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The MMS satellites traversed a ∼6 di‐wide and ∼500 km/s southward reconnection exhaust at the dayside magnetopause on 6 December 2015 and ∼29 di from the associated X‐line region. A narrow ∼0.26–0.34 di layer of enhanced ±3.5 nW/m3 oscillating energy conversion perpendicular to the magnetic field resides in this exhaust. It contained two regions of diverging in‐plane electric fields in general agreement with two clockwise electron flow vortices and a proposed increase of the electron vorticity ∇ × Ve. The layer developed sunward of a unipolar Hall magnetic field for a duskward BM/BL ∼ 0.9 guide field. Each electron flow vortex supported a local ∆BM ∼ 10 nT strengthening of this Hall field. The presence of this electron‐scale layer in a southward exhaust for a duskward guide field is consistent with a two‐dimensional simulation of a similar structure that evolved from an X‐line into a northward exhaust for a similar strength dawnward guide field. Plain Language Summary: Magnetic reconnection is a critical process that drives large‐scale plasma convection in the Earth's magnetosphere. In this letter, we report new direct MMS observations for the presence of two electron flow vortices within a sub‐ion scale energy conversion channel that resides in a dayside magnetopause reconnection exhaust. Each of the two electron‐scale flow vortices is shown to contribute a significant fraction toward the large‐scale Hall magnetic field. Key Points: Electron‐scale 0.3 di layer of oscillating energy conversion and electron vorticity detected in 6 di wide reconnection jet 30 di from EDRTwo regions of in‐plane diverging electric field in energy conversion layer consistent with two measured clockwise electron flow vorticesOut‐of‐plane magnetic field increased ∼10 nT from each passing electron vortex immediately sunward of 50 nT unipolar Hall magnetic field [ABSTRACT FROM AUTHOR]

We present a statistical analysis of electrostatic solitary waves observed aboard Magnetospheric Multiscale spacecraft in the Earth's magnetosheath. Applying single‐spacecraft interferometry to several hundred solitary waves collected in about 2‐minute interval, we show that almost all of them have the electrostatic potential of positive polarity and propagate quasi‐parallel to the local magnetic field with plasma frame velocities of the order of 100 km/s. The solitary waves have typical parallel half‐widths from 10 to 100 m that is between 1 and 10 Debye lengths and typical amplitudes of the electrostatic potential from 10 to 200 mV that is between 0.01% and 1% of local electron temperature. The solitary waves are associated with quasi‐Maxwellian ion velocity distribution functions, and their plasma frame velocities are comparable with ion thermal speed and well below electron thermal speed. We argue that the solitary waves of positive polarity are slow electron holes and estimate the time scale of their acceleration, which occurs due to interaction with ions, to be of the order of one second. The observation of slow electron holes indicates that their lifetime was shorter than the acceleration time scale. We argue that multi‐spacecraft interferometry applied previously to these solitary waves is not applicable because of their too‐short spatial scales. The source of the slow electron holes and the role in electron‐ion energy exchange remain to be established. Plain Language Summary: Earth's magnetosheath is a highly turbulent medium and an ideal natural laboratory for the analysis of plasma turbulence. Spacecraft measurements showed that high‐frequency electric field fluctuations in the Earth's magnetosheath are predominantly electrostatic and consist, particularly, of electrostatic solitary waves with bipolar parallel electric fields. The properties of these electrostatic fluctuations have been largely unaddressed and, moreover, the results of previous studies were inconsistent. In this paper, we present a statistical analysis of electrostatic solitary waves observed aboard Magnetospheric Multiscale in the Earth's magnetosheath. We revealed that most of the solitary waves are Debye‐scale structures with the electrostatic potential of positive polarity and typical amplitudes between 0.01% and 1% of local electron temperature. We demonstrated that the solitary waves must be electron holes, purely kinetic structures produced in a nonlinear stage of various electron‐streaming instabilities. Even more critical is that these structures are slow; their plasma frame velocities are well below electron thermal speed but coincide with the velocities of the bulk of ions. While the source of electrostatic fluctuations in Earth's magnetosheath could not be revealed, the finding that these fluctuations can be slow implies they can facilitate efficient energy exchange between ions and electrons. Key Points: Statistical analysis of 645 solitary waves in the Earth's magnetosheath revealed that 630 of them are electron holesThe electron holes are associated with quasi‐Maxwellian ion velocity distribution functionsThe electron hole velocities are comparable with those of the bulk of ions and well below electron thermal speed [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]

High‐resolution electron and ion velocity distributions, f(v), which consist of N effectively disjoint beams, have been measured by NASA's Magnetospheric Multiscale Mission and in reconnection simulations. Commonly used standard velocity moments assume a single mean‐flow velocity for the entire distribution. This can lead to counterintuitive results for a multibeam f(v). An example is the standard thermal energy density moment (at a given space‐time point) of a pair of equal and opposite cold particle beams. This standard moment is nonzero even though each beam has zero thermal energy density. By contrast, a multibeam moment of two or more cold beams at a given position and time has no thermal energy. A multibeam moment is obtained by taking a standard moment of each beam and then summing over beams. In this paper we will generalize these notions, explore their consequences, and apply them to an f(v) which is a sum of tri‐Maxwellians. Both standard and multibeam energy moments have coherent and incoherent pieces. Examples of incoherent moments are the thermal energy density, the pressure, and the thermal energy flux (enthalpy flux plus heat flux). Corresponding coherent moments are the bulk kinetic energy density, the ram pressure, and the bulk kinetic energy flux. The difference between a standard incoherent moment and its multibeam counterpart will be defined as the "pseudothermal part" of the standard moment. The sum of a pair of corresponding coherent and incoherent moments is the undecomposed moment. Undecomposed standard moments are always equal to the corresponding undecomposed multibeam moments. Key Points: Standard velocity moments of effectively disjoint measured multibeam particle velocity distributions, f(v), can be misleadingSo‐called multibeam moments of multibeam f(v) are found by taking a standard moment of each beam and then summing over beamsStandard thermal moments (pressure, thermal flux, etc.) of multibeam f(v) contain pseudothermal parts which are absent in multibeam moments [ABSTRACT FROM AUTHOR]

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

Interactions between magnetic reconnection inflows and outflows can result in a violent mixing process. In Magnetospheric MultiScale observations of asymmetric, low guide‐field reconnection, highly sheared electron flow paired with sharp density and temperature gradients have been found in association with bursts of strong (≥100 mV/m) electric fields parallel to the ambient magnetic field. It is likely that large spikes in E‖ are part of a dynamic, small‐scale structure which results from mixing between plasmas. In this study, a 1‐D Vlasov simulation with parameters directly comparable to the observed plasma environment and interaction timescale is used to demonstrate that mixing at a sharp boundary between magnetospheric and magnetosheath electrons is qualitatively consistent with measured particle distributions and signatures in E‖. Properties of mixing structures such as net electric potential are estimated and found capable of accelerating electron beams toward the electron diffusion region but are not necessarily sufficient to generate the strongest observed jets. Obliquely propagating lower hybrid drift waves are also present and likely provide most of the energy for acceleration. Drift waves may be responsible for cross‐field transport required to begin the mixing process. We conclude that parallel mixing primarily acts to mediate plasma boundaries, thermalizing electron beams contributing to the high anisotropy (Te‖>Te⊥) electron distributions found in the dayside reconnection magnetospheric inflow region. Key Points: Violent mixing between plasmas is observed within an electron‐scale boundary along the dayside magnetosphere reconnection separatrixConsecutive negative spikes in parallel electric field can be explained by a single oscillating double‐layer‐like structureElectrostatic mixing signatures likely contribute primarily to thermalization and heating rather than linear acceleration toward the X line [ABSTRACT FROM AUTHOR]

Electron phase‐space holes are kinetic plasma structures commonly observed in space plasmas on Debye length scales. Near the Earth's duskside flank at 10 Earth radii, a series of 32 electron holes (EHs) are detected within a 1‐s window on all four Magnetospheric Multiscale spacecraft. The spacecraft separation of <7 km is similar to the expected EH size in this region. Length, width, amplitude, and relative positions are determined for individual EHs using a cylindrically symmetric model fit to Magnetospheric Multiscale E field measurements. The model shows good agreement with observed E fields far from the EH center. Deviations in E⊥ from the model are present near the center, indicating observed EHs have complex, sometimes irregular, internal structure. Perturbation magnetic fields δB modeled assuming an E × B0 electron current reproduce the measured parallel perturbation in most cases, although there is a systematic variation due to geometric and finite gyroradius effects. Many EHs in this event have large amplitude for their size, reaching the theoretical lower limit in length parallel to the background magnetic field, which requires the electron phase‐space density to approach 0 in the center. It is possible that EHs of this type have recently formed, eventually weakening or becoming longer over time. This study provides the most detailed measurements of EHs to date. Their derived properties are largely in agreement with expectations from previous research. It remains unclear whether the few notable differences are due to rapid time evolution or are specific to the local environment. Key Points: For the first time, electron phase‐space hole measurements are unambiguously correlated across four closely spaced spacecraftPerpendicular electric fields can be unexpectedly weak inside electron holes, implying irregular interior structureEstimates of interior phase‐space density approach 0 in many cases, possibly representing an early stage of electron hole evolution [ABSTRACT FROM AUTHOR]

Abstract: We report multispacecraft observations by the Magnetospheric MultiScale (MMS) mission of large (80–155 λDe parallel to B) electrostatic solitary waves (ESWs). Observations of the same ESWs at different positions and times enable unprecedented accuracy in determining the size, velocity, and evolution of these nonlinear structures. This particular event is a short series of negative potential solitary waves in the magnetosheath. The observed structures differ in amplitude and speed, merging or colliding with one another. A Vlasov simulation supports initial wave growth by a cold feature in the electron distribution function. Such cold electrons are likely the result of mixing between the cold magnetospheric and warm magnetosheath plasma along a reconnection separatrix. ESW speed and perpendicular size are inconsistent with ion phase space holes. Solving Poisson's equation for the measured potential, we find a reduction in electron density rather than an enhancement expected from an electron soliton. These ESWs have an unusual combination of properties on both ion and electron scales, likely supported by a complex electron distribution in the nonlinear state. [ABSTRACT FROM AUTHOR]