Fast Reconnection Research Articles

Decay of Kelvin-Helmholtz Vortices at the Earth's Magnetopause Under Pure Southward IMF Conditions.

At the Earth's low‐latitude magnetopause, clear signatures of the Kelvin‐Helmholtz (KH) waves have been frequently observed during periods of the northward interplanetary magnetic field (IMF), whereas these signatures have been much less frequently observed during the southward IMF. Here, we performed the first 3‐D fully kinetic simulation of the magnetopause KH instability under the southward IMF condition. The simulation demonstrates that fast magnetic reconnection is induced at multiple locations along the vortex edge in an early nonlinear growth phase of the instability. The reconnection outflow jets significantly disrupt the flow of the nonlinear KH vortex, while the disrupted turbulent flow strongly bends and twists the reconnected field lines. The resulting coupling of the complex field and flow patterns within the magnetopause boundary layer leads to a quick decay of the vortex structure, which may explain the difference in the observation probability of KH waves between northward and southward IMF conditions.

An Event Study of Simultaneous Earthward and Tailward Reconnection Exhaust Flows in the Earth's Midtail

AbstractWe report an event of two‐satellite measurements of simultaneous earthward and tailward fast flows of ~500 km/s in the midtail at X ~ −63 RE and evaluate magnetic reconnection as a responsible mechanism by comparing the observations with a particle‐in‐cell (PIC) simulation. The two satellites were near midnight separated mainly along the X direction by ~5 RE. As they moved across the current sheet from the northern to southern lobes, the one closer to the Earth crossed the x line with fast flows changing from tailward to earthward, while the other one simultaneously observed tailward flows. The observed plasma and fields showed several key reconnection signatures, including the Walén relation, the fast reconnection rate of ~0.1, the Hall magnetic and electric fields, and counterstreaming electrons in the separatrix, indicating the fast flow was the reconnection exhaust. The observed temporal variations of flow speeds and magnetic fields suggested that the x line was moving tailward to a location between the two satellites and the exhaust was moving up and down. Within the exhaust, plasma pressure was highly anisotropic, and the current sheet can be unstable to the mirror, ion cyclotron, and firehose instabilities. Current sheet flapping and enhanced compressional waves near proton's local gyro frequencies were observed around the current sheet. Comparing with the PIC simulation suggests that the waves were mainly a result of oblique firehose instability.

3D Numerical Simulation of Kink-driven Rayleigh–Taylor Instability Leading to Fast Magnetic Reconnection

Abstract Fast magnetic reconnection involving non-MHD microscale physics is believed to underlie both solar eruptions and laboratory plasma current disruptions. While there is extensive research on both the MHD macroscale physics and the non-MHD microscale physics, the process by which large-scale MHD couples to the microscale physics is not well understood. An MHD instability cascade from a kink to a secondary Rayleigh–Taylor instability in the Caltech astrophysical jet laboratory experiment provides new insights into this coupling and motivates a 3D numerical simulation of this transition from large to small scale. A critical finding from the simulation is that the axial magnetic field inside the current-carrying dense plasma must exceed the field outside. In addition, the simulation verifies a theoretical prediction and experimental observation that, depending on the strength of the effective gravity produced by the primary kink instability, the secondary instability can be Rayleigh–Taylor or mini-kink. Finally, it is shown that the kink-driven Rayleigh–Taylor instability generates a localized electric field sufficiently strong to accelerate electrons to very high energy.

Particle acceleration and the origin of the very high energy emission around black holes and relativistic jets

AbstractParticle acceleration induced by fast magnetic reconnection may help to solve current puzzles related to the interpretation of the very high energy (VHE) and neutrino emissions from AGNs and compact sources in general. Our general relativistic-MHD simulations of accretion disk-corona systems reveal the growth of turbulence driven by MHD instabilities that lead to the development of fast magnetic reconnection in the corona. In addition, our simulations of relativistic MHD jets reveal the formation of several sites of fast reconnection induced by current-driven kink turbulence. The injection of thousands of test particles in these regions causes acceleration up to energies of several PeVs, thus demonstrating the ability of this process to accelerate particles and produce VHE and neutrino emission, specially in blazars. Finally, we discuss how reconnection can also explain the observed VHE luminosity-black hole mass correlation, involving hundreds of non-blazar sources like Perseus A, and black hole binaries.

Magnetic reconnection in partially ionized plasmas.

Magnetic reconnection has been intensively studied in fully ionized plasmas. However, plasmas are often partially ionized in astrophysical environments. The interactions between the neutral particles and ionized plasmas might strongly affect the reconnection mechanisms. We review magnetic reconnection in partially ionized plasmas in different environments from theoretical, numerical, observational and experimental points of view. We focus on mechanisms which make magnetic reconnection fast enough to compare with observations, especially on the reconnection events in the low solar atmosphere. The heating mechanisms and the related observational evidence of the reconnection process in the partially ionized low solar atmosphere are also discussed. We describe magnetic reconnection in weakly ionized astrophysical environments, including the interstellar medium and protostellar discs. We present recent achievements about fast reconnection in laboratory experiments for partially ionized plasmas.

The Critical Role of Collisionless Plasma Energization on the Structure of Relativistic Magnetic Reconnection

Abstract During magnetically dominated relativistic reconnection, inflowing plasma depletes the initial relativistic pressure at the x-line and collisionless plasma heating inside the diffusion region is insufficient to overcome this pressure loss. The resulting significant pressure drop causes a collapse at the x-line, essentially a localization mechanism of the diffusion region necessary for fast reconnection. The extension of this low-pressure region (into the outflow) further explains the bursty nature of antiparallel reconnection because a once opened outflow exhaust can also collapse, which repeatedly triggers secondary tearing islands. However, a stable single x-line reconnection can be achieved when an external guide field exists, since the reconnecting magnetic field component rotates out of the reconnection plane at outflows, providing additional magnetic pressure to keep the exhaust open.

Synchrotron Gamma-Ray Emission Model of the Giant Outburst of Quasar 3C 279 in 2015 June: Fast Reconnection or Stochastic Acceleration with Electromagnetic Cascade?

Abstract We test the synchrotron emission scenario for the very bright gamma-ray flare of blazar 3C 279 observed in 2015 June using time-dependent numerical simulations. A bulk Lorentz factor as high as 100 can bring the synchrotron maximum energy above the GeV energy range. We find two possible solutions for the X-ray to gamma-ray spectrum. One is a prompt electron injection model with a hard power-law index as magnetic reconnection models suggest. Too strong a magnetic field yields too bright a synchrotron X-ray flux due to secondary electron–positron pairs. Even in the prompt electron injection model, the Poynting flux luminosity is at most comparable to the gamma-ray or electron luminosity. Another model is the stochastic acceleration model, which leads to a very unique picture accompanying the electromagnetic cascade and reacceleration of the secondary electron–positron pairs. In this model, the energy budget of the magnetic field is very low compared to gamma-rays and electrons.

Onset of Turbulent Fast Magnetic Reconnection Observed in the Solar Atmosphere

Abstract Fast magnetic reconnection powers explosive events throughout the universe, from gamma-ray bursts to solar flares. Despite its importance, the onset of astrophysical fast reconnection is the subject of intense debate and remains an open question in plasma physics. Here we report high-cadence observations of two reconnection-driven solar microflares obtained by the Interface Region Imaging Spectrograph that show persistent turbulent flows preceding flaring. The speeds of these flows are comparable to the local sound speed initially, suggesting the onset of fast reconnection in a highly turbulent plasma environment. Our results are in close quantitative agreement with the theory of turbulence-driven reconnection as well as with numerical simulations in which fast magnetic reconnection is induced by turbulence.

Determination of a macro- to micro-scale progression leading to a magnetized plasma disruption

We report the observations of a plasma jet evolving through a macro- to micro-scale progression sequence. This leads to a fast magnetic reconnection that results in the jet breaking off from its originating electrode and forming a force-free state. A sausage-like pinching occurs first and squeezes an initially fat, short magnetized jet so that it becomes thin. The thin jet then becomes kink unstable. The lengthening of the jet by the kinking thins the jet even more since the kink is an incompressible instability. When the jet radius becomes comparable to the ion-skin depth, Hall and electron inertial physics become important and establish the environment for a fast magnetic reconnection. This fast reconnection occurs, disrupting the jet and establishing a force-free state. X-ray bursts and whistler waves, evidence of a magnetic reconnection, are observed when the plasma jet breaks off from the electrode. This experimentally observed sequence of successive thinning from pinching followed by kinking is reproduced in a three-dimensional ideal Magnetohydrodynamic (MHD) numerical simulation. The results of the experiment and the numerical simulation, together demonstrate a viable path from macro-scale MHD physics to micro-scale non-MHD physics where fast reconnection occurs.

Control of neoclassical tearing mode by synergetic effects of resonant magnetic perturbation and electron cyclotron current drive in reversed magnetic shear tokamak plasmas

Synergetic effects of resonant magnetic perturbation (RMP) and electron cyclotron current drive (ECCD) on stabilizing neoclassical tearing mode (NTM) in reversed magnetic shear (RMS) tokamak plasmas are numerically investigated based on a set of reduced MHD equations. For the moderate separation, it is found that the explosive burst induced by the fast reconnection of double tearing mode (DTM) in the RMS configuration can be completely suppressed by externally applied RMPs. Zonal flows with strong shear induced by a rotating RMP play an important role in this suppression process. Moreover, turning on ECCD in advance is essential to mitigate the NTM. For the large separation without the explosive burst, two strategies, i.e. a continuous ECCD with static RMP and a modulated ECCD with rotating RMP, are separately investigated. It is shown that when the NTM is decelerated by a relatively slow rotating RMP, the modulated ECCD can have a better stabilizing effect. In addition, the ECCD deposition widths in both radial and helical angle directions, as well as the ECCD on-duty time, are analyzed in detail. The best effectiveness of ECCD is obtained and the relevant physical mechanisms are discussed.

Role of Hall effect on the resistive kink mode in tokamaks

The influence of the Hall effect on the nonlinear evolution of the m/n = 1/1 resistive-kink mode is numerically investigated by the three-dimensional toroidal Hall-MHD code CLT. It is found that the Hall effect can lead to the explosive growth of the resistive-kink mode at the nonlinear stage. The explosive nonlinear growth of the resistive-kink mode mainly results from the structural transition of the current sheet in the Hall-MHD simulations. At the nonlinear stage, the geometry of the current sheet turns into X-type from Y-type, resulting in the significant acceleration of the reconnection process. The fast reconnection induced by the Hall effect may explain the fast crash observed in large tokamaks. We also found that there exists a critical value of the ion inertial length that is resulted from the acceleration of magnetic reconnection due to the decoupling motions of ions and electrons and the suppression effect by the electron diamagnetic rotation in the Hall MHD. The critical value of decreases with increasing thermal conductivity When exceeds a critical value, the peak growth rate decreases with increasing

Effect of out-of-plane driving flow on formation of plasmoids in current sheet system

In the last two decades, a wide variety of plasmoids events have been observed, ranging from space and astrophysical phenomenon to magnetically confined laboratory plasmas, in which there are a lot of evidence of observational plasmoid-like features supported by direct large-scaled computer simulations. A super-Alfvénic instability, named plasmoid instability, occurs in an extended current sheet, when the Lundquist number exceeds a critical value. The large-aspect-ratio current sheet is fragmented by generating, growing, coalescing and ejecting of plasmoids so that this phenomenon has been proposed as a possible mechanism for fast reconnection scenario. This super-Alfvénic plasmoid instability has been usedin the significant new development of reconnection theory, and thus can provide alternative and more convincing mechanism for fast reconnection. In this work, a “driving” kind of shear flow in the out-of-plane direction is imposed on a two-dimensional, three-component magnetohydrodynamic model with a current sheet system to study the dynamic process of the plasmoids in a current sheet system. The effect of the width and strength of the driving flow on the reconnection rate of plasmoids are numerically analyzed in detail. It is found that the plasmoids are easily formed in the case of strong and wide out-of-plane driving flow. The reconnection rate and the number of the plasmoids increase with the driving flow width and/or driving flow strength increasing. In the presence of guiding field, it is found that the symmetry of the plasmoids is broken in the reconnection plane. In addition, for the fixed guiding field, the growth rate of plasmoids increases much faster when the strength of driving flow increases.

The Acceleration and Confinement of Energetic Electrons by a Termination Shock in a Magnetic Trap: An Explanation for Nonthermal Loop-top Sources during Solar Flares

Abstract Nonthermal loop-top sources in solar flares are the most prominent observational signatures that suggest energy release and particle acceleration in the solar corona. Although several scenarios for particle acceleration have been proposed, the origin of the loop-top sources remains unclear. Here we present a model that combines a large-scale magnetohydrodynamic simulation of a two-ribbon flare with a particle acceleration and transport model for investigating electron acceleration by a fast-mode termination shock (TS) at the loop top. Our model provides spatially resolved electron distribution that evolves in response to the dynamic flare geometry. We find a concave-downward magnetic structure located below the flare TS, induced by the fast reconnection downflows. It acts as a magnetic trap to confine the electrons at the loop top for an extended period of time. The electrons are energized significantly as they cross the shock front, and eventually build up a power-law energy spectrum extending to hundreds of kiloelectron volts. We suggest that this particle acceleration and transport scenario driven by a flare TS is a viable interpretation for the observed nonthermal loop-top sources.

3D Turbulent Reconnection: 20 Years After

A theory of 3D turbulent reconnection was published 20 years ago. At that time it was suggested that the fast reconnection requires X-point Petschek-type configuration of magnetic field in the reconnection region, as well as plasma effects that would stabilize this configuration. On the contrary, the turbulent reconnection theory (i) identified 3D Alfvenic turbulence as the cause of fast reconnection, (ii) postulated the ubiquitous nature of turbulent reconnection in astrophysical high-Reynolds number environments, and (iii) identified Y-point volume reconnection as the typical magnetic reconnection on length scales where the magnetohydrodynamic (MHD) approximation is valid. In this article we briefly summarize the status of the 3D turbulent reconnection theory. We show that other alternative models of reconnection have evolved toward supporting the essential features of the original turbulent reconnection theory. We also provide numerical and observational evidence supporting the turbulent reconnection and its consequences, e.g., the breakdown of classical magnetic flux freezing in highly conducting turbulent fluids.

Oscillatory regimes of compressible 2.5D viscous linear magnetic reconnection

We investigate the dynamics and energetics of magnetic reconnection in a general linear magnetohydrodynamic (MHD) model. Our investigation is threefold: First, we formulate a generalized 2.5D linearized MHD system in the presence of viscous, pressure, collisionless, and axial magnetic effects. Second, we find, in accordance with previous studies, that viscous effects, while reducing the rate of reconnection, boost the rate of total energy release. Moreover, viscous dissipation, as opposed to resistive dissipation, is unlikely to be impeded by pressure forces. Third, we compare two different equilibrium axial magnetic field profiles. One profile emulates a quasiseparatrix layer and the other profile emulates a 3D null point. In 2.5D, these profiles actually correspond to a hyperbolic field threaded by an axial field and a null line, respectively. We show evidence that fast reconnection is only attainable in the presence of a null.

Mechanism of Reconnection on Kinetic Scales Based on Magnetospheric Multiscale Mission Observations

Abstract We examine the role that ions and electrons play in reconnection using observations from the Magnetospheric Multiscale (MMS) mission on kinetic ion and electron scales, which are much shorter than magnetohydrodynamic scales. This study reports observations with unprecedented high resolution that MMS provides for magnetic field (7.8 ms) and plasma (30 ms for electrons and 150 ms for ions). We analyze and compare approaches to the magnetopause in 2016 November, to the electron diffusion region in the magnetotail in 2017 July followed by a current sheet crossing in 2018 July. Besides magnetic field reversals, changes in the direction of the flow velocity, and ion and electron heating, MMS observed large fluctuations in the electron flow speeds in the magnetotail. As expected from numerical simulations, we have verified that when the field lines and plasma become decoupled a large reconnecting electric field related to the Hall current (1–10 mV m−1) is responsible for fast reconnection in the ion diffusion region. Although inertial accelerating forces remain moderate (1–2 mV m−1), the electric fields resulting from the divergence of the full electron pressure tensor provide the main contribution to the generalized Ohm’s law at the neutral sheet (as large as 200 mV m−1). In our view, this illustrates that when ions decouple electron physics dominates. The results obtained on kinetic scales may be useful for better understanding the physical mechanisms governing reconnection processes in various magnetized laboratory and space plasmas.

Fast Magnetic Reconnection: Secondary Tearing Instability and Role of the Hall Term

Magnetic reconnection provides the primary source for explosive energy release, plasma heating and particle acceleration in many astrophysical environments. The last years witnessed a revival of interest in the MHD tearing instability as a driver for efficient reconnection. It has been established that, provided the current sheet aspect ratio becomes small enough ($a/L \sim S^{-1/3}$ for a given Lundquist number $S\gg 1$), reconnection occurs on ideal Alfv\'en timescales and becomes independent on $S$. Here we investigate, by means of two-dimensional simulations, the \emph{ideal} tearing instability in the Hall-MHD regime, which is appropriate when the width of the resistive layer $\delta$ becomes comparable to the ion inertial length $d_i$. Moreover, we study in detail the spontaneous development and reconnection of secondary current sheets, which for high $S$ naturally adjust to the ideal aspect ratio and hence their evolution proceeds very rapidly. For moderate low $S$, the aspect ratio tends to the Sweet-Parker scaling ($a/L \sim S^{-1/2}$), in order to fulfill the condition $\delta \ll a$ necessary for the onset of a tearing instability. When the Hall term is included, the reconnection rate of this secondary nonlinear phase is enhanced and, depending on the ratio $d_i/\delta$, can be twice with respect to the pure MHD case, and up to ten times larger than the linear phase. Therefore, the evolution of the tearing instability in thin current sheets in the Hall-MHD regime naturally leads to an explosive disruption of the reconnecting site and to energy release on super-Alfv\'enic timescales, as required to explain astrophysical observations.

First Detection of Plasmoids from Breakout Reconnection on the Sun

Transient collimated plasma ejections (jets) occur frequently throughout the solar corona, in active regions, quiet Sun, and coronal holes. Although magnetic reconnection is generally agreed to be the mechanism of energy release in jets, the factors that dictate the location and rate of reconnection remain unclear. Our previous studies demonstrated that the magnetic breakout model explains the triggering and evolution of most jets over a wide range of scales, through detailed comparisons between our numerical simulations and high-resolution observations. An alternative explanation, the resistive-kink model, invokes breakout reconnection without forming and explosively expelling a flux rope. Here we report direct observations of breakout reconnection and plasmoid formation during two jets in the fan-spine topology of an embedded bipole. For the first time, we observed the formation and evolution of multiple small plasmoids with bidirectional flows associated with fast reconnection in 3D breakout current sheets in the solar corona. The first narrow jet was launched by reconnection at the breakout current sheet originating at the deformed 3D null, without significant flare reconnection or a filament eruption. In contrast, the second jet and release of cool filament plasma were triggered by explosive breakout reconnection when the leading edge of the rising flux rope formed by flare reconnection beneath the filament encountered the preexisting breakout current sheet. These observations solidly support both reconnection-driven jet models: the resistive kink for the first jet, and the breakout model for the second explosive jet with a filament eruption.

Analysis of fast turbulent reconnection with self-consistent determination of turbulence timescale

We present results of a Reynolds-averaged turbulence model simulation on the problem of magnetic reconnection. In the model, in addition to the mean density, momentum, magnetic field, and energy equations, the evolution equations of the turbulent cross-helicity W, turbulent energy K, and its dissipation rate ε are simultaneously solved to calculate the rate of magnetic reconnection for a Harris-type current sheet. In contrast to previous studies based on algebraic modeling, the turbulence time scale is self-determined by the nonlinear evolutions of K and ε, their ratio being a time scale. We compare the reconnection rate produced by our mean-field model to the resistive nonturbulent MHD rate. To test whether different regimes of reconnection are produced, we vary the initial strength of turbulent energy and study the effect on the amount of magnetic flux reconnected in time. We obtain that the time scale of turbulence self-adjusts to always produce fast reconnection.

Fast Recursive Reconnection and the Hall Effect: Hall-MHD Simulations

Abstract Magnetohydrodynamic (MHD) theory and simulations have shown that reconnection is triggered via a fast “ideal” tearing instability in current sheets whose inverse aspect ratio decreases to , with S as the Lundquist number defined by the half-length L of the current sheet (of a thickness of 2a). Ideal tearing, in 2D sheets, triggers a hierarchical collapse via stretching of X-points and recursive instability. At each step, the local Lundquist number decreases, until the subsequent sheet thickness either approaches kinetic scales or the Lundquist number becomes sufficiently small. Here we carry out a series of Hall-MHD simulations to show how the Hall effect modifies recursive reconnection once the ion inertial scale is approached. We show that as the ion inertial length becomes of the order of the inner, singular layer thickness at some step of the recursive collapse, reconnection transits from the plasmoid-dominant regime to an intermediate plasmoid+Hall regime and then to the Hall-dominant regime. The structure around the X-point, the reconnection rate, the dissipation property, and the power spectra are also modified significantly by the Hall effect.