Konrad Steinvall

Magnetic reconnection occurs commonly at current sheets in magnetized plasmas, accelerating electrons to very high energies. Among the various acceleration processes, magnetic-field-aligned (parallel) electric fields can directly accelerate electrons during reconnection. In this study, we map the spatial distribution of parallel electric field structures and characterize their properties during a magnetic reconnection event in the Earth’s magnetotail, using 3D electric field measurements from the Magnetospheric Multiscale mission. The types of observed structures include electron and ion phase-space holes, electrostatic and electromagnetic waves, and double layer-like (DL-like) structures. We determine the properties and locations of the structures, and investigate in detail the effects of the DL-like structures on electron acceleration, trapping, and heating in the separatrix region (SR). Our observations reveal that 71% of all timeable structures propagate toward the X line, including 82% of electron-hole trains and 70% of electrostatic waves. Of the structures that are timeable with all four spacecraft (130 of 264), we find that only five, all in the SR, have a peak potential greater than a tenth of the local electron temperature. Five DL-like structures are found, all of which are located in the SR. One of these DL-like structures causes clear parallel acceleration of an electron beam toward the X line, which subsequently generates large-amplitude electron holes. An electron beam traveling from the X line is observed to reflect off the DL-like structure back toward the X line, contributing to electron trapping and regulating quasi-neutrality in the ion diffusion region.

Magnetic reconnection occurs commonly at current sheets in magnetized plasmas, accelerating electrons to very high energies. Among the various acceleration processes, magnetic-field-aligned (parallel) electric fields can directly accelerate electrons during reconnection. In this study, we map the spatial distribution of parallel electric field structures and characterize their properties during a magnetic reconnection event in the Earth’s magnetotail, using 3D electric field measurements from the Magnetospheric Multiscale mission. The types of observed structures include electron and ion phase-space holes, electrostatic and electromagnetic waves, and double layer-like (DL-like) structures. We determine the properties and locations of the structures, and investigate in detail the effects of the DL-like structures on electron acceleration, trapping, and heating in the separatrix region (SR). Our observations reveal that 82% of electron-hole trains and 74% of electrostatic waves travel toward the X line, consistent with the direction of inflowing electron beams. We find that only 5 of the 264 identified structures, all in the SR, have a peak potential greater than a tenth of the local electron temperature. Five DL-like structures are found, all of which are located in the SR. One of these DL-like structures causes clear parallel acceleration of an electron beam toward the X line, which is subsequently thermalized by large-amplitude electron holes. An electron beam traveling from the X line is observed to reflect off the DL-like structure back toward the X line, contributing to electron trapping and regulating quasi-neutrality in the ion diffusion region.

Magnetic reconnection is a source of energetic particles throughout the universe. How this energization takes place has been the subject of studies for decades. In this work, we use particle-in-cell simulations to study how so-called electron flat-top distributions relate to electron heating by reconnection. These distributions are known to be generated during reconnection by large-scale electric fields. Their formation corresponds to significant bulk heating, and they are therefore believed to play an important role in seeding subsequent Fermi-acceleration processes. Our results show that flat-tops develop in both the reconnection inflow and outflow, with the outflow populations being isotropized by magnetic curvature scattering. By decomposing the electric field into potential and solenoidal parts, we show that the potential part, despite performing zero net work in the x-line frame, reduces the efficiency of subsequent Fermi acceleration by locally depressing electron energy within the outflow jet.

Magnetic reconnection jets in Earth's magnetotail are regions of electron heating and acceleration. They are often associated with dipolarization fronts (DFs), which are characterized by sharp increases in the northward magnetic field component . Electrons can be accelerated at DFs mainly through the betatron and Fermi mechanisms. We perform a statistical study of betatron and Fermi electron rate of energy change at Earthward‐propagating DFs observed by the Magnetospheric Multiscale (MMS) spacecraft. Our results show that betatron acceleration is dominant ahead of DFs, where suprathermal electron fluxes perpendicular to the magnetic field are detected. Up to ion inertial lengths inside the outflow, Fermi acceleration acts on the low‐density electron population, potentially driving them to high energies (∼100 ). Spatially in the magnetotail, betatron acceleration is statistically larger near the most probable X‐line location, while Fermi‐dominated events are observed closer to the Earth.

Electrostatic waves are frequently observed at collisionless shocks and are believed to play a key role in energy dissipation. Recent spacecraft observations have identified a proton‐alpha instability in the Earth's bow shock as a potential mechanism to dissipate the energy associated with a relative drift between protons and alpha particles, . At plasma parameters relevant to this setting, we investigate the dependence of the evolution of this instability on through particle‐in‐cell simulations. We find that the instability produces strong ion heating, increasing ion temperatures by factors of two to five. The electrons, in contrast, experience only minimal heating . The heating is highly anisotropic and is most efficient when the wave vector is at a 45 angle to the alpha flow velocity. To interpret these results, we introduce a geometric model based on Landau resonance interaction bands in velocity space. It explains both the angular dependence and the magnitude of heating through the change in velocity distribution functions. At the same time, the wave properties are consistent with linear theory. Our results demonstrate that the proton‐alpha streaming instability can be an important mechanism to convert the bulk kinetic energy of the alphas into ion heating at collisionless shocks.

Electrostatic waves are frequently observed at collisionless shocks and are believed to play a key role in energy dissipation. Recent spacecraft observations have identified a proton-alpha instability in the Earth’s bow shock as a potential mechanism to dissipate the energy associated with a relative drift between protons and alpha particles, uα. At plasma parameters relevant to this setting, we investigate the dependence of the evolution of this instability on uα through particle-in-cell simulations. We find that the instability produces strong ion heating, increasing ion temperatures by factors of two to five. The electrons, in contrast, experience only minimal heating (~1%). The heating is highly anisotropic and is most efficient when the wave vector is at a 45° angle to the alpha flow velocity. To interpret these results, we introduce a geometric model based on Landau resonance interaction bands in velocity space. It explains both the angular dependence and the magnitude of heating through the change in velocity distribution functions. At the same time, the wave properties are consistent with linear theory. Our results demonstrate that the proton-alpha streaming instability can be an important mechanism to convert the bulk kinetic energy of the alphas into ion heating at collisionless shocks.

We investigate electron heating by magnetic-field-aligned electric fields (E_{∥}) during antiparallel magnetic reconnection in the Earth's magnetotail. Using a statistical sample of 140 reconnection outflows, we infer the acceleration potential associated with E_{∥} from the shape of the electron velocity distribution functions. We show that heating by E_{∥} in the reconnection outflow can reach up to 10 times the inflow electron temperature. We demonstrate that the magnitude of the acceleration potential scales with the inflow Alfvén and electron thermal speeds to maintain quasineutrality in the reconnection region. Our results suggest that, as the inflow plasma parameter β_{e∞} increases, E_{∥} becomes increasingly important to the ion-to-electron energy partition associated with magnetic reconnection.

Electron energisation by magnetic reconnection has historically been studied in the Lagrangian guiding-centre framework. Insights from such studies include that Fermi acceleration in magnetic islands can accelerate electrons to high energies. An alternative Eulerian fluid formulation of electron energisation was recently used to study electron energisation during magnetic reconnection in the absence of magnetic islands. Here, we use particle-in-cell simulations to compare the Eulerian and Lagrangian models of electron energisation in a set-up where reconnection leads to magnetic island formation. We find the largest energisation at the edges of magnetic islands. There, energisation related to the diamagnetic drift dominates in the Eulerian model, while the Fermi related term dominates in the Lagrangian model. The models predict significantly different energisation rates locally. A better agreement is found after integrating over the simulation domain. We show that strong magnetic curvature can break the magnetic moment conservation assumed by the Lagrangian model, leading to erroneous results. The Eulerian fluid model is a complete fluid description and accurately models bulk energisation. However, local measurements of its constituent energisation terms need not reflect locations where plasma is heated or accelerated. The Lagrangian guiding centre model can accurately describe the energisation of particles, but it cannot describe the evolution of the fluid energy. We conclude that while both models can be valid, they describe two fundamentally different quantities, and care should be taken when choosing which model to use.

Magnetic reconnection converts magnetic field energy into particle energy by breaking and reconnecting magnetic field lines. Magnetic reconnection is a kinetic process that generates a wide variety of kinetic waves via wave-particle interactions. Kinetic waves have been proposed to play an important role in magnetic reconnection in collisionless plasmas by, for example, contributing to anomalous resistivity and diffusion, particle heating, and transfer of energy between different particle populations. These waves range from below the ion cyclotron frequency to above the electron plasma frequency and from ion kinetic scales down to electron Debye length scales. This review aims to describe the progress made in understanding the relationship between magnetic reconnection and kinetic waves. We focus on the waves in different parts of the reconnection region, namely, the diffusion region, separatrices, outflow regions, and jet fronts. Particular emphasis is placed on the recent observations from the Magnetospheric Multiscale (MMS) spacecraft and numerical simulations, which have substantially increased the understanding of the interplay between kinetic waves and reconnection. Some of the ongoing questions related to waves and reconnection are discussed.