Marc Swisdak

Pressure–strain interaction is an important channel for converting bulk-flow energy into heat in weakly collisionless plasmas. Its decomposition into compressive and incompressive work, how their relative importance depends on geometry, and how it relates spatially and statistically to electromagnetic work, remain unclear. Previous simulation studies of plasma turbulence and reconnection starting from weakly compressive initial data have typically found that incompressive work continues to dominate over compressive work. In contrast, many magnetospheric multiscale (MMS) mission events in the magnetosheath exhibit stronger compressive contributions. In this study, we investigate energy conversion in collisionless magnetic reconnection using fully kinetic particle-in-cell (PIC) simulations for interchange reconnection and compare the simulation results with MMS observations. The results show that electromagnetic work is highly localized near current layers, whereas pressure–strain work is distributed more broadly. The difference in their peak regions indicates a separation between where field-particle energy conversion occurs and where the bulk flow energy is converted into internal energy. These support a two-step energy conversion pathway for a Vlasov–Maxwell system and its dependence on reconnection geometries. The consistency between simulation and observation results underscores the pressure–strain diagnostics for understanding energy conversion in space plasmas.

Physical processes in solar flares span many orders of magnitude in spatiotemporal scales, so a single unified model accounting for all relevant processes remains computationally intractable. Instead, specialized codes have been developed, targeting specific domains that tackle specific aspects of the solar flare problem. Here, we present a comprehensive framework that links three models to capture flare initiation and energy release, the subsequent particle acceleration, the propagation and thermalization of those particles, and ultimately the heating of the lower solar atmosphere. We use the 2.5D ARMS magnetohydrodynamics (MHD) code to model stressing of the magnetic field in a simulated active region, the formation of a current sheet, magnetic reconnection, and the evolution of newly formed flare loops. We use the conditions in the ARMS current sheet to initialize the kglobal model, which predicts time-dependent nonthermal electron and proton distributions. Finally, we use RADYN+FP to inject these distributions into a newly reconnected ARMS loop and compute subsequent particle transport, atmospheric heating, and radiative emissions. This is the first detailed presentation of a RADYN+FP simulation tracking nonthermal electrons and protons in the same loop. As well as discussing the interesting physics that takes place, we note how this results in white-light flare emission height consistent with observations. This work is a first step toward building a fully 3D flare modeling framework.

Recent observations of solar flares have revealed a distinct “hot onset” phase preceding the impulsive phase, characterized by elevated plasma temperature (>10 MK), a steady increase in emission measure, and the absence of detectable nonthermal emission. The standard flare model attributes the formation of hot flare plasma to chromospheric evaporation driven by energy deposition from nonthermal electrons, and therefore does not naturally explain why thermal heating precedes nonthermal emission. Motivated by recent observations of guide-field evolution in two-ribbon flares and by large-scale MHD simulations, we investigate hot-onset energization using simulations of magnetic reconnection with the kinetic macroscale model kglobal initialized with a spatially nonuniform guide field. Because the efficiency of electron nonthermal energy production from Fermi acceleration decreases with guide-field strength, the evolving guide field produces two distinct energization regimes. During the initial high-guide-field phase, the bulk electron temperature rises to approximately twice its initial value, while nonthermal particle production remains weak. Upon entering the low-guide-field regime, energetic particle production is significantly enhanced, with the population of high-energy electrons growing rapidly. These findings provide a self-consistent physical explanation for the hot onset, demonstrating that the macroscopic evolution of the guide field naturally bridges the hot-onset and impulsive phases of solar flares without requiring a separate trigger mechanism.

Magnetic switchbacks are large-amplitude fluctuations in the interplanetary magnetic field, and appear frequently in the near-Sun solar wind explored recently by Parker Solar Probe: these new observations have prompted many new studies into their properties and origins. Here, we first review what is known about how switchbacks evolve as they travel away from the Sun: both in terms of their expansion-driven growth and their decay due to various processes like turbulence, reconnection, dispersion, parametric instability, and interaction with interplanetary shocks. We then review the current state of knowledge on how switchbacks impact the physics of the solar wind as a whole: in terms of the turbulent cascade, acceleration and heating of the wind, modification of the open solar flux and scattering of energetic particles. Finally, we suggest future studies to further our understanding of switchback evolution and impacts on the heliosphere.

The shape of the heliosphere, regarded as comet-like since the 1960s, has recently been the subject of intense debate in the last decade. There is disagreement as to whether the heliospheric tail extends to ∼10,000 au in a comet-like shape or if it is short (∼400 au) with a split. Energetic neutral atom (ENA) maps from Cassini/Ion and Neutral Camera (INCA) at energies from 5.2 to 13.5 keV revealed a global structure extending from the nose to the heliospheric tail, known as the Belt, whose origin has remained largely unexplored. Here, we use a state-of-the-art multi-ion magnetohydrodynamic model and a novel reconnection simulation to establish that the Belt structure is consistent with a split-tail heliosphere but not with a comet-like heliosphere. In a split-tail heliosphere, there is a region of low β (the ratio of thermal to magnetic pressure) in the downwind direction close to the heliopause. Direct simulations of this region reveal that magnetic reconnection is strong and drives the energetic particles that produce the >5.2 keV ENAs measured by INCA in the low-latitude portion of the Belt. Since the comet-like heliosphere does not produce this low- β region and the resultant reconnection-driven mechanism for the >5.2 keV ENAs, our results indicate that the INCA observations are inconsistent with a comet-like heliosphere. Further, these simulations and analysis establish for the first time that magnetic reconnection in the complex magnetic fields, expected in astrospheres across the Universe, is likely to be a source of energetic particles and radiation.

Two-dimensional particle-in-cell simulations with a realistic mass ratio reveal the mechanisms underlying the generation of out-of-plane magnetic fields in magnetic islands/flux ropes during magnetic reconnection. In the absence of an initial guide field, reconnection produces a large electron temperature anisotropy (∼4.5) that drives the Weibel instability inside magnetic islands and produces strong out-of-plane fields ( B _z / B ₀ ∼ 0.4, greatly exceeding the Hall field) with a regular bipolar structure. A space–time analysis reveals a one-to-one correspondence between the temperature anisotropy and the development of the Weibel magnetic field. The instability relaxes the anisotropy, but island merging leads to anisotropy reemergence and re-excitation. In the presence of a strong ambient guide field ( B _g / B ₀ = 0.5), the electron outflow from the X-point deflects along the separatrices and forms a circular current loop that wraps the flux ropes. This current generates an out-of-plane magnetic field that reinforces the ambient guide field, reaching B _z / B ₀ ∼ 1.4. The current can, in some cases, drive the electron Kelvin–Helmholtz instability, which produces electron vortices and strengthens the magnetic field. These self-generated out-of-plane magnetic fields scatter electrons and reduce their temperature anisotropy, which can potentially affect electron heating via Fermi reflection. The simulation results are supported by spacecraft observations suggesting that magnetic fields can be enhanced within flux ropes in near-Earth space and in the solar wind. The presence of flux-rope core fields can also affect particle energy gain during reconnection in various astrophysical environments.

We investigate the onset conditions for electron-only magnetic reconnection through an analysis of 86 reconnection events observed by the Magnetospheric Multiscale mission in magnetosheath and magnetospheric current sheets. Our study presents the first observational evidence that the occurrence of electron-only reconnection depends on both the magnetic-field shear angle ( θ ) across the current sheet and the cross-sheet electron beta jump, quantified either by the inflow electron beta difference (Δ β _e ) or by a modified electron beta jump ( Δ β e * ) defined by the cross-sheet electron pressure difference normalized by the average of the inflow magnetic pressure in the two inflow regions. We extend the diamagnetic-drift suppression mechanism from standard reconnection to electron-only reconnection by deriving relations applicable to electron-scale current sheets. Using scaling estimates, the electron-only reconnection suppression relation is derived as Δ β e * ≳ 2 ( L / d e ) tan ( θ / 2 ) , while a natural extension of the ion-coupled reconnection suppression criterion is Δ β e ≳ 2 ( L / d e ) tan ( θ / 2 ) . At low Δ β _e and Δ β e * values, reconnection is detected across a large range of magnetic shear (from low to high), whereas high Δ β _e and Δ β e * conditions restrict reconnection to large shear configurations: shear angles as small as θ ≈ 5 ^∘ occur when Δ β _e (or Δ β e * ) ≲0.1. However, only events exceeding θ ≳ 70 ^∘ are found when Δ β _e ≳ 1.0; this apparent restriction weakens when employing Δ β e * , as the corresponding data points shift toward smaller values of the modified electron beta jump. These findings quantitatively support theoretical predictions of reconnection suppression in high- β _e plasmas at low shears, attributed to X-line drift induced by pressure gradients across electron-scale current sheets.

Magnetic switchbacks are large amplitude deflections of the magnetic field within the solar wind. They are Alfvénic in character and so are associated with a spike in velocity and a generally small variation in local plasma density. Early orbits of Parker Solar Probe revealed that the solar wind near the Sun is dominated by these structures, and therefore, they may be playing an important role in the energy budget and acceleration of the young solar wind. In this review, we present an overview of different mechanisms that have been proposed for how switchbacks could be formed. We group the mechanisms by whether they predominantly act in the low solar atmosphere or within the solar wind (in situ). We focus on mechanisms that can create reversals of the ambient magnetic field direction and, thus, account for the most extreme perturbations. The general consensus is that mechanisms in the lower solar atmosphere do not form such reversals on their own but provide the seed perturbations, flows, or particle beams necessary for in situ mechanisms to create switchbacks within the solar wind. Switchback observations thus likely contain an imprint of the coronal source of the seed perturbation or flow, which is evolved further locally by one of several plausible in situ mechanisms. We discuss the strengths and weaknesses of each mechanism and outline future observational and theoretical tests that could help differentiate between them.

We present a first record of significantly enhanced occurrences of magnetic reconnection exhausts as measured by the Wind satellite across a stream interaction region (SIR) at 1 AU from 10:11:20 UT on 4 Jan 2019 to 09:58:00 UT on 5 Jan 2019. The activity is clustered in a slow wind compression regime ahead of the SIR interface with a deflected, compressed fast wind. The 43 exhausts of this 1-day SIR dominate a distribution of 71 exhausts as obtained by a multi-window sliding technique application to the 8-day period on 1-9 Jan 2019. Active current sheets inside the SIR are associated with normal directions mostly near the ecliptic plane and a more azimuthal-than-Parker magnetic field direction at 1 AU. We find that exhausts wider than 500 ion inertial lengths are typically present just upstream and inside this SIR rather than within unperturbed slow and fast winds beyond a shocked solar wind. The observations suggest that plasma and field compressions may be crucial elements in driving a break-up of large-scale current sheets embedded in SIRs into smaller, multi-layered current sheet segments through magnetic reconnection.

Magnetic reconnection at the near-Sun heliospheric current sheet (HCS) dissipates the Parker spiral and converts magnetic energy into kinetic energy of the plasma constituents. Observations at a radial distance of ~16.25 Rs by Parker Solar Probe associated with the encounter 14 (E14) HCS crossing have shown that reconnection-driven particle acceleration mechanisms, likely facilitated by the merging of large-scale flux tubes, are able to accelerate protons up to ~400 keV, which is ≈1000 times greater than the available magnetic energy per particle during this crossing (Desai et al. 2025; Phan et al. 2024). In this paper, we present a detailed analysis of the pitch-angle distributions, differential energy spectra, and maximum energies and spectra of protons and heavy ions (He, O, and Fe) in conjunction with observations of local wave activity during the E14 HCS crossing. Our results show the following: 1) First direct observations of the energization of protons and heavy ions during reconnection. 2) First direct observations that the power-law spectral slopes of heavy ions differ from that of protons, which contradicts previous simulation results where the spectral indices of all ion species are essentially identical. 3) First demonstration that the anisotropies and beams of ions produced during reconnection drive waves in the ion cyclotron range of frequencies. 4) First evidence that the pitch angle scattering of protons is stronger than that of the other ion species and that this might be responsible for the harder spectral slopes of the heavy ions compared with protons. In summary, PSP observations during the E14 HCS crossing provide strong evidence for in-situ reconnection-driven acceleration of protons and heavy ions at the near-Sun HCS that will need to be fully accounted for by contemporary reconnection-driven energization models.