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Magnetic reconnection in current sheets is a magnetic-to-particle energy conversion process that is fundamental to many space and laboratory plasma systems. In the standard model of reconnection, this process occurs in a minuscule electron-scale diffusion region 1 , 2 . On larger scales, ions couple to the newly reconnected magnetic-field lines and are ejected away from the diffusion region in the form of bi-directional ion jets at the ion Alfvén speed 3 – 5 . Much of the energy conversion occurs in spatially extended ion exhausts downstream of the diffusion region 6 . In turbulent plasmas, which contain a large number of small-scale current sheets, reconnection has long been suggested to have a major role in the dissipation of turbulent energy at kinetic scales 7 – 11 . However, evidence for reconnection plasma jetting in small-scale turbulent plasmas has so far been lacking. Here we report observations made in Earth’s turbulent magnetosheath region (downstream of the bow shock) of an electron-scale current sheet in which diverging bi-directional super-ion-Alfvénic electron jets, parallel electric fields and enhanced magnetic-to-particle energy conversion were detected. Contrary to the standard model of reconnection, the thin reconnecting current sheet was not embedded in a wider ion-scale current layer and no ion jets were detected. Observations of this and other similar, but unidirectional, electron jet events without signatures of ion reconnection reveal a form of reconnection that can drive turbulent energy transfer and dissipation in electron-scale current sheets without ion coupling. Observations of electron-scale current sheets in Earth’s turbulent magnetosheath reveal electron reconnection without ion coupling, contrary to expectations from the standard model of magnetic reconnection.

Make and break for magnetic field reconnections The process of magnetic reconnection, which drives many explosive events in space including solar and stellar flares, involves the breakage and reconnection of magnetic field lines in a plasma. Exactly how this happens is not clear, but ion-electron drag arising from turbulence (or anomalous resistivity) and thermal momentum transport have been widely invoked. Che et al . report computer simulations showing that neither of these two mechanisms controls how magnetic field lines reconnect. Rather, they find that when current layers formed during magnetic reconnection become too intense, they disintegrate and spread into a complex web of filaments that causes the rate of reconnection to increase abruptly. During magnetic reconnection, the field lines must break and reconnect to release the energy that drives solar and stellar flares 1 , 2 and other explosive events in space 3 and in the laboratory 4 . Exactly how this happens has been unclear, because dissipation is needed to break magnetic field lines and classical collisions are typically weak. Ion–electron drag arising from turbulence 5 , dubbed ‘anomalous resistivity’, and thermal momentum transport 6 are two mechanisms that have been widely invoked. Measurements of enhanced turbulence near reconnection sites in space 7 , 8 and in the laboratory 9 , 10 support the anomalous resistivity idea but there has been no demonstration from measurements that this turbulence produces the necessary enhanced drag 11 . Here we report computer simulations that show that neither of the two previously favoured mechanisms controls how magnetic field lines reconnect in the plasmas of greatest interest, those in which the magnetic field dominates the energy budget. Rather, we find that when the current layers that form during magnetic reconnection become too intense, they disintegrate and spread into a complex web of filaments that causes the rate of reconnection to increase abruptly. This filamentary web can be explored in the laboratory or in space with satellites that can measure the resulting electromagnetic turbulence.

Magnetic fields in plasmas can rapidly rearrange themselves in a process known as magnetic reconnection, which releases energy and accelerates particles. Torbert et al. used the Magnetospheric Multiscale (MMS) mission to probe a reconnection event in Earth's magnetotail—the region of plasma downstream from the planet as it moves through the solar wind. MMS has previously studied reconnection in the upstream magnetopause, but a different orbit was used to study the magnetotail, where the symmetry of the process is different. The authors measured plasma properties on scales of the electron dynamics, leading to insights that will apply in other regions where magnetic reconnection occurs. Science , this issue p. 1391 Magnetic reconnection in Earth’s magnetotail is observed on electron dynamics scales. Magnetic reconnection is an energy conversion process that occurs in many astrophysical contexts including Earth’s magnetosphere, where the process can be investigated in situ by spacecraft. On 11 July 2017, the four Magnetospheric Multiscale spacecraft encountered a reconnection site in Earth’s magnetotail, where reconnection involves symmetric inflow conditions. The electron-scale plasma measurements revealed (i) super-Alfvénic electron jets reaching 15,000 kilometers per second; (ii) electron meandering motion and acceleration by the electric field, producing multiple crescent-shaped structures in the velocity distributions; and (iii) the spatial dimensions of the electron diffusion region with an aspect ratio of 0.1 to 0.2, consistent with fast reconnection. The well-structured multiple layers of electron populations indicate that the dominant electron dynamics are mostly laminar, despite the presence of turbulence near the reconnection site.

Magnetic reconnection is a fundamental physical process in plasmas whereby stored magnetic energy is converted into heat and kinetic energy of charged particles. Reconnection occurs in many astrophysical plasma environments and in laboratory plasmas. Using measurements with very high time resolution, NASA's Magnetospheric Multiscale (MMS) mission has found direct evidence for electron demagnetization and acceleration at sites along the sunward boundary of Earth's magnetosphere where the interplanetary magnetic field reconnects with the terrestrial magnetic field. We have (i) observed the conversion of magnetic energy to particle energy; (ii) measured the electric field and current, which together cause the dissipation of magnetic energy; and (iii) identified the electron population that carries the current as a result of demagnetization and acceleration within the reconnection diffusion/dissipation region.

The fast solar wind that fills the heliosphere originates from deep within regions of open magnetic field on the Sun called ‘coronal holes’. The energy source responsible for accelerating the plasma is widely debated; however, there is evidence that it is ultimately magnetic in nature, with candidate mechanisms including wave heating 1 , 2 and interchange reconnection 3 – 5 . The coronal magnetic field near the solar surface is structured on scales associated with ‘supergranulation’ convection cells, whereby descending flows create intense fields. The energy density in these ‘network’ magnetic field bundles is a candidate energy source for the wind. Here we report measurements of fast solar wind streams from the Parker Solar Probe (PSP) spacecraft 6 that provide strong evidence for the interchange reconnection mechanism. We show that the supergranulation structure at the coronal base remains imprinted in the near-Sun solar wind, resulting in asymmetric patches of magnetic ‘switchbacks’ 7 , 8 and bursty wind streams with power-law-like energetic ion spectra to beyond 100 keV. Computer simulations of interchange reconnection support key features of the observations, including the ion spectra. Important characteristics of interchange reconnection in the low corona are inferred from the data, including that the reconnection is collisionless and that the energy release rate is sufficient to power the fast wind. In this scenario, magnetic reconnection is continuous and the wind is driven by both the resulting plasma pressure and the radial Alfvénic flow bursts. Measurements of fast solar wind streams from the Parker Solar Probe spacecraft provide strong evidence for the interchange reconnection mechanism being responsible for accelerating the fast solar wind.

Coulomb collisions provide plasma resistivity and diffusion but in many low-density astrophysical plasmas such collisions between particles are extremely rare. Scattering of particles by electromagnetic waves can lower the plasma conductivity. Such anomalous resistivity due to wave-particle interactions could be crucial to many processes, including magnetic reconnection. It has been suggested that waves provide both diffusion and resistivity, which can support the reconnection electric field, but this requires direct observation to confirm. Here, we directly quantify anomalous resistivity, viscosity, and cross-field electron diffusion associated with lower hybrid waves using measurements from the four Magnetospheric Multiscale (MMS) spacecraft. We show that anomalous resistivity is approximately balanced by anomalous viscosity, and thus the waves do not contribute to the reconnection electric field. However, the waves do produce an anomalous electron drift and diffusion across the current layer associated with magnetic reconnection. This leads to relaxation of density gradients at timescales of order the ion cyclotron period, and hence modifies the reconnection process. It is suggested that waves can provide both diffusion and resistivity that can potentially support the reconnection electric field in low-density astrophysical plasmas. Here, the authors show, using direct spacecraft measurements, that the waves contribute to anomalous diffusion but do not contribute to the reconnection electric field.

Electrons gain kinetic energy by reflecting from the ends of the contracting 'magnetic islands' that form as reconnection proceeds. The repetitive interaction of electrons with many islands allows large numbers to be efficiently accelerated to high energy. A long-standing problem in the study of space and astrophysical plasmas is to explain the production of energetic electrons as magnetic fields ‘reconnect’ and release energy. In the Earth's magnetosphere, electron energies reach hundreds of thousands of electron volts (refs 1 –3 ), whereas the typical electron energies associated with large-scale reconnection-driven flows are just a few electron volts. Recent observations further suggest that these energetic particles are produced in the region where the magnetic field reconnects 4 . In solar flares, upwards of 50 per cent of the energy released can appear as energetic electrons 5 , 6 . Here we show that electrons gain kinetic energy by reflecting from the ends of the contracting ‘magnetic islands’ that form as reconnection proceeds. The mechanism is analogous to the increase of energy of a ball reflecting between two converging walls—the ball gains energy with each bounce. The repetitive interaction of electrons with many islands allows large numbers to be efficiently accelerated to high energy. The back pressure of the energetic electrons throttles reconnection so that the electron energy gain is a large fraction of the released magnetic energy. The resultant energy spectra of electrons take the form of power laws with spectral indices that match the magnetospheric observations.

Three-dimensional particle simulations of magnetic reconnection reveal the development of turbulence driven by intense electron beams that form near the magnetic x-line and separatrices. The turbulence collapses into localized three-dimensional nonlinear structures in which the electron density is depleted. The predicted structure of these electron holes compares favorably with satellite observations at Earth's magnetopause. The birth and death of these electron holes and their associated intense electric fields lead to strong electron scattering and energization, whose understanding is critical to explaining why magnetic explosions in space release energy so quickly and produce such a large number of energetic electrons.

The heating of ions downstream of the x‐line during magnetic reconnection is explored using full‐particle simulations, test particle simulations, and analytic analysis. Large‐scale particle simulations reveal that the ion temperature increases sharply across the boundary layer that separates the upstream plasma from the Alfvénic outflow. This boundary layer, however, does not take the form of a classical switch‐off shock as discussed in the Petschek reconnection model, so the particle heating cannot be calculated from the magnetohydrodynamic, slow‐shock prediction. Test particle trajectories in the fields from the simulations reveal that ions crossing the narrow boundary into the exhaust instead behave like pickup particles: they gain both a directed outflow and an effective thermal speed given by the flow speed v0 of the exhaust. The detailed dynamics of these particles are explored by taking 1‐D cuts of the simulation data across the exhaust, transforming to the deHoffman‐Teller frame, and calculating explicitly the increment in the temperature, miv02/3, with mi, the ion mass. We compare the model predictions with the temperature increment in solar wind exhausts measured by the ACE and Wind spacecraft, confirming that the temperature increment is proportional to the ion mass. The Wind data from 22 high‐shear exhaust encounters confirm the scaling of the proton temperature increment with the square of the exhaust velocity. However, the temperature increments are consistently lower than the model prediction. Implications for understanding the production of high‐energy ions in flares and the broader universe are discussed.

Using particle‐in‐cell simulations and kinetic theory, we explore the current‐driven turbulence and associated electron heating in the dissipation region during 3D magnetic reconnection with a guide field. At late time the turbulence is dominated by the Buneman and lower hybrid instabilities. Both produce electron holes that co‐exist but have very different propagation speeds. The associated scattering of electrons by the holes enhances electron heating in the dissipation region.