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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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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]

We examine a Dipolarization Front (DF) event with an embedded electron diffusion region (EDR), observed by the Magnetospheric Multiscale (MMS) spacecraft on 08 September 2018 at 14:51:30 UT in the Earth's magnetotail by applying multi‐scale multipoint analysis methods. In order to study the large‐scale context of this DF, we use conjunction observations of the Cluster spacecraft together with MMS. A polynomial magnetic field reconstruction technique is applied to MMS data to characterize the embedded electron current sheet including its velocity and the X‐line exhaust opening angle. Our results show that the MMS and Cluster spacecraft were located in two counter‐rotating vortex flows, and such flows may distort a flux tube in a way that the local magnetic shear angle is increased and localized magnetic reconnection may be triggered. Using multi‐point data from MMS we further show that the local normalized reconnection rate is in the range of R ∼ 0.16 to 0.18. We find a highly asymmetric electron in‐ and outflow structure, consistent with previous simulations on strong guide‐field reconnection events. This study shows that magnetic reconnection may not only take place at large‐scale stable magnetopause or magnetotail current sheets but also in transient localized current sheets, produced as a consequence of the interaction between the fast Earthward flows and the Earth's dipole field. Plain Language Summary: Magnetic Reconnection is a key energy conversion process, where magnetic energy is converted into kinetic energy of plasma particles. During this process the magnetic field topology changes and the plasma particles decouple from the magnetic field in the so‐called diffusion region and get accelerated, forming a fast outflow jet. Over the last decades, hints arise that reconnection can take place at many different places in the magnetosphere and also very locally and intermittently. Fast plasma flows in the Magnetotail, moving toward the Earth, are assumed to be a consequence of magnetic reconnection, and are often accompanied by dipolar‐shaped magnetic flux bundles, embedded into them. The leading edges of such flux bundles are called dipolarization fronts (DF). In this work, we investigate a DF event, which hosts a diffusion region. First, we study the large‐scale characteristics of the DF, by utilizing data from both the Magnetospheric Multiscale (MMS) and the Cluster mission, that observe different regions of the event almost simultaneously. Second, we performed a 3D magnetic field reconstruction technique and compared the results to MMS data, to investigate the event on small scales. Key Points: A thin current sheet inside a dipolarization front, embedded in a diverging flow is analyzed using a polynomial reconstruction techniqueTransient reconnection event is detected in a high magnetic shear region, where the magnetic field is deflected due to duskward fast plasma flowThe reconstructed current sheet has a guide field of ∼1.8 the reconnecting component with normalized reconnection rate between 0.16 and 0.18 [ABSTRACT FROM AUTHOR]

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

We demonstrate that any plasmaspheric/cold ions accelerated in the vicinity of the magnetopause boundary, can proxy the local magnetopause motion over many minutes. The timeseries of this motion capture local structures such as waves on the boundary. We determine cold ion velocities normal to full magnetopause boundary crossings for three events with varying distances to the predicted reconnection X‐line, thus, providing a proof‐of‐concept study demonstrating the potential for using cold ion velocities to track magnetopause motion over a long period of time. Obtaining the time history of the (local) motion of the magnetopause relative to the spacecraft is determined by integrating the bulk (<100 eV for H+) ion velocities normal to the boundary. Timeseries of these tracked cold ion accelerations may be used to investigate boundary layer thicknesses, potential wave structures on the magnetopause, and their evolution beyond the boundary crossing. This method generally tracks magnetopause motion out to distances of ∼1–2RE away from the spacecraft during quasi‐steady space weather conditions. Key Points: Energized plasmaspheric cold ions effectively track the magnetopause motion continuously over many minutesThe magnetopause location and velocity are tracked reliably out to distances of ∼1–2 RE from the spacecraft given fairly consistent cold ion detectabilityOne of the 3 events shows quasi‐periodic magnetopause motion suggesting that this technique reveals wave propagation along the boundary [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]

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. [ABSTRACT FROM AUTHOR]

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

Downstream of the Earth's bow shock, the magnetosheath can exhibit strong turbulence. This turbulent cascade is associated with electron‐scale current sheets where magnetic reconnection can occur. We present data from NASA's Magnetospheric Multiscale mission during a turbulent magnetosheath interval that includes two electron‐scale reconnecting current sheets, one with higher guide field and one that is closer to antiparallel. We examine the characteristics of energy conversion and the role of parallel electric fields and waves in that energy conversion. The high‐guide field reconnection event shows energy conversion dominated by electric fields and currents parallel to the background magnetic field, while the lower guide field event is perpendicular dominated, consistent with observations of laminar reconnecting current sheets in the magnetosheath. Both current sheets exhibit signs of a parallel electric field acceleration channel, and the higher guide field event shows whistler‐mode waves at the x‐line. Both current sheets show variation in dissipation along the n‐direction, with perpendicular dissipation on one side and parallel on the other. Between the two is a dispersion of pitch angles from 90° to 0° or 180°. This dispersion is likely caused by the acceleration channel and rapidly suppresses whistler waves, suggesting whistler waves do not contribute to the dissipation in guide field reconnection. Key Points: Two magnetic reconnection events are observed in the turbulent magnetosheath with different guide fieldBoth current sheets exhibit a parallel electric field acceleration channel that changes the anisotropy of the electronsThis acceleration channel is sufficient to suppress whistler waves near the diffusion region [ABSTRACT FROM AUTHOR]

We present observations in Earth's magnetotail by the Magnetospheric Multiscale spacecraft that are consistent with magnetic field annihilation, rather than magnetic topology change, causing fast magnetic‐to‐electron energy conversion in an electron‐scale current sheet. Multi‐spacecraft analysis for the magnetic field reconstruction shows that an electron‐scale magnetic island was embedded in the observed electron diffusion region (EDR), suggesting an elongated shape of the EDR. Evidence for the annihilation was revealed in the form of the island growing at a rate much lower than expected for the standard X‐type geometry of the EDR, which indicates that magnetic flux injected into the EDR was not ejected from the X‐point or accumulated in the island, but was dissipated in the EDR. This energy conversion process is in contrast to that in the standard EDR of a reconnecting current sheet where the energy of antiparallel magnetic fields is mostly converted to electron bulk‐flow energy. Fully kinetic simulation also demonstrates that an elongated EDR is subject to the formation of electron‐scale magnetic islands in which fast but transient annihilation can occur. Consistent with the observations and simulation, theoretical analysis shows that fast magnetic diffusion can occur in an elongated EDR in the presence of nongyrotropic electron effects. We suggest that the annihilation in elongated EDRs may contribute to the dissipation of magnetic energy in a turbulent collisionless plasma. Plain Language Summary: Magnetic reconnection in electric current sheets is the key to fast release of magnetic energy in many space and astrophysical plasma systems, such as during magnetospheric substorms and solar flares. Establishing the mechanism by which magnetic energy is converted to particle energy in the reconnection process is the key to understanding the large‐scale impacts of reconnection, including energy partition and particle acceleration. It is generally believed that an electron‐scale diffusion region (EDR), where a magnetic‐to‐electron energy conversion occurs, has an X‐type magnetic field geometry around which the energy of antiparallel magnetic fields injected into the EDR is mostly converted to the bulk‐flow energy of electrons by magnetic tension of reconnected field lines. Contrary to this standard X‐type magnetic field geometry of reconnection, we report observations in Earth's magnetotail by NASA's Magnetospheric Multiscale spacecraft showing that the EDR can be highly elongated. The important and surprising consequence of the observed elongated shape of the EDR is that the fast energy conversion in the EDR can be caused mostly by magnetic field annihilation, rather than magnetic topology change. The fast collisionless annihilation that we discovered is fundamentally different from the classical magnetic field annihilation due to collisional and wave‐induced resistivity. Key Points: Multi‐spacecraft observations consistent with magnetic field annihilation in an electron diffusion region (EDR) of magnetotail reconnectionMagnetic field reconstruction suggests that an electron‐scale magnetic island was embedded in the EDR with elongated shapeTheoretical analysis shows that fast collisionless magnetic diffusion can occur in the elongated part of EDR with nongyrotropic electrons [ABSTRACT FROM AUTHOR]

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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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

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

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]

Magnetopause diamagnetic currents arise from density and temperature driven pressure gradients across the boundary layer. While theoretically recognized, the temperature contributions to the magnetopause current system have not yet been systematically studied. To bridge this gap, we used a database of Magnetospheric Multiscale magnetopause crossings to analyze diamagnetic current densities and their contributions across the dayside and flank magnetopause. Our results indicate that the ion temperature gradient component makes up to 37% of the ion diamagnetic current density along the magnetopause and typically opposes the classical Chapman‐Ferraro current direction, interfering destructively with the density gradient component, thus lowering the total diamagnetic current density. This effect is most pronounced on the flank magnetopause. The electron diamagnetic current was found to be 5–14 times weaker than the ion diamagnetic current on average. Plain Language Summary: The solar wind represents a continuous outflow of charged particles from the Sun's upper atmosphere into the solar system. Upon reaching Earth's magnetosphere, the solar wind's dynamic pressure is balanced by the magnetic pressure of Earth's magnetic field in a boundary layer known as the magnetopause. This boundary layer represents the entry point of the solar wind's energy into Earth's magnetosphere and upper atmosphere, playing a crucial role in energy transport throughout the interconnected system. Plasma density and temperature differences across the boundary layer generate an electric current that supports the magnetopause. In this paper, we clarify the physical mechanism of the magnetopause current by using high‐resolution data from NASA's Magnetospheric Multiscale mission. We found a significant ion temperature contribution to the magnetopause current not identified in previous studies. Our results also indicated that the plasma electrons' contribution to the magnetopause current was significantly smaller than the ion contribution. Key Points: The magnetopause diamagnetic current is composed of opposing density and temperature gradient generated componentsThe temperature gradient contributes up to 37% of the ion diamagnetic current density along the magnetopauseThe temperature component typically opposes the classical Chapman‐Ferraro current direction [ABSTRACT FROM AUTHOR]

The near-Earth plasma sheet becomes cold and dense under northward interplanetary magnetic field (IMF) condition, which suggests efficient solar wind plasma entry into the magnetosphere across the magnetopause for northward IMF and a possible contribution of ionospheric oxygen ion outflow. The cold and dense characteristics of the plasma sheet are more evident in the magnetotail flank regions that are the interface between cold solar wind plasma and hot magnetospheric plasma. Several physical mechanisms have been proposed to explain the solar wind plasma entry across the magnetopause and resultant formation of the cold-dense plasma sheet (CDPS) in the tail flank regions. However, the transport path of the cold-dense plasma inside the magnetotail has not been understood yet. Here, we present a case study of the CDPS in the dusk magnetotail by magnetospheric multiscale (MMS) spacecraft under strongly northward IMF and high-density solar wind conditions. The ion distribution function consists of high- and low-energy components, and the low-energy one intermittently shows energy dispersion in the directions parallel and antiparallel to the local magnetic field. The time-of-flight analysis of the energy-dispersed low-energy ions suggests that these ions originate in the region farther down the tail, move along the magnetic field toward the ionosphere and then come back to the magnetotail by the mirror reflection. The pitch-angle dispersion analysis gives consistent results on the traveling time and path length of the energy-dispersed ions. Based on these observations, we discuss possible generation mechanisms of the energy-dispersed structure of the low-energy ions during the northward IMF. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

Electrostatic solitary waves (ESWs) are a type of nonlinear time‐domain plasma structure (TDS) generally defined by bipolar electric fields and propagation parallel to the local magnetic field. Formation mechanisms for TDSs in the magnetosphere have been studied extensively and are associated with plasma boundary layers and the braking of bursty bulk flows (BBFs). However, the rapid timescales over which these TDSs occur (<2 ms) make them infeasible to count by eye over large time periods. Furthermore, high‐cadence data are not always available. The Solitary Wave Detector (SWD) on NASA's Magnetospheric Multiscale (MMS) mission quantifies the occurrence and amplitude of TDS throughout the constellation's orbit; analysis of burst (65 kS/s) parallel electric field data indicates that the SWD captures approximately 60% of all bipolar TDS encountered in the tail region, enabling large‐scale examination of their occurrence. Maps of TDS occurrence rates during several years of the MMS mission were generated from SWD data, showing enhanced TDS density in the tail region between 6 and 9 Re; enhance occurrence in or near shocks; and an unexpected enhancement in the dawn side of the tail and in the radiation belt. Key Points: The MMS Solitary Wave Detector records time‐domain structures in Earth's magnetosphereSWD maps highlight regions like the Van Allen belts and the bursty bulk flow braking regionThe SWD shows intriguing nonlinear activity in the shocks, magnetotail, flank, and dawn‐side outer radiation belt [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]

We compare plasma measurements in the solar wind between the Magnetospheric MultiScale Mission's (MMS) Fast Plasma Investigation (FPI) and those obtained near the Earth‐Sun L1 Lagrangian point from OMNI. FPI is an instrument designed to investigate plasma processes such as magnetic reconnection, acceleration, and turbulence. Since 2017, MMS supports solar wind campaigns where the plasma can have a higher Mach number, a cooler temperature, and a narrower beam compared with the MMS primary regions of interest. With FPI's firmware now optimized for these campaigns, the comparison with OMNI aids the interpretation of solar wind measurements. From these comparisons, we can make suggestions for both fast and burst‐survey telemetry modes. From fast mode intervals, we find first, that the FPI ion density can be lower than the OMNI proton density. However, the FPI electron density agrees well with the OMNI proton density. Second, due to the solar wind's cooler proton temperature and narrow‐angle, the FPI ion temperature is overestimated. Thus, using the OMNI proton temperature is suggested for plasma parameters such as the ion β, the ratio of ion thermal to magnetic pressure. Third, the FPI ion velocity is well‐estimated when compared with the OMNI proton velocity. In burst mode, the ion density and temperature have similar characteristics but to a lesser degree. Spin effects observed in all these plasma moments in burst mode intervals are reduced with the methods discussed in this paper. The results are summarized in a table that includes linear fit parameters and their defined errors. Key Points: Plasma measurements in the solar wind from Magnetospheric MultiScale Mission's Fast Plasma Investigation (FPI) are compared with the OMNI solar wind databaseThe electron density and ion velocity show good agreement with OMNI's proton density and velocity respectivelyThe FPI ion density is underestimated and FPI ion temperature is overestimated when compared to OMNI [ABSTRACT FROM AUTHOR]

Observations by the Magnetospheric Multiscale spacecraft (MMS) of an unusual layer, located between the dayside magnetosheath and the magnetosphere, alternating with encounters with the magnetosheath during an extended time period between December 31, 2015 and January 01, 2016, when the interplanetary magnetic field was strongly southward and the Earth's dipole tilt large and negative, are presented. It appears to have been magnetically connected to both magnetosphere and magnetosheath. The layer appears to be located mostly on closed field lines and was bounded by a rotational discontinuity (RD) at its magnetosheath edge and by the magnetosphere on its earthward side. A separatrix layer, with heated magnetosheath electrons streaming unidirectionally along the field lines, was present sunward of the RD. We infer that the layer was started by a dominant reconnection site well north of the spacecraft and that it may have gained additional width, from a large drop in solar wind density and ram pressure, which preceded the beginning of the event by more than an hour. Relative to the magnetosheath, in which the magnetic field was strongly southward, this unusual layer was characterized by a less southward, more dawnward magnetic field of lower magnitude. The plasma density and flow speed in the region were lower than in the magnetosheath, albeit with Alfvénic jetting occurring at the magnetosheath edge as well as at the magnetospheric edge of the layer. The closing of the magnetic field lines requires the existence of another reconnection site, located southward/tailward of MMS. Key Points: Magnetopause encounter for strongly southward interplanetary magnetic field, low solar wind Alfvén Mach number, and large dipole tiltPersistent and broad magnetopause layer with magnetospheric O+ and heated magnetosheath plasmaInferred dominant reconnection site near northern cusp, far from the Magnetospheric Multiscale spacecraft location [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]

Dipolarization fronts (DFs) have been suggested as crucial energy conversion sites contributing significantly to global energy transfer in the magnetosphere. However, energy partitioning of DF‐driven energy transfer remains hitherto elusive. Using high‐cadence data from MMS spacecraft, we present a detailed investigation of energy flux densities at two DFs with/without surface ripples. We find that during both DF intervals, electron enthalpy flux increases dramatically, carries the greatest energy, and well correlates with local energy conversion. Poynting flux also increases but contributes to a relatively smaller portion. Ion enthalpy flux which in magnitude is slightly smaller than electron enthalpy flux barely changes. Particle kinetic energy and heat fluxes are negligible. Strong difference in energy fluxes observed by different spacecraft is found at the rippled DF, indicating three‐dimensional energy transport. These results indicate that energy budgets at the DFs are dominated by electron physics, rather than ion dynamics suggested by previous studies. Plain Language Summary: Dipolarization fronts, earthward‐propagating transient magnetic structures, have been well documented to be crucial energy conversion sites contributing significantly to global energy transfer in the magnetosphere. However, owing to the limited capacity of previous spacecraft measurements, energy partition of energy transfer driven by the fronts remains not well understood so far. In this study, taking advantage of high‐resolution measurements from MMS which measure particle velocity distributions orders of magnitude faster than previous spacecraft, we present the first in‐situ investigation of energy flux densities at the fronts in the magnetotail. We reveal that energy budgets at the fronts are dominated by electron enthalpy flux and redistributed by electron flow moving along front ripples. Key Points: Electron enthalpy and Poynting fluxes carry the greatest energy and well correlate with local energy conversionIon energy flux which in magnitude is slightly smaller than electron energy flux barely changes across the frontsEnergy transport becomes three‐dimensional due to rippling effect induced by front surface waves [ABSTRACT FROM AUTHOR]

Magnetic reconnection is a fundamental process in collisionless space plasma environment, and plasma waves relevant to the kinetic interactions can have a significant impact on the multiscale behavior of reconnection. Here, we present Magnetospheric Multiscale (MMS) observations during an encounter of an X line of symmetric magnetic reconnection in the magnetotail. The X line is characterized by reversals of ion and electron jets and electromagnetic fields, agyrotropic electron velocity distribution functions (VDFs), and an electron‐scale current sheet. MMS observe large‐amplitude nonlinear upper‐hybrid (UH) waves on both sides of the neutral line, and the wave amplitudes have highly localized distribution along the normal direction. The inbound meandering electrons drive the UH waves, releasing the free energy stored from the reconnection electric field along the meandering trajectories. The interaction between the meandering electrons and the UH waves may modify the balance of the reconnection electric field around the X line. Plain Language Summary: The electron‐scale kinetic physics in the electron diffusion region (EDR) controls how magnetic field lines break and reconnect. Electron crescent, an indicator of EDR, can drive high‐frequency electrostatic waves around EDR. For the first time, the upper‐hybrid (UH) waves are observed on both sides of the X line and we show the direct association between the UH waves and the reconnection electric field. The strong wave‐electron interaction can change the electron‐scale dynamics and may modify the reconnection electric field. This study demonstrates that the UH waves may play an important role in controlling the reconnection rate. Key Points: Large amplitude nonlinear upper‐hybrid (UH) waves are observed on both inflow sides of an X lineThe UH waves are driven by the inbound meandering electronsThe UH waves may dissipate a significant part of the meandering electron energy gained from the reconnection electric field [ABSTRACT FROM AUTHOR]

We use Magnetospheric Multiscale (MMS) data to investigate the energy dissipation in a magnetopause reconnection electron diffusion region (EDR) event with moderate guide field. The four MMS spacecraft were separated by about 10 km so that comparative study among spacecraft within the EDR can be implemented. Similar magnetic field and electric current properties at each spacecraft indicate the formation of a quasi‐homogeneous magnetic and current structure in the diffusion region. However, we find that the energy dissipations detected by each spacecraft are still different due to the temporal or spatial effect of the out‐of‐plane reconnection electric field (EM) within the dissipation region. Our study suggests that the nonuniform or unsteady energy dissipation in the reconnection EDR may be a universal process. Plain Language Summary: Magnetic reconnection is a fundamental physical process during which magnetic topologies are changed and magnetic energy is transferred to plasma. Although the effects and structures created by reconnection can be very large in scales, reconnection is actually triggered within a small diffusion region. In the electron diffusion region, energy dissipation is a significant issue, which needs to be investigated further. Previous studies have shown that energy dissipation can be localized and oscillatory due to plasma waves or turbulent environment. Our study shows that the energy dissipations are still nonuniform or unsteady even under diffusion region with quasi‐homogeneous magnetic field and electric current structure. Thus, we suggest that the nonuniform or unsteady energy dissipation in the reconnection dissipation region is a universal process. Key Points: A quasi‐homogeneous magnetic field and current structure in the reconnection electron diffusion region at the magnetopause is observedThe reconnection electric field (EM) in the electron diffusion region is nonuniform among the four spacecraftThe energy dissipations are nonuniform due to the temporal or spatial effect of the reconnection electric field [ABSTRACT FROM AUTHOR]

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 (fw is the wave frequency and fce is 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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

We present a statistical analysis of >2,100 bipolar electrostatic solitary waves (ESWs) collected from 10 quasi‐perpendicular Earth's bow shock crossings by Magnetospheric Multiscale spacecraft. We developed and implemented a correction procedure for reconstruction of actual electric fields, velocities, and other properties of ESW, whose spatial scales are typically comparable with or smaller than spatial distance between voltage‐sensitive probes. We found that more than 95% of the ESW are of negative polarity with amplitudes typically below a few Volts and 0.1Te (5–30 V or 0.1–0.3Te for a few percent of ESW), spatial scales of 10–100 m or λD–10λD, and velocities from a few tens to a few hundred km/s that is on the order of local ion‐acoustic speed. The spatial scales of ESW are correlated with local Debye length λD. The ESW have electric fields generally oblique to magnetic field and they propagate highly oblique to shock normal N; more than 80% of ESW propagate within 30° of the shock plane LM. In the shock plane, ESW typically propagates within a few tens of degrees of local magnetic field projection BLM and preferentially opposite to N × BLM. We argue that the ESW of negative polarity are ion holes produced by ion‐ion streaming instabilities. We estimate ion hole lifetimes to be 10–100 ms, or 1–10 km in terms of traveling distance. The revealed statistical properties will be useful for quantitative studies of electron thermalization in the Earth's bow shock. Plain Language Summary: The Earth's bow shock is a natural laboratory for in‐situ analysis of plasma processes in supercritical collisionless shock waves. The current consensus is that quasi‐static magnetic and electric fields play a major role in electron heating, while scattering by waves results in thermalization of electron velocity distribution functions shaped by quasi‐static fields. Among various waves observed in the Earth's bow shock, electrostatic fluctuations deserve particular attention, because they are always present in the shock transition region. This study is focused on experimental analysis of electrostatic fluctuations in the Earth's bow shock using 3D electric field measurements by the Magnetospheric Multiscale (MMS) spacecraft. We presented a statistical analysis of electrostatic solitary waves (ESWs) using a data set of more than 2,000 solitary waves selected in 10 Earth's bow shock crossings. In contrast to previous interpretations, we showed that ESWs in the Earth's bow shock are predominantly ion phase‐space holes, which is a strong indication that electrostatic fluctuations in the Earth's bow shock are produced by ion‐ion streaming instabilities. We obtained statistical distributions of various properties of ion holes, which will be of value for quantifying electron thermalization by electrostatic fluctuations in collisionless shocks. Key Points: More than 95% of bipolar electrostatic structures in the Earth's bow shock are ion holesIon holes typically have electric fields oriented oblique to local magnetic fieldWe estimated lifetimes of ion holes to be 10–100 ms or 1–10 km in terms of traveling distance [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 (

The Magnetospheric Multiscale (MMS) spacecraft observed many enhancements of electromagnetic ion cyclotron (EMIC) waves in an event in the late afternoon outer magnetosphere. These enhancements occurred mainly in the troughs of magnetic field intensity associated with a compressional ultralow frequency (ULF) wave. The ULF wave had a period of ∼2–5 min (Pc5 frequency range) and was almost static in the plasma rest frame. The magnetic and ion pressures were in antiphase. They are consistent with mirror‐mode type structures. We apply the Wave‐Particle Interaction Analyzer method, which can quantitatively investigate the energy transfer between hot anisotropic protons and EMIC waves, to burst‐mode data obtained by the four MMS spacecraft. The energy transfer near the cyclotron resonance velocity was identified in the vicinity of the center of troughs of magnetic field intensity, which corresponds to the maxima of ion pressure in the compressional ULF wave. This result is consistent with the idea that the EMIC wave generation is modulated by ULF waves, and preferential locations for the cyclotron resonant energy transfer are the troughs of magnetic field intensity. In these troughs, relatively low resonance velocity due to the lower magnetic field intensity and the enhanced hot proton flux likely contribute to the enhanced energy transfer from hot protons to the EMIC waves by cyclotron resonance. Due to the compressional ULF wave, regions of the cyclotron resonant energy transfer can be narrow (only a few times of the gyroradii of hot resonant protons) in magnetic local time. Key Points: Electromagnetic ion cyclotron wave enhancements were detected in a compressional ultralow frequency (ULF) waveTroughs of magnetic field intensity of the ULF wave are preferential locations for the cyclotron resonant energy transferDue to compressional ULF wave, regions of the cyclotron resonant energy transfer can be narrow in magnetic local time [ABSTRACT FROM AUTHOR]

This article presents 18 magnetic reconnection electron diffusion region (EDR) candidates found using a neural network algorithm with the Magnetospheric Multiscale Mission phase 1a data at the Earth's dayside magnetopause. These new candidates are compared to the 32 previously reported dayside EDRs listed in Webster et al. (2018), https://doi.org/10.1029/2018ja025245, which constitute the training database of our algorithm. One of the main parameters used is a scalar quantity called "MeanRL" which is based on the asymmetry of the electron velocity distribution function and better identifies electron agyrotropy in the plane perpendicular to the magnetic field. In the light of the new EDR candidates found, we discuss and analyze the sign of the energy dissipation during the reconnection process and the distinction between the inner and outer EDRs, with 40% of the candidates showing negative or oscillating dissipation. We also present in details one of the new identified EDR candidates. Key Points: We report 18 new electron diffusion region (EDR) candidates close to the Earth magnetopause in the Magnetospheric Multiscale Mission (MMS) phase 1a data using a neural networkThe algorithm makes use of a scalar quantity called "MeanRL" to identify the electron perpendicular agyrotropy typical of EDRs from MMS distribution functionsWe analyze and discuss the geometry of EDR based on energy dissipation signatures [ABSTRACT FROM AUTHOR]

We report observations of the ion dynamics inside an Alfvén branch wave that propagates near the reconnecting dayside magnetopause. The measured frequency, wave normal angle and polarization are consistent with the predictions of a dispersion solver. The magnetospheric plasma contains hot protons (keV), cold protons (eV), plus some heavy ions. While the cold protons follow the magnetic field fluctuations and remain frozen‐in, the hot protons are at the limit of magnetization. The cold protons exchange energy back and forth, adiabatically, with the wave fields. The cold proton velocity fluctuations contribute to balance the Hall term fluctuations in Ohm's law, and the wave E field has small ellipticity and right‐handed polarization. The dispersion solver indicates that increasing the cold proton density facilitates propagation and amplification of these waves at oblique angles, as for the observed wave. Plain Language Summary: The Earth's magnetosphere is a very dilute cloud of charged particles that are trapped in the Earth's magnetic field. This cloud is surrounded by the solar wind, another very dilute gas that flows supersonically throughout the solar system. These two plasmas can couple to each other via magnetic reconnection, a fundamental plasma process that occurs at the dayside region of the interface between the two plasmas. When reconnection occurs, large amounts of energy and particles enter the magnetosphere, driving the near Earth space dynamics and generating, for instance, aurorae. The magnetospheric plasma sources are the solar wind and the Earth's ionosphere. Multiple plasma populations can be found inside the Earth's magnetosphere, depending on the plasma origin and its time history, as well as the magnetospheric forcing of the solar wind. In this study, we show how the presence of multiple particle populations at the interface between the solar wind and the magnetosphere modifies the properties of the waves that propagate there. Waves are known to play a fundamental role in converting energy and heating these very dilute charged gas clouds. Key Points: In situ observation of different dynamics of cold (eV) and hot (keV) protons inside an electromagnetic ion cyclotron waveWave number estimation shows that cold protons behave as fluid while hot protons interact at kinetic scalesMagnetized cold protons modify the Ohm's law balance and favor propagation at a large wave normal angle [ABSTRACT FROM AUTHOR]

Flux ropes are a proposed site for particle energization during magnetic reconnection, with several mechanisms proposed. Here, Magnetospheric Multiscale mission observations of magnetic mirror structures on the edge of two ion‐scale magnetopause flux ropes are presented. Donut‐shaped features in the electron pitch angle distributions provide evidence for electron trapping in the structures. Furthermore, both events show trapping with extended 3D structure along the body of the flux rope. Potential formation mechanisms, such as the magnetic mirror instability, are examined and the evolutionary states of the structures are compared. Pressure and force analysis suggest that such structures could provide an important electron acceleration mechanism for magnetopause flux ropes, and for magnetic reconnection more generally.Key Points: Magnetospheric Multiscale observations show magnetic mirror structures on the edge of magnetopause flux ropesMirror structures trap electrons and have extended 3D structureEvolution of mirror structures could facilitate particle acceleration [ABSTRACT FROM AUTHOR]

Electromagnetic ion cyclotron (EMIC) waves play important roles in particle loss processes in the magnetosphere. Determining the evolution of EMIC waves as they propagate and how this evolution affects wave-particle interactions requires accurate knowledge of the wave vector, k. We present a technique using the curl of the wave magnetic field to determine k observationally, enabled by the unique configuration and instrumentation of the Magnetospheric MultiScale (MMS) spacecraft. The wave curl analysis is demonstrated for synthetic arbitrary electromagnetic waves with varying properties typical of observed EMIC waves. The method is also applied to an EMIC wave interval observed by MMS on October 28, 2015. The derived wave properties and k from the wave curl analysis for the observed EMIC wave are compared with the Waves in Homogenous, Anisotropic, Multi-component Plasma (WHAMP) wave dispersion solution and with results from other single- and multi-spacecraft techniques. We find good agreement between k from the wave curl analysis, k determined from other observational techniques, and k determined from WHAMP. Additionally, the variation of k due to the time and frequency intervals used in the wave curl analysis is explored. This exploration demonstrates that the method is robust when applied to a wave containing at least 3-4 wave periods and over a rather wide frequency range encompassing the peak wave emission. These results provide confidence that we are able to directly determine the wave vector properties using this multi-spacecraft method implementation, enabling systematic studies of EMIC wave k properties with MMS. [ABSTRACT FROM AUTHOR]

Flux transfer events (FTEs) are transient phenomena produced by magnetic reconnection at the dayside magnetopause typically under southward interplanetary magnetic field (IMF) conditions. They are usually thought of as magnetic flux ropes with helical structures forming through patchy, unsteady or multiple X‐line reconnection. While the IMF often has a non‐zero BY component, its impacts on the FTE flux rope helicity remain unknown. We survey Magnetospheric Multiscale (MMS) observations of FTE flux ropes during years 2015–2017 and investigate the solar wind conditions prior to the events. By fitting a force‐free flux rope model, we select 84 events with good fits and obtain the helicity sign (i.e., handedness) of the flux ropes. We find that positive (negative) helicity flux ropes are mainly preceded by positive (negative) BY component. This finding is compatible with flux ropes formed through a multiple X‐lines mechanism. Plain Language Summary: The Earth's near‐space environment is very dynamic with transient phenomena triggered by interaction between the solar wind and the Earth's magnetopause. The solar wind carries along an interplanetary magnetic field (IMF) whose orientation determines the dynamics of the interaction. When the IMF is southward, magnetic reconnection can be triggered at the Earth's magnetopause on the dayside. A flux transfer event (FTE) is a transient portal that allows the bursty transfer of solar wind into the Earth's magnetosphere. An FTE is envisaged as a twisted magnetic field structure with helical field that looks like a rope. We study the relationship between the IMF orientations and the twist direction of an ensemble of FTEs observed by NASA's Magnetospheric Multiscale mission, by modeling FTEs as magnetic flux ropes. We found that the flux rope twist direction is controlled by the IMF orientation, such that the rope is twisted in the left‐handed or right‐handed sense depending on the east‐west component of the IMF. This result supports the formation of FTEs by a multiple reconnection mechanism. Key Points: The helicity sign of 84 flux transfer events (FTEs) is studied using force‐free flux rope model fittingRight‐handed (left‐handed) FTE flux ropes are mostly preceded by positive (negative) interplanetary magnetic field (IMF) BYThis IMF BY control of the helicity sign is compatible with a multiple X‐line formation mechanism [ABSTRACT FROM AUTHOR]

The Earth's magnetotail contains a current sheet separating the anti‐Sunward field of the southern lobe from the sunward‐pointing northern lobe. Herein, we report tail current sheets that are supported only by electron currents. We examine one electron‐only current sheet in detail and briefly discuss 10 others. Three current sheets are interpreted in terms of the time‐evolution of reconnection onset. These current sheets show evidence of parallel electron heating, perpendicular ion heating, and current sheet expansion. These features are consistent with electron and ion behavior during traditional "electron‐ion" reconnection. Ground‐based and in‐situ data show that electron‐ion reconnection occurs shortly after each "pre‐ion reconnection" electron‐only reconnection event. This suggests that electron‐only reconnection can act as a precursor to electron‐ion reconnection. We note that five events occur shortly after a period of electron‐ion reconnection, which suggests that electron‐only reconnection is more than merely a precursor to ion reconnection. Plain Language Summary: Magnetic reconnection is a key process in conversion of magnetic energy to kinetic and thermal energy in space and laboratory plasmas. The Magnetosphere Multiscale (MMS) mission is designed to study the physics of magnetic reconnection with unparalleled time and spatial resolution. In this letter, we present several MMS observations of electron‐supported current sheets that do not show signatures of typical magnetic reconnection, dubbed "Electron‐Only" reconnection. We use three events to show that "electron‐only" reconnection can lead to "electron‐ion" magnetic reconnection. We use six events to suggest that "electron‐only" reconnection occurs in more regimes than merely during the onset of "electron‐ion" reconnection. Key Points: Eleven electron‐only reconnection events observed by Magnetosphere Multiscale in the near‐Earth magnetotailThree events are snapshots in a time evolution into "electron‐ion" reconnectionFive events occur after "electron‐ion" reconnection; Electron‐only reconnection is more than a precursor to "electron‐ion" reconnection [ABSTRACT FROM AUTHOR]

We investigate agyrotropic electron distributions from two magnetopause events observed by magnetospheric multiscale (MMS) spacecraft. Agyrotropic electron distributions can be generated by the finite electron gyration at an electron‐scale boundary, and the electric field normal to this boundary usually contributes to the electron acceleration to make the agyrotropic distributions more apparent. The effect of the electric field becomes important only when it is sufficiently strong and local, meaning its electrostatic potential is comparable to or larger than the electron temperature, and its width is smaller than the electron thermal gyroradius, so that this electric field can directly accelerate part of the electrons out of the original core to form agyrotropic electron distributions. Also, we reproduce the measured electron "finger" structures from test particle simulations, which can be effectively suppressed by increasing the sampling rate of the electron measurement. Plain Language Summary: Agyrotropic electron distributions reveal valuable information of electron dynamics at electron scales, and the generation of these distributions have been extensively studied. In this study, we provide a new possibility to generate agyrotropic electron distributions with a strong localized electric field, which can accelerate part of electrons out of the original electron core to form agyrotropic distributions. As such large‐amplitude small‐scale electric field fluctuations are frequently observed in turbulent plasma environments, we suggest that more agyrotropic electron distributions can be observed with high temporal resolution measurements. Key Points: Agyrotropic electron distributions formed by the finite electron gyration at electron‐scale boundaries and by direct acceleration of a strong local electric field are presentedThese different processes to generate agyrotropic electron distributions are confirmed by test particle simulationsThe agyrotropic "finger" distribution measured by magnetospheric multiscale mission/fast plasma investigation can be effectively suppressed by increasing the sampling rate [ABSTRACT FROM AUTHOR]

Understanding the physical mechanisms responsible for the cross‐scale energy transport and plasma heating from solar wind into the Earth's magnetosphere is of fundamental importance for magnetospheric physics and for understanding these processes in other places in the universe with comparable plasma parameter ranges. This paper presents observations from the Magnetosphere Multiscale (MMS) mission at the dawn‐side high‐latitude dayside boundary layer on February 25, 2016 between 18:55 and 20:05 UT. During this interval, MMS encountered both the inner and outer boundary layers with quasiperiodic low frequency fluctuations in all plasma and field parameters. The frequency analysis and growth rate calculations are consistent with the Kelvin‐Helmholtz instability (KHI). The intervals within the low frequency wave structures contained several counter‐streaming, low‐ (0–200 eV) and mid‐energy (200 eV–2 keV) electrons in the loss cone and trapped energetic (70–600 keV) electrons in alternate intervals. The counter‐streaming electron intervals were associated with large‐magnitude field‐aligned Poynting fluxes. Burst mode data at the large Alfvén velocity gradient revealed a strong correlation between counter streaming electrons, enhanced parallel electron temperatures, strong anti‐field aligned wave Poynting fluxes, and wave activity from sub‐proton cyclotron frequencies extending to electron cyclotron frequency. Waves were identified as Kinetic Alfvén waves but their contribution to parallel electron heating was not sufficient to explain the >100 eV electrons, and rapid nonadiabatic heating of the boundary layer as determined by the characteristic heating frequency, derived here for the first time.Plain Language Summary: Electrons, The Riders of the Space Hurricane: Earth's magnetic field forms a barrier in the solar wind, called the magnetosphere, which provides some shielding against solar radiation and galactic cosmic rays. However, this shield can be penetrated by process called magnetic reconnection, and secondary processes created by giant "fluid‐scale" space hurricanes (typically 20,000–36,000 km in wave length) aka Kelvin‐Helmholtz (KH) waves that are whipped along the magnetic barrier by solar wind flow. One of the puzzling problems of the Earth's magnetosphere is that it is so hot: both electrons and ions are heated to tens of millions of degrees when they get transported from solar wind through the Earth's magnetic barrier. This article shows observations of multiscale wave structures, spanning the fluid‐scales, ion scales and electron scales detected by the NASA's magnetosphere multiscale mission consisting of four satellites. We show how these large‐scale waves contain ion and electron scale waves that are able to produce some of the observed electron heating and acceleration. We "fingerprint" the exact plasma wave modes (tornadoes) inside the space hurricane that are responsible for resonantly whipping and transferring the wave energy to the electrons surfing the wave.Key Points: Magnetosphere multiscale observed periodic low frequency waves, likely Kelvin‐Helmholtz instability, at the dawn‐flank high‐latitude boundary layerHigher frequency waves within the low frequency waves were associated with enhanced Poynting flux and parallel electron heatingWaves close to proton cyclotron frequency were identified as Kinetic Alfvén waves, and were evaluated to provide partial heating [ABSTRACT FROM AUTHOR]

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]

Energy and momentum can be transferred from the magnetosphere to the ionosphere through field-aligned currents (FACs), which show seasonal variations due to seasonal variation of the ionospheric conductance. Using Magnetospheric Multiscale Mission observations, we investigate the characteristics of kinetic-scale (close to local ion inertial length) FACs in the plasma sheet boundary layer (PSBL), such as occurrence rate, magnitude, and carriers. The results show that: (1) the occurrence rates of FACs exhibit a trident distribution; they are also larger in the northern hemisphere than in the southern hemisphere, especially for the earthward FACs. (2) Distribution patterns vs. plasma β are different for the beam-type and the flow-type FACs. Flow-type FACs are observed closer to the central plasma sheet (higher β), and their occurrence rate decreases linearly toward the tail lobe (lower β), while the beamtype FACs are mainly and equally distributed within 0.1 < β < 1.0. (3) The magnitude of FACs shows little dependence on β, and generally increases closer to the Earth. (4) Occurrence rate and magnitude of FACs both increase during active periods, consistent with ionospheric observations. (5) The main carriers for FACs in the PSBL are thermal electrons, while cold electrons also contribute occasionally, especially during active periods. Hemispheric asymmetry of occurrence rate and dependence on geomagnetic activity of the kinetic-scale FACs in the PSBL are consistent with FACs at ionospheric altitudes. This fact demonstrates that kinetic-scale FACs are significant in magnetosphere-ionosphere coupling and illustrates the possible ionospheric feedback effects to the magnetosphere in the nightside. [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]

The region surrounding the reconnection separatrix consists of many particle and wave transient features (electron, cold and hot ion beams, Hall E&B fields, kinetic Alfvén, LH, etc. waves) whose pattern and parameters may vary depending on the distance from active neutral line. We study nine quick MMS entries into the plasma sheet boundary layer (PSBL) from the tail lobe to address the meso‐scale pattern and other characteristics of phenomena for active separatrix crossings as deduced from particle observations. The outermost thin layer (a fraction of ion inertial scale, di) of low‐density plasma consists of accelerated electron beams and lobe cold ions and displays density depletions (EBL region). It is followed by hot proton beam (PBL region) in which the plasma density grows from lobe‐like towards plasma sheet‐like values; the beam energy‐dispersion is used to estimate the distance from the active neutral line. Thin (usually ≤ di) region containing intense Hall‐like Ez perturbations (HR) usually overlaps with EBL and PBL regions. It often includes correlated B perturbations suggesting the Alfvén wave‐related transport from the reconnection source; the estimated Alfvénic ratio δE/(VA δB) varied between 0.3 and 1.3 in studied examples. The HR is associated with profound plasma property changes, including the heating of cold ion beams in its innermost part, it hosts intense structured field‐aligned currents and intense E‐field fluctuations. Surprisingly, most of abovementioned findings are valid for crossings observed at large distances from the reconnection region (exceeding a few tens Re or >100 di) except for longer time‐scales and larger spatial scales of the pattern.Key Points: Survey of nine separatrix crossings during active reconnection with X‐line distance estimated using proton beam dispersionStrong Hall‐type Ez region overlaps with hot proton beam, hosts Alfvénic E&B variations, intense FACs, cold ions, and strong E fluctuationsSimilar patterns of distant (>100 di) and mid‐distance crossings (<30–50 di) except for their longer duration and larger spatial scales [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]

Decomposing the electric field (E) into the contributions from generalized Ohm's law provides key insight into both nonlinear and dissipative dynamics across the full range of scales within a plasma. Using high-resolution, multispacecraft measurements of three intervals in Earth's magnetosheath from the Magnetospheric Multiscale mission, the influence of the magnetohydrodynamic, Hall, electron pressure, and electron inertia terms from Ohm's law, as well as the impact of a finite electron mass, on the turbulent E spectrum are examined observationally for the first time. The magnetohydrodynamic, Hall, and electron pressure terms are the dominant contributions to E over the accessible length scales, which extend to scales smaller than the electron gyroradius at the greatest extent, with the Hall and electron pressure terms dominating at sub-ion scales. The strength of the nonideal electron pressure contribution is stronger than expected from linear kinetic Alfvén waves and a partial antialignment with the Hall electric field is present, linked to the relative importance of electron diamagnetic currents in the turbulence. The relative contribution of linear and nonlinear electric fields scale with the turbulent fluctuation amplitude, with nonlinear contributions playing the dominant role in shaping E for the intervals examined in this study. Overall, the sum of the Ohm's law terms and measured E agree to within ~20% across the observable scales. These results both confirm general expectations about the behavior of E in turbulent plasmas and highlight features that should be explored further theoretically. Plain Language Summary Complex turbulent motions are observed in plasmas throughout the Universe and act to transfer energy from large-scale fluctuations to small-scale fluctuations, which can be more easily dissipated into the thermal energy of the particles. Electric fields in these plasmas play a central role in enabling the exchange of energy between the magnetic field and the motion of the charged particles and are, therefore, important for disentangling the complex nonlinear dynamics and dissipative processes. Using cutting-edge, high-resolution, multispacecraft measurements from NASA's Magnetospheric Multiscale mission, we decompose the electric field in Earth's turbulent magnetosheath into the various terms from generalized Ohm's law, which governs the behavior of the electric field across the wide range of length scales in the plasma. The results confirm a number of general expectations about the relative behavior of the terms in Ohm's law, as well as highlight several new features that are significant for understanding the nonlinear behavior and turbulent dissipation at different scales within the plasma. [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]

Lower hybrid waves are investigated at the magnetosheath separatrix region in asymmetric guide field reconnection by using the Magnetospheric Multiscale (MMS) mission. Three of the four MMS spacecraft observe clear wave activities around the lower hybrid frequency across the magnetosheath separatrix, where a density gradient is present. The observed waves are consistent with generation by the lower hybrid drift instability. The characteristic properties of these waves include the following: (1) the waves propagate toward the x‐line in the spacecraft frame due to the large out‐of‐plane magnetic field, which is in the same direction of the diamagnetic drift of the x‐line; (2) the wave potential is about 20% of the electron temperature. These drift waves effectively produce cross‐field particle diffusion, enabling the transport of magnetosheath electrons into the exhaust region. At last, we suggest that the lower hybrid waves at the magnetosheath separatrix region represent some unique features of asymmetric guide field reconnection, which is different from that widely observed at the magnetospheric side of magnetopause reconnection. Plain Language Summary: Magnetic reconnection is a fundamental process of explosive energy conversion in space, and one important unresolved issue during this process is how plasma waves impact the magnetic reconnection. Different types of waves have been found and investigated during reconnection, including kinetic Alfvén waves, lower hybrid waves, whistler waves, upper hybrid waves, and parallel electrostatic waves. Among these waves, lower hybrid waves, taken as a basic feature of 3‐D asymmetric reconnection, are frequently observed at the magnetospheric side. In this study, we present new observations from the Magnetospheric Multiscale (MMS) mission, showing that the lower hybrid waves can also be found at the magnetosheath separatrix in asymmetric guide field reconnection, which enable the cross‐field particle diffusion from the magnetosheath to the exhaust. These results can help deepen our understanding of the roles of plasma waves in reconnection. Key Points: Lower hybrid waves are observed at the magnetosheath separatrix region in asymmetric guide field reconnectionProperties of these waves are presented and compared with that at the magnetospheric sideThese waves lead to effective cross‐field particle diffusion from the magnetosheath to the reconnection exhaust [ABSTRACT FROM AUTHOR]

We report a Magnetospheric Multiscale (MMS) mission observation of magnetic reconnection occurring at the edge of a large‐scale magnetic flux rope (MFR), the cross‐section of which was about 2 RE. The MFR was observed at the duskside in the Earth's magnetotail and was highly oblique with its axis approximately along the XGSM direction. We find an electron‐scale current sheet near the edge of this MFR. The Hall magnetic and electric field, super‐Alfvénic electron outflow, parallel electric field, and positive energy dissipation were observed associated with the current sheet, which indicates that MMS detected a reconnecting current sheet with a large guide field. Interestingly, ions were not coupled in this reconnection, akin to the electron‐only reconnection observed in the turbulent magnetosheath. We further find that the electron‐only reconnection is commonly associated with a macroscopic MFR. This result will shed new light on understanding the multiscale coupling associated with an MFR in space plasmas. Key Points: MMS observed an electron‐only reconnection at the edge of a macroscopic magnetic flux rope in the magnetotailElectron‐only reconnection is ubiquitous in space plasma [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]