We present statistical analysis of 16,903 current sheets (CS) observed over 641 days aboard Ulysses spacecraft at 5 AU. We show that the magnetic field rotates across CSs through some shear angle, while only weakly varies in magnitude. The CSs are typically asymmetric with statistically different, though only by a few percent, magnetic field magnitudes at the CS boundaries. The dataset is classified into about 90.6\% non-bifurcated and 9.4\% bifurcated CSs. Most of the CSs are proton kinetic-scale structures with the half-thickness of non-bifurcated and bifurcated CSs within respectively 200–2,000 km and 500–5,000 km or 0.5–$5\lambda_{p}$ and 0.7–$15\lambda_{p}$ in units of local proton inertial length. The amplitude of the current density, mostly parallel to magnetic field, is typically within 0.05–0.5 nA/m$^{2}$ or 0.04–$0.4J_{A}$ in units of local Alfv\’{e}n current density. The CSs demonstrate approximate scale-invariance with the shear angle and current density amplitude scaling with the half-thickness, $\Delta \theta\approx 16.6^{\circ}\;(\lambda/\lambda_{p})^{0.34}$ and $J_0/J_{A}\approx 0.14\;(\lambda/\lambda_{p})^{-0.66}$. The matching of the magnetic field rotation and compressibility observed within the CSs against those in ambient solar wind indicate that the CSs are produced by turbulence, inheriting thereby its scale-invariance and compressibility. The estimated asymmetry in plasma beta between the CS boundaries is shown to be insufficient to suppress magnetic reconnection through the diamagnetic drift of X-line, but magnetic reconnection is probably suppressed by other processes. The presented results will be of value for future comparative analysis of CSs observed at different distances from the Sun.

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 these 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 non-EDR magnetopause crossings - crossings that either passed through the reconnection exhausts or through the non-reconnecting magnetopause. 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 non-EDR 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 current sheet.

The Wind spacecraft is a critical element in NASA’s Heliophysics System Observatory (HSO) – a fleet of spacecraft created to understand the dynamics of the sun-Earth system – owing to the combination of its longevity (>25 years in service), its diverse compliment of instrumentation, and high resolution and accurate measurements. Wind has over 55 selectable public data products with over ~1100 total data variables (including OMNI data products) on SPDF/CDAWeb alone. These data have led to paradigm shifting results in studies of statistical solar wind trends, magnetic reconnection, large-scale solar wind structures, kinetic physics, electromagnetic turbulence, the Van Allen radiation belts, coronal mass ejection topology, interplanetary and interstellar dust, the lunar wake, solar radio bursts, solar energetic particles, and extreme astrophysical phenomena such as gamma-ray bursts. This review introduces the mission and instrument suites then discusses examples of the contributions by Wind to these scientific topics that emphasize its importance to both the fields of heliophysics and astrophysics.

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 collisionless reconnection, 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.

We investigate the onset of magnetic reconnection, utilizing a fully kinetic Particle-In-Cell (PIC) simulation. Characteristic features of the electron phase-space distributions immediately before reconnection onset are identified. These include signatures of pressure non-gyrotropy in the velocity distributions, and lemon shaped distributions in the in-plane velocity directions. Further, we explain how these features form through particle energization by the out-of-plane electric field. Identification of these features in the distributions can aid in analysis of data where clear signatures of ongoing reconnection are not yet present.

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.

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.

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.

We report observations of reconnection exhausts and associated ion and electron separatrix layers in and around the Heliospheric Current Sheet (HCS) during PSP Encounters 08 and 07, at 16 Rs and 20 Rs. HCS reconnection accelerated protons to almost twice the solar wind speed and increased the proton core energy by a factor of ~3, due to the Alfvén speed being comparable to the solar wind flow speed at these near-Sun distances. During E08, accelerated protons were found to have leaked out of the exhaust along separatrix field lines, appearing as field-aligned energetic proton beams in a broad region outside the HCS. Concurrent dropouts of strahl electrons, indicating disconnection from the Sun, provide further evidence for the HCS being the source of the beams. Around the HCS in E07, there were also proton beams but without electron strahl dropouts, indicating that their origin was non-local and closer to the Sun.

We report results of our analysis of a solar wind reconnecting current sheet (RCS) and its solar wind magnetic hole observed on 20 November 2018. In the solar wind, the normal vector to the current sheet plane makes an angle of 32° with the Sun-Earth line. A combination of tilted current sheet plane and foreshock effects cause an asymmetric interaction with the bow shock, such that the structure arrives at the quasi-perpendicular side of the bow shock before the quasi-parallel side. The solar wind flow slowdown and deflection during the bow shock crossing significantly disrupt the reconnection exhausts within the RCS. Unlike localized magnetosheath jets, the solar wind RCS has a global impact on the bow shock and the magnetopause. Plasma flow deflection in the magnetosheath also increases with the passage of the RCS. The magnetic field strength inside the magnetic hole decreases by ~69 percent in the solar wind, with a similar depression rate observed inside the magnetosheath due to this structure. The ion density and temperature both increase within the current sheet to form a roughly pressure balanced structure. Field rotation and change in the dynamic pressure during this event modify the reconnection zones at the magnetopause and cause an inward motion of this boundary.

In situ spacecraft missions are powerful assets to study processes that occur in space plasmas. One of their main limitations, however, is extrapolating such local measurements to the global scales of the system. To overcome this problem at least partially, multi-point measurements can be used. There are several multi-spacecraft missions currently operating in the Earth’s magnetosphere, and the simultaneous use of the data collected by them provides new insights into the large-scale properties and evolution of magnetospheric plasma processes. In this work, we focus on studying the Earth’s magnetopause using a conjunction between the MMS and Cluster fleets, when both missions skimmed the magnetopause for several hours at distant locations during radial IMF conditions. The observed magnetopause positions as a function of the evolving solar wind conditions and compared to model predictions of the magnetopause. We observe an inflation of the magnetosphere (˜0.7 RE), consistent with magnetosheath pressure decrease during radial IMF conditions, which is less pronounced on the flank (< 0.2 RE). There is observational evidence of magnetic reconnection in the subsolar region for the whole encounter, and in the dusk flank for the last portion of the encounter, suggesting that reconnection was extending more than 15 RE. However, reconnection jets were not always observed, suggesting that reconnection was patchy, intermittent or both. Shear flows reduce the reconnection rate up to ˜30% in the dusk flank according to predictions, and the plasma ß enhancement in the magnetosheath during radial IMF favors reconnection suppression by the diamagnetic drift.

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 current densities and their contributions across the dayside and flank magnetopause. Our results indicate that the ion temperature gradient component makes up to 38% 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 to 16 times weaker than the ion diamagnetic current on average.