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This research examines the plasma processes under penetration of the plasma clouds (plasmoids) across the magnetopause which is modeled as a tangential discontinuity (TD). Cases with the parallel magnetic field in both sides out of the TD are under investigation. Plasma parameters and magnetic field were chosen from the MMS mission and other spacecraft observations. The results are important for understanding the following basic space plasma physics problems: (a) plasma cloud deformation and strong phase mixing with magnetospheric plasma; (b) the transfer of mass, momentum and energy of magnetosheath and magnetic cloud plasma into magnetospheric plasmas; (c) necessary conditions for plasma cloud penetration via the magnetopause; (d) wave generation by plasma clouds inside the magnetopause. Plasma clouds (fragments) experience a significant slowdown when moving through magnetosheath plasmasWave‐particle interactions provide a strong phase mixing between plasma cloudsand magnetosheath, and magnetospheric plasmas3‐D hybrid modeling and spacecraft observations provethe mass, momentum and energy transport from the magnetosheath tomagnetosphere Plasma clouds (fragments) experience a significant slowdown when moving through magnetosheath plasmas Wave‐particle interactions provide a strong phase mixing between plasma cloudsand magnetosheath, and magnetospheric plasmas 3‐D hybrid modeling and spacecraft observations provethe mass, momentum and energy transport from the magnetosheath tomagnetosphere

The Vlasov equation describes collisionless plasmas in the continuum limit and applies to many fundamental plasma energization phenomena. Because this equation governs the evolution of plasma in six-dimensional phase space, studies of its structure have mostly been limited to numerical or analytical methods. Here terms of the Vlasov equation are determined from observations of electron phase-space density gradients measured by the four Magnetospheric Multiscale spacecraft in the vicinity of magnetic reconnection at Earth’s magnetopause. We identify which electrons in velocity space substantially support the electron pressure divergence within electron-scale current layers. Furthermore, we isolate and characterize the effects of density, velocity and temperature gradients on the velocity-space structure and dynamics of these electrons. Unipolar, bipolar and ring structures in the electron phase-space density gradients are compared to a simplified Maxwellian model and correspond to localized gradients in density, velocity and temperature, respectively. These structures have implications for the ability of collisionless plasmas to maintain kinetic Vlasov equilibrium. The results provide a kinetic perspective relevant to how the electron pressure divergence may develop to violate the electron frozen-in condition and sustain electron-scale energy conversion processes, such as the reconnection electric field, in collisionless space plasma environments.

We present a new method for determining the main relevant features of the local magnetic field configuration, based entirely on the knowledge of the magnetic field gradient four‐spacecraft measurements. The method, named “magnetic configuration analysis” (MCA), estimates the spatial scales on which the magnetic field varies locally. While it directly derives from the well‐known magnetic directional derivative and magnetic rotational analysis procedures (Shi et al., 2005, htpps://doi.org/10.1029/2005GL022454; Shen et al., 2007, https://doi.org/10.1029/2005JA011584), MCA was specifically designed to address the actual magnetic field geometry. By applying MCA to multispacecraft data from the Magnetospheric Multiscale (MMS) satellites, we perform both case and statistical analyses of local magnetic field shape and dimensionality at very high cadence and small scales. We apply this technique to different near‐Earth environments and define a classification scheme for the type of configuration observed. While our case studies allow us to benchmark the method with those used in past works, our statistical analysis unveils the typical shape of magnetic configurations and their statistical distributions. We show that small‐scale magnetic configurations are generally elongated, displaying forms of cigar and blade shapes, but occasionally being planar in shape like thin pancakes (mostly inside current sheets). Magnetic configurations, however, rarely show isotropy in their magnetic variance. The planar nature of magnetic configurations and, most importantly, their scale lengths strongly depend on the plasma β parameter. Finally, the most invariant direction is statistically aligned with the electric current, reminiscent of the importance of electromagnetic forces in shaping the local magnetic configuration. We define and use a four‐spacecraft method on MMS data to study the magnetic field configuration at small scales in near Earth regionsCase studies demonstrate the ability of the method to measure local magnetic configuration, benchmarking the method on previous studiesStatistically, low plasma betas are correlated with large length scales and slightly more frequent planar shapes in magnetic configurations

We investigate the kinetic structure of electron‐scale current sheets found in the vicinity of the magnetopause and embedded in the magnetosheath within the reconnection exhaust. A new technique for computing terms of the Vlasov equation using Magnetospheric Multiscale (MMS) measurements is presented and applied to study phase space density gradients and the kinetic origins of the electron pressure divergence found within these current sheets. Crescent‐shaped structures in ∇⊥2fegive rise to bipolar and quadrupolar signatures in v·∇femeasured near the maximum ∇·Peinside the current layers. The current density perpendicular to the magnetic field is strong (J⊥∼2 μA/m2), and the thickness of the current layers ranges from 3 to 5 electron inertial lengths. The electron flows supporting the current layers mainly result from the combination of E×Band diamagnetic drifts. We find nonzero J·E′within the current sheets even though they are observed apart from typical diffusion region signatures. We discovered how to use data from outer space to measure what is known as the Vlasov equation, held by many to be the most important equation in plasma physics. Beginning with Ludwig Boltzmann's insights from the late 1800s regarding microscopic motions of ordinary gases, Anatoly Vlasov in 1945 applied Boltzmann's ideas to understand the nature of electrified gases called plasmas. What has prevented researchers from measuring the Vlasov equation for over 100 years since Boltzmann? The difficulty is that Vlasov's equation lives in phase space, which enlists no less than seven dimensions — three for position (x, y, z), three for velocity (vx, vy, vz), and one for time (t) — thus, for over a century, no experiment has been designed to sufficiently and accurately resolve each of these dimensions. Today, instruments comprising the revolutionary Fast Plasma Investigation onboard NASA's four spacecraft Magnetospheric Multiscale mission, flying over 40,000 miles away from Earth through regions where the magnetic fields of the Earth and Sun collide, equip space scientists with data collected at higher resolution than ever before achieved — high enough to at last resolve terms in the Vlasov equation for the first time in history, which we demonstrate here. Bipolar and quadrupolar structures in the spatial gradient term of the electron Vlasov equation were measured for the first time with MMSElectron‐scale current layers exhibit alternating velocity space crescent structures in electron phase space density gradient distributionsEnergy conversion was observed in thin diamagnetic current layers in the magnetopause reconnection exhaust far from the diffusion region

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. 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. 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

The unprecedented detail of measurements by the four Magnetospheric Multiscale (MMS) spacecraft enable deeper investigation of quasi‐perpendicular collisionless shocks. We compare shock normals, planarities, and Normal Incidence Frame cross‐shock potentials determined from electric field measurements and proxies, for a subcritical interplanetary shock and a supercritical bow shock. The subcritical shock's cross‐shock potential was 26±6V. The shock scale was 33 km, too short to allow comparison with proxies from ion moments. Proxies from electron moments provided potential estimates of 40±5V. Shock normals from magnetic field minimum variance analysis were nearly identical, indicating a planar front. The supercritical shock's cross‐shock potential was estimated to be from 290 to 440 V from the different spacecraft measurements, with shock scale 120 km. Reflected ions contaminated the ion‐based proxies upstream, whereas electron‐based proxies yielded reasonable estimates of 250±50V. Shock normals from electric field maximum variance analysis differed, indicating a rippled front. An important problem in shock physics is understanding how the incoming plasma flow is thermalized across the shock. The role of the cross‐shock electric field has not been well studied. We compare measurements and implicit estimates of cross‐shock potential for a quasi‐perpendicular weak (low Mach) shock and a quasi‐perpendicular strong (moderate/high Mach) shock using data from the four Magnetospheric Multiscale satellites. The weak shock had lower cross‐shock potential in the Normal Incidence Frame (about 30 V) than the strong shock (about 300 V). We also estimated the potential deduced from ion and electron data. Electron‐based estimates agreed reasonably well with the measurements, but ion‐based estimates encountered problems. The weak shock was too short compared to the ion data sampling period, while the strong shock reflected ions back into the upstream flow. Data from individual spacecraft indicated that the surface of the strong shock was not flat but rippled, one reason why its measured potential showed such a broad range. We compared cross‐shock potential structure of two quasi‐perpendicular shocks (low/high Mach) in Magnetospheric Multiscale mission dataThe Normal Incidence Frame cross‐shock potential was 26 +/‐ 6 V for low‐Mach shock, 290‐440 V for high‐Mach shock, similar to electron proxiesAt the high‐Mach shock, ion proxies were spoiled by ions reflected off the rippled front

We report Magnetospheric Multiscale observations of reconnection in a thin current sheet at the interface of interlinked flux tubes carried by converging reconnection jets at Earth's magnetopause. The ion skin depth‐scale width of the interface current sheet and the non‐frozen‐in ions indicate that Magnetospheric Multiscale crossed the reconnection layer near the X‐line, through the ion diffusion region. Significant pileup of the reconnecting component of the magnetic field in this and three other events on approach to the interface current sheet was accompanied by an increase in magnetic shear and decrease in Δβ, leading to conditions favorable for reconnection at the interface current sheet. The pileup also led to enhanced available magnetic energy per particle and strong electron heating. The observations shed light on the evolution and energy release in 3‐D systems with multiple reconnection sites. The Earth and the solar wind magnetic fields interconnect through a process called magnetic reconnection. The newly reconnected magnetic field lines are strongly bent and accelerate particles, similar to a rubber band in a slingshot. In this paper we have used observations from NASA's Magnetospheric MultiScale spacecraft to investigate what happens when two of these slingshot‐like magnetic field lines move toward each other and get tangled up. We found that the two bent magnetic field lines tend to orient themselves perpendicular to each other as they become interlinked and stretched, similar to what rubber bands would do. This reorientation allows the interlinked magnetic fields to reconnect again, releasing part of the built‐up magnetic energy as strong electron heating. The results are important because they show how interlinked magnetic fields, which occur in many solar and astrophysics contexts, reconnect and produce enhanced electron heating, something that was not understood before. Magnetic flux pileup observed upstream of reconnecting current sheet at the interface of converging reconnection jetsMagnetic flux pileup was accompanied by increase in magnetic shear and decrease in Δβ, leading to conditions favorable for reconnectionMagnetic flux pileup leads to enhanced available magnetic energy per particle and strong electron heating

The most intense currents in the solar wind are carried by magnetic field discontinuities. The formation of such structures depends on whether they are rotational or tangential discontinuities. To distinguish between these two types, variations of plasma characteristics should be studied. We analyze data from a set of discontinuities observed by the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun and Magnetospheric Multiscale missions in the near‐Earth solar wind. We show that these discontinuities have properties characteristic of both tangential and rotational discontinuities: (1) Jumps in the tangential velocity component are well correlated with jumps in the Alfven speed, and (2) Electron density and temperature vary significantly across these discontinuities. Suprathermal electron pitch angle distributions change across discontinuities, revealing the importance of electron kinetics to discontinuity structure. Investigation of the discontinuity formation needs to go beyond magnetohydrodynamics theory and highly likely involves both ion and electron kinetics. The most intense currents in the solar wind are carried by magnetic field discontinuities, coherent structures that contribute significantly to plasma heating and magnetic field fluctuations. Although investigation of discontinuities was started with the first spacecraft in situ measurements in the solar wind, we still do not know the primary mechanism of the discontinuity formation. Conclusions of the simplified fluid models of discontinuities often do not describe the observed discontinuity properties. In this study we investigate the nonfluid (kinetic) properties of these discontinuities. Such investigation becomes possible owing to unprecedented detailed plasma measurements provided by the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun and Magnetospheric Multiscale missions in the near‐Earth solar wind. Our findings show that observed discontinuity classification needs to go beyond fluid theory and highly likely involves both ion and electron kinetics. Magnetic field discontinuities in solar wind are investigated using multispacecraft observationsObserved discontinuities have properties characteristic of both rotational and tangential discontinuitiesElectron pitch angle distributions vary sharply across the observed discontinuities

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. 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

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 Bnsignatures 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. 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

Earth and its magnetosphere are immersed in the supersonic flow of the solar-wind plasma that fills interplanetary space. As the solar wind slows and deflects to flow around Earth, or any other obstacle, a ‘bow shock’ forms within the flow. Under almost all solar-wind conditions, planetary bow shocks such as Earth’s are collisionless, supercritical shocks, meaning that they reflect and accelerate a fraction of the incident solar-wind ions as an energy dissipation mechanism1,2, which results in the formation of a region called the ion foreshock3. In the foreshock, large-scale, transient phenomena can develop, such as ‘hot flow anomalies’4–9, which are concentrations of shock-reflected, suprathermal ions that are channelled and accumulated along certain structures in the upstream magnetic field. Hot flow anomalies evolve explosively, often resulting in the formation of new shocks along their upstream edges5,10, and potentially contribute to particle acceleration11–13, but there have hitherto been no observations to constrain this acceleration or to confirm the underlying mechanism. Here we report observations of a hot flow anomaly accelerating solar-wind ions from roughly 1–10 kiloelectronvolts up to almost 1,000 kiloelectronvolts. The acceleration mechanism depends on the mass and charge state of the ions and is consistent with first-order Fermi acceleration14,15. The acceleration that we observe results from only the interaction of Earth’s bow shock with the solar wind, but produces a much, much larger number of energetic particles compared to what would typically be produced in the foreshock from acceleration at the bow shock. Such autogenous and efficient acceleration at quasi-parallel bow shocks (the normal direction of which are within about 45 degrees of the interplanetary magnetic field direction) provides a potential solution to Fermi’s ‘injection problem’, which requires an as-yet-unexplained seed population of energetic particles, and implies that foreshock transients may be important in the generation of cosmic rays at astrophysical shocks throughout the cosmos.

We consider intense electrostatic solitary waves (ESW) observed in a supercritical quasi‐perpendicular Earth's bow shock crossing by the Magnetospheric Multiscale Mission. The ESW have spatial scales of a few tens of meters (a few Debye lengths) and propagate oblique to a local quasi‐static magnetic field with velocities from a few tens to a few hundred kilometers per second in the spacecraft frame. Because the ESW spatial scales are comparable to the separation between voltage‐sensitive probes, correction factors are used to compute the ESW electric fields. The ESW have electric fields with amplitudes exceeding 600 mV/m (oriented oblique to the local magnetic field) and negative electrostatic potentials with amplitudes of a few tenths of the electron temperature. The negative electrostatic potentials indicate that the ESW are not electron phase space holes, while interpretation in terms of ions phase space holes is also questionable. Whatever is their nature, we show that due to the oblique electric field orientation the ESW are capable of efficient pitch‐angle scattering and isotropization of thermal electrons. Due to the negative electrostatic potentials the ESW Fermi reflects a significant fraction of the thermal electrons streaming from upstream (downstream) back to upstream (downstream) region, thereby affecting the shock dynamics. The role of the ESW in electron heating is discussed. Processes governing electron thermalization across shock waves are not entirely understood. The high resolution particle and 3‐D electric field measurements provided by the Magnetospheric Multiscale Mission make it possible to study the Earth's bow shock that is an excellent laboratory for addressing the electron thermalization across supercritical shock waves. Previous observations showed that electron heating across the bow shock is generally governed by macroscopic cross‐shock electrostatic field. On the other hand, the role of the turbulence observed across the bow shock in the electron thermalization has remained unclear. In this letter we consider a particular bow shock crossing by the Magnetospheric Multiscale Mission and focus on the role of the high amplitude electrostatic solitary waves in the electron thermalization process. We accurately estimate the electrostatic solitary wave parameters and show that due to electric fields oriented oblique to a local DC magnetic field and negative electrostatic potentials with amplitudes of a few tenths of the electron temperature, these Debye‐scale structures are capable of efficient pitch angle scattering, Fermi reflection, and isotropization of thermal electrons. Electrostatic solitary waves (ESW) across the bow shock are Debye‐scale structures with electric field oriented oblique to magnetic fieldESW have negative electrostatic potential (they are not electron phase space holes) with amplitude of a few tenths of electron temperatureESW are capable of efficient pitch angle scattering, Fermi reflection, and isotropization of thermal electrons

The occurrence of spatially and temporally variable reconnection at the Earth's magnetopause leads to the complex interaction of magnetic fields from the magnetosphere and magnetosheath. Flux transfer events (FTEs) constitute one such type of interaction. Their main characteristics are (1) an enhanced core magnetic field magnitude and (2) a bipolar magnetic field signature in the component normal to the magnetopause, reminiscent of a large‐scale helicoidal flux tube magnetic configuration. However, other geometrical configurations which do not fit this classical picture have also been observed. Using high‐resolution measurements from the Magnetospheric Multiscale mission, we investigate an event in the vicinity of the Earth's magnetopause on 7 November 2015. Despite signatures that, at first glance, appear consistent with a classic FTE, based on detailed geometrical and dynamical analyses as well as on topological signatures revealed by suprathermal electron properties, we demonstrate that this event is not consistent with a single, homogenous helicoidal structure. Our analysis rather suggests that it consists of the interaction of two separate sets of magnetic field lines with different connectivities. This complex three‐dimensional interaction constructively conspires to produce signatures partially consistent with that of an FTE. We also show that, at the interface between the two sets of field lines, where the observed magnetic pileup occurs, a thin and strong current sheet forms with a large ion jet, which may be consistent with magnetic flux dissipation through magnetic reconnection in the interaction region. We characterized the scale, geometry, and propagation of an ion scale current structure resulting from the interaction between interlaced flux tubesSome signatures of magnetic reconnection are found at the interaction interfaceThe intrinsic properties of this event are inconsistent with a single, homogenous helicoidal magnetic structure as expected from a typical flux transfer event (FTE)

We report Magnetospheric Multiscale observations of electron pressure gradient electric fields near a magnetic reconnection diffusion region using a new technique for extracting 7.5 ms electron moments from the Fast Plasma Investigation. We find that the deviation of the perpendicular electron bulk velocity from E× Bdrift in the interval where the out‐of‐plane current density is increasing can be explained by the diamagnetic drift. In the interval where the out‐of‐plane current is transitioning to in‐plane current, the electron momentum equation is not satisfied at 7.5 ms resolution. Diamagnetic drift explains out‐of‐plane current in regions of deviation from frozen‐in fluxPerpendicular crescents exist in regions where electrons are diamagnetically driftingNew technique for extracting 7.5 ms electron moments produces reliable data

Mirror mode waves are ubiquitous in the Earth's magnetosheath, in particular behind the quasi‐perpendicular shock. Embedded in these nonlinear structures, intense lion roars are often observed. Lion roars are characterized by whistler wave packets at a frequency ∼100 Hz, which are thought to be generated in the magnetic field minima. In this study, we make use of the high time resolution instruments on board the Magnetospheric MultiScale mission to investigate these waves and the associated electron dynamics in the quasi‐perpendicular magnetosheath on 22 January 2016. We show that despite a core electron parallel anisotropy, lion roars can be generated locally in the range 0.05–0.2fceby the perpendicular anisotropy of electrons in a particular energy range. We also show that intense lion roars can be observed up to higher frequencies due to the sharp nonlinear peaks of the signal, which appear as sharp spikes in the dynamic spectra. As a result, a high sampling rate is needed to estimate correctly their amplitude, and the latter might have been underestimated in previous studies using lower time resolution instruments. We also present for the first‐time 3‐D high time resolution electron velocity distribution functions in mirror modes. We demonstrate that the dynamics of electrons trapped in the mirror mode structures are consistent with the Kivelson and Southwood (1996) model. However, these electrons can also interact with the embedded lion roars: first signatures of electron quasi‐linear pitch angle diffusion and possible signatures of nonlinear interaction with high‐amplitude wave packets are presented. These processes can lead to electron untrapping from mirror modes. Intense lion roars are observed in mirror modes by high time resolution instruments on board Magnetospheric MultiScale missionNonlinear lion roars are observed up to 0.4fcedue to their high amplitude, which may have been underestimated in previous studiesPossible signatures of linear and nonlinear resonant interaction between lion roars and electrons, which can be untrapped from mirror modes

An electron diffusion region (EDR) in magnetic reconnection with a guide magnetic field approximately 0.2 times the reconnecting component is encountered by the four Magnetospheric Multiscale spacecraft at the Earth's magnetopause. The distinct substructures in the EDR on both sides of the reconnecting current sheet are visualized with electron distribution functions that are 2 orders of magnitude higher cadence than ever achieved to enable the following new findings: (1) Motion of the demagnetized electrons plays an important role to sustain the reconnection current and contributes to the dissipation due to the nonideal electric field, (2) the finite guide field dominates over the Hall magnetic field in an electron‐scale region in the exhaust and modifies the electron flow dynamics in the EDR, (3) the reconnection current is in part carried by inflowing field‐aligned electrons in the magnetosphere part of the EDR, and (4) the reconnection electric field measured by multiple spacecraft is uniform over at least eight electron skin depths and corresponds to a reconnection rate of approximately 0.1. The observations establish the first look at the structure of the EDR under a weak but not negligible guide field. Motion of demagnetized electrons is key to the reconnection current and collisionless dissipation under the observed guide fieldThe guide field modifies the out‐of‐plane magnetic field pattern and the electron flow dynamics thereinThe reconnection electric field is measured to be ~0.1 and uniform over the interspacecraft separation in the magnetosheath EDR and inflow

Taking advantage of high‐resolution measurements from the MMS mission, we find evidence for a complete Hall system in the exhaust of asymmetric magnetic reconnection 40 Didownstream of the X line. The investigation of the fine structure of the Hall system reveals that it displays features in the exhaust similar to those reported previously in the ion diffusion region by simulations and observations. This finding confirms the importance of particle‐scale processes in the reconnection exhaust as well. On the magnetospheric side of the exhaust, electrons are strongly accelerated by parallel electric fields. This process significantly contributes to feed the Hall current system, resulting in a nonnegligible Hall magnetic field signature on this side despite an otherwise lower density. Calculation of the induced out‐of‐plane magnetic field by in‐plane currents (based on Biot‐Savart law) provides direct quantitative evidence for the process of Hall magnetic field generation by the Hall current system. A strong normal Hall electric field is present only on the magnetospheric side of the exhaust region, consistent with previous works. Multipoint data analysis shows that the ion pressure gradient in the ion momentum equation produces this Hall electric field. This global pattern of the Hall system can be explained by kinetic Alfvén wave theory. Quantification of Hall magnetic field induction by the measured Hall current system using Biot‐Savart's lawIon pressure gradients support a normal Hall electric field across the exhaust regionHall system pattern consistent with kinetic Alfven wave theory also in the exhaust

This paper presents Magnetospheric Multiscale mission (MMS) observations of the exhaust region in the vicinity of the central reconnection site in Earth's magnetopause current sheet. High‐time‐resolution measurements of field and particle distributions enable us to explore the fine structure of the diffusion region near the X line. Ions are decoupled from the magnetic field throughout the entire current sheet crossing. Electron jets flow downstream from the X line at speeds greater than the E× Bdrift velocity. At/around the magnetospheric separatrix, large‐amplitude electric fields containing field‐aligned components accelerate electrons along the magnetic field toward the X line. Near the neutral sheet, crescent‐shaped electron distributions appear coincident with (1) an out‐of‐plane electric field whose polarity is opposite to that of the reconnection electric field and (2) the energy transfer from bulk kinetic to field energy. The observations indicate that MMS passed through the edge of an elongated electron diffusion region (EDR) or the outer EDR in the exhaust region. MMS observations of the fine structure of the reconnection exhaust region close to an X lineThe parallel crescent distributions and associated field signatures suggest MMS passage through the outer EDR downstream of an X lineNear the magnetospheric separatrix large‐amplitude electric fields including field‐aligned components accelerate electrons toward the X line

The role and properties of lower hybrid waves in the ion diffusion region and magnetospheric inflow region of asymmetric reconnection are investigated using the Magnetospheric Multiscale (MMS) mission. Two distinct groups of lower hybrid waves are observed in the ion diffusion region and magnetospheric inflow region, which have distinct properties and propagate in opposite directions along the magnetopause. One group develops near the ion edge in the magnetospheric inflow, where magnetosheath ions enter the magnetosphere through the finite gyroradius effect and are driven by the ion‐ion cross‐field instability due to the interaction between the magnetosheath ions and cold magnetospheric ions. This leads to heating of the cold magnetospheric ions. The second group develops at the sharpest density gradient, where the Hall electric field is observed and is driven by the lower hybrid drift instability. These drift waves produce cross‐field particle diffusion, enabling magnetosheath electrons to enter the magnetospheric inflow region thereby broadening the density gradient in the ion diffusion region. Two groups of lower hybrid waves are observed in the ion diffusion and magnetospheric inflow regionsIn the magnetospheric inflow region lower hybrid waves develop when cold magnetospheric ions are present and can heat cold ionsIn the diffusion region lower hybrid waves develop at the density gradient and can cause cross‐field particle diffusion

Data compression has been a staple of imaging instruments for years. Recently, plasma measurements have utilized compression with relatively low compression ratios. The Fast Plasma Investigation (FPI) on board the Magnetospheric Multiscale (MMS) mission generates data roughly 100 times faster than previous plasma instruments, requiring a higher compression ratio to fit within the telemetry allocation. This study investigates the performance of a space‐based compression standard employing a Discrete Wavelet Transform and a Bit Plane Encoder (DWT/BPE) in compressing FPI plasma count data. Data from the first 6 months of FPI operation are analyzed to explore the error modes evident in the data and how to adapt to them. While approximately half of the Dual Electron Spectrometer (DES) maps had some level of loss, it was found that there is little effect on the plasma moments and that errors present in individual sky maps are typically minor. The majority of Dual Ion Spectrometer burst sky maps compressed in a lossless fashion, with no error introduced during compression. Because of induced compression error, the size limit for DES burst images has been increased for Phase 1B. Additionally, it was found that the floating point compression mode yielded better results when images have significant compression error, leading to floating point mode being used for the fast survey mode of operation for Phase 1B. Despite the suggested tweaks, it was found that wavelet‐based compression, and a DWT/BPE algorithm in particular, is highly suitable to data compression for plasma measurement instruments and can be recommended for future missions. Phase 1A FPI data show that DIS is unaffected by compression error and that DES has induced error, with small effect on momentsCompression error effects can be well characterized into known error modesChanges for Phase 1B for FPI include a larger compression size limit for DES bursts and for fast survey to use floating point compression

Temporal, spatial, and velocity‐space variations of electron phase space density are measured observationally and compared for the first time using the four magnetospheric multiscale (MMS) spacecraft at Earth's magnetopause. Equipped with these unprecedented spatiotemporal measurements offered by the MMS tetrahedron, we compute each term of the electron Vlasov equation that governs the evolution of collisionless plasmas found throughout the universe. We demonstrate how to use single spacecraft measurements to improve the resolution of the electron pressure gradient that supports nonideal parallel electric fields, and we develop a model to intuit the types of kinetic velocity‐space signatures that are observed in the Vlasov equation terms. Furthermore, we discuss how the gradient in velocity‐space sheds light on plasma energy conversion mechanisms and wave‐particle interactions that occur in fundamental physical processes such as magnetic reconnection and turbulence. Measuring spatial and temporal variations of space plasmas usually requires choosing between the following two approaches: (a) measure how the quantity of interest changes in time as the plasma flows past a single spacecraft, or (b) compare measurements of the quantity gathered from multiple, spatially separated spacecraft. The first approach requires measurements at two different times from the same spatial location, while the second requires simultaneous measurements taken from multiple spatial points. There are advantages and disadvantages to each of these existing approaches. While single‐spacecraft measurements may be gathered at high time resolution, a known limitation of single‐point data sets is the inability to distinguish between spatial and temporal variations: both a thin, slow‐moving structure and a thick, fast‐moving one could produce the same measured time series of a quantity when sampled at only a single spatial location. In many situations, multipoint measurements, such as those provided by NASA's Magnetospheric Multiscale (MMS) four‐spacecraft mission enable us to overcome that limitation; however, oftentimes electron‐scale structures of interest are even smaller than the close inter‐spacecraft separation of MMS, which means typical techniques for estimating spatial gradients from the four spacecraft also become inaccurate for those events. In this paper, we present a new approach for quantifying variations in a collisionless plasma that only requires information about the plasma particles and fields taken at a singlepoint in time and space. Temporal, spatial, and velocity‐space derivatives of electron phase space density are computed and compared for the first time using the Magnetospheric Multiscale spacecraftSingle‐spacecraft measurements show how parallel electric fields are balanced by electron pressure gradients at Earth's magnetopauseA simplified form of the electron distribution function provides intuition for interpreting nongyrotropic velocity‐space structures Temporal, spatial, and velocity‐space derivatives of electron phase space density are computed and compared for the first time using the Magnetospheric Multiscale spacecraft Single‐spacecraft measurements show how parallel electric fields are balanced by electron pressure gradients at Earth's magnetopause A simplified form of the electron distribution function provides intuition for interpreting nongyrotropic velocity‐space structures

Localized kinetic‐scale regions of strong current are believed to play an important role in plasma thermalization and particle acceleration in turbulent plasmas. We present a detailed study of a strong localized current, 4900 nA m−2, located at a fast plasma jet observed in the magnetosheath downstream of a quasi‐parallel shock. The thickness of the current region is ∼3 ion inertial lengths and forms at a boundary separating magnetosheath‐like and solar wind‐like plasmas. On ion scales the current region has the shape of a sheet with a significant average normal magnetic field component but shows strong variations on smaller scales. The dynamic pressure within the magnetosheath jet is over 3 times the solar wind dynamic pressure. We suggest that the current sheet is forming due to high velocity shears associated with the jet. Inside the current sheet we observe local electron acceleration, producing electron beams, along the magnetic field. However, there is no clear sign of ongoing reconnection. At higher energies, above the beam energy, we observe a loss cone consistent with part of the hot magnetosheath‐like electrons escaping into the colder solar wind‐like plasma. This suggests that the acceleration process within the current sheet is similar to the one that occurs at shocks, where electron beams and loss cones are also observed. Therefore, electron beams observed in the magnetosheath do not have to originate from the bow shock but can also be generated locally inside the magnetosheath. MMS observations of strong ion scale current sheet forming at a magnetosheath jetLocal field‐aligned electron acceleration at current sheetThe acceleration mechanism is similar to the one observed at shocks

The Magnetospheric Multiscale mission has demonstrated the frequent presence of reconnection exhausts at thin current sheets within Kelvin‐Helmholtz (KH) waves at the flank magnetopause. Motivated by these recent observations, we performed a statistical analysis of the boundary layers on the magnetosheath side of all KH current sheets on 8 September 2015. We show 86% consistency between the exhaust flows and particle leakage in the magnetosheath boundary layers but further highlight the very frequent presence of additional boundary layer signatures that do not come from the locally observed reconnection exhausts. These additional electron and ion boundary layers, of various durations and at various positions with respect to the leading and trailing boundaries of the KH waves, signal connections to reconnection sites at other locations. Based on the directionality and extent of these layers, we provide an interpretation whereby complex magnetic topologies can arise within KH waves from the combination of reconnection in the equatorial plane and at midlatitudes in the Southern and Northern Hemispheres, where additional reconnection sites are expected to be triggered by the three‐dimensional field lines interweaving induced by the KH waves at the flanks (owing to differential flow and magnetic field shear with latitude). The present event demonstrates that the three‐dimensional development of KH waves can induce plasma entry (through reconnection at both midlatitude and equatorial regions) already sunward of the terminator where the instability remains in its linear stage. Statistical study of magnetosheath boundary layers at Kelvin‐Helmholtz wave boundariesParticle leakage statistics consistent with local reconnection exhausts within KH wavesAdditional signatures suggest complex topologies with reconnection sites at higher latitudes

On 15 August 2015, MMS (Magnetospheric Multiscale mission), skimming the dusk magnetopause, detected an isolated region of an increased magnetic strength and bipolar Bn, indicating a flux transfer event (FTE). The four spacecraft in a tetrahedron allowed for investigations of the shape and motion of the FTE. In particular, high‐resolution particle data facilitated our exploration of FTE substructures and their magnetic connectivity inside and surrounding the FTE. Combined field and plasma observations suggest that the core fields are open, magnetically connected to the northern magnetosphere from which high‐energy particles leak; ion “D” distributions characterize the axis of flux ropes that carry old‐opened field lines; counterstreaming electrons superposed by parallel‐heated components populate the periphery surrounding the FTE; and the interface between the core and draped regions contains a separatrix of newly opened magnetic field lines that emanate from the X line above the FTE. Multilayers of open and draped fields in and around the FTE exhibit rapid‐varying connectivities to the magnetosphere and the FTEThe interface between the core and draped regions contains a separatrix of newly opened magnetic field linesD‐shaped ion distributions feature the axis of flux ropes that carry old‐opened field lines

We present multipoint observations of earthward moving dipolarization fronts and energetic particle injections from NASA's Magnetospheric Multiscale mission with a focus on electron acceleration. From a case study during a substorm on 02 August 2015, we find that electrons are only accelerated over a finite energy range, from a lower energy threshold at ~7–9 keV up to an upper energy cutoff in the hundreds of keV range. At energies lower than the threshold energy, electron fluxes decrease, potentially due to precipitation by strong parallel electrostatic wavefields or initial sources in the lobes. Electrons at energies higher than the threshold are accelerated cumulatively by a series of impulsive magnetic dipolarization events. This case demonstrates how the upper energy cutoff increases, in this case from ~130 keV to >500 keV, with each dipolarization/injection during sustained activity. We also present a simple model accounting for these energy limits that reveals that electron energization is dominated by betatron acceleration. Electron acceleration in substorm injections can be from a cumulative effect of a series of magnetic dipolarization eventsInjected electron acceleration is most consistent with betatron acceleration over only a finite energy range, from ~10 to a few 100 keVInjections of electrons between ~100 eV and ~10 keV result in a decrease in flux and phase space density as a function of energy

We present observations of asymmetric magnetic reconnection showing evidence of electron demagnetization in the electron outflow. The observations were made at the magnetopause by the four Magnetospheric Multiscale (MMS) spacecraft, separated by ∼15 km. The reconnecting current sheet has negligible guide field, and all four spacecraft likely pass close to the electron diffusion region just south of the X line. In the electron outflow near the X line, all four spacecraft observe highly structured electron distributions in a region comparable to a few electron gyroradii. The distributions consist of a core with T∥>T⊥and a nongyrotropic crescent perpendicular to the magnetic field. The crescents are associated with finite gyroradius effects of partly demagnetized electrons. These observations clearly demonstrate the manifestation of finite gyroradius effects in an electron‐scale reconnection current sheet. An electron scale reconnecting current sheet is observedCrescent‐shaped electron distributions are observed in the electron outflowThe crescents are associated with finite electron gyroradius effects

Observations by the four Magnetospheric Multiscale spacecraft are used to investigate the Hall physics of a magnetopause magnetic reconnection separatrix layer. Inside this layer of currents and strong normal electric fields, cold (eV) ions of ionospheric origin can remain frozen‐in together with the electrons. The cold ions reduce the Hall current. Using a generalized Ohm's law, the electric field is balanced by the sum of the terms corresponding to the Hall current, the v× Bdrifting cold ions, and the divergence of the electron pressure tensor. A mixture of hot and cold ions is common at the subsolar magnetopause. A mixture of length scales caused by a mixture of ion temperatures has significant effects on the Hall physics of magnetic reconnection. Verification at high resolution that cold ions introduce a new length scale at the magnetopauseVerification at high resolution that cold ions modify the Hall physics of magnetic reconnection and reduce Hall currentsSeparatrix normal electric field balanced by the Hall term, the cold ion term, and the divergence of the electron pressure tensor

Both electron cyclotron harmonic (ECH) waves and whistler mode chorus waves resonate with electrons of the Earth's plasma sheet in the energy range from tens of eV to several keV and produce the electron diffuse aurora at ionospheric altitudes. Interaction of these superthermal electrons with the neutral atmosphere leads to the production of secondary electrons (E??600?eV) initial energies will travel back through the loss cone, become trapped in the magnetosphere, and redistribute the energy content of the magnetosphere-ionosphere system. Thus, scattering of the secondary electrons by ECH and whistler mode chorus waves leads to an increase of the fraction of superthermal electron energy deposited into the core magnetospheric plasma. Magnetosphere-ionosphere coupling is a key in forming the diffuse auroral electron distribution functionWe present the solution of the Boltzmann-Landau kinetic equationIt is important to consider the coupled ionosphere-magnetosphere system

Collisionless space plasma turbulence can generate reconnecting thin current sheets as suggested by recent results of numerical magnetohydrodynamic simulations. The Magnetospheric Multiscale (MMS) mission provides the first serious opportunity to verify whether small ion‐electron‐scale reconnection, generated by turbulence, resembles the reconnection events frequently observed in the magnetotail or at the magnetopause. Here we investigate field and particle observations obtained by the MMS fleet in the turbulent terrestrial magnetosheath behind quasi‐parallel bow shock geometry. We observe multiple small‐scale current sheets during the event and present a detailed look of one of the detected structures. The emergence of thin current sheets can lead to electron scale structures. Within these structures, we see signatures of ion demagnetization, electron jets, electron heating, and agyrotropy suggesting that MMS spacecraft observe reconnection at these scales. We detect electron scale current sheetWe observe electron jetsWe observe particle heating and energy dissipation

We present Magnetospheric Multiscale (MMS) mission measurements during a full magnetopause crossing associated with an enhanced southward ion flow. A quasi‐steady magnetospheric whistler mode wave emission propagating toward the reconnection region with quasi‐parallel and oblique wave angles is detected just before the opening of the magnetic field lines and the detection of escaping energetic electrons. Its source is likely the perpendicular temperature anisotropy of magnetospheric energetic electrons. In this region, perpendicular and parallel currents as well as the Hall electric field are calculated and found to be consistent with the decoupling of ions from the magnetic field and the crossing of a magnetospheric separatrix region. On the magnetosheath side, Hall electric fields are found smaller as the density is larger but still consistent with the decoupling of ions. Intense quasi‐parallel whistler wave emissions are detected propagating both toward and away from the reconnection region in association with a perpendicular anisotropy of the high‐energy part of the magnetosheath electron population and a strong perpendicular current, which suggests that in addition to the electron diffusion region, magnetosheath separatrices could be a source region for whistler waves. A quasi‐steady whistler mode wave emission is detected on the magnetospheric side, just before the opening of the magnetic field linesHall electric fields are calculated and found to be consistent with the decoupling of ions from the magnetic fieldThe source of the whistler mode waves is likely the perpendicular temperature anisotropy of the energetic part of the electron distribution

The structure of asymmetric magnetopause reconnection is explored with multiple point and high‐time‐resolution ion velocity distribution observations from the Magnetospheric Multiscale mission. On 9 September 2015, reconnection took place at the magnetopause, which separated the magnetosheath and the magnetosphere with a density ratio of 25:2. The magnetic field intensity was rather constant, even higher in the asymptotic magnetosheath. The reconnected field line region had a width of approximately 540 km. In this region, streaming and gyrating ions are discriminated. The large extension of the reconnected field line region toward the magnetosheath can be identified where a thick layer of escaping magnetospheric ions was formed. The scale of the magnetosheath side of the reconnected field line region relative to the scale of its magnetospheric side was 4.5:1. The structure of magnetopause reconnection is determined with ion distribution observationsStreaming and gyrating ions are discriminated in the reconnected field line regionThe large extension of the reconnected field line region toward the magnetosheath

We report Magnetospheric Multiscale observations of macroscopic and electron‐scale current layers in asymmetric reconnection. By intercomparing plasma, magnetic, and electric field data at multiple crossings of a reconnecting magnetopause on 22 October 2015, when the average interspacecraft separation was ~10 km, we demonstrate that the ion and electron moments are sufficiently accurate to provide reliable current density measurements at 30 ms cadence. These measurements, which resolve current layers narrower than the interspacecraft separation, reveal electron‐scale filamentary Hall currents and electron vorticity within the reconnection exhaust far downstream of the X line and even in the magnetosheath. Slightly downstream of the X line, intense (up to 3 μA/m2) electron currents, a super‐Alfvénic outflowing electron jet, and nongyrotropic crescent shape electron distributions were observed deep inside the ion‐scale magnetopause current sheet and embedded in the ion diffusion region. These characteristics are similar to those attributed to the electron dissipation/diffusion region around the X line. Demonstrate unprecedented MMS measurements of current density at 30 ms, capable of resolving current layers on electron skin depth scalesEvidence for electron‐scale filamentary Hall currents in exhaust and at its boundaries ~70 ion skin depths downstream of the X lineNongyrotropic crescent shape electron distributions in electron current layer embedded in the ion diffusion region near the X line

We present measurements from the Magnetospheric Multiscale (MMS) mission taken during a reconnection event on the dayside magnetopause which includes a passage through an electron diffusion region (EDR). The four MMS satellites were separated by about 10 km such that estimates of gradients and divergences allow a reasonable estimate of terms in the generalized Ohm's law, which is key to investigating the energy dissipation during reconnection. The strength and character of dissipation mechanisms determines how magnetic energy is released. We show that both electron pressure gradients and electron inertial effects are important, but not the only participants in reconnection near EDRs, since there are residuals of a few mV/m (~30–50%) of E+ Ue× B(from the sum of these two terms) during the encounters. These results are compared to a simulation, which exhibits many of the observed features, but where relatively little residual is present. Pressure Gradient and Inertial Terms in Ohm's law evaluated in an Electron Diffusion RegionIon and electron Frozen‐in Condition observed to be broken

We report observations from the Magnetospheric Multiscale satellites of large-amplitude, parallel, electrostatic waves associated with magnetic reconnection at the Earth's magnetopause. The observed waves have parallel electric fields (E||) with amplitudes on the order of 100?mV/m and display nonlinear characteristics that suggest a possible net E||. These waves are observed within the ion diffusion region and adjacent to (within several electron skin depths) the electron diffusion region. They are in or near the magnetosphere side current layer. Simulation results support that the strong electrostatic linear and nonlinear wave activities appear to be driven by a two stream instability, which is a consequence of mixing cold (<10?eV) plasma in the magnetosphere with warm (~100?eV) plasma from the magnetosheath on a freshly reconnected magnetic field line. The frequent observation of these waves suggests that cold plasma is often present near the magnetopause. Magnetospheric Multiscale observations of large-amplitude, parallel, electrostatic waves associated with magnetic reconnectionSimulations support that the strong electrostatic linear and nonlinear wave activities appear to be driven by a two-stream instabilityThe frequent observation of these waves suggests that cold plasma is often present near the magnetopause

We report evidence for reconnection between colliding reconnection jets in a compressed current sheet at the center of a magnetic flux rope at Earth's magnetopause. The reconnection involved nearly symmetric inflow boundary conditions with a strong guide field of two. The thin (2.5 ion-skin depth (di) width) current sheet (at ~12 didownstream of the X line) was well resolved by MMS, which revealed large asymmetries in plasma and field structures in the exhaust. Ion perpendicular heating, electron parallel heating, and density compression occurred on one side of the exhaust, while ion parallel heating and density depression were shifted to the other side. The normal electric field and double out-of-plane (bifurcated) currents spanned almost the entire exhaust. These observations are in good agreement with a kinetic simulation for similar boundary conditions, demonstrating in new detail that the structure of large guide field symmetric reconnection is distinctly different from antiparallel reconnection. First report of magnetic reconnection in a compressed thin current sheet between colliding jets at the center of a magnetic flux ropeLarge guide field causes asymmetries in plasma and field structures in the reconnection layerExcellent agreement in plasma and field profiles between observations and 2-D particle-in-cell simulation

We present Magnetospheric Multiscale observations of an electron-scale current sheet and electron outflow jet for asymmetric reconnection with guide field at the subsolar magnetopause. The electron jet observed within the reconnection region has an electron Mach number of 0.35 and is associated with electron agyrotropy. The jet is unstable to an electrostatic instability which generates intense waves with E?amplitudes reaching up to 300 mV m-1and potentials up to 20% of the electron thermal energy. We see evidence of interaction between the waves and the electron beam, leading to quick thermalization of the beam and stabilization of the instability. The wave phase speed is comparable to the ion thermal speed, suggesting that the instability is of Buneman type, and therefore introduces electron-ion drag and leads to braking of the electron flow. Our observations demonstrate that electrostatic turbulence plays an important role in the electron-scale physics of asymmetric reconnection. Electron outflow jet of asymmetric magnetic reconnection is observedElectron jet is unstable to Buneman-type instability generating intense electrostatic wavesThe instability leads to thermalization and braking of the electron jet

The Kelvin-Helmholtz (KH) instability at the Earth's magnetopause is predominantly excited during northward interplanetary magnetic field (IMF). Magnetic reconnection due to KH waves has been suggested as one of the mechanisms to transfer solar wind plasma into the magnetosphere. We investigate KH waves observed at the magnetopause by the Magnetospheric Multiscale (MMS) mission; in particular, we study the trailing edges of KH waves with Alfvénic ion jets. We observe gradual mixing of magnetospheric and magnetosheath ions at the boundary layer. The magnetospheric electrons with energy up to 80 keV are observed on the magnetosheath side of the jets, which indicates that they escape into the magnetosheath through reconnected magnetic field lines. At the same time, the low-energy (below 100 eV) magnetosheath electrons enter the magnetosphere and are heated in the field-aligned direction at the high-density edge of the jets. Our observations provide unambiguous kinetic evidence for ongoing reconnection due to KH waves. Magnetosheath electrons enter magnetosphere and magnetospheric electrons escape due to reconnectionField-aligned heating of magnetosheath electrons at the high-density edges of the jetsGradual ion mixing across the magnetopause

The four Magnetospheric Multiscale (MMS) spacecraft recorded the first direct evidence of reconnection exhausts associated with Kelvin-Helmholtz (KH) waves at the duskside magnetopause on 8 September 2015 which allows for local mass and energy transport across the flank magnetopause. Pressure anisotropy-weighted Walén analyses confirmed in-plane exhausts across 22 of 42 KH-related trailing magnetopause current sheets (CSs). Twenty-one jets were observed by all spacecraft, with small variations in ion velocity, along the same sunward or antisunward direction with nearly equal probability. One exhaust was only observed by the MMS-1,2 pair, while MMS-3,4 traversed a narrow CS (1.5 ion inertial length) in the vicinity of an electron diffusion region. The exhausts were locally 2-D planar in nature as MMS-1,2 observed almost identical signatures separated along the guide-field. Asymmetric magnetic and electric Hall fields are reported in agreement with a strong guide-field and a weak plasma density asymmetry across the magnetopause CS. First direct evidence of equatorial plane exhausts at 22 of 42 compressed KH-related current sheetsMMS observed an equal probability of inward and outward directed exhausts along the magnetopauseMMS observed asymmetric Hall magnetic and electric fields consistent with a strong guide-field and a weak density asymmetry

At 02:13 UT on 18 November 2015 when the geomagnetic dipole was tilted by -27°, the MMS spacecraft observed southward reconnection jets near the subsolar magnetopause under southward and dawnward interplanetary magnetic field conditions. Based on four-spacecraft estimations of the magnetic field direction near the separatrix and the motion and direction of the current sheet, the location of the reconnection line was estimated to be ~1.8 REor further northward of MMS. The Geotail spacecraft at GSM Z~1.4 REalso observed southward reconnection jets at the dawnside magnetopause 30–40?min later. The estimated reconnection line location was northward of GSM Z~2 RE. This crossing occurred when MMS observed purely southward magnetic fields in the magnetosheath. The simultaneous observations are thus consistent with the hypothesis that the dayside magnetopause reconnection line shifts from the subsolar point toward the northern (winter) hemisphere due to the effect of geomagnetic dipole tilt. The magnetopause reconnection line shifts to the winter hemisphere under southward IMF conditionsThe reconnection line was 1.8 REor further northward of GSM Z?=?0 plane for IMF By?=?Bz?=?-4?nTFor purely southward IMF, the reconnection line was northward of GSM Z~2 REat GSM Y?=?-6.5 RE

We report Magnetospheric Multiscale observations of high‐frequency electrostatic waves in the vicinity of the reconnection ion diffusion region on the dayside magnetopause. The ion diffusion region is identified during two magnetopause crossings by the Hall electromagnetic fields, the slippage of ions with respect to the magnetic field, and magnetic energy dissipation. In addition to electron beam modes that have been previously detected at the separatrix on the magnetospheric side of the magnetopause, we report, for the first time, the existence of electron cyclotron harmonic waves at the magnetosheath separatrix. Broadband waves between the electron cyclotron and electron plasma frequencies, which were probably generated by electron beams, were found within the magnetopause current sheet. Contributions by these high‐frequency waves to the magnetic energy dissipation were negligible in the diffusion regions as compared to those of lower‐frequency waves. Ion diffusion region was identified by MMS spacecraft in the Earth's dayside magnetopauseHigh‐frequency electrostatic waves were observed in the vicinity of the ion diffusion regionEnergy dissipated by these high‐frequency waves is negligible compared to that by the lower‐frequency process

We report on field‐aligned current observations by the four Magnetospheric Multiscale (MMS) spacecraft near the plasma sheet boundary layer (PSBL) during two major substorms on 23 June 2015. Small‐scale field‐aligned currents were found embedded in fluctuating PSBL flux tubes near the separatrix region. We resolve, for the first time, short‐lived earthward (downward) intense field‐aligned current sheets with thicknesses of a few tens of kilometers, which are well below the ion scale, on flux tubes moving equatorward/earthward during outward plasma sheet expansion. They coincide with upward field‐aligned electron beams with energies of a few hundred eV. These electrons are most likely due to acceleration associated with a reconnection jet or high‐energy ion beam‐produced disturbances. The observations highlight coupling of multiscale processes in PSBL as a consequence of magnetotail reconnection. Multipoint multiscale observations of field‐aligned currents during storm time substormsSmall‐scale intense field‐aligned currents are found embedded in PSBL flux tubes near separatrix regionField‐aligned low‐energy electron beams correlate with short‐lived localized field‐aligned currents

We present observations on 2 October 2015 when the Geotail spacecraft, near the Earth's equatorial plane, and the Magnetospheric Multiscale (MMS) spacecraft, at midsouthern latitudes, simultaneously encountered southward jets from dayside magnetopause reconnection under southward interplanetary magnetic field conditions. The observations show that the equatorial reconnection site under modest solar wind Alfvén Mach number conditions remained active almost continuously for hours and, at the same time, extended over a wide range of local times (≥4 h). The reconnection jets expanded toward the magnetosphere with distance from the reconnection site. Geotail, closer to the reconnection site, occasionally encountered large‐amplitude mesoscale flux transfer events (FTEs) with durations about or less than 1 min. However, MMS subsequently detected no or only smaller‐amplitude corresponding FTE signatures. It is suggested that during quasi‐continuous spatially extended reconnection, mesoscale FTEs decay as the jet spatially evolves over distances between the two spacecraft of ≥350 ion inertial lengths. Evidence of quasi‐continuous and spatially extended magnetopause reconnection for southward IMFBroadening toward the magnetosphere of the reconnection jet with distance from the X lineMesoscale flux transfer events decay as the jet evolves with distance from the X line

New Magnetospheric Multiscale (MMS) observations of small‐scale (~7 ion inertial length radius) flux transfer events (FTEs) at the dayside magnetopause are reported. The 10 km MMS tetrahedron size enables their structure and properties to be calculated using a variety of multispacecraft techniques, allowing them to be identified as flux ropes, whose flux content is small (~22 kWb). The current density, calculated using plasma and magnetic field measurements independently, is found to be filamentary. Intercomparison of the plasma moments with electric and magnetic field measurements reveals structured non‐frozen‐in ion behavior. The data are further compared with a particle‐in‐cell simulation. It is concluded that these small‐scale flux ropes, which are not seen to be growing, represent a distinct class of FTE which is generated on the magnetopause by secondary reconnection. Ion‐scale flux ropes are observed during magnetopause reconnectionThe largely force‐free flux ropes exhibit filamentary currents and nonideal ion behaviorSmall flux content and comparison with simulation indicate a secondary reconnection origin

In this letter the structure of the ion diffusion region of magnetic reconnection at Earth's magnetopause is investigated using the Magnetospheric Multiscale (MMS) spacecraft. The ion diffusion region is characterized by a strong DC electric field, approximately equal to the Hall electric field, intense currents, and electron heating parallel to the background magnetic field. Current structures well below ion spatial scales are resolved, and the electron motion associated with lower hybrid drift waves is shown to contribute significantly to the total current density. The electron heating is shown to be consistent with large‐scale parallel electric fields trapping and accelerating electrons, rather than wave‐particle interactions. These results show that sub‐ion scale processes occur in the ion diffusion region and are important for understanding electron heating and acceleration. The ion diffusion region of asymmetric reconnection is investigatedElectron‐scale currents develop in the ion diffusion regionElectron heating is consistent with electron trapping by DC electric fields

We present a new simulation results of the cloud dynamics in the ambient magnetospheric plasma on the large time and spatial scales. It was assumed that these impulsive structures observed by the MMS spacecraft originally were created because of the reconnection at the magnetopause. Our new 3‐D hybrid kinetic modeling on the large time and spatial scales captures several of these processes: an excitation of the electromagnetic waves (whistler and shear‐Alfvén waves) and plasma instabilities (mirror and flute); a formation of shock waves, and collapsing diamagnetic cavity; particle acceleration. A strong overshoot in plasma density profile was observed in the modeling and MMS observation at the interface between the cloud and magnetospheric plasma. The cloud expansion into ambient magnetospheric plasma causes the flute waves connected with excitation of the Rayleigh‐Taylor instability observed at the overshoot in plasma density profile across the external magnetic field. The modeling demonstrates a formation of the whistler waves at the initial stage which propagate in the external magnetic field direction. At the later stage, a formation of shear‐Alfvén waves was observed. Dense plasma structures moving through supersonic and sub‐Alfvenic magnetospheric flows were captured in high‐resolution MMS observationsOur research provides for the first time insights into the wave‐particle interactions in the plasma cloud environment3‐D hybrid modeling and MMS observation were used for the study of the plasma cloud dynamics Dense plasma structures moving through supersonic and sub‐Alfvenic magnetospheric flows were captured in high‐resolution MMS observations Our research provides for the first time insights into the wave‐particle interactions in the plasma cloud environment 3‐D hybrid modeling and MMS observation were used for the study of the plasma cloud dynamics

Decades after the introduction of top‐hat electrostatic analyzers (ESAs) to experimental space plasma physics, photoelectron noise in apertures of low‐energy electron instruments continues to be an issue in instrument performance. The photoelectrons at each portion of measured phase space distribution can be mapped through laboratory and in‐flight testing, and have been for many missions. In flight, the photoelectrons are a response to dynamic solar EUV spectral intensities which vary over the course of a solar cycle and even within a solar rotation. This paper outlines three solar irradiance proxies that can be used to scale a map of photo‐electron locations in phase space in order to account for dynamic solar activity. These proxies apply to top‐hat ESAs coated with Ebonol‐C (the most common coating) used by missions such as MMS, Solar Orbiter, the Van Allen Probes, and Juno. These proxies are discovered by searching possible wavelengths for correlation with an independent algorithm for MMS/FPI (Magnetospheric Multiscale Mission/Fast Plasma Instrument) which automatically determines the scaling factor. The three wavelengths that serve as viable proxies are found to be: 17.5, 30.5, and 37.5 nm. Space instruments measuring electron distributions have a noise source originating from photoelectrons caused by solar EUV. The photoelectrons are closely associated with the spectrum of solar EUV, which is dynamic over a solar cycle and even within a solar rotation. This work shows that three wavelengths (17.5, 30.5, and 37.5 nm) serve as good proxies for the entire relevant EUV spectrum that generates the noise associated with the photoelectrons. These proxy wavelengths work well when the instrument is a top‐hat electrostatic analyzer (ESA) coated with Ebonol‐C, which is a commonly used coating. Scale factors to adjust a background photoelectron map for low‐energy top‐hat electron ESAs should be updated multiple times per monthA method is presented to use proxy spectral irradiances to estimate the scale factorThree wavelengths suitable for use as proxies with this method are found Scale factors to adjust a background photoelectron map for low‐energy top‐hat electron ESAs should be updated multiple times per month A method is presented to use proxy spectral irradiances to estimate the scale factor Three wavelengths suitable for use as proxies with this method are found

Abstract We present both statistical and case studies of magnetosheath interaction with the high-latitude magnetopause on the basis of Interball-1 and other ISTP spacecraft data. We discuss those data along with recently published results on the topology of cusp-magnetosheath transition and the roles of nonlinear disturbances in mass and energy transfer across the high-latitude magnetopause. For sunward dipole tilts, a cusp throat is magnetically open for direct interaction with the incident flow that results in the creation of a turbulent boundary layer (TBL) over an indented magnetopause and downstream of the cusp. For antisunward tilts, the cusp throat is closed by a smooth magnetopause; demagnetized ‘plasma balls’ (with scale ∼ few RE, an occurrence rate of ∼25% and trapped energetic particles) present a major magnetosheath plasma channel just inside the cusp. The flow interacts with the ‘plasma balls’ via reflected waves, which trigger a chaotization of up to 40% of the upstream kinetic energy. These waves propagate upstream of the TBL and initiate amplification of the existing magnetosheath waves and their cascade-like decays during downstream passage throughout the TBL. The most striking feature of the nonlinear interaction is the appearance of magnetosonic jets, accelerated up to an Alfvenic Mach number of 3. The characteristic impulsive local momentum loss is followed by decelerated Alfvenic flows and modulated by the TBL waves; momentum balance is conserved only on time scales of the Alfvenic flows (1/fA ∼12 min). Wave trains at fA∼1.3 mHz are capable of synchronizing interactions throughout the outer and inner boundary layers. The sonic/Alfvenic flows, bounded by current sheets, control the TBL spectral shape and result in non-Gaussian statistical characteristics of the disturbances, indicating the fluctuation intermittency. We suggest that the multi-scale TBL processes play at least a comparable role to that of macro-reconnection (remote from or in the cusp) in solar wind energy transformation and population of the magnetosphere by the magnetosheath plasma. Secondary micro-reconnection constitutes a necessary chain at the small-scale (∼ion gyroradius) edge of the TBL cascades. The thick TBL transforms the flow energy, including deceleration and heating of the flow in the open throat, ‘plasma ball’ and the region downstream of the cusp.

We revisit Interball‐Tail and Magion‐4 observations of the dusk side magnetospheric boundary on February 15–16, 1996. The observed transient behavior of the boundary can be interpreted in terms of surface waves or as the manifestation of isolated magnetosheath plasma entities embedded in the magnetosphere. We examine the arguments for each of these interpretations with high time resolution magnetic field and plasma data and by exploiting the dual‐satellite nature of the observations. We find strong evidence for magnetic field and flow vortices near the magnetospheric boundary and hence for the existence of flux tubes with helicoidal field lines; such structures can be associated with both interpretations. The cross‐correlation between the dual satellite observations and the apparent periodicity strongly suggest a Kelvin‐Helmholtz surface wave, although other interpretations are not impossible. In any case, the observations for this particular event allow us to derive constraints on surface wave generation mechanisms and on scenarios that could account for the presence of isolated plasma elements in the magnetosphere.

The Magnetospheric Multiscale spacecraft observations in the burst mode allow the determination of the characteristics of resonant electrons interacting with quasi‐parallel whistler waves during prolonged dipolarizations in the near Earth magnetotail at −22 RE< X≤ −8 RE. We have detected 163 whistler bursts observed during 48 dipolarization events. The bursts were registered within ∼13 min following the dipolarization onset when the burst mode observations were available. In the majority of events, electrons with energies Wres≥ 10 keV and pitch angles αres∼ 100°–130° and αres∼ 50°–80°made the maximum positive contribution to the growth rate of whistler waves propagating quasi‐parallel and antiparallel to the ambient magnetic field, respectively. Our analysis shows that electrons with Wres∼ 10–20 keV could potentially be scattered into the loss cone by the low frequency whistler waves with fw∼ (0.05–0.2)fce(fwis the wave frequency and fceis the electron gyrofrequency). The electrons that are scattered into the loss cone can contribute to electron precipitation within ∼13 min following the dipolarization onset. We suggest that whistler waves that are excited due to cyclotron instability driven by the temperature anisotropy of suprathermal electrons (≥2 keV) may, in turn, affect electron distribution in this energy range. Specifically, lower energy resonant electrons transfer a part of their kinetic energy to waves, while more energetic electrons absorb wave energy increasing their kinetic energy. This may lead to the transformation of higher‐energy part of electron distribution from Maxwellian to the power law shape. Magnetic dipolarization is an important process in the magnetotail dynamics which manifests in transformation of initially stretched tail‐like configuration into the dipole‐like configuration. The dipolarization is followed by various processes of energy dissipation and transformation including plasma heating and acceleration as well as generation of different wave modes. In collisionless plasma, wave‐particle interactions play a key role in energy exchange between different populations of plasma particles. Also particle interactions with waves may cause particle scattering into the loss cone and their precipitation in the auroral region. The processes of wave‐particle interactions usually occur at very short time scales, and their investigation “in situ” requires observations with high time resolution. In this study, we use the Magnetospheric Multiscale spacecraft observations in burst mode and determine the characteristics of resonant electrons interacting with quasi‐parallel whistler waves at time scales of the order of a few seconds. We have found that quasi‐parallel whistler waves can be generated in the course of dipolarization due to cyclotron instability driven by the temperature anisotropy of suprathermal electrons (≥2 keV). Once have been generated, these waves, in turn, affect electron distribution in this energy range. Specifically, lower energy resonant electrons transfer a part of their kinetic energy to waves, while more energetic electrons absorb wave energy increasing their kinetic energy. We also found that some fraction of suprathermal electrons can be potentially scattered into the loss cone by the low‐frequency whistler waves. Such electrons can contribute to electron precipitation within ∼13 min following the dipolarization onset. Perpendicular anisotropy of electrons with energies more than 10 keV is responsible for whistler waves generation during dipolarizationsWhistler waves play an important role in energy exchange between different parts of electron spectrum in energy range more than 2 keVElectrons with energies 10–20 keV can be scattered into the loss cone due to their interaction with the low‐frequency whistler waves Perpendicular anisotropy of electrons with energies more than 10 keV is responsible for whistler waves generation during dipolarizations Whistler waves play an important role in energy exchange between different parts of electron spectrum in energy range more than 2 keV Electrons with energies 10–20 keV can be scattered into the loss cone due to their interaction with the low‐frequency whistler waves