Collisionless Magnetic Reconnection Research Articles

Kinetic Verification of the Stochastic Ion Heating Mechanism in Collisionless Magnetic Reconnection

Abstract The origin of anomalous, non-classical ion heating during magnetic reconnection has been a longstanding problem. It is verified via fully kinetic analyses and particle-in-cell simulations that stochastic heating is the main ion heating mechanism in collisionless magnetic reconnection up to moderate guide fields. Strong in-plane Hall electric fields that form during reconnection render ion motions chaotic and de facto broaden the ion distribution function. The mechanism is consistent with numerous observed features of ion heating in reconnection, such as the preferential heating of ions with higher mass-to-charge ratios and the non-conservation of the ion magnetic moment.

Collisionless Magnetic Reconnection and Waves: Progress Review

Magnetic reconnection is a fundamental process whereby microscopic plasma processes cause macroscopic changes in magnetic field topology, leading to explosive energy release. Waves and turbulence g ...

Global Ten‐Moment Multifluid Simulations of the Solar Wind Interaction with Mercury: From the Planetary Conducting Core to the Dynamic Magnetosphere

AbstractFor the first time, we explore the tightly coupled interior‐magnetosphere system of Mercury by employing a three‐dimensional ten‐moment multifluid model. This novel fluid model incorporates the nonideal effects including the Hall effect, electron inertia, and tensorial pressures that are critical for collisionless magnetic reconnection; therefore, it is particularly well suited for investigating collisionless magnetic reconnection in Mercury's magnetotail and at the planet's magnetopause. The model is able to reproduce the observed magnetic field vectors, field‐aligned currents, and cross‐tail current sheet asymmetry (beyond magnetohydrodynamic approach), and the simulation results are in good agreement with spacecraft observations. We also study the magnetospheric response of Mercury to a hypothetical extreme event with an enhanced solar wind dynamic pressure, which demonstrates the significance of induction effects resulting from the electromagnetically coupled interior. More interestingly, plasmoids (or flux ropes) are formed in Mercury's magnetotail during the event, indicating the highly dynamic nature of Mercury's magnetosphere.

On the Energy Conversion Rate during Collisionless Magnetic Reconnection

Abstract Magnetic reconnection efficiently converts magnetic energy into kinetic and thermal energy of plasmas. The electric field at the X-line, which represents the reconnection rate, is commonly used to measure how fast the reconnection proceeds. However, the energy conversion rate (ECR) has rarely been investigated. Using a 2.5D particle-in-cell simulation, we have examined the temporal evolution of the ECR in collisionless reconnection. It is found that the ECR reaches peak significantly later than the reconnection rate does. This is because the energy conversion primarily occurs at the reconnection fronts rather than at the X-line. With the increase of the inflow density, both the reconnection rate and the conversion rate decrease. The presence of a guide field leads to the reduction of both the reconnection rate and the conversion rate, though reconnection remains fast. We further find that ECR does not depend on the mass ratio but is sensitive to the length of the simulation domain.

Dynamic patterns of self-organization inflow in collisionless magnetic reconnection

We investigate the evolution of reconnection inflow using a fully kinetic approach. Three types of inflow are detailed, namely the collapse inflow, the vortex inflow and the reverse inflow. They are formed dynamically at different stages of reconnection via self-organizing processes, but are closely interrelated with each other. The reconnection starts from a small perturbation, which can trigger off a chain of pressure-induced collapses propagating into the inflow region. The pressure gradient results in the collapse inflow toward the reconnection site. Then due to the continuous injection of hot plasma carried by the reconnection outflows, the expanding exhaust causes its adjacent region to be compressed. The combined effects of the compression and the reflection of conducting walls lead to the formation of the vortex inflow. Subsequently, the reverse inflow develops gradually within the exhaust. Under the modulation of these inflows, the reconnection rate shows a transient oscillation. We also discussed the possible occurrence of the self-organization inflow available in different contexts.

Turbulence and Particle Acceleration in Collisionless Magnetic Reconnection: Effects of Temperature Inhomogeneity across Pre-reconnection Current Sheet

Abstract Magnetic reconnection is an important process in various collisionless plasma environments because it reconfigures the magnetic field and releases magnetic energy to accelerate charged particles. Its dynamics depend critically on the properties of the pre-reconnection current sheet. One property in particular, cross-sheet temperature inhomogeneity, which is ubiquitous throughout the heliosphere, has been shown to increase reconnection outflow speed, energy conversion efficiency, and secondary island formation rate using two-dimensional particle-in-cell simulations. Here we expand upon these findings, considering two cases with a long, thin current sheet, one with homogeneous temperature and one with inhomogeneous temperature across the current sheet. In the inhomogeneous temperature case, numerous secondary islands form continuously, which increases current sheet turbulence (well-developed cascade power spectra) at large wavenumbers. Current density, energy conversion, dissipation, and acceleration of high-energy particles are also enhanced relative to the homogenous temperature case. Our results suggest that inhomogeneous temperature profiles, which are realistic, need to be incorporated into studies of turbulence and particle acceleration in collisionless magnetic reconnection.

Formation of electron depletion layer and parallel electric field in the separatrix region of anti-parallel magnetic reconnection**Project supported by the National Natural Science Foundation of China (Grant Nos. 41774169, 41527804, and 41804159), the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant No. QYZDJ-SSW-DQC010), and the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No.

It is generally accepted that during collisionless magnetic reconnection, electrons flow toward the X line in the separatrix region, and then an electron depletion layer is formed. In this paper, with two-dimensional (2D) particle-in-cell (PIC) simulation, we investigate the characteristics of the separatrix region during magnetic reconnection. In addition to the electron depletion layer, we find that there still exists an electric field parallel to the magnetic field in the separatrix region. Because a reduced ion-to-electron mass ratio and light speed are usually used in PIC simulation models, we also change these parameters to analyze the characteristics of the separatrix region. It is found that the increase in the ion-to-electron mass ratio makes the electron depletion layer and the parallel electric field more obvious, while the influence of light speed is less pronounced.

A Parametric Study of the Structure of Hall Magnetic Field Based on Kinetic Simulations. I. Anti-parallel Magnetic Reconnection in an Asymmetric Current Sheet

Abstract The generation of the Hall magnetic field is considered one of the most important characteristics in collisionless magnetic reconnection. Here, in this paper, with two-dimensional particle-in-cell simulations, we study the structure of the Hall magnetic field generated during anti-parallel magnetic reconnection in an asymmetric current sheet. A quadrupolar structure of the Hall magnetic field is first formed, and then it evolves into a hexapolar structure with the proceeding magnetic reconnection. In the quadrupolar structure of the Hall magnetic field, the quadrants on the side of the current sheet with the weaker magnetic field (the dominant Hall magnetic field) are much stronger than those on the other side, and they can cross the center of the current sheet. With the increase of the ratio of the magnetic fields or the decrease of the density ratio (here, the ratio is defined as the values between the side with the stronger magnetic field to that with the weaker magnetic field), the tendency will become more salient. However, with a decrease of the temperature ratio, the tendency reverses. With the proceeding reconnection, two ribbons with an enhanced Hall magnetic field are generated in the region below the dominant Hall magnetic field, and then a hexapolar structure of the Hall magnetic field is formed, which will become stronger with an increase of the ratio of the magnetic field or decrease of the density, while the effect of the temperature asymmetry is much weaker than that of the magnetic field and density asymmetry. The generation of the Hall magnetic field can be explained by the in-plane current carried mainly by the electrons.

Energy transfer and electron energization in collisionless magnetic reconnection for different guide-field intensities

Electron dynamics and energization are one of the key components of magnetic field dissipation in collisionless reconnection. In 2D numerical simulations of magnetic reconnection, the main mechanism that limits the current density and provides an effective dissipation is most probably the electron pressure tensor term, which has been shown to break the frozen-in condition at the x-point. In addition, the electron-meandering-orbit scale controls the width of the electron dissipation region, where the electron temperature has been observed to increase both in recent Magnetospheric Multiple-Scale (MMS) observations and in laboratory experiments, such as the Magnetic Reconnection Experiment (MRX). By means of two-dimensional full-particle simulations in an open system, we investigate how the energy conversion and particle energization depend on the guide field intensity. We study the energy transfer from the magnetic field to the plasma in the vicinity of the x-point and close downstream regions, and E·J and the threshold guide field separating two regimes where either the parallel component, E||J||, or the perpendicular component, E⊥·J⊥, dominate the energy transfer, confirming recent MRX results and also consistent with MMS observations. We calculate the energy partition between fields and kinetic and thermal energies of different species, from electron to ion scales, showing that there is no significant variation for different guide field configurations. Finally, we study possible mechanisms for electron perpendicular heating by examining electron distribution functions and self-consistently evolved particle orbits in high guide field configurations.

Fast Ion Heating in Transient Collisionless Magnetic Reconnection via an Intrinsic Stochastic Mechanism

Abstract Stochastic heating has been known to be a powerful ion heating mechanism in the solar wind, atmosphere, and flares. In this Letter, we show that stochastic ion heating is inherent to transient collisionless magnetic reconnection. The explanation exploits the connected nature of electron canonical vorticity to show analytically that the in-plane electric and magnetic fields in a typical reconnection geometry satisfy the condition for stochastic heating of ions. Electron fluid simulations, test ion simulations, and comparisons to experiments all support the existence of this mechanism.

Energy Partition between Ion and Electron of Collisionless Magnetic Reconnection

Abstract The plasma heating during collisionless magnetic reconnection is investigated using particle-in-cell simulations. We analyze the time evolution of the plasma temperature associated with the motion of the reconnecting flux tube, where the plasma temperature is defined as the second-order moment of the velocity distribution function in the simulation frame/in the center of the flux tube frame, and we show that the plasma heating during magnetic reconnection can be separated into two distinct stages: the nonadiabatic heating stage, in which the magnetic field lines are just reconnecting in the X-type diffusion region, and the adiabatic heating stage, in which the flux tube is shrinking after two flux tubes merge. During the adiabatic heating stage, the plasma temperature T can be approximated by TV γ−1 = const., where γ = 5/3 is the specific heat, and V is the volume of the flux tube. In the nonadiabatic heating stage, we found numerically that the ratio of the increment of the ion temperature to that of the electron temperature can be approximated by , where m i and m e are the ion and electron masses, respectively. We also present a theoretical model based on a magnetic-diffusion-dominated reconnection to explain the simulation result.

Multi-Scale Kinetic Simulation of Magnetic Reconnection With Dynamically Adaptive Meshes

Magnetic reconnection is essentially a multi-scale phenomenon, driven by kinetic process in microscopic region and enabling explosive energy conversion from magnetic field energy to plasma kinetic energy in large area. It has been poorly understood how the kinetic process around the x-line connects to the magnetohydrodynamics (MHD) scale process in the reconnection downstream region. The present study has investigated the energy conversion process in the region far downstream of the x-line, by means of the particle-in-cell (PIC) simulation with the adaptive mesh refinement (AMR). The AMR-PIC model enables efficient kinetic simulation of multi-scale phenomena using dynamically adaptive meshes. It is found that the ion energy gain dominates in the reconnection region and is carried out mainly in the exhaust center rather than the exhaust boundaries. The simulation results suggest that the energy conversion process in collisionless magnetic reconnection is significantly different from that in the MHD reconnection model in which most energy conversion occurs at slow mode shocks formed at the exhaust boundaries.

The reduction of magnetic reconnection outflow jets to sub-Alfvénic speeds

The outflow velocity of jets produced by collisionless magnetic reconnection is shown to be reduced by the ion exhaust temperature in fully kinetic particle in cell simulations and in situ satellite observations. We derive a scaling relationship for the outflow velocity based on the upstream Alfvén speed and the parallel ion exhaust temperature, which is verified in kinetic simulations and observations. The outflow speed reduction is shown to be due to the firehose instability criterion, and so, for large enough guide fields, this effect is suppressed and the outflow speed reaches the upstream Alfvén speed based on the reconnecting component of the magnetic field.

Particle acceleration in relativistic magnetic reconnection with strong inverse-Compton cooling in pair plasmas

ABSTRACT Particle-in-cell (PIC) simulations have shown that relativistic collisionless magnetic reconnection drives non-thermal particle acceleration (NTPA), potentially explaining high-energy (X-ray/γ-ray) synchrotron and/or inverse Compton (IC) radiation observed from various astrophysical sources. The radiation back-reaction force on radiating particles has been neglected in most of these simulations, even though radiative cooling considerably alters particle dynamics in many astrophysical environments where reconnection may be important. We present a radiative PIC study examining the effects of external IC cooling on the basic dynamics, NTPA, and radiative signatures of relativistic reconnection in pair plasmas. We find that, while the reconnection rate and overall dynamics are basically unchanged, IC cooling significantly influences NTPA: the particle spectra still show a hard power law (index ≥ −2) as in non-radiative reconnection, but transition to a steeper power law that extends to a cooling-dependent cut-off. The steep power-law index fluctuates in time between roughly −3 and −5. The time-integrated photon spectra display corresponding power laws with indices ≈−0.5 and ≈−1.1, similar to those observed in hard X-ray spectra of accreting black holes.

Effective Resistivity in Collisionless Magnetic Reconnection

An effective resistivity relevant to collisionless magnetic reconnection (MR) in plasma is presented. It is based on the argument that pitch angle scattering of electrons in the small electron diffusion region around the X line can lead to an effective, resistivity in collisionless plasma. The effective resistivity so obtained is in the form of a power law of the local plasma and magnetic field parameters. Its validity is confirmed by direct collisionless particle-in-cell (PIC) simulation. The result agrees very well with the resistivity (obtained from available data) of a large number of environments susceptible to MR: from the intergalactic and interstellar to solar and terrestrial to laboratory fusion plasmas. The scaling law can readily be incorporated into existing collisional magnetohydrodynamic simulation codes to investigate collisionless MR, as well as serve as a guide to ab initio theoretical investigations of the collisionless MR process.

Temperature gradient driven heat flux closure in fluid simulations of collisionless reconnection

Recent efforts to include kinetic effects in fluid simulations of plasmas have been very promising. Concerning collisionless magnetic reconnection, it has been found before that damping of the pressure tensor to isotropy leads to good agreement with kinetic runs in certain scenarios. An accurate representation of kinetic effects in reconnection was achieved in a study by Wang et al. (Phys. Plasmas, vol. 22, 2015, 012108) with a closure derived from earlier work by Hammett and Perkins (Phys. Rev. Lett., vol. 64, 1990, 3019). Here, their approach is analysed on the basis of heat flux data from a Vlasov simulation. As a result, we propose a new local closure in which heat flux is driven by temperature gradients. This way, a more realistic approximation of Landau damping in the collisionless regime is achieved. Previous issues are addressed and the agreement with kinetic simulations in different reconnection set-ups is improved significantly. To the authors’ knowledge, the new fluid model is the first to perform well in simulations of the coalescence of large magnetic islands.

Kinetic simulations of relativistic magnetic reconnection with synchrotron and inverse Compton cooling

First results are presented from kinetic numerical simulations of relativistic collisionless magnetic reconnection in a pair plasma that include radiation reaction from both synchrotron and inverse Compton (IC) processes, motivated by non-thermal high-energy astrophysical sources, including in particular blazars. These simulations are initiated from a configuration known as ‘ABC fields’ that evolves due to coalescence instability and generates thin current layers in its linear phase. Global radiative efficiencies, instability growth rates, time-dependent radiation spectra, lightcurves, variability statistics and the structure of current layers are investigated for a broad range of initial parameters. We find that the IC radiative signatures are generally similar to the synchrotron signatures. The luminosity ratio of IC to synchrotron spectral components, the Compton dominance, can be modified by more than one order of magnitude with respect to its nominal value. For very short cooling lengths, we find evidence for modification of the temperature profile across the current layers, no systematic compression of plasma density and very consistent profiles of the scalar product$\boldsymbol{E}\boldsymbol{\cdot }\boldsymbol{B}$of electric field$\boldsymbol{E}$and magnetic field$\boldsymbol{B}$. We decompose the profiles of$\boldsymbol{E}\boldsymbol{\cdot }\boldsymbol{B}$with the use of the Vlasov momentum equation, demonstrating a contribution from radiation reaction at the thickness scale consistent with the temperature profile.

Electron Physics in 3‐D Two‐Fluid 10‐Moment Modeling of Ganymede's Magnetosphere

AbstractWe studied the role of electron physics in 3‐D two‐fluid 10‐moment simulation of Ganymede's magnetosphere. The model captures nonideal physics like the Hall effect, electron inertia, and anisotropic, nongyrotropic pressure effects. A series of analyses were carried out: (1) The resulting magnetic field topology and electron and ion convection patterns were investigated. The magnetic fields were shown to agree reasonably well with in situ measurements by the Galileo satellite. (2) The physics of collisionless magnetic reconnection were carefully examined in terms of the current sheet formation and decomposition of generalized Ohm's law. The importance of pressure anisotropy and nongyrotropy in supporting the reconnection electric field is confirmed. (3) We compared surface “brightness” morphology, represented by surface electron and ion pressure contours, with oxygen emission observed by the Hubble Space Telescope. The correlation between the observed emission morphology and spatial variability in electron/ion pressure was demonstrated. Potential extension to multi‐ion species in the context of Ganymede and other magnetospheric systems is also discussed.

An intuitive two-fluid picture of spontaneous 2D collisionless magnetic reconnection and whistler wave generation

An intuitive and physical two-fluid picture of spontaneous 2D collisionless magnetic reconnection and whistler wave generation is presented in the framework of 3D electron-magnetohydrodynamics. In this regime, canonical circulation (Q=me∇×u+qeB) flux tubes can be defined in analogy to magnetic flux tubes in ideal magnetohydrodynamics. Following the 3D behavior of these Q flux tubes provides a new perspective on collisionless reconnection—a perspective that has been hard to perceive via examinations of 2D projections. This shows that even in a 2D geometry with an ignorable coordinate, a 3D examination is essential for a full comprehension of the process. Intuitive answers are given to three main questions in collisionless reconnection: why is reconnection spontaneous, why do particles accelerate extremely fast, and why are whistler waves generated? Possible extensions to other regimes are discussed.

Collisionless magnetic reconnection in curved spacetime and the effect of black hole rotation

Magnetic reconnection in curved spacetime is studied by adopting a general relativistic magnetohydrodynamic model that retains collisionless effects for both electron-ion and pair plasmas. A simple generalization of the standard Sweet-Parker model allows us to obtain the first order effects of the gravitational field of a rotating black hole. It is shown that the black hole rotation acts as to increase the length of azimuthal reconnection layers, per se leading to a decrease of the reconnection rate. However, when coupled to collisionless thermal-inertial effects, the net reconnection rate is enhanced with respect to what would happen in a purely collisional plasma due to a broadening of the reconnection layer. These findings identify an underlying interaction between gravity and collisionless magnetic reconnection in the vicinity of compact objects.