Magnetic Reconnection Diffusion Region Research Articles

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

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

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

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

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

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

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.

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

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

MAVEN Observations of Tailward Reconnection Front in the Martian Magnetotail

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

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

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

Spatial evolution of magnetic reconnection diffusion region structures with distance from the X-line

We report Magnetospheric Multiscale four-spacecraft observations of a thin reconnecting current sheet with weakly asymmetric inflow conditions and a guide field of approximately twice the reconnecting magnetic field. The event was observed at the interface of interlinked magnetic field lines at the flank magnetopause when the maximum spacecraft separation was 370 km and the spacecraft covered ∼1.7 ion inertial lengths (di) in the reconnection outflow direction. The ion-scale spacecraft separation made it possible to observe the transition from electron-only super ion-Alfvénic outflow near the electron diffusion region (EDR) to the emergence of sub-Alfvénic ion outflow in the ion diffusion region (IDR). The EDR to IDR evolution over a distance less than 2 di also shows the transition from a near-linear reconnecting magnetic field reversal to a more bifurcated current sheet as well as significant decreases in the parallel electric field and dissipation. Both the ion and electron heating in this diffusion region event were similar to the previously reported heating in the far downstream exhausts. The dimensionless reconnection rate, obtained four different ways, was in the range of 0.13–0.27. This event reveals the rapid spatial evolution of the plasma and electromagnetic fields through the EDR to IDR transition region.

In Situ Observations of the Ion Diffusion Region in the Venusian Magnetotail

AbstractUtilizing the in situ observations of the Analyzer of Space Plasmas and Energetic Atoms (ASPERA‐4) and the magnetometer (MAG) onboard Venus Express, we report evidence of crossing the ion diffusion region of magnetic reconnection. Evidence of magnetic reconnection is based on two cases recorded by Venus Express in the Venusian magnetotail. In both cases, the variations of the magnetic field components are consistent with the classical picture of crossing the ion diffusion region. Our evidence is also supported by observed ion flows whose directions and speeds agree with expected characteristics of reconnection outflow. The reconnection crossing cases demonstrate that magnetic reconnection can appear in both ± E‐hemispheres in the Venusian magnetotail. The Venusian magnetotail provides a unique space laboratory for the investigation of the reconnection process.

Electron Bernstein waves driven by electron crescents near the electron diffusion region

The Magnetospheric Multiscale (MMS) spacecraft encounter an electron diffusion region (EDR) of asymmetric magnetic reconnection at Earth’s magnetopause. The EDR is characterized by agyrotropic electron velocity distributions on both sides of the neutral line. Various types of plasma waves are produced by the magnetic reconnection in and near the EDR. Here we report large-amplitude electron Bernstein waves (EBWs) at the electron-scale boundary of the Hall current reversal. The finite gyroradius effect of the outflow electrons generates the crescent-shaped agyrotropic electron distributions, which drive the EBWs. The EBWs propagate toward the central EDR. The amplitude of the EBWs is sufficiently large to thermalize and diffuse electrons around the EDR. The EBWs contribute to the cross-field diffusion of the electron-scale boundary of the Hall current reversal near the EDR.

Inner and outer electron diffusion region of antiparallel collisionless reconnection: Density dependence

We study inflow density dependence of substructures within electron diffusion region (EDR) of collisionless symmetric magnetic reconnection. We perform a set of 2.5D particle-in-cell simulations which start from a Harris current layer with a uniform background density nb. A scan of nb ranging from 0.02 n0 to 2 n0 of the peak current layer density (n0) is studied keeping other plasma parameters the same. Various quantities measuring reconnection rate, EDR spatial scales, and characteristic velocities are introduced. We analyze EDR properties during quasisteady stage when the EDR length measures saturate. Consistent with past kinetic simulations, electrons are heated parallel to the B field in the inflow region. The presence of the strong parallel anisotropy acts twofold: (1) electron pressure anisotropy drift gets important at the EDR upstream edge in addition to the E×B drift speed and (2) the pressure anisotropy term −∇·P(e)/(ne) modifies the force balance there. We find that the width of the EDR demagnetization region and EDR current are proportional to the electron inertial length ∼de and ∼denb0.22, respectively. Magnetic reconnection is fast with a rate of ∼0.1 but depends weakly on density as ∼nb−1/8. Such reconnection rate proxies as EDR geometrical aspect or the inflow-to-outflow electron velocity ratio are shown to have different density trends, making electric field the only reliable measure of the reconnection rate.

Electron Vorticity Indicative of the Electron Diffusion Region of Magnetic Reconnection.

While vorticity defined as the curl of the velocity has been broadly used in fluid and plasma physics, this quantity has been underutilized in space physics due to low time resolution observations. We report Magnetospheric Multiscale (MMS) observations of enhanced electron vorticity in the vicinity of the electron diffusion region of magnetic reconnection. On 11 July 2017 MMS traversed the magnetotail current sheet, observing tailward‐to‐earthward outflow reversal, current‐carrying electron jets in the direction along the electron meandering motion or out‐of‐plane direction, agyrotropic electron distribution functions, and dissipative signatures. At the edge of the electron jets, the electron vorticity increased with magnitudes greater than the electron gyrofrequency. The out‐of‐plane velocity shear along distance from the current sheet leads to the enhanced vorticity. This, in turn, contributes to the magnetic field perturbations observed by MMS. These observations indicate that electron vorticity can act as a proxy for delineating the electron diffusion region of magnetic reconnection.

Observations of Short-period Current Sheet Flapping Events in the Earth's Magnetotail

Abstract The flapping motion of the current sheet, with the period from several minutes to tens of minutes, is one common dynamic phenomenon in the planetary magnetotail. This Letter reports on one current sheet flapping event with the short semi-period of ∼6 s on 2017 July 17 in the Earth’s magnetotail for the first time using the Magnetospheric Multiscale (MMS) mission. This short time period flapping event consists of five consecutive crossings of the current sheet. Based on a multipoint analysis of the MMS, it is found that the first four crossings propagated duskward and belong to kink-like flapping, and the fifth crossing belongs to steady flapping. The current sheet flapping was embedded in the diffusion region of magnetic reconnection, which was identified by the well-organized Hall electromagnetic field. The period of current sheet flapping was modulated by the reconnection electric field and perpendicular plasma flow, indicating that this flapping motion may be triggered by the periodical unsteady magnetic reconnection.

Localized and Intense Energy Conversion in the Diffusion Region of Asymmetric Magnetic Reconnection

AbstractWe analyze a high‐resolution simulation of magnetopause reconnection observed by the Magnetospheric Multiscale mission and explain the occurrence of strongly localized dissipation with an amplitude more than an order of magnitude larger than expected. Unlike symmetric reconnection, wherein reconnection of the ambient reversed magnetic field drives the dissipation, we find that the annihilation of the self‐generated, out‐of‐plane (Hall) magnetic field plays the dominant role. Electrons flow along the magnetosheath separatrices, converge in the diffusion region, and jet past the X‐point into the magnetosphere. The resulting accumulation of negative charge generates intense parallel electric fields that eject electrons along the magnetospheric separatrices and produce field‐aligned beams. Many of these features match Magnetospheric Multiscale observations.

Assessing the Time Dependence of Reconnection With Poynting's Theorem: MMS Observations

AbstractWe investigate the time dependence of electromagnetic‐field‐to‐plasma energy conversion in the electron diffusion region of asymmetric magnetic reconnection. To do so, we consider the terms in Poynting's theorem. In a steady state there is a perfect balance between the divergence of the electromagnetic energy flux and the conversion between electromagnetic field and particle energy . This energy balance is demonstrated with a particle‐in‐cell simulation of reconnection. We also evaluate each of the terms in Poynting's theorem during an observation of a magnetopause reconnection region by Magnetospheric Multiscale (MMS). We take the equivalence of both sides of Poynting's theorem as an indication that the errors associated with the approximation of each term with MMS data are small. We find that, for this event, balance between is only achieved for a small fraction of the energy conversion region at/near the X‐point. Magnetic energy was rapidly accumulating on either side of the current sheet at roughly 3 times the predicted energy conversion rate. Furthermore, we find that while and are observed, as is expected for reconnection, the energy accumulation is driven by the overcompensation for by . We note that due to the assumptions necessary to do this calculation, the accurate evaluation of may not be possible for every MMS‐observed reconnection event; but, if possible, this is a simple approach to determine if reconnection is or is not in a steady state.

Quantifying the Effect of Non-Larmor Motion of Electrons on the Pressure Tensor.

In space plasma, various effects of magnetic reconnection and turbulence cause the electron motion to significantly deviate from their Larmor orbits. Collectively these orbits affect the electron velocity distribution function and lead to the appearance of the "non-gyrotropic" elements in the pressure tensor. Quantification of this effect has important applications in space and laboratory plasma, one of which is tracing the electron diffusion region (EDR) of magnetic reconnection in space observations. Three different measures of agyrotropy of pressure tensor have previously been proposed, namely, A∅ e , Dng and Q. The multitude of contradictory measures has caused confusion within the community. We revisit the problem by considering the basic properties an agyrotropy measure should have. We show that A∅ e , Dng and Q are all defined based on the sum of the principle minors (i.e. the rotation invariant I 2) of the pressure tensor. We discuss in detail the problems of I 2-based measures and explain why they may produce ambiguous and biased results. We introduce a new measure AG constructed based on the determinant of the pressure tensor (i.e. the rotation invariant I 3) which does not suffer from the problems of I 2-based measures. We compare AG with other measures in 2 and 3-dimension particle-in-cell magnetic reconnection simulations, and show that AG can effectively trace the EDR of reconnection in both Harris and force-free current sheets. On the other hand, A∅ e does not show prominent peaks in the EDR and part of the separatrix in the force-free reconnection simulations, demonstrating that A∅ e does not measure all the non-gyrotropic effects in this case, and is not suitable for studying magnetic reconnection in more general situations other than Harris sheet reconnection.

Evidence for Secondary Flux Rope Generated by the Electron Kelvin-Helmholtz Instability in a Magnetic Reconnection Diffusion Region

Secondary flux ropes are suggested to play important roles in energy dissipation and particle acceleration during magnetic reconnection. However, their generation mechanism is not fully understood. In this Letter, we present the first direct evidence that a secondary flux rope was generated due to the evolution of an electron vortex, which was driven by the electron Kelvin-Helmholtz instability in an ion diffusion region as observed by the Magnetospheric Multiscale mission. The subion scale (less than the ion inertial length) flux rope was embedded within the electron vortex, which contained a secondary electron diffusion region at the trailing edge of the flux rope. We propose that intense electron shear flow produced by reconnection generated the electron Kelvin-Helmholtz vortex, which induced a secondary reconnection in the exhaust of the primary X line and then led to the formation of the flux rope. This result strongly suggests that secondary electron Kelvin-Helmholtz instability is important for reconnection dynamics.

Electron Crescent Distributions as a Manifestation of Diamagnetic Drift in an Electron-Scale Current Sheet: Magnetospheric Multiscale Observations Using New 7.5ms Fast Plasma Investigation Moments.

We report Magnetospheric Multiscale observations of electron pressure gradient electric fields near a magnetic reconnection diffusion region using a new technique for extracting 7.5 ms electron moments from the Fast Plasma Investigation. We find that the deviation of the perpendicular electron bulk velocity from E × B drift in the interval where the out‐of‐plane current density is increasing can be explained by the diamagnetic drift. In the interval where the out‐of‐plane current is transitioning to in‐plane current, the electron momentum equation is not satisfied at 7.5 ms resolution.

Intermittent energy dissipation by turbulent reconnection

AbstractMagnetic reconnection—the process responsible for many explosive phenomena in both nature and laboratory—is efficient at dissipating magnetic energy into particle energy. To date, exactly how this dissipation happens remains unclear, owing to the scarcity of multipoint measurements of the “diffusion region” at the sub‐ion scale. Here we report such a measurement by Cluster—four spacecraft with separation of 1/5 ion scale. We discover numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O‐lines) and the separatrices. Inside each current filament, kinetic‐scale turbulence is significantly increased and the energy dissipation, E′ ⋅ j, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X‐lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O‐lines but not X‐lines.

Electron currents and heating in the ion diffusion region of asymmetric reconnection

AbstractIn this letter the structure of the ion diffusion region of magnetic reconnection at Earth's magnetopause is investigated using the Magnetospheric Multiscale (MMS) spacecraft. The ion diffusion region is characterized by a strong DC electric field, approximately equal to the Hall electric field, intense currents, and electron heating parallel to the background magnetic field. Current structures well below ion spatial scales are resolved, and the electron motion associated with lower hybrid drift waves is shown to contribute significantly to the total current density. The electron heating is shown to be consistent with large‐scale parallel electric fields trapping and accelerating electrons, rather than wave‐particle interactions. These results show that sub‐ion scale processes occur in the ion diffusion region and are important for understanding electron heating and acceleration.