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This paper gives an overview of the insights into the magnetic reconnection process obtained by in-situ measurements across current sheets found in planetary magnetospheres and the solar wind. Emphasis is placed on results that might be of interest to the study of reconnection in regions where no in-situ observations are available. These results include the role of symmetric versus asymmetric boundary conditions, the identification of the onset conditions, the reconnection rates, and the spatial and temporal scales. Special attention is paid to observations in the so-called diffusion region surrounding the reconnection sites, where ions and eventually also electrons become demagnetized and reconnection is initiated.

Particles are energized—heated and accelerated to nonthermal energies—in laboratory, space, solar, and astrophysical plasmas. In collisionless plasmas, ion and electron temperatures are often unequal and cannot be fully understood within the framework of magnetohydrodynamics (MHD). In this context, a relation, Δεs = qsVBLs, for each species can be useful, where Δεs is the energy gain for species s, measured in the plasma rest frame, relative to the upstream region of shocks and magnetic reconnection; qs is the charge; V is the plasma bulk flow speed; B is the magnetic field strength; and Ls is a characteristic length scale of energization. From this relation, we recently derived semiempirical scalings for ion and electron temperature increases across shocks and magnetic reconnection in Earth’s plasma environment. However, it remains unclear how broadly these scalings apply. Here we show that the same scalings explain temperature increases in other plasma environments such as laboratory experiments, planetary magnetospheres, solar flares, and supernova remnant shocks. Combined with another recent report that the maximum energy of particles in various plasma environments follows the same relation when Ls is taken as the system size, our results indicate that Δεs = qsVBLs provides a novel framework that universally captures particle energization—both heating and acceleration to nonthermal energies. Additionally, the scaling captures the essential MHD trends while revealing systematic deviations that point to kinetic effects beyond fluid models, highlighting promising directions for theoretical and simulation studies.

Particles are heated efficiently through energy conversion processes, such as shocks and magnetic reconnection, in collisionless plasma environments. While empirical scaling laws for the temperature increase have been obtained, the precise mechanism of energy partition between ions and electrons remains unclear. Here we show, based on coupled theoretical and observational scaling analyses, that the temperature increase, ΔT, depends linearly on three factors: the available magnetic energy per particle, the Alfvén Mach number (or reconnection rate), and the characteristic spatial scale L. Based on statistical data sets obtained from Earth’s plasma environment, we find that L is on the order of: (1) the ion gyroradius for ion heating at shocks; (2) the ion inertial length for ion heating in magnetic reconnection; and (3) the hybrid inertial length for electron heating in both shocks and magnetic reconnection. With these scales, we derive the ion-to-electron ratios of temperature increases as ΔTi/ΔTe=(3βi/2)1/2(mi/me)1/4 for shocks and ΔTi/ΔTe=(mi/me)1/4 for magnetic reconnection, where βi is the ion plasma beta and mi and me are the ion and electron particle masses, respectively. We anticipate that this study will serve as a starting point for a better understanding of particle heating in space plasmas, enabling more sophisticated modeling of its scaling and universality.

Electrons are accelerated to high, nonthermal energies during explosive energy-release events in space, such as magnetic reconnection. However, the properties and acceleration mechanisms of relativistic electrons directly associated with the reconnection X-line are not well understood. This study utilizes Magnetospheric Multiscale (MMS) measurements to analyze the flux and spectral features of subrelativistic to relativistic (∼80–560 keV) electrons during a magnetic reconnection event in Earth’s magnetotail. This event provided a unique opportunity to measure the electrons directly energized by the X-line as MMS stayed in the separatrix layer, where the magnetic field directly connects to the X-line, for approximately half of the observation period. Our analysis revealed that the fluxes of relativistic electrons were clearly enhanced within the separatrix layer, and the highest flux was directed away from the X-line, which suggested that these electrons originated directly from the X-line. Spectral analysis showed that these relativistic electrons deviated from the main plasma sheet population and exhibited an “ankle” feature similar to that observed in galactic cosmic rays. The contribution of “ankle” electrons to the total electron energy density increased from 0.1% to 1% in the separatrix layer though the spectral slopes did not exhibit clear variations. Further analysis indicated that while these relativistic electrons originated from the X-line, they experienced a nonnegligible degree of scattering during transport. These findings provide clear evidence that magnetic reconnection in Earth’s magnetotail can efficiently energize relativistic electrons directly at the X-line, providing new insights into the complex processes governing electron dynamics during magnetic reconnection.

Magnetic reconnection is an explosive energy release event. It plays an important role in accelerating particles to high non‐thermal energies. These particles often exhibit energy spectra characterized by a power‐law distribution. However, the partitioning of energy between thermal and non‐thermal components, and between ions and electrons, remains unclear. This study provides estimates of energy partition based on a statistical analysis of magnetic reconnection events in Earth's magnetotail using data from the Magnetospheric Multiscale mission. Ions are up to 10 times more energetic than electrons but have softer spectra. We found for both ions and electrons that, as the average energy of particles (temperature) increases, their energy spectra become softer (steeper) and thus, the fraction of energy carried by the non‐thermal components decreases. These results challenge existing theories of particle acceleration through magnetotail reconnection. Plain Language Summary Magnetic reconnection events are explosive processes that energize plasma and are accompanied by high‐speed plasma flows. During these events, both electrons and ions are heated and accelerated, resulting in thermal and non‐thermal components, respectively. We conducted a statistical analysis of reconnection events in Earth's magnetotail to explore how energy is distributed between electrons and protons, and between their thermal and non‐thermal components. Our findings show that as the average energy of particles (temperature) increases, the fraction of energy in the non‐thermal particles decreases. We also observed that this fraction is lower for ions, even though they are up to 10 times more energetic than electrons. We discuss the implications of these results. Key Points Despite having softer energy spectra (power‐law index κ$\\kappa $≳ 5), ions carry more non‐thermal energy density than electrons (κ$\\kappa $of 4–6) The non‐thermal energy fraction for both ions (10%–50%) and electrons (30%–60%) rarely exceeds 50% Both ions and electrons tend to have softer spectra for higher values of average energy of particles

From 10 to 12 May 2024, a series of coronal mass ejections led to one of the strongest geomagnetic storms of the century, referred to as the Mother's Day or Gannon Storm. MMS's position on the dayside magnetosphere on 11 May provided observations of a strongly driven and compressed ∼7RE$\\left(\\sim 7\\ {R}_{E}\\right)$reconnecting magnetopause. Because of the driving conditions, the magnetopause became saturated with O+${O}^{+}$outflows that dominated the mass density of the plasma environment. In the reconnecting magnetopause, MMS observes signatures of parallel electron heating on the magnetopause's magnetosheath side, but anomalous and significant electron cooling, especially from the perpendicular electron temperature on the magnetosphere side, possibly driven by additional mechanisms besides reconnection. Even with the strong driving and O+${O}^{+}$outflows, we find an expected (0.19±0.04)$(0.19\\pm 0.04)$normalized reconnection rate for the primary exhaust, indicating insensitivity to these conditions. The unnormalized rate, however, is atypically large and scales with the driving conditions.

A layer of nearly stagnant cold dense plasma is observed by THEMIS spacecraft in a closed field region immediately inside the dayside magnetopause near the low‐latitude boundary layer on 3 June 2007. Using the OpenGGCM global MHD magnetosphere numerical model, we successfully reproduce this observed cold dense plasma layer in the simulation. The simulation results show that reconnection first occurs poleward of the cusp in the northern hemisphere, creating new open field lines extending southward and forming an open field layer; then subsequently occurs in the other hemisphere, creating new closed field lines that capture the magnetosheath plasma and form the dayside cold dense plasma layer. In this event, the open layer and the skin of the cold dense plasma layer have a southward tangential flow while the inner part of the cold dense plasma layer has a more stagnant and more turbulent flow.

On behalf of the journal, AGU, and the scientific community, the editors of Geophysical Research Letters would like to sincerely thank those who reviewed manuscripts for us in 2023. The hours reading and commenting on manuscripts not only improve the manuscripts, but also increase the scientific rigor of future research in the field. With the advent of AGU's data policy, many reviewers have also helped immensely to evaluate the accessibility and availability of data, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. We received 4,512 submissions in 2023 and 5,112 reviewers contributed to their evaluation by providing 8,587 reviews in total. We deeply appreciate their contributions. Plain Language Summary Individuals in italics provided three or more reviews for GRL in 2023. Key Points The editors thank the 2023 peer‐reviewers

We present conjugate observations of ionospheric small‐scale magnetic perturbations (dB)$(dB)$and GPS scintillations from the Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) and the Canadian High Arctic Ionospheric Network (CHAIN) during a storm‐time substorm on 9 August 2025 in the nightside auroral region. Rotational transverse dB$dB$of up to ∼1,000 nT, associated with filamentary field‐aligned currents (FACs) exceeding 150 μA/m2${\\upmu }\\mathrm{A}/{\\mathrm{m}}^{2}$on spatial scales of ∼1–16 km, were colocated with extreme, localized auroral scintillations (σϕ>${\\sigma }_{\\phi } > $10 rad, S4≃${S}_{4}\\simeq $0.3) and ∼10 dB‐Hz intensity reduction. The most intense scintillations lasted ∼1.5 s and coincided with peaks in auroral electrojets, Pi2 pulsations, and enhanced dTEC (∼12 TECU). The sheet‐like morphology and spatiotemporal correlation with auroral electrojets suggest that Farley–Buneman processes likely play a dominant role in scintillation formation, with additional contributions from FAC‐driven and gradient‐drift instabilities.