Reconnection Diffusion Region Research Articles

Effect of the Reconnection Electric Field on Electron Distribution Functions in the Diffusion Region of Magnetotail Reconnection

AbstractElectron distribution functions in the electron diffusion region during symmetric magnetic reconnection are investigated by means of theory and fully kinetic simulations. Crescent‐like striations are formed in distribution functions in the velocity plane perpendicular to the magnetic field. Using an analytical current sheet, we solve the equation of motion for electrons, and derive the shape of a crescent distribution, as a function of the distance from the neutral line, field gradients, and the reconnection electric field. Each crescent is tilted in the velocity plane because of the acceleration by the reconnection electric field, and multiple stripes appear due to multiple meandering bounces. Applying the theory to distribution functions observed in Earth's magnetotail, we deduce the amplitude of the reconnection electric field.

Energy Conversion and Partition in the Asymmetric Reconnection Diffusion Region

AbstractWe investigate the energy conversion and partition in the asymmetric reconnection diffusion region using two‐dimensional particle‐in‐cell simulations and Magnetosphere Multiscale (MMS) mission observations. Under an upstream condition with equal temperatures in the two inflow regions, the simulation analysis indicates that the energy partition between ions and electrons depends on the distance from the X‐line. Within the central electron diffusion region (EDR), nearly all dissipated electromagnetic field energies are converted to electrons. From the EDR to the ion diffusion region (IDR) scales, the rate of the electron energy gain decreases to be lower than that of ions. A magnetopause reconnection event inside the IDR observed by MMS shows comparable ion and electron energy gains, consistent with the simulation result in the transition region from EDR to IDR. At the EDR scale, the electron energization is mainly by the reconnection electric field (Er); in‐plane electric fields (Exz) provide additional positive contributions near the X‐line and do negative work on electrons beyond the EDR. The guide field reduces the electron energization by both Er and Exz in the EDR. For ion energization, Er and Exz have comparable contributions near the time of the peak reconnection rate, while Exz dominants at later time. At the IDR scale, the guide field causes asymmetry in the amount of the energy gain and energization mechanisms between two exhausts but does not have significant effects on energy partition. Our study advances understanding of ion and electron energization in asymmetric reconnect IDRs.

Oblique Ion‐Scale Magnetotail Flux Ropes Generated by Secondary Tearing Modes

AbstractWe examine Magnetospheric Multiscale observations of three ion‐scale magnetic flux ropes (diameter ~6–9 ion inertial lengths) in the Earth's magnetotail reconnection diffusion region with a small guide field. During the event, the electron reconnection exhaust reversed direction together with the reversal of normal magnetic field component. After that, the three ion‐scale flux ropes were observed near the ion diffusion region, in which flat‐top distributions were found at the field‐aligned electrons (<2 keV). The axis orientation and motion velocity of the flux ropes are determined by minimum variance analysis of the electric field component along the flux rope axis, using single‐spacecraft measurements. Results show that the three flux rope axes form 13°, 55°, and 55° to the guide field direction and are mostly tilted toward the direction of the reconnecting field. These results are also validated by multiple‐spacecraft analysis methods. It is found that the tilted angles agree reasonably well with the predicted angles for secondary tearing modes, suggesting that these ion‐scale flux ropes are likely generated by secondary tearing instabilities as those recently found in the three‐dimensional kinetic simulations of turbulent magnetic reconnection.

In Situ Observation of Magnetic Reconnection Between an Earthward Propagating Flux Rope and the Geomagnetic Field

AbstractIt has been proposed that, in the near‐Earth magnetotail, earthward propagating flux ropes can merge with the Earth's dipole magnetic field and dissipate its magnetic energy. However, the reconnection diffusion region related to this process has not been identified. Here we report the first in situ observation of magnetic reconnection between an earthward propagating flux rope and the closed magnetic field lines connecting to Earth. Magnetospheric Multiscale (MMS) spacecraft crossed a vertical current sheet between the leading edge of the flux rope (negative Bz) and the geomagnetic field (positive Bz). The subion‐scale current sheet, super‐Alfvénic electron outflow, Hall magnetic and electric field, conversion of magnetic energy to plasma energy (J·E > 0), and magnetic null were observed during the crossing. All the above signatures indicate that MMS detected the reconnection diffusion region. This result is also relevant to other planets with intrinsic magnetosphere.

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.

Wave Phenomena and Beam-Plasma Interactions at the Magnetopause Reconnection Region.

This paper reports on Magnetospheric Multiscale observations of whistler mode chorus and higher‐frequency electrostatic waves near and within a reconnection diffusion region on 23 November 2016. The diffusion region is bounded by crescent‐shaped electron distributions and associated dissipation just upstream of the X‐line and by magnetic field‐aligned currents and electric fields leading to dissipation near the electron stagnation point. Measurements were made southward of the X‐line as determined by southward directed ion and electron jets. We show that electrostatic wave generation is due to magnetosheath electron beams formed by the electron jets as they interact with a cold background plasma and more energetic population of magnetospheric electrons. On the magnetosphere side of the X‐line the electron beams are accompanied by a strong perpendicular electron temperature anisotropy, which is shown to be the source of an observed rising‐tone whistler mode chorus event. We show that the apex of the chorus event and the onset of electrostatic waves coincide with the opening of magnetic field lines at the electron stagnation point.

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.

Turbulent heating due to magnetic reconnection

Dissipation of plasma turbulent energy is a phenomenon having significant implications for the heating of the solar corona and solar wind. While processes involving linear wave damping, stochastic heating, and reconnection have been postulated as contributors to heating mechanisms, the relative role that they play is not currently understood. In this manuscript, we establish a theoretical framework for applying reconnection heating predictions to turbulent systems. Kinetic particle-in-cell (PIC) simulations are used to study heating due to reconnection, and these results are then adapted to a turbulent medium. First, the factors controlling the heating of plasmas in reconnection exhausts are examined using laminar reconnection simulations; predictions for heating are determined which require only the plasma conditions just upstream of the reconnection diffusion region as input. The laminar predictions are then applied to PIC simulations of turbulence. Key assumptions are: (1) the plasma conditions just upstream of the diffusion region are consistent with Kolmogorov scaling of turbulent fluctuations at the ion inertial scale and (2) the statistics of the numbers of reconnecting x-lines do not vary significantly between the various turbulent simulations. We find that the reconnection theory predicts quite well the scaling of the ratio of ion to electron heating, in which the statistics of the turbulent reconnection sites are expected to roughly cancel. Separate ion and electron heating rates scale differently from the theory, however. This suggests that the statistics of the turbulent reconnection (e.g., number of x-lines, percentage of x-lines reconnecting) is playing an important role in determining the ion and electron heating.

Instability of Agyrotropic Electron Beams near the Electron Diffusion Region.

During a magnetopause crossing the Magnetospheric Multiscale spacecraft encountered an electron diffusion region (EDR) of asymmetric reconnection. The EDR is characterized by agyrotropic beam and crescent electron distributions perpendicular to the magnetic field. Intense upper-hybrid (UH) waves are found at the boundary between the EDR and magnetosheath inflow region. The UH waves are generated by the agyrotropic electron beams. The UH waves are sufficiently large to contribute to electron diffusion and scattering, and are a potential source of radio emission near the EDR. These results provide observational evidence of wave-particle interactions at an EDR, and suggest that waves play an important role in determining the electron dynamics.

Survey of magnetic reconnection signatures in the Martian magnetotail with MAVEN

AbstractWe conduct an extended survey of reconnection signatures observed in the Martian magnetotail by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Although it is well established that magnetic reconnection plays a fundamental role in the dynamics of intrinsic magnetospheres, the role of reconnection in the dynamics of induced magnetospheres remains poorly understood. Based on comprehensive plasma and field measurements by MAVEN in the Martian magnetotail, we first identified 776 current sheet crossings and then selected 34 crossings with Hall magnetic field signatures, which are indicative of the ion diffusion region of tail reconnection. For the majority of the identified Hall field events, we observe ion flow enhancements in the directions consistent with the reconnection outflow directions expected from the Hall magnetic field polarity. The reconnection signatures are preferentially observed in the −E hemisphere of the near‐Mars magnetotail at ∼1–2 Mars radii downstream from Mars. We have found no strong correlation of the likelihood of observing reconnection signatures with local crustal field strengths or with upstream drivers. The duty cycle of tail reconnection is estimated to be ∼1–10% or even higher. The MAVEN observations suggest that magnetic reconnection can play an important role in the dynamics of the Martian magnetotail.

Large‐scale characteristics of reconnection diffusion regions and associated magnetopause crossings observed by MMS

AbstractThe Magnetospheric Multiscale (MMS) mission was designed to make observations in the very small electron diffusion region (EDR), where magnetic reconnection takes place. From a data set of over 4500 magnetopause crossings obtained in the first phase of the mission, MMS had encounters near or within 12 EDRs. These 12 events and associated magnetopause crossings are considered as a group to determine if they span the widest possible range of external and internal conditions (i.e., in the solar wind and magnetosphere). In addition, observations from MMS are used to determine if there are multiple X‐lines present and also to provide information on X‐line location relative to the spacecraft. These 12 events represent nearly the widest possible range of conditions at the dayside magnetopause. They occur over a wide range of local times and magnetic shear angles between the magnetosheath and magnetospheric magnetic fields. Most show evidence for multiple reconnection sites.

Hodographic approach for determining spacecraft trajectories through magnetic reconnection diffusion regions

AbstractWe develop an algorithm that finds a trajectory through simulations of magnetic reconnection along which input Magnetospheric Multiscale (MMS) spacecraft observations are matched. Using two‐dimensional particle‐in‐cell simulations of asymmetric reconnection, the method is applied to a magnetopause electron diffusion region (EDR) encountered by the MMS spacecraft to facilitate interpretation of the event based on fully kinetic models. The recently discovered crescent‐shaped electron velocity distributions measured by MMS in the EDR are consistent with simulation distributions at the corresponding time along the computed trajectory.

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.

Lower hybrid waves in the ion diffusion and magnetospheric inflow regions

AbstractThe role and properties of lower hybrid waves in the ion diffusion region and magnetospheric inflow region of asymmetric reconnection are investigated using the Magnetospheric Multiscale (MMS) mission. Two distinct groups of lower hybrid waves are observed in the ion diffusion region and magnetospheric inflow region, which have distinct properties and propagate in opposite directions along the magnetopause. One group develops near the ion edge in the magnetospheric inflow, where magnetosheath ions enter the magnetosphere through the finite gyroradius effect and are driven by the ion‐ion cross‐field instability due to the interaction between the magnetosheath ions and cold magnetospheric ions. This leads to heating of the cold magnetospheric ions. The second group develops at the sharpest density gradient, where the Hall electric field is observed and is driven by the lower hybrid drift instability. These drift waves produce cross‐field particle diffusion, enabling magnetosheath electrons to enter the magnetospheric inflow region thereby broadening the density gradient in the ion diffusion region.

Two-stage bulk electron heating in the diffusion region of anti-parallel symmetric reconnection

Electron bulk energization in the diffusion region during anti-parallel symmetric reconnection entails two stages. First, the inflowing electrons are adiabatically trapped and energized by an ambipolar parallel electric field. Next, the electrons gain energy from the reconnection electric field as they undergo meandering motion. These collisionless mechanisms have been described previously, and they lead to highly structured electron velocity distributions. Nevertheless, a simplified control-volume analysis gives estimates for how the net effective heating scales with the upstream plasma conditions in agreement with fully kinetic simulations and spacecraft observations.

Electron energization and mixing observed by MMS in the vicinity of an electron diffusion region during magnetopause reconnection

AbstractMeasurements from the Magnetospheric Multiscale (MMS) mission are reported to show distinct features of electron energization and mixing in the diffusion region of the terrestrial magnetopause reconnection. At the ion jet and magnetic field reversals, distribution functions exhibiting signatures of accelerated meandering electrons are observed at an electron out‐of‐plane flow peak. The meandering signatures manifested as triangular and crescent structures are established features of the electron diffusion region (EDR). Effects of meandering electrons on the electric field normal to the reconnection layer are detected. Parallel acceleration and mixing of the inflowing electrons with exhaust electrons shape the exhaust flow pattern. In the EDR vicinity, the measured distribution functions indicate that locally, the electron energization and mixing physics is captured by two‐dimensional reconnection, yet to account for the simultaneous four‐point measurements, translational invariant in the third dimension must be violated on the ion‐skin‐depth scale.

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.