Asymmetric Reconnection Research Articles

Impact of the Out‐Of‐Plane Flow Shear on Magnetic Reconnection at the Flanks of Earth's Magnetopause

AbstractMagnetic reconnection changes the magnetic field topology and facilitates the energy and particle exchange at magnetospheric boundaries such as the Earth's magnetopause. The flow shear perpendicular to the reconnecting plane prevails at the flank magnetopause under southward interplanetary magnetic field conditions. However, the effect of the out‐of‐plane flow shear on asymmetric reconnection is an open question. In this study, we utilize kinetic simulations to investigate the impact of the out‐of‐plane flow shear on asymmetric reconnection. By systematically varying the flow shear strength, we analyze the flow shear effects on the reconnection rate, the diffusion region structure, and the energy conversion rate. We find that the reconnection rate increases with the upstream out‐of‐plane flow shear, and for the same upstream conditions, it is higher at the dusk side than at the dawn side. The diffusion region is squeezed in the outflow direction due to magnetic pressure which is proportional to the square of the Alfvén Mach number of the shear flow. The out‐of‐plane flow shear increases the energy conversion rate , and for the same upstream conditions, the magnitude of is larger at the dusk side than at the dawn side. This study reveals that out‐of‐plane flow shear not only enhances the reconnection rate but also significantly boosts energy conversion, with more pronounced effects on the dusk‐side flank than on the dawn‐side flank. These insights pave the way for better understanding the solar wind‐magnetosphere interactions.

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.

Density and Magnetic Field Asymmetric Kelvin‐Helmholtz Instability

AbstractThe Kelvin‐Helmholtz (KH) instability can transport mass, momentum, magnetic flux, and energy between the magnetosheath and magnetosphere, which plays an important role in the solar‐wind‐magnetosphere coupling process for different planets. Meanwhile, strong density and magnetic field asymmetry are often present between the magnetosheath (MSH) and magnetosphere (MSP), which could affect the transport processes driven by the KH instability. Our magnetohydrodynamics simulation shows that the KH growth rate is insensitive to the density ratio between the MSP and the MSH in the compressible regime, which is different than the prediction from linear incompressible theory. When the interplanetary magnetic field (IMF) is parallel to the planet's magnetic field, the nonlinear KH instability can drive a double mid‐latitude reconnection (DMLR) process. The total double reconnected flux depends on the KH wavelength and the strength of the lower magnetic field. When the IMF is anti‐parallel to the planet's magnetic field, the nonlinear interaction between magnetic reconnection and the KH instability leads to fast reconnection (i.e., close to Petschek reconnection even without including kinetic physics). However, the peak value of the reconnection rate still follows the asymmetric reconnection scaling laws. We also demonstrate that the DMLR process driven by the KH instability mixes the plasma from different regions and consequently generates different types of velocity distribution functions. We show that the counter‐streaming beams can be simply generated via the change of the flux tube connection and do not require parallel electric fields.

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.

Kinetic and Shear Alfvén Waves in a Large Guide-field Reconnection at Earth's Magnetopause

A large guide-field (∼1.1B o) reconnection X-line, observed by the Magnetospheric Multiscale spacecraft during an outbound magnetopause crossing, is studied for Alfvén waves. Here B o is the reconnecting field magnitude. The current sheet thickness of the magnetopause was ∼2.6 ion inertial lengths (∼269 km), where field-aligned counter-streaming electrons were observed and Hall electromagnetic fields were identified. A remarkable finding was that a kinetic Alfvén wave (KAW) was seen in the magnetopause upstream region after a shear Alfvén wave (SAW) was encountered in the magnetopause layer. The presence of both the SAW and KAW near the reconnection X-line is for the first time reported. In the spacecraft frame of reference, the SAW has a dominant frequency at ∼0.74 Hz, while the KAW has two dominant frequencies at ∼0.38 and ∼0.64 Hz. The wave energy for KAW and SAW was mostly carried away from the reconnection site by the Poynting flux parallel to the magnetic field. The parallel temperatures for ions and electrons were increased at KAW. The peaks of T ∥/T ⊥ for ions were located near the wave peaks, while the ratio peaks for electrons were near the wave troughs. Our findings suggest that KAWS and SAWs can be generated by asymmetric reconnection with a large guide field.

Multiscale Magnetosphere‐Ionosphere Coupling During Stormtime: A Case Study of the Dawnside Current Wedge

AbstractA characteristic feature of the main phase of geomagnetic storms is the dawn‐dusk asymmetric depression of low‐ and mid‐latitude ground magnetic fields, with largest depression in the dusk sector. Recent work has shown, using data taken from hundreds of storms, that this dawn‐dusk asymmetry is strongly correlated with enhancements of the dawnside westward electrojet and this has been interpreted as a “dawnside current wedge” (DCW). Its ubiquity suggests it is an important aspect of stormtime magnetosphere‐ionosphere (MI) coupling. In this work we simulate a moderate geomagnetic storm to investigate the mechanisms that give rise to the formation of the DCW. Using synthetic SuperMAG indices we show that the model reproduces the observed phenomenology of the DCW, namely the correlation between asymmetry in the low‐latitude ground perturbation and the dawnside high‐latitude ground perturbation. We further show that these periods are characterized by the penetration of mesoscale bursty bulk flows (BBFs) into the dawnside inner magnetosphere. In the context of this event we find that the development of the asymmetric ring current, which inflates the dusk‐side magnetotail, leads to asymmetric reconnection and dawnward‐biased flow bursts. This results in an eastward expansion and multiscale enhancement of the dawnside electrojet. The electrojet enhancement extends across the dawn quadrant with localized enhancements associated with the wedgelet current systems of the penetrating BBFs. Finally, we connect this work with recent studies that have shown rapid, localized ground variability on the dawnside which can lead to hazardous geomagnetically induced currents.

Topological analysis of three-dimensional magnetic reconnection in SPERF-AREX for simulated magnetopause events

A scientific research device for geospace physics, the Space Plasma Environment Research Facility (SPERF), is going to be in operation this year at the Harbin Institute of Technology, China. One of its laboratory simulation platforms, the Asymmetric Reconnection EXperiment (AREX), is designed to investigate the asymmetric magnetic reconnection relevant to the process in the dayside magnetopause of the Earth. As a ground-based experimental platform, AREX emphasizes three-dimensional (3D) asymmetric driving features with topological similarity relevant to dayside reconnection. Thus, in this paper, we focus on topological analysis based on numerical simulation of AREX reconnection processes. 3D topological features in various scenarios relevant to magnetopause reconnection in different interplanetary magnetic field (IMF) conditions are investigated by adjusting setup parameters of plasma sources and magnetic coils. The simulation results reveal that the plasma distribution of asymmetric reconnection in SPERF-AREX is analogous to that at the magnetopause. Various types of the 3D magnetic topology relevant to typical IMF conditions, including the X-line and the A–B null chain structures, are, respectively, identified and analyzed in detail.

Kinetics of Cold Ions in Asymmetric Reconnection: Particle‐In‐Cell Simulation

AbstractCold ions from Earth's ionosphere and plasmasphere have frequently been observed at the magnetopause, where they impact reconnection and get energized in the process. The behavior of cold ions in different regions of magnetopause reconnection is not well understood. This study investigates their kinetics in asymmetric reconnection through a 2.5‐D fully kinetic simulation. The simulation starts with cold ions being present only in the magnetosphere. We find that the velocity distribution functions (VDFs) of cold ions in different reconnection regions are composed of two types of particles, distinguished by their ability to penetrate the magnetosheath. One type produces ring‐shaped distributions in the magnetosheath due to meandering motion, while the other generates ring‐shaped distributions in the magnetospheric separatrix as a result of gyromotion. The reconnection electric field Ey has a negative effect on one type and a positive effect on the other. Moreover, the Hall electric field Ez can stepwise accelerate the meandering particles. The cold ion temperature increases significantly in the magnetosheath as compared to its original temperature, the enhancement is a kinetic effect as a result of the ring‐shaped velocity distribution. On the other hand, in the remaining regions, cold ions experience bulk heating, resulting in noticeable temperature elevation. These findings further emphasize the necessity to examine the VDF for an all‐round comprehension of the particle heating mechanism and contribute to a better understanding of cold ion dynamics at the Earth's magnetopause.

Electron Dynamics in Strong Turbulence within the Diffusion Region during Magnetopause Magnetic Reconnection

Magnetic reconnection has been proposed to play an important role in energy dissipation in space plasma. The diffusion region is an essential place for generation of energetic electrons. However, the mechanism responsible for the generation of these energetic electrons in such a confined region remains elusive. Here a diffusion region of asymmetric reconnection is observed at the turbulent magnetopause. The diffusion region is a rather structured region where two dynamical filamentary currents at subion scale were observed. Intense electron flow and nonideal electric field inside the filamentary currents induced significant energy dissipation. Concurrently, the electron parallel temperature between the current layers increases from 58 to 80 eV. Direct evidence indicates that the electrons inside the three-dimensional diffusion region are accelerated by the parallel electric fields inside the current layers and are effectively heated by the turbulence between them.

Kinetics of Cold Ions in Asymmetric Reconnection: Particle-in-Cell Simulation

Kinetics of Cold Ions in Asymmetric Reconnection: Particle-in-Cell Simulation

Kinetic Simulation of Cold Ion Heating in Asymmetric Reconnection

AbstractCold ions from the plasmasphere can reach the Earth's magnetopause through the plasma wind and plume, affecting the reconnection at the magnetopause, such as changing the reconnection rate and influencing the energy budget. Recent observations found that cold ions were substantially heated by magnetopause reconnection. Although a few mechanisms have been proposed to account for the cold ion acceleration at the magnetopause, the dominant mechanism remains unclear. In this paper, we perform 2.5‐dimensional fully kinetic simulations to study cold ion heating in asymmetric reconnection at Earth's magnetopause. We find that cold ions absorb about 10%–25% of the total released magnetic energy, which is predominantly converted into the thermal energy of cold ions. The proportion of the energy gained by cold ions increases with the increment of the initial density ratio of the cold ions to hot ions in the magnetosphere. Cold ions are primarily accelerated by the Hall electric field at the magnetospheric separatrix region, while the out‐of‐plane electric field does negative work. The increase in thermal energy of cold ions is mainly caused by stochastic heating, that is, the viscous heating associated with the non‐gyrotropic pressure tensor, , contributes the most. The work done by pressure is most significant around the magnetosheath separatrix region. These results can significantly deepen our understanding of the cold ion dynamics at Earth's magnetopause.

3D simulation of lower-hybrid drift waves in strong guide field asymmetric reconnection in laboratory experiments

The magnetic reconnection experiment has recently seen short wavelength ( k ρ e ∼ 1) lower-hybrid waves near the electron diffusion region in strong guide field reconnection. Based on plasma parameters from the experiment, we perform a three-dimensional fully kinetic simulation in order to investigate the generation of the lower-hybrid waves and their effects on the reconnection process. We find that the low-beta regions around the reconnection site are unstable to the lower-hybrid drift instability propagating in the outflow direction, driven by the difference between the electron and ion outflows. The waves modify the electron distributions, leading to periodic opening and closing of gaps in electron velocity space, and provide a small contribution to the anomalous resistivity. Finally, the simulation results are discussed in the context of space observations and laboratory experiments.

Dataset for "3D simulations of lower-hybrid drift waves in strong guide field asymmetric reconnection in laboratory experiments"

Dataset for "3D simulations of lower-hybrid drift waves in strong guide field asymmetric reconnection in laboratory experiments"

Kinetic Simulation of Cold Ion Heating in Asymmetric Reconnection

Kinetic Simulation of Cold Ion Heating in Asymmetric Reconnection

Kinetic Simulation of Cold Ion Heating in Asymmetric Reconnection

Kinetic Simulation of Cold Ion Heating in Asymmetric Reconnection

Energy Transport during 3D Small-scale Reconnection Driven by Anisotropic Plasma Turbulence

Energy dissipation in collisionless plasmas is a long-standing fundamental physics problem. Although it is well known that magnetic reconnection and turbulence are coupled and transport energy from system-size scales to subproton scales, the details of the energy distribution and energy dissipation channels remain poorly understood. Especially, the energy transfer and transport associated with 3D small-scale reconnection that occurs as a consequence of a turbulent cascade is unknown. We use an explicit fully kinetic particle-in-cell code to simulate 3D small-scale magnetic reconnection events forming in anisotropic and decaying Alfvénic turbulence. We identify a highly dynamic and asymmetric reconnection event that involves two reconnecting flux ropes. We use a two-fluid approach based on the Boltzmann equation to study the spatial energy transfer associated with the reconnection event and compare the power density terms in the two-fluid energy equations with standard energy-based damping, heating, and dissipation proxies. Our findings suggest that the electron bulk flow transports thermal energy density more efficiently than kinetic energy density. Moreover, in our turbulent reconnection event, the energy density transfer is dominated by plasma compression. This is consistent with turbulent current sheets and turbulent reconnection events, but not with laminar reconnection.

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.

An 18.3 MJ charging and discharging pulsed power supply system for the Space Plasma Environment Research Facility (SPERF): The integrated control subsystem.

The Space Plasma Environment Research Facility uses a coil system with the corresponding pulsed power supply (PPS) system to generate a very flexible magnetosphere-like magnetic configuration. Its purpose is to investigate the 3D asymmetric reconnection and the processes of trapping, acceleration, and transport of energetic charged particles restrained in a dipole magnetic field configuration, as well as the physical mechanism of the dipolarization front in the magnetotail. The control and monitoring function of the PPS system is realized by the integrated control subsystem, which adopts a two-layer network structure of the control layer and device layer and is developed based on the Experimental Physics and Industrial Control System framework. The control layer includes a remote control system that consists of an engineer station and an operator station and the data storage system. Both the engineer station and operator station are developed by Control System Studio. The data storage system is based on the combination of the Hierarchical Data Format 5 database and MySQL database, and the data management software of the data storage system is developed based on LabVIEW. The synchronous trigger device, the safety interlocking device, the local controller of each set of PPS, and the module controller of each discharge module are the device layer. Their hardware is designed and developed based on the Field Programmable Gate Array, and their software is based on the Quartus II platform and programmed with the Verilog Hardware Description Language language. The function of the integrated control subsystem is verified by the discharge test of the PPS system.

Asymmetrically Varying Guide Field During Magnetic Reconnection: Particle‐In‐Cell Simulations

AbstractUsing fully kinetic particle‐in‐cell modeling, we investigate how magnetic reconnection responds to a varying guide field in one of the inflow regions. We find that the reconnection rate varies significantly when the orientation of the magnetic field changes between being strictly antiparallel and having a guide field. These variations are fairly consistent with the scaling relation for asymmetric reconnection developed by Cassak and Shay (2007). However, the rate is also found to be nonlinearly modulated by changes in the ion inflow velocity. The spatio‐temporal change in the inflow velocity arises as the magnetic forces reconfigure to regions of different magnetic field strengths. The variations in the inflow magnetic field configuration allow for different gradients in the magnetic field, leading to asymmetries in the magnetic tension force. By momentum conservation, this facilitates asymmetries in the inflow velocity, which in turn affects the flux transport into the reconnection site. The outflow is found to be less laminar when the inflow varies, and various signatures of the inflow variations are identified in the outflow.

Whistler waves generated by nongyrotropic and gyrotropic electron beams during asymmetric guide field reconnection

Using a two-dimensional particle-in-cell simulation of asymmetric reconnection with a guide field whose strength is 0.3 times the reconnecting magnetic field, we study electron distribution functions and wave intensities in the diffusion region, focusing on the electron diffusion region (EDR). Wave activities with frequencies below the electron cyclotron frequency are observed, and these are whistler waves propagating almost anti-parallel to the magnetic field. The waves are concentrated near the magnetospheric separatrix away from the X line, but the wave activity also spreads through the EDR near the X line. The reconnection outflows are asymmetric in the outflow direction in the magnetospheric side, and the wave intensity is stronger in the side of the faster electron outflow. We study the whistler waves using the fast Fourier transform, analyses of electron velocity distribution functions, and the dispersion solver calculation. Along the magnetospheric separatrix in the stronger outflow side, highly anisotropic electron beams exist with super-Alfvénic drift speeds. The dispersion analysis shows that there are two modes: a temperature anisotropy mode and a beam mode. Outside the EDR, the whistler wave intensity is highest near the separatrix, but the wave intensity decreases if we move away from the separatrix toward the magnetic neutral line because of the increase in the electron population near zero parallel velocity. In the EDR, in the velocity plane perpendicular to the magnetic field, ring/crescent electron distribution functions are observed. Near the X-line, the wave power is enhanced where nongyrotropic electrons contribute to increase the perpendicular temperature anisotropy.