Antiparallel Reconnection Research Articles

Numerical analysis of three-dimensional magnetopause-like reconnection properties by Hall MHD simulation for SPERF-AREX

The ground-based device, the Space Plasma Environment Research Facility (SPERF), is established for experimentally simulating magnetosphere plasma processes, with one of its major components, asymmetric reconnection experiment (AREX), for three-dimensional physics relevant to dayside asymmetric magnetopause reconnection. As an outstanding property of fast magnetic reconnection in collisionless plasmas, the Hall effect and its geometric features can be experimentally investigated in SPERF-AREX with various magnetic configurations related to different driven scenarios for simulating interplanetary magnetic field (IMF) conditions. In this work, the Hall effect and its geometric characteristics in such proposed experiments are numerically studied based on a Hall MHD model. The simulation results reveal that in the X-line geometry relevant to southward IMFs, the Hall field features in cross section perpendicular to the X-line are mostly analogous to typical two-dimensional Hall quadrupole structures, clearly an “anti-parallel reconnection” feature. In the separator (A-B null-line) geometry relevant to arbitrary IMF orientations, along the separator between magnetic nulls, the magnetic field configuration near a magnetic null also demonstrates the typical quadrupolar pattern. However, the pattern is distorted away (>10di, here di=c/ωpi is the ion inertial length) from the nulls, in a way similar to that in “component reconnection.” Furthermore, the Hall effect induces a dawn-dusk asymmetry for both the X-line and the separator geometries.

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.

Phase-space Analysis of Ordered and Disordered Nonthermal Ion Energization during Magnetic Reconnection

Anomalous ion heating is frequently observed to accompany magnetic reconnection, yet there is little consensus on its origin. Instead of the usual velocity-space analysis, we use phase-space analysis to exhaustively explain how ions are nonthermally energized during collisionless, antiparallel magnetic reconnection. There are both ordered and disordered aspects in the process; the former is explained in terms of conservative quantities, and the latter is explained by demonstrating chaos through a direct calculation of Lyapunov exponents. The former induces “multibeam-like heating” in all three directions, whereas the latter induces stochastic bulk heating. Profiles of the ion temperature tensor components during reconnection can be easily understood by the phase-space distributions of ions in different motional stages.

The force balance of electrons during kinetic anti-parallel magnetic reconnection

Fully kinetic simulations are applied to the study of 2D anti-parallel reconnection, elucidating the dynamics by which the electron fluid maintains force balance within both the ion diffusion region (IDR) and the electron diffusion region (EDR). Inside the IDR, magnetic field-aligned electron pressure anisotropy (pe∥≫pe⊥) develops upstream of the EDR. Compared to previous investigations, the use of modern computer facilities allows for simulations at the natural proton to electron mass ratio mi/me=1836. In this high-mi/me-limit, the electron dynamics change qualitatively, as the electron inflow to the EDR is enhanced and mainly driven by the anisotropic pressure. Using a coordinate system with the x-direction aligned with the reconnecting magnetic field and the y-direction aligned with the central current layer, it is well known that for the much studied 2D laminar anti-parallel and symmetric scenario the reconnection electric field at the X-line must be balanced by the ∂pexy/∂x and ∂peyz/∂z off-diagonal electron pressure stress components. We find that the electron anisotropy upstream of the EDR imposes large values of ∂pexy/∂x within the EDR, and along the direction of the reconnection X-line, this stress cancels with the stress of a previously determined theoretical form for ∂peyz/∂z. The electron frozen-in law is instead broken by pressure tensor gradients related to the direct heating of the electrons by the reconnection electric field. The reconnection rate is free to adjust to the value imposed externally by the plasma dynamics at larger scales.

The Role of a Strong Out‐Of‐Plane Magnetic Field at the X‐Line in Antiparallel Magnetic Reconnection as a Guide Field

AbstractThe magnetic reconnection process substantially changes the configuration of the magnetic field in plasma and explosively releases magnetic energy. The asymptotic magnetic field lines during magnetic reconnection events can be strictly antiparallel or only have one antiparallel component, and the magnetic reconnection features are distinct under different asymptotic magnetic fields. Here, we provide the first observational evidence demonstrating the existence of a strong out‐of‐plane magnetic field at the X‐line in antiparallel reconnection. The analyses described herein show that this strong out‐of‐plane magnetic field significantly changes the structure of the diffusion region, thereby altering the reconnection features in terms of particle dynamics and energy dissipation by way of a guide field. A new pattern of the normal electric field near the X‐line is responsible for producing this out‐of‐plane magnetic field. Our findings suggest that, in addition to asymptotic conditions, local processes inside the diffusion region can also determine whether magnetic reconnection events develop as antiparallel reconnection or component reconnection.

Super-Fermi acceleration in multiscale MHD reconnection

We investigate the Fermi acceleration of charged particles in 2D MHD anti-parallel plasmoid reconnection, finding a drastic enhancement in energization rate ε ̇ over a standard Fermi model of ε ̇ ∼ ε. The shrinking particle orbit width around a magnetic island due to E → × B → drift produces a ε ̇ ∥ ∼ ε ∥ 1 + 1 / 2 χ power law with χ ∼ 0.75. The increase in the maximum possible energy gain of a particle within a plasmoid due to the enhanced efficiency increases with the plasmoid size and is by multiple factors of 10 in the case of solar flares and much more for larger plasmas. Including the effects of the non-constant E → × B → drift rates leads to further variation in power law indices from ≳ 2 to ≲ 1, decreasing with plasmoid size at the time of injection. The implications for energetic particle spectra are discussed alongside applications to 3D plasmoid reconnection and the effects of a guide field.

Ion and electron acoustic bursts during anti-parallel magnetic reconnection driven by lasers

Ion and electron acoustic bursts during anti-parallel magnetic reconnection driven by lasers

Pressure–strain interaction. III. Particle-in-cell simulations of magnetic reconnection

How energy is converted into thermal energy in weakly collisional and collisionless plasma processes, such as magnetic reconnection and plasma turbulence, has recently been the subject of intense scrutiny. The pressure–strain interaction has emerged as an important piece, as it describes the rate of conversion between bulk flow and thermal energy density. In two companion studies, we presented an alternate decomposition of the pressure–strain interaction to isolate the effects of converging/diverging flow and flow shear instead of compressible and incompressible flow, and we derived the pressure–strain interaction in magnetic field-aligned coordinates. Here, we use these results to study pressure–strain interaction during two-dimensional anti-parallel magnetic reconnection. We perform particle-in-cell simulations and plot the decompositions in both Cartesian and magnetic field-aligned coordinates. We identify the mechanisms contributing to positive and negative pressure–strain interaction during reconnection. This study provides a roadmap for interpreting numerical and observational data of the pressure–strain interaction, which should be important for studies of reconnection, turbulence, and collisionless shocks.

Kinetic simulations verifying reconnection rates measured in the laboratory, spanning the ion-coupled to near electron-only regimes

The rate of reconnection characterizes how quickly flux and mass can move into and out of the reconnection region. In the Terrestrial Reconnection EXperiment (TREX), the rate at which the antiparallel asymmetric reconnection occurs is modulated by the presence of a shock and a region of flux pileup in the high-density inflow. Simulations utilizing a generalized Harris-sheet geometry have tentatively shown agreement with TREX's measured reconnection rate scaling relative to system size, which is indicative of the transition from ion-coupled toward electron-only reconnection. Here, we present simulations tailored to reproduce the specific TREX geometry, which confirm both the reconnection rate scale and the shock jump conditions previously characterized experimentally in TREX. The simulations also establish an interplay between the reconnection layer and the Alfvénic expansions of the background plasma associated with the energization of the TREX drive coils; this interplay has not yet been experimentally observed.

First-principles theory of the rate of magnetic reconnection in magnetospheric and solar plasmas

The rate of magnetic reconnection is of the utmost importance in a variety of processes because it controls, for example, the rate energy is released in solar flares, the speed of the Dungey convection cycle in Earth’s magnetosphere, and the energy release rate in harmful geomagnetic substorms. It is known from numerical simulations and satellite observations that the rate is approximately 0.1 in normalized units, but despite years of effort, a full theoretical prediction has not been obtained. Here, we present a first-principles theory for the reconnection rate in non-relativistic electron-ion collisionless plasmas, and show that the same prediction explains why Sweet-Parker reconnection is considerably slower. The key consideration of this analysis is the pressure at the reconnection site (i.e., the x-line). We show that the Hall electromagnetic fields in antiparallel reconnection cause an energy void, equivalently a pressure depletion, at the x-line, so the reconnection exhaust opens out, enabling the fast rate of 0.1. If the energy can reach the x-line to replenish the pressure, the exhaust does not open out. In addition to heliospheric applications, these results are expected to impact reconnection studies in planetary magnetospheres, magnetically confined fusion devices, and astrophysical plasmas.

Influences of IMF By Polarity on Dayside Electron Precipitation in Terms of Energy Channels

AbstractThe By component of the interplanetary magnetic field (IMF) has long been suggested to play a crucial role in modulating precipitating electrons that create the dayside aurora. However, the effects of the By polarity on dayside precipitating electrons have never been examined in terms of energy channels. The present study statistically analyzes these effects using number fluxes of four energy channels. Our results show that the flux response of the dayside electron precipitation to the By polarity is not fixed in different energy channels. We infer that the response in the energy range of 688–1,000 eV is associated with interhemispheric field‐aligned currents, while the response in the energy range of 154–224 eV is likely related to the direct entry of magnetosheath electrons via antiparallel reconnection. In addition, local time distributions of precipitating electrons reveal two types of preexisting hemispheric asymmetry regardless of the By polarities. The first (second) type, which appears in the 06–12 magnetic local time (MLT) (12–18 MLT) sector, indicates higher (lower) flux in the Southern Hemisphere than in the Northern Hemisphere. All of the present results improve our understanding at the dayside aurora and the IMF By effects on it, which would not have been retrieved without the use of individual energy channels.

3D Reconnection Geometries With Magnetic Nulls: Multispacecraft Observations and Reconstructions

AbstractMagnetic reconnection is universal in various plasma environments like the solar atmosphere, planetary magnetosphere, and laboratory to trigger explosive phenomena. Its nature and magnetic structure in two dimensions (2D) have been abundantly analyzed. The three‐dimensional (3D) characteristics of reconnection are different from 2D, and the mysteries of its 3D geometry are under exploration. A critical character in 3D is the presence of magnetic nulls, which is essential when the magnetic field topology is changing. The four identical spacecraft from the Cluster mission enable several methods for analyzing and visualizing the 3D nature of the magnetic structure. In this paper, we focus on reviewing the magnetic topology of the reconnection geometry containing 3D magnetic null points. Applying the fitting‐reconstruction method to Cluster data, magnetic null pairs or multiple magnetic nulls linked by separators or helically wrapped spines are found in the reconnection diffusion region and turbulent outflow region. The reconstructed magnetic structures show that the 2D features are parts of local approximations of the 3D geometries. The 2D regimes of antiparallel and component reconnection can simultaneously occur in 3D separator reconnection. The flux ropes are formed along the spine line or in a region enclosed by wrapped fan surfaces in the presence of spiral null points.

Reconstruction of the Electron Diffusion Region With Inertia and Compressibility Effects.

A method based on electron magnetohydrodynamics (EMHD) for the reconstruction of steady, two‐dimensional plasma and magnetic field structures from data taken by a single spacecraft, first developed by Sonnerup et al. (2016), https://doi.org/10.1002/2016ja022430, is extended to accommodate inhomogeneity of the electron density and temperature, electron inertia effects, and guide magnetic field in and around the electron diffusion region (EDR), the central part of the magnetic reconnection region. The new method assumes that the electron density and temperature are constant along, but may vary across, the magnetic field lines. We present two models for the reconstruction of electron streamlines, one of which is not constrained by any specific formula for the electron pressure tensor term in the generalized Ohm's law that is responsible for electron unmagnetization in the EDR, and the other is a modification of the original model to include the inertia and compressibility effects. Benchmark tests using data from fully kinetic simulations show that our new method is applicable to both antiparallel and guide‐field (component) reconnection, and the electron velocity field can be better reconstructed by including the inertia effects. The new EMHD reconstruction technique has been applied to an EDR of magnetotail reconnection encountered by the Magnetospheric Multiscale spacecraft on 11 July 2017, reported by Torbert et al. (2018), https://doi.org/10.1126/science.aat2998 and reconstructed with the original inertia‐less version by Hasegawa et al. (2019), https://doi.org/10.1029/2018ja026051, which demonstrates that the new method better performs in recovering the electric field and electron streamlines than the original version.

Observations of an Electron‐Cold Ion Component Reconnection at the Edge of an Ion‐Scale Antiparallel Reconnection at the Dayside Magnetopause

AbstractSolar wind parameters play a dominant role in reconnection rate, which controls the solar wind‐magnetosphere coupling efficiency at Earth's magnetopause. Besides, low‐energy ions from the ionosphere, frequently detected on the magnetospheric side of the magnetopause, also affect magnetic reconnection. However, the specific role of low‐energy ions in reconnection is still an open question under active discussion. In the present work, we report in situ observations of a multiscale, multi‐type magnetopause reconnection in the presence of low‐energy ions using NASA's Magnetospheric Multiscale data on September 11, 2015. This study divides ions into cold (10–500 eV) and hot (500–30,000 eV) populations. The observations can be interpreted as a secondary reconnection dominated by electrons and cold ions (mainly in plane) located at the edge of an ion‐scale reconnection (mainly in plane). This analysis demonstrates a dominant role of cold ions in the secondary reconnection without hot ions' response. Cold ions and electrons are accelerated and heated by the secondary process. The case study provides observational evidence for the simultaneous operation of antiparallel and component reconnection. Our results imply that the pre‐accelerated and heated cold ions and electrons in the secondary reconnection may participate in the primary ion‐scale reconnection affecting the solar wind‐magnetopause coupling and the complicated magnetic field topology could affect the reconnection rate.

On the Presence and Thermalization of Cold Ions in the Exhaust of Antiparallel Symmetric Reconnection

Using fully kinetic 2.5 dimensional particle-in-cell simulations of anti-parallel symmetric magnetic reconnection, we investigate how initially cold ions are captured by the reconnection process, and how they evolve and behave in the exhaust. We find that initially cold ions can remain cold deep inside the exhaust. Cold ions that enter the exhaust downstream of active separatrices, closer to the dipolarization front, appear as cold counter-streaming beams behind the front. In the off-equatorial region, these cold ions generate ion-acoustic waves that aid in the thermalization both of the incoming and outgoing populations. Closest to the front, due to the stronger magnetization, the ions can remain relatively cold during the neutral plane crossing. In the intermediate exhaust, the weaker magnetization leads to enhanced pitch angle scattering and reflection. Cold ions that enter the exhaust closer to the X line, at active separatrices, evolve into a thermalized exhaust. Here, the cold populations are heated through a combination of thermalization at the separatrices and pitch angle scattering in the curved magnetic field around the neutral plane. Depending on where the ions enter the exhaust, and how long time they have spent there, they are accelerated to different energies. The superposition of separately thermalized ion populations that have been accelerated to different energies form the hot exhaust population.

The influence of collision angle for viscous vortex reconnection

We revisit the mechanism of viscous vortex reconnection by considering the collision of vortex rings over a range of initial collision angles and Reynolds numbers. While the overall reconnection process is similar to anti-parallel vortex reconnection, we find that collision angle exerts significant influence over the process, altering the evolution of various global and local quantities. The collision angle primarily manipulates the “pyramid” process, a recently identified stretching mechanism proposed by Moffatt and Kimura [“Towards a finite-time singularity of the Navier-Stokes equations Part 1. Derivation and analysis of dynamical system,” J. Fluid Mech., 861, 930–967 (2019)] to be a potential pathway for finite-time singularity of Euler’s equations, during the approach stage of the rings. However, the “pyramid” process is short-lived for viscous vortices. The present work shows that the “pyramid” process is arrested by parallelization of the colliding vortices, wherein contact of the colliding vortices halts their motion toward each other at the pyramid apex, allowing the rest of the vortex tube to “catch up,” breaking the pyramid structure. Parallelization marks the transition to a second phase of stretching, where the colliding vortices remain parallel. Vorticity amplification from pyramid stretching is significantly stronger than for its parallel counterpart, and is thus the dominant factor determining reconnection properties. Based upon the findings in this study, we conjecture that the parallelization process is the primary mechanism that prevents the finite-time singularity through the pyramid process. Critically, the Reynolds number scaling for the reconnection rate differs depending on the collision angle, which challenges the conjecture of universal Reynolds number scaling in the literature.

Scaling theory of three-dimensional magnetic reconnection spreading

We develop a first-principles scaling theory of the spreading of three-dimensional (3D) magnetic reconnection of finite extent in the out of plane direction. This theory addresses systems with or without an out of plane (guide) magnetic field, and with or without Hall physics. The theory reproduces known spreading speeds and directions with and without guide fields, unifying previous knowledge in a single theory. New results include the following: (1) reconnection spreads in a particular direction if an x-line is induced at the interface between reconnecting and non-reconnecting regions, which is controlled by the out of plane gradient of the electric field in the outflow direction. (2) The spreading mechanism for anti-parallel collisionless reconnection is convection, as is known, but for guide field reconnection it is magnetic field bending. We confirm the theory using 3D two-fluid and resistive-magnetohydrodynamics simulations. (3) The theory explains why anti-parallel reconnection in resistive-magnetohydrodynamics does not spread. (4) The simulation domain aspect ratio, associated with the free magnetic energy, influences whether reconnection spreads or convects with a fixed x-line length. (5) We perform a simulation initiating anti-parallel collisionless reconnection with a pressure pulse instead of a magnetic perturbation, finding spreading is unchanged rather than spreading at the magnetosonic speed as previously suggested. The results provide a theoretical framework for understanding spreading beyond systems studied here and are important for applications including two-ribbon solar flares and reconnection in Earth's magnetosphere.

Plasma heating and current sheet structure in anti-parallel magnetic reconnection

A theoretical model and an analytic theory of current sheet structure are presented for understanding anti-parallel driven magnetic reconnection in 2-1/2 dimension in collisionless plasmas. The theoretical model provides formulation to compute the current sheet y-profiles by specifying the profiles of electron and ion flow velocities Vex(x,y) and Vix(x,y). The current sheet solutions depend on the plasma density nin, merging magnetic field B0, ion velocity vi, and electron velocity ve in the upstream and the Sevz=Vez/Vdz parameter where Vez is the electron velocity accelerated by the reconnection electric field Ez in the electron orbit meandering region, Vdz≃cEy/Bx is the E→×B→ drift velocity as electrons enter the orbit meandering region, Bx is the merging magnetic field, and Ey is the electrostatic electric field. With simplifying assumptions on the y-profiles of Vex and Vix, we have also developed an analytic theory of the current sheet structure. Analytic expressions for the anomalous resistivity, the electrostatic potential drop, and the maximum Ey amplitude Emax are obtained. The analytic results agree reasonably well with both the particle-in-cell simulation results and the numerical solutions of the theoretical model. The ions energy gain due to the potential drop is ∝B02/nin. The electron energy gain is ∝(B02/8πnin)Sevz. The B02/nin scaling of the average ion and electron energy gains are consistent with laboratory experiments and space plasma observations.

Zero helicity of Seifert framed defects

Line defects are one-dimensional phase singularities (forming knots and links) that arise in a variety of physical systems. In these systems, isophase surfaces (Seifert surfaces) have the phase defects as boundary, and these Seifert surfaces define a framing of the normal bundle of each link component. We define the individual helicity for each component of a link singularity, and prove that each individual helicity is zero if and only if there exists a Seifert framing for the link. We extend these results to multi-armed defects. We prove that under anti-parallel reconnection of defect strands total twist is conserved.

Direct evidence for magnetic reconnection at the boundaries of magnetic switchbacks with Parker Solar Probe

Context.The first encounters of Parker Solar Probe (PSP) with the Sun revealed the presence of ubiquitous localised magnetic deflections in the inner heliosphere; these structures, often called switchbacks, are particularly striking in solar wind streams originating from coronal holes.Aims.We report the direct piece of evidence for magnetic reconnection occurring at the boundaries of three switchbacks crossed by PSP at a distance of 45 to 48 solar radii to the Sun during its first encounter.Methods.We analyse the magnetic field and plasma parameters from the FIELDS and Solar Wind Electrons Alphas and Protons instruments.Results.The three structures analysed all show typical signatures of magnetic reconnection. The ion velocity and magnetic field are first correlated and then anti-correlated at the inbound and outbound edges of the bifurcated current sheets with a central ion flow jet. Most of the reconnection events have a strong guide field and moderate magnetic shear, but one current sheet shows indications of quasi anti-parallel reconnection in conjunction with a magnetic field magnitude decrease by 90%.Conclusions.Given the wealth of intense current sheets observed by PSP, reconnection at switchback boundaries appears to be rare. However, as the switchback boundaries accomodate currents, one can conjecture that the geometry of these boundaries offers favourable conditions for magnetic reconnection to occur. Such a mechanism would thus contribute in reconfiguring the magnetic field of the switchbacks, affecting the dynamics of the solar wind and eventually contributing to the blending of the structures with the regular wind as they propagate away from the Sun.