data in a volume roughly 10 times smaller than techniques used for previous space missions with low error introduced. Key Points: Wavelet compression can be used to compress plasma count data with an error of the order of Poisson noiseWavelet compression is more efficient in lower density, lower temperature plasmasElectron data compression is less efficient than ion data, and the efficiency varies according to the plasma regime explored [ABSTRACT FROM AUTHOR]

High‐resolution multispacecraft magnetic field measurements from the Magnetospheric Multiscale mission's flux‐gate magnetometer are employed to examine statistical properties of plasma turbulence in the terrestrial magnetosheath and in the solar wind. Quantities examined include wave number spectra; structure functions of order two, four, and six; probability density functions of increments; and scale‐dependent kurtoses of the magnetic field. We evaluate the Taylor frozen‐in approximation by comparing single‐spacecraft time series analysis with direct multispacecraft measurements, including evidence based on comparison of probability distribution functions. The statistics studied span spatial scales from the inertial range down to proton and electron scales. We find agreement of spectral estimates using three different methods, and evidence of intermittent turbulence in both magnetosheath and solar wind; however, evidence for subproton‐scale coherent structures, seen in the magnetosheath, is not found in the solar wind. Plain Language Summary: The unique measurement capabilities of the Magnetospheric Multiscale mission are employed to investigate turbulence in the terrestrial magnetosheath and in the solar wind. These statistical analyses extend our knowledge of the turbulent environment in these near‐Earth space plasmas. Key Points: Structure functions up to 6th order are computed in magnetosheath and solar wind; differences are noted in the statistics in the two casesAgreement between single‐ and two‐spacecraft results confirms Taylor "frozen‐in" hypothesis at subproton scalesStrong subproton‐scale intermittency is obtained for magnetosheath, but not for solar wind [ABSTRACT FROM AUTHOR]

Magnetic flux ropes of various scale sizes have been observed at the Earth's magnetopause for four decades. These multiple structures resulting from reconnection have complex internal field and plasma signatures, and evolve as they propagate along the dayside magnetopause. Here plasma and magnetic field observations from the Magnetospheric Multiscale (MMS) mission are used to describe a different type of large‐scale multiple reconnection, magnetic flux rope‐like structure at the Earth's magnetopause. These observations show at least two X lines separated by many Earth radii. Unlike smaller‐scale flux ropes or flux transfer events, these multiple X lines are stationary and consist of primary and secondary X lines. The secondary X line is either transient in time or does not reconnect all of the magnetic flux that reconnects at the primary X line. Several examples of these large‐scale reconnection structures are tabulated. These examples indicate that this type of structure may be common at the magnetopause at least for a narrow range of interplanetary magnetic field clock angles. Plain Language Summary: Magnetic reconnection at the Earth's magnetopause is the driver for the interaction between the solar wind and Earth, that is, space weather. This reconnection is usually considered to occur along a long reconnection X line across the magnetopause. However, the observations here show that there are multiple X lines at the magnetopause where reconnection is occurring. These X lines do not appear to move and there appears to be a primary X line and at least one secondary X line. Key Points: Multiple reconnection X lines are observed at the Earth's magnetopauseTheir particle signatures indicate that the X lines are quasi‐stationary and far apart from one another [ABSTRACT FROM AUTHOR]

We report MMS observations of the ion‐scale flux transfer events (FTEs) that may involve two main X lines and tearing instability between the two X lines. The four spacecraft detected multiple isolated regions with enhanced magnetic field strength and bipolar Bn signatures normal to the nominal magnetopause, indicating FTEs. The currents within the FTEs flow mostly parallel to B, and the magnetic tension force is balanced by the total pressure gradient force. During these events, the plasma bulk flow velocity was directed southward. Detailed analysis of the magnetic and electric field and plasma moments variations suggests that the FTEs were initially embedded within the exhaust region north of an X line but were later located southward/downstream of a subsequent X line. The cross sections of the individual FTEs are in the range of ~2.5–6.8 ion inertial lengths. The observations suggest the formation of multiple secondary FTEs. The presence of an X line in the exhaust region southward of a second X line results from the southward drift of an old X line and the reformation of a new X line. The current layer between the two X lines is unstable to the tearing instability, generating multiple ion‐scale flux‐rope‐type secondary islands. Key Points: MMS observation of ion‐scale flux ropes in the reconnection outflow regionThe initial X line is embedded in the exhaust region downstream of a second X lineFlux transfer events (FTE) are formed between the two X lines due to tearing instability [ABSTRACT FROM AUTHOR]

Abstract: Three flux ropes associated with near‐Earth magnetotail reconnection are analyzed using Magnetospheric Multiscale observations. The flux ropes are Earthward propagating with sizes from ∼3 to 11 ion inertial lengths. Significantly different axial orientations are observed, suggesting spatiotemporal variability in the reconnection and/or flux rope dynamics. An electron‐scale vortex, associated with one of the most intense electric fields (E) in the event, is observed within one of the flux ropes. This E is predominantly perpendicular to the magnetic field (B); the electron vortex is frozen‐in with E × B drifting electrons carrying perpendicular current and causing a small‐scale magnetic enhancement. The vortex is ∼16 electron gyroradii in size perpendicular to B and potentially elongated parallel to B. The need to decouple the frozen‐in vortical motion from the surrounding plasma implies a parallel E at the structure's ends. The formation of frozen‐in electron vortices within reconnection‐generated flux ropes may have implications for particle acceleration. [ABSTRACT FROM AUTHOR]

Abstract: It has been proposed that, in the near‐Earth magnetotail, earthward propagating flux ropes can merge with the Earth's dipole magnetic field and dissipate its magnetic energy. However, the reconnection diffusion region related to this process has not been identified. Here we report the first in situ observation of magnetic reconnection between an earthward propagating flux rope and the closed magnetic field lines connecting to Earth. Magnetospheric Multiscale (MMS) spacecraft crossed a vertical current sheet between the leading edge of the flux rope (negative Bz) and the geomagnetic field (positive Bz). The subion‐scale current sheet, super‐Alfvénic electron outflow, Hall magnetic and electric field, conversion of magnetic energy to plasma energy (J·E > 0), and magnetic null were observed during the crossing. All the above signatures indicate that MMS detected the reconnection diffusion region. This result is also relevant to other planets with intrinsic magnetosphere. [ABSTRACT FROM AUTHOR]

We report observations from the Magnetospheric Multiscale satellites of reconnecting current sheets in the magnetosheath over a range of out‐of‐plane "guide" magnetic field strengths. The currents exhibit nonideal energy conversion in the electron frame of reference, and the events are within the ion diffusion region within close proximity (a few electron skin depths) to the electron diffusion region. The study focuses on energy conversion on the electron scale only. At low guide field (antiparallel reconnection), electric fields and currents perpendicular to the magnetic field dominate the energy conversion. Additionally, electron distributions exhibit significant nongyrotropy. As the guide field increases, the electric field parallel to the background magnetic field becomes increasingly strong, and the electron nongyrotropy becomes less apparent. We find that even with a guide field less than half the reconnecting field, the parallel electric field and currents dominate the dissipation. This suggests that parallel electric fields are more important to energy conversion in reconnection than previously thought and that at high guide field, the physics governing magnetic reconnection are significantly different from antiparallel reconnection. Key Points: We observe ion diffusion regions and electron jets in the magnetosheath for multiple eventsThe dissipation due to the parallel electric field becomes more significant at increasing guide magnetic fieldThis may be associated with a dissipative electric field along the direction of the electron jet [ABSTRACT FROM AUTHOR]

NASA’s magnetospheric multiscale (MMS) mission was successfully launched in March 2015. The scientific objectives of MMS are to explore and understand fundamental plasma physics processes in the earth’s magnetosphere: magnetic reconnection, particle acceleration, and turbulence. The region of scientific interest of MMS is in a tenuous plasma environment where the positive spacecraft potential may reach an equilibrium as high as several tens of volts. The active spacecraft potential control (ASPOC) instrument neutralizes the spacecraft potential by releasing the positive charge produced by indium ion emitters. While the method has successfully been applied to other spacecraft such as Cluster and Double Star, new developments in the design of the emitters and the electronics are enabling lower spacecraft potentials and higher reliability compared to previous missions. In this paper, we report the initial results from the tests of the ASPOC performance during the commissioning phase and discuss the different effects on the particle and field instruments observed at different plasma environments in the magnetosphere. [ABSTRACT FROM PUBLISHER]

Magnetic reconnection at Earth's magnetopause is usually described by two different scenarios, antiparallel and component reconnection. The Maximum Magnetic Shear model combines these two scenarios at specific connection points known as the Knee regions. Using a database of confirmed magnetopause reconnection locations observed during Phase 1a of the Magnetospheric Multiscale mission, a recent study showed that the model predicts the reconnection locations correctly within 2 Earth radii 80% of the time. The study also revealed and confirmed the existence of anomalies, that is, a specific set of conditions/parameters for which the observed reconnection locations are significantly different than the predicted locations. The first anomaly, described in an earlier study, occurs during the equinoxes for events with interplanetary magnetic field (IMF) clock angles around 120° and 240°. Another previously unknown anomaly was found for events observed around December that also occurs for the same IMF clock angle ranges as the first anomaly. Several of the anomalous December events were observed in the dawn sector Knee region and show that a combination of a large dipole tilt with an IMF clock angle of about 140° causes the magnetopause antiparallel reconnection region to line up along the draped IMF. That causes a deflection of the Knee points and the predicted reconnection location. These events demonstrate that magnetic reconnection at the Earth's dayside magnetopause preferentially occurs in the antiparallel locations and only occurs along component reconnection line segments when the draped IMF field lines at the magnetopause no longer contact an antiparallel reconnection region. Key Points: Antiparallel magnetic reconnection is dominant at the Earths magnetopauseAntiparallel magnetic reconnection seems to control where component reconnection will occur [ABSTRACT FROM AUTHOR]

Magnetic reconnection is an energy conversion process that occurs in many astrophysical contexts including Earth's magnetosphere, where the process can be investigated in situ by spacecraft. On 11 July 2017, the four Magnetospheric Multiscale spacecraft encountered a reconnection site in Earth's magnetotail, where reconnection involves symmetric inflow conditions. The electron-scale plasma measurements revealed (i) super-Alfvénic electron jets reaching 15,000 kilometers per second; (ii) electron meandering motion and acceleration by the electric field, producing multiple crescent-shaped structures in the velocity distributions; and (iii) the spatial dimensions of the electron diffusion region with an aspect ratio of 0.1 to 0.2, consistent with fast reconnection. The well-structured multiple layers of electron populations indicate that the dominant electron dynamics are mostly laminar, despite the presence of turbulence near the reconnection site. [ABSTRACT FROM AUTHOR]

Particle acceleration by plasma waves and spontaneous wave generation are fundamental energy and momentum exchange processes in collisionless plasmas. Such wave-particle interactions occur ubiquitously in space. We present ultrafast measurements in Earth’s magnetosphere by the Magnetospheric Multiscale spacecraft that enabled quantitative evaluation of energy transfer in interactions associated with electromagnetic ion cyclotron waves. The observed ion distributions are not symmetric around the magnetic field direction but are in phase with the plasma wave fields. The wave-ion phase relations demonstrate that a cyclotron resonance transferred energy from hot protons to waves, which in turn nonresonantly accelerated cold He+ to energies up to ~2 kilo–electron volts. These observations provide direct quantitative evidence for collisionless energy transfer in plasmas between distinct particle populations via wave-particle interactions. [ABSTRACT FROM AUTHOR]

The article discusses the launch by the U.S. National Aeronautics and Space Administration (NASA) of the Magnetospheric Multiscale (MMS) mission to advance understanding of magnetic reconnection. Topics covered include the need for high-resolution measurements to capture the microphysics of reconnection, the reconnection site encountered by the MMS tetrahedrone on the dayside magnetopause and the primary evidence for magnetic dissipation.

The same time domain structures (TDS) have been observed on two Magnetospheric Multiscale Satellites near Earth's dayside magnetopause. These TDS, traveling away from the X line along the magnetic field at 4000 km/s, accelerated field-aligned ~5 eV electrons to ~200 eV by a single Fermi reflection of the electrons by these overtaking barriers. Additionally, the TDS contained both positive and negative potentials, so they were a mixture of electron holes and double layers. They evolve in ~10 km of space or 7 ms of time and their spatial scale size is 10-20 km, which is much larger than the electron gyroradius (<1 km) or the electron inertial length (4 km at the observation point, less nearer the X line). [ABSTRACT FROM AUTHOR]