Particle acceleration during the coalescence of two magnetic loops in electron-ion plasmas

To investigate how magnetic reconnection (MR) accelerates electrons to a power-law energy spectrum in solar flares, we explore the scaling of a kinetic model proposed by Che & Zank (CZ) and compare it to observations. Focusing on thin current sheet MR particle-in-cell (PIC) simulations, we analyze the impact of domain size on the evolution of the electron Kelvin–Helmholtz instability (EKHI). We find that the duration of the growth stage of the EKHI ( tG∼Ωe−1 ) is short and remains nearly unchanged because the electron gyrofrequency Ω e is independent of domain size. The quasi-steady stage of the EKHI (t MR) dominates the electron acceleration process and scales linearly with the size of the simulations as L/v A0, where v A0 is the Alfvén speed. We use the analytical results obtained by CZ to calculate the continuous temporal evolution of the electron energy spectra from PIC simulations and linearly scale them to solar flare observational scales. For the first time, an electron acceleration model predicts the sharp two-stage transition observed in typical soft–hard–harder electron energy spectra, implying that the electron acceleration model must be efficient with an acceleration timescale that is a small fraction of the duration of solar flares. Our results suggest that we can use PIC MR simulations to investigate the observational electron energy spectral evolution of solar flares if the ratio t MR/t G is sufficiently small, i.e., ≲10%.

The heating of electrons in collisionless magnetic reconnection is explored in particle-in-cell simulations with non-zero guide fields so that electrons remain magnetized. In this regime, electric fields parallel to B accelerate particles directly, while those perpendicular to B do so through gradient-B and curvature drifts. The curvature drift drives parallel heating through Fermi reflection, while the gradient B drift changes the perpendicular energy through betatron acceleration. We present simulations in which we evaluate each of these mechanisms in space and time in order to quantify their role in electron heating. For a case with a small guide field (20% of the magnitude of the reconnecting component), the curvature drift is the dominant source of electron heating. However, for a larger guide field (equal to the magnitude of the reconnecting component) electron acceleration by the curvature drift is comparable to that of the parallel electric field. In both cases, the heating by the gradient B drift is negligible in magnitude. It produces net cooling because the conservation of the magnetic moment and the drop of B during reconnection produce a decrease in the perpendicular electron energy. Heating by the curvature drift dominates in the outflow exhausts where bent field lines expand to relax their tension and is therefore distributed over a large area. In contrast, the parallel electric field is localized near X-lines. This suggests that acceleration by parallel electric fields may play a smaller role in large systems where the X-line occupies a vanishing fraction of the system. The curvature drift and the parallel electric field dominate the dynamics and drive parallel heating. A consequence is that the electron energy spectrum becomes extremely anisotropic at late time, which has important implications for quantifying the limits of electron acceleration due to synchrotron emission. An upper limit on electron energy gain that is substantially higher than earlier estimates is obtained by balancing reconnection drive with radiative loss.

Magnetic reconnection driven by a capacitor coil target is an innovative way to investigate low-β magnetic reconnection in the laboratory, where β is the ratio of particle thermal pressure to magnetic pressure. Low-β magnetic reconnection frequently occurs in the Earth’s magnetosphere, where the plasma is characterized by β ≲ 0.01. In this paper, we analyze electron acceleration during magnetic reconnection and its effects on the electron energy spectrum via particle-in-cell simulations informed by parameters obtained from experiments. We note that magnetic reconnection starts when the current sheet is down to about three electron inertial lengths. From a quantitative comparison of the different mechanisms underlying the electron acceleration in low-β reconnection driven by coil targets, we find that the electron acceleration is dominated by the betatron mechanism, whereas the parallel electric field plays a cooling role and Fermi acceleration is negligible. The accelerated electrons produce a hardened power-law spectrum with a high-energy bump. We find that injecting electrons into the current sheet is likely to be essential for further acceleration. In addition, we perform simulations for both a double-coil co-directional magnetic field and a single-coil one to eliminate the possibility of direct acceleration of electrons beyond thermal energies by the coil current. The squeeze between the two coil currents can only accelerate electrons inefficiently before reconnection. The simulation results provide insights to guide future experimental improvements in low-β magnetic reconnection driven by capacitor coil targets.

By means of fully kinetic simulations, we investigate electron acceleration during magnetic reconnection in a nonrelativistic proton--electron plasma with conditions similar to solar corona and flares. We demonstrate that reconnection leads to a nonthermally dominated electron acceleration with a power-law energy distribution in the nonrelativistic low-$\beta$ regime but not in the high-$\beta$ regime, where $\beta$ is the ratio of the plasma thermal pressure and the magnetic pressure. The accelerated electrons contain most of the dissipated magnetic energy in the low-$\beta$ regime. A guiding-center current description is used to reveal the role of electron drift motions during the bulk nonthermal energization. We find that the main acceleration mechanism is a \textit{Fermi}-type acceleration accomplished by the particle curvature drift motion along the electric field induced by the reconnection outflows. Although the acceleration mechanism is similar for different plasma $\beta$, low-$\beta$ reconnection drives fast acceleration on Alfv\'enic timescales and develops power laws out of thermal distribution. The nonthermally dominated acceleration resulting from magnetic reconnection in low-$\beta$ plasma may have strong implications for the highly efficient electron acceleration in solar flares and other astrophysical systems.

The plasma of the Solar corona is practically collisionless. This imposes an important role of waves and micro-turbulent interactions with particles for plasma transport, dissipation, stability, etc, since binary particles collisions are inefficient. As a result the Solar corona is also a natural laboratory for testing basic plasma turbulent phenomena. Current sheets (CS), through which magnetic energy can be released, are ubiquitous in this environment. Different from many other plasmas, the presence of large guide magnetic fields plays a crucial role in the corona. In general, CS are prone to a large number of macro and micro-instabilities, the more the thinner they become. CS can lead to or are formed as consequence of magnetic reconnection. This is a fundamental physical process in the Universe that converts magnetic into other forms of energy which goes along with change of the magnetic connectivity. In this context, we aim towards an appropriate characterization of the influence of the guide magnetic field on the instabilities of and magnetic reconnection through CS, for which in coronal plasmas small scale kinetic effects are essential. In order to investigate the essential nonlinear properties, we use fully kinetic Particle-in-Cell (PIC) numerical simulations to adequately describe the collisionless solar coronal plasma. The kinetic approach is necessary to properly describe coupling of scales in collisionless magnetic reconnection and to provide, in the end, macroscopic transport properties appropriately describing the coronal plasma. In order to validate our methods, we also analyze the limits cases of zero (antiparallel configuration) and infinite guide fields (gyrokinetic theory) for comparison. In the case of antiparallel Harris CS, we find several instabilities driven by temperature anisotropy which might be numerically induced when more realistic parameters (high mass ratios) are used in PIC simulations. We reveal that they may mimic real collisional physical processes, and we show how they can be efficiently avoided by choosing appropriate numerical parameters, such as the shape functions (interpolation schemes). Next, we analyze the instabilities of Harris CS in the presence of small guide fields. We develop methods to calculate spatial and temporal derivatives, as well as averages, for a proper calculation of the mechanisms supporting the reconnected electric field. Our methods, more accurate than previously used ones, reveals the appearance of additional terms in the mean field generalized Ohm's law at the edge of the magnetic islands, arising from the interaction with electromagnetic fluctuations. Our findings reveal cross-field streaming and pressure gradient driven instabilities, causing plasma heating, particle acceleration and turbulence. In the third and last part, we analyze instabilities of force free CS in the presence of large guide fields, which we compare with the results of gyrokinetic simulations. For beta_i=0.01, we find that gyrokinetic simulations model sufficiently well the regions close to the reconnection X point for guide fields b_g>5, and practically everywhere for b_g>30. But only a fully kinetic PIC simulation can reveal, e.g., the physics of secondary magnetic islands for moderate guide fields b_g>5, where macro and micro instabilities driven by shear flow and streamings generate magnetic fields, particle heating and acceleration besides of high frequency electromagnetic fluctuations. For beta_i~1, the applicability of gyrokinetic simulation is much less restricted, in the sense that the convergence to the PIC simulations results requires even lower guide fields. Our results have important implications for understanding the role of current sheet instabilities in the solar corona and their macroscopic consequences for the overall dynamics and energy conversion processes. This also applies for astrophysical collisionless plasmas, as well as laboratory plasmas and nuclear fusion facilities on Earth.

Quantitative analysis the role of turbulence in magnetic self-generating and organization, plasma self-feeding and sustaining, which closely related to the evolution of neutral points and flux ropes, is the most important to understand magnetic energy release-conversion, plasma heating, charged particles energization and acceleration in 3D large temporal-spatial scale turbulent magnetic reconnection (LTSTMR; observed current sheets thickness to electron characterize length, electron Larmor radius for low- and electron inertial length for high-, ratio in 10E10 ~ 10E11 order; observed evolution time to electron gyroperiod ratio in 10E10 ~10E11 order) that cover from Earth's magnetosphere to solar eruption and other astrophysical phenomena. As the first part of this work, a relativistic hybrid particle-in-cell & lattice Boltzmann (HPIC-LBM) model that describes the continue kinetic-dynamic-hydro full coupled temporal-spatial scale LTSTMR evolution process. First, based on the governing equations for resistive relativistic Magnetohydrodynamics (MHD), the relativistic discrete distribution function for magnetic field (D3Q7), electric field (D3Q13), electromagnetic field (D3Q13), as well as charged particle (D3Q19) and neutral particle (D3Q27) of different species plasma are obtained for RHPIC-LBM lattice grid. Then, the RHPIC-LBM numerical process, the algorithm, the code flow chart as well as GPU-CPU heterogeneous frame work is described. Last, the present model is tested and validated on Tianhe-2 from National Supercomputer Center in Guang Zhou (NSCC-GZ) with 10,000 ~ 100,000 CPU cores & 50 ~ 120 hours per case for investigating solar atmosphere activities LTSTMR from pico-scale (10E-2 m ~10E5 m), nano-scale (10E5 m ~10E6 m), micro-scale (10E6 m ~ 10E7m), macro-scale (10E7 m ~ 10E8 m) and large-scale (10E8 m ~10E9 m). All simulation results are consistent with observations. Based on the model and code developed in first part, we investigated the turbulence of helical magnetic structure, current density vector fields, magnetic fields self-generating and organization, plasma self-feeding and sustaining, ion and electron acceleration of 3D LTSTMR with 100,000 CPU cores on Tianhe-2 in the second part. By simulation evidences, we discovered and confirmed following results: i) Slipping magnetic reconnection (MR) exists in the adjacent magnetic field lines (MFLs) compress-stretch-slip process on the quasi separatrix layers (QSLs, drastic change MFL linkage span) and adjacent MFLs cut-break-reconnect MR exists on the separatrix surfaces (SLs) that are consistent to observations; ii) The slipping MR and the MFLs cut-break-reconnect MR is closed link with the oblique and resistive tearing instabilities, respectively. The 1st type MR forms O-type neutral points, and the 2nd type MR forms X-type neutral points. The magnetic energy conversion is dominated by turbulence induced oblique instabilities (O-type) in 3D model instead of resistive tearing instabilities (X-type) in 2D/2.5D model, which are very well consistent with 3D observations; iii) The magnetic energy conversion is produced in the interaction of plasmoid-to-flux rope, plasmoid-to-plasmoid, and flux ropes-to-flux ropes. The turbulent acceleration is an independent acceleration mechanism in LTSTMR, induced by the interaction of waves-to-waves and waves-to-particles, and is different from original hybrid acceleration mechanism (composed of parallel electric fields, betatron, shock and Fermi) ; iv) The particles can be energized and accelerated in a longer time scale (~30 s) and can be accelerated to relativistic energy bin after pre-accelerated by Fermi-Betatron-shock wave acceleration which are agreed with observations.

Recent observations and simulations indicate that solar flares undergo extremely complex 3D evolution, making 3D particle transport models essential for understanding electron acceleration and interpreting flare emissions. In this study, we investigate this problem by solving Parker’s transport equation with 3D MHD simulations of solar flares. By examining energy conversion in the 3D system, we evaluate the roles of different acceleration mechanisms, including reconnection current sheet (CS), termination shock (TS), and supra-arcade downflows (SADs). We find that large-amplitude turbulent fluctuations are generated and sustained in the 3D system. The model results demonstrate that a significant number of electrons are accelerated to hundreds of keV and even a few MeV, forming power-law energy spectra. These energetic particles are widely distributed, with concentrations at the TS and in the flare looptop region, consistent with results derived from recent hard X-ray (HXR) and microwave (MW) observations. By selectively turning particle acceleration on or off in specific regions, we find that the CS and SADs effectively accelerate electrons to several hundred keV, while the TS enables further acceleration to MeV. However, no single mechanism can independently account for the significant number of energetic electrons observed. Instead, the mechanisms work synergistically to produce a large population of accelerated electrons. Our model provides spatially and temporally resolved electron distributions in the whole flare region and at the flare footpoints, enabling synthetic HXR and MW emission modeling for comparison with observations. These results offer important insights into electron acceleration and transport in 3D solar flare regions.

The role of turbulent acceleration and heating in the fractal magnetic reconnection of solar flares is still not clear, especially at the X-point in the diffusion region. At virtual test aspect, it is hardly to quantitatively analyze the vortex generation, turbulence evolution, particle acceleration and heating in the magnetic islands coalesce in fractal manner, formatting into largest plasmid and ejection process in diffusion region through classical magnetohydrodynamics numerical method. With the development of physical particle numerical method (particle in cell method [PIC] , Lattice Boltzmann method [LBM] ) and high performance computing technology in recently two decades. Kinetic-dynamic simulation has developed into an effectively manner to exploring the role of magnetic field and electric field turbulence in charged particles acceleration and heating process, since all the physical aspects relating to turbulent reconnection are taken into account. In this paper, the LBM based lattice DxQy grid and extended distribution are added into charged-particles-to-grid-interpolation of PIC based finite difference time domain scheme and Yee Grid, the hybrid PIC-LBM simulation tool is developed to investigating turbulence acceleration on TIANHE-2. The actual solar coronal condition (L≈105Km,B≈50-500G,T≈5×106K, n≈108-109, mi/me≈500-1836) is applied to study the turbulent acceleration and heating in solar flare fractal current sheet. At stage I, magnetic islands shrink due to magnetic tension forces, the process of island shrinking halts when the kinetic energy of the accelerated particles is sufficient to halt the further collapse due to magnetic tension forces, the particle energy gain is naturally a large fraction of the released magnetic energy. At stage II and III, the particles from the energized group come in to the center of the diffusion region and stay longer in the area. In contract, the particles from non energized group only skim the outer part of the diffusion regions. At stage IV, the magnetic reconnection type nano-plasmoid (200km) stop expanding and carrying enough energy to eject particles as constant velocity. Last, the role of magnetic field turbulence and electric field turbulence in electron and ion acceleration at the diffusion regions in solar flare fractural current sheet is given.

Solar flares are known to be prolific electron accelerators, yet identifying the mechanism(s) for such efficient electron acceleration in solar flare (and similar astrophysical settings) presents a major challenge. This is due in part to a lack of observational constraints related to conditions in the primary acceleration region itself. Accelerated electrons with energies above ∼20 keV are revealed by hard X-ray (HXR) bremsstrahlung emission, while accelerated electrons with even higher energies manifest themselves through radio gyrosynchrotron emission. Here, we show, for a well-observed flare on 2017 September 10, that a combination of RHESSI HXR and and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) EUV observations provides a robust estimate of the fraction of the ambient electron population that is accelerated at a given time, with an upper limit of ≲10−2 on the number density of nonthermal (≥20 keV) electrons, expressed as a fraction of the number density of ambient protons in the same volume. This upper limit is about 2 orders of magnitude lower than previously inferred from microwave observations of the same event. Our results strongly indicate that the fraction of accelerated electrons in the coronal region at any given time is relatively small but also that the overall duration of the HXR emission requires a steady resupply of electrons to the acceleration site. Simultaneous measurements of the instantaneous accelerated electron number density and the associated specific electron acceleration rate provide key constraints for a quantitative study of the mechanisms leading to electron acceleration in magnetic reconnection events.

By means of fully kinetic particle-in-cell simulations, we study whether the proton-to-electron mass ratio m i /m e influences the energy spectrum and underlying acceleration mechanism during magnetic reconnection. While kinetic simulations are essential for studying particle acceleration during magnetic reconnection, a reduced m i /m e is often used to alleviate the demanding computing resources, which leads to artificial scale separation between electron and proton scales. Recent kinetic simulations with high mass ratios have suggested new regimes of reconnection, as electron pressure anisotropy develops in the exhaust region and supports extended current layers. In this work, we study whether different m i /m e changes the particle acceleration processes by performing a series of simulations with different mass ratio (m i /m e = 25–400) and guide field strength in a low-β plasma. We find that mass ratio does not strongly influence reconnection rate, magnetic energy conversion, ion internal energy gain, plasma energization processes, ion energy spectra, and the acceleration mechanisms for high-energy ions. Simulations with different mass ratios are different in electron acceleration processes, including electron internal energy gain, electron energy spectrum, and the acceleration efficiencies for high-energy electrons. We find that high-energy electron acceleration becomes less efficient when the mass ratio gets larger because the Fermi-like mechanism associated with particle curvature drift becomes less efficient. These results indicate that when particle curvature drift dominates high-energy particle acceleration, the further the particle kinetic scales are from the magnetic field curvature scales (∼d i ), the weaker the acceleration will be.

<p>Turbulence, the self-generated turbulence by plasmas and magnetic field collective interaction, has been found to play an essential role in energizing charged particles in the large-scale reconnecting current sheet in the major solar eruption.</p><p>The typical large-scale CME/Flare events involve sudden bursts of particle acceleration from the sudden release of magnetic energy in a few minutes to a few tens of minutes. The X-rays emission and gamma rays burst produced by the combined result from the interactions of electrons, hydrogen, helium, and other heavier ions. Space and laboratory researchers are more inclined to believe that turbulence acceleration is belonged to shock acceleration. Solar and astrophysics researchers are more inclined to believe that turbulence acceleration is an independent acceleration mechanism that belongs to the flare acceleration. The evidence in both theories and observations from solar atmosphere activities shows that the acceleration is related to nonlinear resonant wave-particle interaction (e.g., Landau acceleration). So far, many-particle acceleration models consider turbulence acceleration as an effective way of generating energetic electrons, protons, and heavier ions. However, the detailed role of turbulence in this process remains unclear. More effort needs to invest in looking into particle accelerations by turbulence that occurs over a large range of the scale in space from the inertial scale of individual particles to the MHD scale.</p><p>In this work, applying the statistical treatment of plasma physics, combing with filter theory of turbulence, the actual ratio of the proton mass to the electron mass, and mass-to-charge ratios, we investigate the interaction of charged particles with the turbulent electric field and magnetic field in the large-scale CME/flare current sheet by applying the</p><p>We found the significant Langmuir turbulence acceleration (LTA) through the nonlinear resonant wave-particle interaction in the diffusion region via tracking the trajectories and analyzing the energy spectrum of energetic protons and electrons. The results show that protons and electrons could be efficiently accelerated simultaneously and that the way of LTA is similar to that of the shock acceleration}} but is much more efficient than the shock acceleration. This indicates that large-scale reconnection is a good candidate for the mechanism for the efficient acceleration of protons and electrons in the major solar eruption.</p><p>The acceleration of heavy ion considered Helium (3He/4He) and other heavy elements in 3He-rich flares burst would explore in the follow-up work series.</p><p>URL: https://pan.cstcloud.cn/s/drEdcjIaT8E</p>

Magnetic reconnection is a fundamental physical process in various astrophysical, space, and laboratory environments. Many pieces of evidence for magnetic reconnection have been uncovered. However, its specific processes that could be fragmented and turbulent have been short of direct observational evidence. Here, we present observations of a super-hot current sheet during the SOL2017-09-10T X8.2-class solar flare that display the fragmented and turbulent nature of magnetic reconnection. As bilateral plasmas converge toward the current sheet, significant plasma heating and nonthermal motions are detected therein. Two oppositely directed outflow jets are intermittently expelled out of the fragmenting current sheet, whose intensity shows a power-law distribution in the spatial frequency domain. The intensity and velocity of the sunward outflow jets also display a power-law distribution in the temporal frequency domain. The length-to-width ratio of current sheet is estimated to be larger than the theoretical threshold and thus ensures its occurrence. The observations therefore suggest that fragmented and turbulent magnetic reconnection occurs in the long stretching current sheet.

The authors propose a model describing physical processes of solar flares based on resistive reconnection of magnetic field subject to continuous increase of magnetic shear in the arcade. The individual flaring process consists of magnetic reconnection of arcade field lines, generation of magnetic islands in the magnetic arcade, and coalescence of magnetic islands. When a magnetic arcade is sheared (either by foot point motion or by flux emergence), a current sheet is formed and magnetic reconnection can take place to form a magnetic island. A continuing increase of magnetic shear can trigger a new reconnection process and create a new island in the under lying arcade below the magnetic island. The new born island rises faster than the preceding island and merges with it to form one island. Before completing the island merging process, the new born island exhibits two phases of rising motion: a first phase with a slower rising speed and a second phase with a faster rising speed. The flare plasma heating occurs mainly due to magnetic reconnection in the current sheet under the new born island. The new born island represents the X-ray plasma ejecta which shows two phases of rising motion observed by Yohkoh [Ohyama and Shibata (1997)]. The first phase with slower new born island rising speed corresponds to the early phase of reconnection of line-tied field in the underlying current sheet and is considered as the preflare phase. In the second phase, the island coalescence takes place, and the underlying current sheet is elongated so that the line-tied arcade field reconnection rate is enhanced. This phase is interpreted as the impulsive phase or the flash phase of flares. The obtained reconnection electric field is large enough to accelerate electrons to an energy level higher than 10 keV, which is necessary for observed hard X-ray emissions. After merging of the islands is completed, magnetic reconnection continues in the current sheet under the integrated island for a longer period, which is considered as the main phase of flares. The sequence of all these processes is repeated with some time interval while a shear-increasing motion continues. The authors propose that these repetitive flaring processes constitute a set of homologous flares.

We propose a model describing physical processes of solar flares based on resistive reconnection of magnetic field subject to continuous increase of magnetic shear in the arcade. The individual flaring process consists of magnetic reconnection of arcade field lines, generation of magnetic islands in the magnetic arcade, and coalescence of magnetic islands. When a magnetic arcade is sheared (either by footpoint motion or by flux emergence), a current sheet is formed and magnetic reconnection can take place to form a magnetic island. A continuing increase of magnetic shear can trigger a new reconnection process and create a new island in the underlying arcade below the magnetic island. The newborn island rises faster than the preceding island and merges with it to form one island. Before completing the island merging process, the newborn island exhibits two phases of rising motion: a first phase with a slower rising speed and a second phase with a faster rising speed. The flare plasma heating occurs mainly due to magnetic reconnection in the current sheet under the newborn island. The newborn island represents the X-ray plasma ejecta which shows two phases of rising motion observed by Yohkoh (Ohyama and Shibata, 1997). The first phase with slower newborn island rising speed corresponds to the early phase of reconnection of line-tied field in the underlying current sheet and is considered as the preflare phase. In the second phase, the island coalescence takes place, and the underlying current sheet is elongated so that the line-tied arcade field reconnection rate is enhanced. This phase is interpreted as the impulsive phase or the flash phase of flares. The obtained reconnection electric field is large enough to accelerate electrons to an energy level higher than 10 keV, which is necessary for observed hard X-ray emissions. After merging of the islands is completed, magnetic reconnection continues in the current sheet under the integrated island for a longer period, which is considered as the main phase of flares. The sequence of all these processes is repeated with some time interval while a shear-increasing motion continues. We propose that these repetitive flaring processes constitute a set of homologous flares.

In quasi-parallel shock waves, turbulence occurs in the shock transition region due to instabilities such as the ion-ion beam instability, which eventually bends magnetic field lines and current sheets are produced. There are two types of current sheets in the shock turbulence region: reconnecting current sheets and non-reconnecting current sheets. In the Earth’s bow shock, NASA’s Magnetospheric Multiscale (MMS) has been observing many current sheets, some of which show evidence of magnetic reconnection and energetic accelerated particles.  We study electron acceleration in the Earth’s quasi-parallel bow shock by means of 2D particle-in-cell (PIC) simulation. We discuss differences in properties in reconnecting and non-reconnecting current sheets. Reconnecting current sheets and magnetic islands produced by reconnection show significant heating and energetic particles, and several acceleration mechanisms work in these regions: Fermi acceleration, Hall electric field acceleration, and island betatron acceleration. We also demonstrate that electrons are energized in non-reconnecting current sheets. In some regions in turbulence, an elongated, extending current sheet is formed, and electrons can be accelerated by the perpendicular electric field inside the non-reconnecting current sheet. We compare the efficiency between the acceleration mechanisms in reconnection regions and non-reconnecting current sheets.