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 appx. 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 6-9 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: 8 • The change in electron kinetic entropy per particle is calculated for 22 shock cross-9 ings having wide range of shock conditions 10 • The entropy change displays a strong dependence on the electron beta parame-11 ter 12 • The entropy change corresponds to an adiabatic index γ e = 1.595 ± 0.036 13 Corresponding author: Martin Lindberg, [email protected] 14 We use Magnetospheric Multiscale (MMS) data to study electron kinetic entropy across 15 Earth’s quasi-perpendicular bow shock. We have selected 22 shock crossings covering a 16 wide range of shock conditions. Measured distribution functions are calibrated and cor-17 rected for spacecraft potential, secondary electron contamination, lack of measurements 18 at the lowest energies and electron density measurements based on the plasma frequency 19 measurements. The change in electron kinetic entropy per particle is calculated for 22 20 shock crossings. 20 out of 22 crossings display an increase in the electron kinetic entropy 21 per particle ranging between 0.1-1.4 k B while two crossings display a slight decrease of 22-0.06 k B. We observe that the change in electron kinetic entropy, ∆S e , displays a strong 23 dependence on the change in electron temperature, ∆T e , and the upstream electron plasma 24 beta, β e. Shocks with high ∆T e are found to have high ∆S e. Shocks with low upstream 25 electron plasma betas are associated to higher ∆S e than shocks with large electron plasma 26 beta. We show that the calculated entropy per particle is strictly less than the maximum 27 state of entropy obtained using a Maxwellian distribution function. The resulting change 28 in electron kinetic entropy per particle ∆S e , density ∆n e and temperature ∆T e is used 29 to determine a value for the adiabatic index of electrons. We find that an adiabatic in-30 dex of γ e = 1.595 ± 0.036 describes the observations best.

Magnetopause Kelvin-Helmholtz (KH) waves are believed to mediate solar wind plasma transport via small-scale mechanisms. Vortex-induced reconnection (VIR) was predicted in simulations and recently observed using NASA’s Magnetospheric Multiscale (MMS) mission data. Flux Transfer Events (FTEs) produced by VIR at multiple locations along the periphery of KH waves were also predicted in simulations but detailed observations were still lacking. Here we report MMS observations of an FTE-type structure in a KH wave trailing edge during KH activity on 5 May 2017 on the dawnside flank magnetopause. The structure is characterised by (1) bipolar magnetic BY variation with enhanced core field BZ and (2) enhanced total pressure with dominant magnetic pressure. The cross-section size of the FTE is found to be consistent with vortex-induced flux ropes predicted in the simulations. Unexpectedly, we observe an ion jet (VY), electron parallel heating, ion and electron density enhancements, and other signatures that can be interpreted as a reconnection exhaust at the FTE central current sheet. Moreover, pitch angle distributions of suprathermal electrons on either side of the current sheet show different properties, indicating different magnetic connectivities. This FTE-type structure may thus alternatively be interpreted as two interlaced flux tubes with reconnection at the interface as reported by Kacem et al. (2018) and Øieroset et al. (2019). The structure may be the result of interaction between two flux tubes, likely produced by multiple VIR at the KH wave trailing edge, and constitutes a new class of phenomenon induced by KH waves.

The unprecedented spatiotemporal resolution of the sixty-four dual electron and ion spectrometers comprising the Fast Plasma Investigation (FPI) onboard the four Magnetospheric Multiscale (MMS) spacecraft enables us to compute terms of the Vlasov equation and thus compare MMS measurements directly to kinetic theory. Here we discuss our techniques for determining spatial and velocity-space gradients of the ion and electron distribution function from the skymaps provided by FPI. We present initial results validating and comparing the contributions from ∂f/∂t, v·∇f, and a·∇vf for a variety of kinetic plasma contexts including both reconnection and non-reconnection events, such as electron diffusion regions (EDRs), magnetic holes, and thin electron-scale current sheets. The ability to resolve gradients of the distribution function motivates comparison of MMS observations to predictions from gyro-kinetic theory and particle-in-cell (PIC) simulations as an approach for determining which physical mechanism is responsible for generating ion and electron crescent distributions observed in association with EDRs and thin boundary layers.

Magnetic reconnection is responsible for the major reconfigurations of the magnetosphere that lead to energy transport and deposition into the ionosphere. The fast rate at which magnetic energy is converted to plasma kinetic energy is likely enabled by the polarization Hall electric field that results from the separation of ions and electrons at small scales. Signatures of Hall fields have played a key role in identifying and studying reconnection, but the density of accumulated charge has not been quantified. We use the 4-point measurements of the Magnetospheric Multiscale mission to compute the divergence of the electric field and present the first observations of charge density in the diffusion region of magnetic reconnection. We show how it ties into the Hall system, discuss measurement uncertainties, analyze quality estimates, and make comparisons to 2D simulations. Charge density is briefly presented for other phenomena, and ranges from 2% or less of the background plasma density for magnetic reconnection and electron-scale magnetic holes and peaks to upwards of 4% for electron phase space holes.

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 (MMS) magnetopause crossings to analyze diamagnetic currents and their contributions across the dayside and flank magnetopause. Our results indicate that the ion temperature gradient component makes up to 30% of the ion diamagnetic current 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. This effect is most pronounced on the flank magnetopause. The electron diamagnetic current was found to be 4 to 12 times weaker than the ion diamagnetic current on average.

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 $B_Y$ component, its impacts on the FTE flux rope helicity remain unknown. We survey Magnetospheric Multiscale (MMS) observations of FTE flux ropes during the 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 a positive (negative) $B_Y$ component. This finding is compatible with flux ropes formed through a multiple X-line mechanism.

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 elongated electron-scale current sheet. This energy conversion process is in contrast to that in the standard electron diffusion region (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 energy conversion is dominated by the annihilation. 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. The discovery of the annihilation-dominated EDR reveals a new form of energy conversion in the collisionless reconnection process.

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 does 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.

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 by only electron currents. We examine one electron-only current sheet in detail, and briefly discuss ten 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.

In order to determine particle velocities and electric field in the frame of the magnetic structure, one first needs to determine the velocity of the magnetic structure in the frame of the spacecraft observations. Here, we show how to determine a two dimensional magnetic structure velocity for the magnetic reconnection event observed in the magnetotail by the Magnetospheric Multiscale (MMS) spacecraft on 11 July 2017. We use two different multi-spacecraft methods, Spatio-Temporal Difference (STD) and the recently developed polynomial reconstruction method. Both of these methods use the magnetic field measurements, and the reconstruction technique also uses the current density measured by the particle instrument. We find rough agreement between the results of our methods and with other velocity determinations previously published. We also explain a number of features of STD and show that the polynomial reconstruction technique is only likely to be valid within a distance of two spacecraft spacings from the centroid of the MMS spacecraft. Both of these methods are susceptible to contamination by magnetometer calibration errors.

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 Magnetosphere Multi-Scale (MMS) mission at the dawn-side high-latitude dayside boundary layer on 25th of February, 2016 between 18:55-20:05 UT. During this interval MMS encountered both inner and outer boundary layer with quasi-periodic 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 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. Wave intervals also showed high energy populations of O+ ions, likely of ionospheric or ring current origin. The counter-streaming electron intervals were associated with a large-magnitude field-aligned Poynting fluxes. Burst mode data at the large Alfven 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 Alfven waves but their contribution to parallel electron heating was not sufficient to explain the > 100 eV electrons, and rapid non-adiabatic heating of the boundary layer as determined by the characteristic heating frequency, derived here for the first time.