Magnetic Reconnection Region Research Articles

Observational Evidence of Particle Acceleration by Relativistic Magnetic Reconnection in Gamma-Ray Bursts

Gamma-ray bursts (GRBs) as the most energetic explosions in the modern universe have been studied for over half a century, but the physics of the particle acceleration and radiation responsible for their observed spectral behaviors are still not well understood. Based on the comprehensive analysis of the pulse properties in both bright GRB 160625B and GRB 160509A, for the first time, we identify evidence of particle acceleration by relativistic magnetic reconnection from the evolutionary behavior of the two spectral breaks E p and E cut. Meanwhile, the adiabatic cooling process of the emitting particles in the magnetic reconnection regions produces a relation between the spectral index and the flux. We also discuss the physics behind spectral energy correlations. Finally, we argue that the identification of an anticorrelation between E cut and L iso may open a new avenue for diagnostics of the physics of the particle acceleration and radiation in a variety of astrophysical sources.

The role of an in-plane electric field during the merging formation of spherical tokamak plasmas

Axial merging of two torus plasmas is utilized as a center-solenoid free start-up scheme for a high-beta spherical tokamak (ST) plasma, in which magnetic reconnection under a strong guide field plays dominant roles in energy conversion and equilibrium formation. The ion heating source in magnetic reconnection is the plasma outflow with E×B drift velocity in the downstream region where the reconnected field lines flow out. Since the inductive reconnection electric field is almost parallel to the magnetic field, particularly in the inboard-side downstream region of magnetic reconnection under a strong guide field, a large electrostatic field in the poloidal plane is spontaneously formed to sustain steady plasma outflow motion in the downstream region. In ST plasma merging experiments, the self-generated electrostatic field in the downstream region does not always balance with the inductive electric field to make the total electric field strictly perpendicular to the total magnetic field. The excess electrostatic field will provide an even faster outflow plasma velocity than the magnetic field line motion and a quick reversal of the toroidal plasma current to form convex flux surfaces.

Electron acceleration in the electron dissipation region of asymmetrical magnetic reconnection driven by ultra-intensity lasers

We performed 3D Particle-In-Cell simulations to study electron acceleration in the electron dissipation region of asymmetrical electron magnetic reconnection driven by ultra-intensity lasers, which is similar to the Earth’s magnetosphere reconnection process. Within the electron dissipation region, electrons exhibit a nonthermal distribution, and as the asymmetry increases, the power-law spectrum becomes steeper. Remarkably, the electron spectrum closely resembles a delta distribution, arising from the intense acceleration imparted by the reconnection electric field near the X-line. Both parallel electric field acceleration and the Betatron acceleration mechanism play pivotal roles in this reconnection process. Furthermore, as the magnetic reconnection asymmetry intensifies, the parallel electric acceleration mechanism becomes stronger near the X-point region, whereas the Betatron acceleration mechanism wanes, primarily concentrated in the outflow region.

Anomalous Resistivity and Electron Heating by Lower Hybrid Drift Waves during Magnetic Reconnection with a Guide Field.

The lower hybrid drift wave (LHDW) has been a candidate for anomalous resistivity and electron heating inside the electron diffusion region of magnetic reconnection. In a laboratory reconnection layer with a finite guide field, quasielectrostatic LHDW (ES-LHDW) propagating along the direction nearly perpendicular to the local magnetic field is excited in the electron diffusion region. ES-LHDW generates large density fluctuations (δn_{e}, about 25% of the mean density) that are correlated with fluctuations in the out-of-plane electric field (δE_{Y}, about twice larger than the mean reconnection electric field). With a small phase difference (∼30°) between two fluctuating quantities, the anomalous resistivity associated with the observed ES-LHDW is twice larger than the classical resistivity and accounts for 20% of the mean reconnection electric field. After we verify the linear relationship between δn_{e} and δE_{Y}, anomalous electron heating by LHDW is estimated by a quasilinear analysis. The estimated electron heating is about 2.6±0.3 MW/m^{3}, which exceeds the classical Ohmic heating of about 2.0±0.2 MW/m^{3}. This LHDW-driven heating is consistent with the observed trend of higher electron temperatures when the wave amplitude is larger. Presented results provide the first direct estimate of anomalous resistivity and electron heating power by LHDW, which demonstrates the importance of wave-particle interactions in magnetic reconnection.

Small-scale Field-aligned Currents in the Magnetopause Boundary Layer

Based on high-resolution measurements from the Magnetospheric Multiscale mission from 2015 May to 2018 June, we statistically investigate the properties of small-scale field-aligned currents (SFACs) in the magnetopause boundary layer. A total of 2235 SFACs are successfully identified. The durations of SFACs mainly fall between 0.2 and 0.3 s. Over 90% of SFACs have a width of less than 1 ion inertia length and are primarily distributed from 5 to 25 electron inertia lengths, implying that the SFACs belong to the kinetic-scale current layer. The main carriers of SFACs are electrons, and over 70% of SFACs exhibit net energy dissipation (i.e., J · E ′ > 0) with the majority of energy dissipation taking place in the parallel direction. SFACs are widely distributed spatially, and the occurrence rate of SFACs is higher in the boundary layer closer to the magnetosphere. Additionally, less than half of the total SFACs are identified in well-known structures, including the magnetic reconnection region, flux transfer event, Kelvin–Helmholtz vortex, and exhaust region, and 54% of the SFACs are in the “others” unknown structures. These results improve our comprehension of the current system at the magnetopause and the roles of SFACs in the coupling between the solar wind and magnetosphere.

Turbulent Magnetic Reconnection as an Acceleration Mechanism in Earth’s Magnetotail

Using electric and magnetic fields measured by the Magnetospheric Multiscale (MMS) mission, we construct a test-particle simulation of a turbulent magnetic reconnection region to investigate observed ion acceleration. We identify three types of energized ions: (1) ion jets, (2) Speiser-like energized ions—both of which carry significant energy but do not produce a strong energetic (>80 keV) tail in the ion distribution—and (3) a separate but sizable population of ions that are accelerated to significantly higher energies (>80 keV) by the turbulent fields. The majority of ions that undergo energization by the turbulent fields cross the magnetic null plane multiple times. By preferentially energizing these particles, the turbulence creates a separate population of ions that mostly exits in the dawn direction of the magnetotail and forms a high-energy power-law tail in the ion flux-energy distribution. We also find that the highest acceleration energies are limited by the size of the turbulent region (with respect to ion gyroradii). Since turbulence is widespread in astrophysical plasmas and has no a priori limit on scale size, the MMS observations suggest turbulence may have a significant role in particle acceleration.

Three-dimensional crescent-shaped ion velocity distributions created by magnetic reconnection in the presence of a guide field

By means of theory and particle simulations, crescent-shaped ion velocity distributions in the outflow region of symmetric magnetic reconnection in the presence of a guide magnetic field are investigated. Assuming a spatial one-dimensional electromagnetic field, a theoretical model accounting for the shape of crescents is derived. First, following the earlier theoretical models suggested by Bessho et al. [Geophys. Res. Lett. 43, 1828–1836 (2016)] and Zenitani et al. [J. Geophys. Res.: Space Phys. 122, 7396–7413 (2017)], we derive a theoretical model for 2D velocity distributions based on the conservation of the canonical momentum and the energy. The 2D theoretical model exhibits a two-dimensional structure of crescent-shaped velocity distributions and further demonstrates that no magnetic field reversal is required in the formation of crescents, although many researchers have considered that magnetic field reversal plays an essential role. Next, we construct a theoretical model for 3D velocity distributions based not only on the conservation of the canonical momentum in two directions and the energy but also on the conservation of the magnetic moment and the kinetic energy in the moving frame with the reconnection outflow speed, which are applied when the guide field ratio is high. The 3D theoretical model derives a three-dimensional structure of crescents by combination of an antiparallel magnetic field and a guide magnetic field comparable to or greater than the reconnection field. Both of the 2D and 3D theoretical models are consistent with ion velocity distributions found in our particle simulations.

Analysis of the Energy Flux Density near Electron Diffusion Region of Asymmetric Magnetic Field Reconnection

Magnetic reconnection is a crucial energy conversion process in plasmas, and it is important to study the forms of energy conversion and their distribution in this process. Previous research has focused mainly on the energy fluxes in symmetric reconnection, while the study of asymmetric reconnection at the Earth’s magnetopause, especially in terms of statistical analysis of multiple events, has not been reported before. Therefore, 10 magnetic reconnection events at the magnetopause observed by MMS (Magnetospheric Multiscale) satellite that passed through the electron diffusion region were used for analysis. Although the contribution of different energy flux varies case by case, our results show that in most events, the ion enthalpy flux is dominant, followed by the Poynting flux. The ion heat flux is slightly smaller than the Poynting flux, while the sum of ion kinetic energy flux, electron enthalpy flux and electron heat flux only accounts for less than 10% of the total energy flux. By projecting the normalized energy flux of all events into BL−V(i,L) plane (in LMN coordinate), we find that the energy flux in M direction is comparable to the energy flux in L direction and also there is an “anti-correlated” relationship between the ion velocity and ion heat flux around reconnection diffusion region, consistent with previous studies.

First Observation of Kinetic Alfvén Waves behind Reconnection Front in Terrestrial Magnetotail

Reconnection fronts, also known as dipolarization fronts (DFs), and kinetic Alfvén waves (KAWs) are two of the widely observed structures and waves in the terrestrial magnetotail, but their correlation remains elusive in previous spacecraft measurements. Using high-resolution Magnetospheric Multiscale data, here we provide the first observational identification of KAWs behind the DF. These low-frequency dispersive KAWs propagate quasi-perpendicular to the magnetic field at phase speeds slower than local Alfvén speed, and are highly correlated with high-frequency whistlers propagating obliquely to the magnetic field. Specifically, whistlers appear at local magnetic field minima (wave troughs of KAWs) and disappear at local magnetic field maxima (wave crests of KAWs), highly consistent with the source region of whistler waves previously reported near equator and dayside magnetopause. Our study also suggests that the KAWs behind DFs could originate from the diffusion region of magnetic reconnection. These findings improve our understanding of the relationship between DFs, KAWs, and whistler waves in terrestrial magnetotail.

Multiline Stokes Synthesis of Ellerman Bombs: Obtaining Seamless Information from Photosphere to Chromosphere

Magnetic reconnection in the lower atmosphere is a critical process in determining the chromospheric dynamics, such as Ellerman bombs and UV bursts. Because the heating of the atmosphere significantly depends on the ionization degree and plasma β, which varies with height, it is essential to diagnose the height at which the magnetic reconnection takes place. Multiwavelength spectropolarimetry is a powerful solution to fulfill this requirement. We verify the diagnostic capabilities and usefulness of near-infrared multiwavelength spectropolarimetric observations for understanding magnetic reconnection phenomena by synthesizing the Stokes vector from a realistic magnetohydrodynamic simulation. The analysis considers two magnetic reconnection regions occurring at different heights. In the case of magnetic reconnection at low altitude, both red- and blueshifted components originating from reconnection bidirectional flow are identified in the photospheric lines, Fe i 8468 Å, K i 7664 Å, and K i 7698 Å. In the case of magnetic reconnection at high altitudes, chromospheric lines, Ca ii 8498 Å and 8542 Å, show emission due to the heating that occurs at the upper part of the formation layer. These results suggest that multiwavelength spectropolarimetric observations are capable of distinguishing the height where magnetic reconnection occurs.

On a plasma sheath with a small normal magnetic field separating regions of oppositely directed magnetic field

The Harris-sheet model provides an elegant solution to the kinetic plasma equation for a steady state 1D current sheet geometry separating regions with oppositely directed magnetic field. However, adding just a small normal magnetic field to the Harris configuration yields thermal streaming of particles into and out of the current sheet, fundamentally changing the form of its kinetic description. The action variable, Jz, associated with the oscillatory orbit motion perpendicular to the current sheet is well conserved and can be applied for solving the kinetic equation in the 1D sheet geometry that includes a small normal magnetic field. Revisiting this problem, we develop a new formalism that permits numerical solutions to be readily obtained for general upstream/asymptotic electron and ion distributions. In particular, we consider the case of isotropic ion pressure and anisotropic bi-Maxwellian electrons. The current sheets are then supported by electron pressure anisotropy. Furthermore, the total current across a particular sheet is set by the fire-hose condition based on the electron pressures normalized by the asymptotic magnetic field pressure. Analytical approximations are obtained for the numerical solutions expressed in terms of the asymptotic electron temperature anisotropy and the ion temperature. We discuss a preliminary application of the framework to the electron diffusion region of anti-parallel magnetic reconnection.

The Solar Origin of an In Situ Type III Radio Burst Event

Solar type III radio bursts are generated by beams of energetic electrons that travel along open magnetic field lines through the corona and into interplanetary space. However, understanding the source of these electrons and how they escape into interplanetary space remains an outstanding topic. Here we report multi-instrument, multiperspective observations of an interplanetary type III radio burst event shortly after the second perihelion of the Parker Solar Probe (PSP). This event was associated with a solar jet that produced an impulsive microwave burst event recorded by the Expanded Owens Valley Solar Array. The type III burst event also coincided with the detection of enhanced in situ energetic electrons recorded by both PSP at 0.37 au and WIND at 1 au, which were located very closely on the Parker spiral longitudinally. The close timing association and magnetic connectivity suggest that the in situ energetic electrons originated from the jet’s magnetic reconnection region. Intriguingly, microwave imaging spectroscopy results suggest that the escaping energetic electrons were injected into a large opening angle of about 90°, which is at least nine times broader than the apparent width of the jet spire. Our findings provide an interpretation for the previously reported, longitudinally broad spatial distribution of flare locations associated with prompt energetic electron events and have important implications for understanding the origin and distribution of energetic electrons in interplanetary space.

Turbulence, Particle Acceleration, and Heating Due To the Lower Hybrid Mode Near Magnetic Reconnection Regions in Magnetopause

AbstractMMS mission by NASA has observed lower hybrid waves (LHWs) and associated turbulence near the reconnection regions at Earth's magnetopause. The presented work studies LHWs (electromagnetic mode) and turbulence in the presence of magnetic islands (at the reconnection sites) in magnetopause. A nonlinear model representing electromagnetic 3D LHW has been developed. We have considered that nonlinear effects are due to the ponderomotive force. The dynamical equation, thus obtained, is solved using numerical methods and computer simulation. For spatial integration, we have used the pseudo‐spectral method and the finite difference method is used to study temporal evolution. The simulation results show the spatiotemporal development of the LHW localized structures and current sheets and confirm the presence of turbulence. Using a semi‐analytic model, we have determined the scale size of current sheets and localized structures and shown that these scale sizes depend upon the lower hybrid power. The turbulent power spectra have also been studied. The power scaling of k−2.8 (approximately) and k−2.1 (approximately) are observed along perpendicular and parallel directions, respectively. The power law scaling of turbulence generation has been used to study the formation of the thermal tail of energetic electrons. The fractional diffusion method has also been exploited to determine electron acceleration and plasma heating. We anticipate LHW may be responsible for the acceleration of the energetic electrons and anomalous heating in magnetopause.

Chaos-induced Resistivity in Collisionless Magnetic Reconnection Region

Collisionless magnetic reconnection, which converts the magnetic energy into the kinetic energy of plasma particles via the heating or acceleration, has been believed widely to be able to explain various eruptive phenomena such as solar flares and geomagnetic storms. However, the microphysical mechanism of anomalous resistivity in the collisionless magnetic reconnection is still an unsolved fundamental problem. Among the many physical mechanisms of anomalous resistivity generation, chaos-induced resistivity based on the chaos of the charged particle orbits near the magnetic neutral point is not the most popular formation mechanism, but its microscopic physical picture is the clearest. This paper first briefly reviews the early research and physical model of the chaos-induced resistivity in collisionless magnetic reconnection region, introduces the recent research progress of the chaos-induced resistivity, and expounds the future research direction of the chaos-induced resistivity.

Particle acceleration with magnetic reconnection in large-scale RMHD simulations – I. Current sheet identification and characterization

ABSTRACT We present a new algorithm for the identification and physical characterization of current sheets and reconnection sites in 2D and 3D large-scale relativistic magnetohydrodynamic numerical simulations. This has been implemented in the pluto code and tested in the cases of a single current sheet, a 2D jet, and a 3D unstable plasma column. Its main features are (i) a computational cost that allows its use in large-scale simulations and (ii) the capability to deal with complex 2D and 3D structures of the reconnection sites. In the performed simulations, we identify the computational cells that are part of a current sheet by a measure of the gradient of the magnetic field along different directions. Lagrangian particles, which follow the fluid, are used to sample plasma parameters before entering the reconnection sites that form during the evolution of the different configurations considered. Specifically, we track the distributions of the magnetization parameter σ and the thermal to magnetic pressure ratio β that – according to particle-in-cell simulation results – control the properties of particle acceleration in magnetic reconnection regions. Despite the fact that initial conditions of the simulations were not chosen ‘ad hoc’, the 3D simulation returns results suitable for efficient particle acceleration and realistic non-thermal particle distributions.

Electron magnetohydrodynamics Grad–Shafranov reconstruction of the magnetic reconnection electron diffusion region

We present a study of the electron magnetohydrodynamics Grad–Shafranov (GS) reconstruction of the electron diffusion region (EDR) of magnetic reconnection. Two-dimensionality of the magnetoplasma configuration and steady state are the two basic assumptions of the GS reconstruction technique, which represent the method’s fundamental limitations. The present study demonstrates that the GS reconstruction can provide physically meaningful results even when these two assumptions, which are hardly fulfilled in spacecraft observations, are violated. This conclusion is supported by the reconstruction of magnetic configurations of two EDRs, encountered by the Magnetospheric Multiscale (MMS) Mission on July 11, 2017 and September 8, 2018. Here, the former event exhibited a violation of two-dimensionality, and the latter event exhibited a violation of steady state. In both cases, despite the deviations from the ideal model configuration, reasonable reconstruction results are obtained by implementing the herein introduced compressible GS reconstruction model. In addition to the discussed fundamental limitations, all existing versions of the GS reconstruction technique rely on a number of minor simplifying assumptions, which restrict the model scope and efficiency. We study the prospects for further model improvement and generalization analytically. Our analysis reveals that nearly all these minor limitations can be overcome by using a polynomial MMS-tailored reconstruction technique in the space of rotationally invariant variables instead of Cartesian coordinates.

Agyrotropic Electron Distributions in the Terrestrial Foreshock Transients

AbstractAgyrotropic electron distributions are frequently taken as an indicator of electron diffusion regions of magnetic reconnection. However, they have also been found at electron‐scale boundaries of the non‐reconnecting magnetopause and are generated by the electron finite gyroradius effect. Here, we present magnetospheric multiscale observations of agyrotropic electron distributions in the foreshock region. These distributions are generated by the electron finite gyroradius effect after magnetic curvature scattering at a thin electron‐scale boundary. Meanwhile, the signatures of magnetic reconnection are absent at this boundary. The test‐particle simulation is adopted to verify the generation of the agyrotropic electron distributions by assuming one‐dimensional magnetic geometry. These observations suggest that agyrotropic electron distributions can be more widely formed at electron‐scale boundaries in space plasma environment.

Non-thermal broadening of IRIS Fe XXI line caused by turbulent plasma flows in the magnetic reconnection region during solar eruptions

Magnetic reconnection is the key mechanism for energy release in solar eruptions, where the high-temperature emission is the primary diagnostic for investigating the plasma properties during the reconnection process. Non-thermal broadening of high-temperature lines has been observed in both the reconnection current sheet (CS) and flare loop-top regions by UV spectrometers, but its origin remains unclear. In this work, we use a recently developed three-dimensional magnetohydrodynamic (MHD) simulation to model magnetic reconnection in the standard solar flare geometry and reveal highly dynamic plasma flows in the reconnection regions. We calculate the synthetic profiles of the Fe XXI 1354 Å line observed by the Interface Region Imaging Spectrograph (IRIS) spacecraft by using parameters of the MHD model, including plasma density, temperature, and velocity. Our model shows that the turbulent bulk plasma flows in the CS and flare loop-top regions are responsible for the non-thermal broadening of the Fe XXI emission line. The modeled non-thermal velocity ranges from tens of km s−1 to more than two hundred km s−1, which is consistent with the IRIS observations. Simulated 2D spectral line maps around the reconnection region also reveal highly dynamic downwflow structures where the high non-thermal velocity is large, which is consistent with the observations as well.

MAVEN Observations of Tailward Reconnection Front in the Martian Magnetotail

AbstractIntense electromagnetic energy conversion and local magnetic flux transport occur at reconnection fronts (RFs) formed by magnetic reconnection. Tailward RFs (TRFs), characterized by sharply enhanced negative magnetic field normal components accompanied by tailward plasma flows, are the tailward propagating dipolarization fronts in the Earth's magnetotail. So far RFs and TRFs are observed merely at magnetized planets. Using data from Mars Atmosphere and Volatile EvolutioN, we report, for the first time, the observations of TRF embedded in a current sheet (possibly diffusion region of magnetic reconnection) in the Martian magnetotail. At the TRF, O+ and O2+ are accelerated in tailward direction; behind the TRF, the perpendicular electron fluxes enhance at the energy of 251–905 eV, which should be caused by betatron acceleration. The energized plasmas indicate that the electromagnetic energy converts to plasma kinetic energy around the TRF. Our observations suggest that RFs could occur in the planetary magnetotail without the global intrinsic dipole magnetic field.

Diffusive scattering of energetic electrons by intense whistler-mode waves in an inhomogeneous plasma

Electron resonant interactions with electromagnetic whistler-mode waves play an important role in electron flux dynamics in various space plasma systems: planetary radiation belts, bow shocks, solar wind and magnetic reconnection regions. Two key wave characteristics determining the regime of wave–particle interactions are the wave intensity and the wave coherency. The classical quasi-linear diffusion approach describes well electron diffusion by incoherent and low-amplitude waves, whereas the nonlinear resonant models describe electron phase bunching and trapping by highly coherent intense waves. This study is devoted to the investigation of the regime of electron resonant interactions with incoherent but intense waves. Although this regime is characterized by electron diffusion, we show that diffusion rates scale linearly with the wave amplitude,$D\propto B_w$, in contrast to the quasi-linear diffusion scaling$D_{QL}\propto B_w^2$. Using observed wave amplitude distributions, we demonstrate that the quasi-linear diffusion model significantly overestimates electron scattering by incoherent, but intense whistler-mode waves. We discuss the results obtained in the context of simulations of long-term electron flux dynamics in space plasma systems.