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We present EUV solar observations showing evidence for omnipresent jetting activity driven by small-scale magnetic reconnection at the base of the solar corona. We argue that the physical mechanism that heats and drives the solar wind at its source is ubiquitous magnetic reconnection in the form of small-scale jetting activity (a.k.a. jetlets). This jetting activity, like the solar wind and the heating of the coronal plasma, is ubiquitous regardless of the solar cycle phase. Each event arises from small-scale reconnection of opposite-polarity magnetic fields producing a short-lived jet of hot plasma and Alfvén waves into the corona. The discrete nature of these jetlet events leads to intermittent outflows from the corona, which homogenize as they propagate away from the Sun and form the solar wind. This discovery establishes the importance of small-scale magnetic reconnection in solar and stellar atmospheres in understanding ubiquitous phenomena such as coronal heating and solar wind acceleration. Based on previous analyses linking the switchbacks to the magnetic network, we also argue that these new observations might provide the link between the magnetic activity at the base of the corona and the switchback solar wind phenomenon. These new observations need to be put in the bigger picture of the role of magnetic reconnection and the diverse form of jetting in the solar atmosphere.

In particular during the descending phase of the solar cycle, Alfvén waves in the high-speed solar wind streams are a major form of interplanetary disturbances. The fluctuating southward interplanetary magnetic field (IMF) of Alfvén waves has been suggested to induce geomagnetic activities through intermittent magnetic reconnection at the magnetopause. In this study, we provide in situ observational evidence for dayside magnetopause reconnection induced by such interplanetary Alfvén waves. Using multipoint conjunction observations, we show that the IMF B z from interplanetary Alfvén waves is transmitted through and amplified by the Earth’s bow shock. Associated with the intensified southward B z to the magnetopause, in situ signatures of magnetic reconnection are detected. Repetitively, interplanetary Alfvén waves transmit the intensified B z to the magnetosheath, leading to intervals of large magnetic shear angles across the magnetopause and magnetopause reconnection. Such intervals are promptly followed by hundreds of nanoTesla (nT) increases in the auroral electrojet indices (AE and AU) within 10–20 minutes. These observations are confirmed in multiple events in corotating interaction region-driven geomagnetic storms. To put the observations into context, we propose a phenomenological model of a strongly driven substorm. The substorm electrojet is linked to the enhanced magnetopause reconnection in the short timescale of re-establishing the ionosphere electric field and the two-cell convection. These results provide insights on the temporal patterns of solar wind magnetosphere–ionosphere coupling, especially during the descending phase of the solar cycle.

Understanding the early evolution of coronal mass ejections (CMEs), in particular their initiation, is the key to forecasting solar eruptions and induced disastrous space weather. Although many initiation mechanisms have been proposed, a full understanding of CME initiation, which is identified as a slow rise of CME progenitors in kinematics before impulsive acceleration, remains elusive. Here, with a state-of-the-art thermal magnetohydrodynamics simulation, we determine a complete CME initiation route in which multiple mainstream mechanisms occur in sequence yet are tightly coupled. The slow rise is first triggered and driven by the developing hyperbolic flux tube (HFT) reconnection. Subsequently, the slow rise continues as driven by the coupling of the HFT reconnection and the early development of torus instability. The end of the slow rise, i.e., the onset of the impulsive acceleration, is induced by the start of the fast magnetic reconnection coupled with the torus instability. These results unveil that CME initiation is a complicated process involving multiple physical mechanisms, thus being hardly resolved by a single initiation mechanism.

Partial filament eruptions have often been observed; however, the physical mechanisms that lead to filament splitting are not yet fully understood. In this study, we present a unique event of a partial filament eruption that undergoes two distinct splitting processes. The first process involves vertical splitting and is accompanied by brightenings inside the filament, which may result from internal magnetic reconnection within the filament. Following the first splitting process, the filament is separated into an upper part and a lower part. Subsequently, the upper part undergoes a second splitting, which is accompanied by a coronal blowout jet. An extrapolation of the coronal magnetic field reveals a hyperbolic flux tube structure above the filament, indicating the occurrence of breakout reconnection that reduces the constraining field above. Consequently, the filament is lifted up, but at a nonuniform speed. The high-speed part reaches the breakout current sheet to generate the blowout jet, while the low-speed part falls back to the solar surface, resulting in the second splitting. In addition, continuous brightenings are observed along the flare ribbons, suggesting the occurrence of a slipping reconnection process. This study presents, for the first time, the unambiguous observation of a two-stage filament-splitting process, advancing our understanding of the complex dynamics of solar eruptions.

Parker Solar Probe observations are analyzed for the presence of reconnection exhausts across current sheets (CSs) within R < 0.26 au during encounters 4–11. Exhausts are observed with nearly equal probability at all radial distances with a preference for quiescent Tp < 0.80 MK plasmas typical of a slow-wind regime. High Tp > 0.80 MK plasmas of a fast wind characterized by significant transverse fluctuations rarely support exhausts irrespective of the CS width. Exhaust observations demonstrate the presence of local temperature gradients across several CSs with a higher-Tp plasma on locally closed fields and a lower-Tp plasma on locally open field lines for an interchange-type reconnection. A CS geometry analysis directly supports the property that X-lines bisect the magnetic field rotation θ-angle, whether the fields and plasmas are asymmetric or not, to maximize reconnection rates and available magnetic energy. The CS normal width d cs distributions suggest that a multiscale reconnection process through nested layers of bifurcated CSs may be responsible for observed power-law distributions beyond the median d cs ∼ 1000 km with an exponential d cs distribution present for ion kinetic dissipation scales below this median. Magnetic field shear θ-angles are essentially identical at R < 0.26 and 1 au with medians at θ ∼ 55° near the Sun and θ ∼ 65° at 1 au. In contrast, the tangential flow shear distributions are different near and far from the Sun. A bimodal flow shear angle distribution is present near the Sun with strong shear flow magnitudes. This distribution is modified with radial distance toward a relatively flat distribution of weaker flow shear magnitudes.

Electron‐only magnetic reconnection, a novel type of reconnection where only electron dynamics is involved, has recently been observed in turbulent plasmas in the Earth's magnetosphere. In this letter, using particle‐in‐cell simulations, we demonstrate that electron‐only reconnection can be created via laser‐plasma interactions. The reconnection current sheet is carried by electrons, and its width is at the electron inertial scale. Moreover, only electron outflow is observed, while the ion outflow is negligible. The duration of the reconnection is found to be within the electron scale, thus there is no sufficient time for ions to respond. The energy conversion during electron‐only reconnection mainly occurs in the vicinity of the X‐line, and is dominated along the perpendicular direction. By analyzing the Poynting flux patterns, we find that the reconnection electric field dominates the whole energy conversion. This study advances our knowledge of the energy dissipation mechanism in the electron‐only reconnection regime. Plain Language Summary Magnetic reconnection is a fundamental energy converting process that occurs in space and laboratory plasma. Generally, magnetic reconnection involves both ion dynamics and electron dynamics. Recently, a new type of magnetic reconnection has been discovered in the Earth's turbulent magnetosheath and magnetotail, which is called electron‐only reconnection. In electron‐only reconnection, only electron outflow is observed since there is no time and/or space for ions to couple. In this study, by performing numerical simulations, we create the electron‐only reconnection via laser‐plasma interaction, and investigate the energy conversion during reconnection in detail. We find that both the spatial and temporal scale of the reconnection is within the electron scale. During magnetic reconnection, energy is transferred from the electromagnetic fields to the electrons, mainly occurring near the X‐line. The reconnection electric field plays a crucial role in energy conversion. This study improves our understanding of how energy is converted to electrons in electron‐only reconnection. Key Points Simulations show that electron‐only magnetic reconnection can be created via laser‐plasma interactions Both the spatial and temporal scales of the reconnection are at the electron scale, and only electron outflow is observed Energy conversion mainly occurs in the vicinity of the X‐line, and is driven by the reconnection electric field

Solar flares may be the best-known examples of the explosive conversion of magnetic energy into bulk motion, plasma heating, and particle acceleration via magnetic reconnection. The energy source for all flares is the highly sheared magnetic field of a filament channel above a polarity inversion line (PIL). During the flare, this shear field becomes the so-called reconnection guide field (i.e., the nonreconnecting component), which has been shown to play a major role in determining key properties of the reconnection, including the efficiency of particle acceleration. We present new high-resolution, three-dimensional, magnetohydrodynamics simulations that reveal the detailed evolution of the magnetic shear/guide field throughout an eruptive flare. The magnetic shear evolves in three distinct phases: shear first builds up in a narrow region about the PIL, then expands outward to form a thin vertical current sheet, and finally is transferred by flare reconnection into an arcade of sheared flare loops and an erupting flux rope. We demonstrate how the guide field may be inferred from observations of the sheared flare loops. Our results indicate that initially the guide field is larger by about a factor of 5 than the reconnecting component, but it weakens by more than an order of magnitude over the course of the flare. Instantaneously, the guide field also varies spatially over a similar range along the three-dimensional current sheet. We discuss the implications of the remarkable variability of the guide field for the timing and localization of efficient particle acceleration in flares.

The results of simulations of magnetic reconnection accompanied by electron and proton heating and energization in a macroscale system are presented. Both species form extended power-law distributions that extend nearly three decades in energy. The primary drive mechanism for the production of these nonthermal particles is Fermi reflection within evolving and coalescing magnetic flux ropes. While the power-law indices of the two species are comparable, the protons overall gain more energy than electrons, and their power law extends to higher energy. The power laws roll into a hot thermal distribution at low energy with the transition energy occurring at lower energy for electrons compared with protons. A strong guide field diminishes the production of nonthermal particles by reducing the Fermi drive mechanism. In solar flares, proton power laws should extend down to tens of keV, far below the energies that can be directly probed via gamma-ray emission. Thus, protons should carry much more of the released magnetic energy than expected from direct observations.

We report that kink-driven magnetic reconnection serves as an eruption mechanism for a laboratory jet. A flux rope is formed and becomes a filamentary jet. The jet becomes unstable due to kink instability when the Kruskal–Shafranov instability criterion is met, leading to an inflow of reconnecting fields. As a result of kink-driven magnetic reconnection, ions are substantially energized, resulting in enhanced acceleration of the jet. Based on the evidence observed from this laboratory experiment, we propose that kink-driven magnetic reconnection might act as a key driver for laboratory jet eruptions and might be relevant to solar jets associated with kink instability and magnetic reconnection.

We investigate the development of tearing-mode instability using the highest-resolution 2D magnetohydrodynamic simulations of reconnecting current sheets performed on a uniform grid, for Lundquist numbers of 103 ≤ S ≤ 5 × 105, reaching up to 65,5362 grid cells. We demonstrate a Sweet–Parker scaling of the reconnection rate Vrec ∼ S−1/2 up to Lundquist numbers S ∼ 104. For larger values of Lundquist number, between 2 × 104 ≤ S ≤ 2 × 105, plasmoid formation sets in, leading to a slight enhancement of the reconnection rate, Vrec ∼ S−1/3, consistent with the prediction from linear-tearing-mode-induced reconnection, indicating that reconnection remains resistivity-dependent and therefore slow. In this range of S-values, the plasmoids do not undergo a merger cascade, as they are rapidly advected out of the reconnection layer. Only for S > 2 × 105, we observe the nonlinear development of the tearing-mode instability, with plasmoid coalescence and a saturation of the reconnection rate at Vrec/VA ∼ 0.01. At such high S, however, the corresponding Reynolds number is large, reaching Re > 2000 even on scales comparable to the current-sheet thickness. We therefore conclude that, in astrophysical systems, it is essential to account for the dominant influence of turbulence and 3D effects in the reconnection process.