Reconnection Outflow Region Research Articles

Correlations of Plasma Properties Between the Upstream Magnetosheath and the Downstream Outflow Region of Magnetopause Reconnection

AbstractThe impact of upstream conditions on magnetopause reconnection has been an intriguing question in solar wind‐magnetosphere coupling. In this study, we conduct a statistical analysis of plasma properties in the reconnection outflow region and the associated upstream solar wind/magnetosheath. We observe that the normalized ion density (N/Nsw) decreases and the flow speed (V/Vsw) increases in the upstream magnetosheath with distance from the subsolar point, consistent with previous models and observations. The magnetic field strength (|B|), ion density (N), and ion bulk speed (|V|) in the upstream magnetosheath exhibit close correlations with those in the reconnection outflow region. This upstream‐downstream correlation likely arises from the process of forming reconnection outflows, where most upstream ions cross the separatrix and mix with ion outflow already accelerated near the X‐line. High‐speed part of reconnection outflow is mostly located on the magnetosphere side of the magnetopause current layer, with outflow velocities peaking close to the upstream magnetosheath Alfvén speed. The spatial extent of high‐speed outflow is greater in conditions of lower solar wind Alfvén Mach number (MA,sw). Additionally, the southward magnetic field in the magnetosheath and |B| of magnetopause current layer are larger in the cases of lower MA,sw. These findings indicate a close connection of plasma properties between the outflow region of magnetopause reconnection and the upstream magnetosheath.

Generation of relativistic electrons at the termination shock in the solar flare region

Context. Solar flares are accompanied by an enhanced emission of electromagnetic waves from the radio up to the γ-ray range. The associated hard X-ray and microwave radiation is generated by energetic electrons. These electrons play an important role, since they carry a substantial part of the energy released during a flare. The flare is generally understood as a manifestation of magnetic reconnection in the corona. The so-called standard CSHKP model is one of the most widely accepted models for eruptive flares. The solar flare event on September 10, 2017 offers us a unique opportunity to study this model. The observations from the Expanded Owens Valley Solar Array (EOVSA) show that ≈1.6 × 104 electrons with energies > 300 keV are generated in the flare region. Aims. There are signatures in solar radio and extreme ultraviolet (EUV) observations as well as numerical simulations that a “termination shock” (TS) appears in the magnetic reconnection outflow region. Electrons accelerated at the TS can be considered to generate the loop-top hard X-ray sources. In contrast to previous studies, we investigate whether the heating of the plasma at the TS provides enough relativistic electrons needed for the hard X-ray and microwave emission observed during the solar X8.2 flare on September 10, 2017. Methods. We studied the heating of the plasma at the TS by evaluating the jump in the temperature across the shock by means of the Rankine–Hugoniot relationships under coronal circumstances measured during the event on September 10, 2017. The part of relativistic electrons was calculated in the heated downstream region. Results. In the magnetic reconnection outflow region, the plasma is strongly heated at the TS. Thus, there are enough energetic electrons in the tail of the electron distribution function (EDF) needed for the microwave and hard X-ray emission observed during the event on September 10, 2017. Conclusions. The generation of relativistic electrons at the TS is a possible mechanism of explaining the enhanced microwave and hard X-ray radiation emitted during flares.

Crater Structure Behind Reconnection Front

AbstractMagnetic reconnection is the physical process that converts the energy from the fields to the plasmas in space, astrophysical and laboratory plasmas. The Reconnection front (RF) is the structure generated in the reconnection outflow region and participates in the energy release budget. Here, we first report a novel crater structure of magnetic field behind the RF, which is well supported by both the in‐situ observations from the Magnetospheric Multiscale mission and kinetic particle‐in‐cell simulations. The theoretical explanations from the simulations suggests that the formation of the crater structure is possibly due to that high‐speed outflow electron jet from inner electron diffusion region constantly strikes the RF. From another perspective, the crater structure is the continuous impact of the electron jet. Our results can establish a new understanding of the RF and energy conversion during magnetic reconnection.

Three-dimensional crescent-shaped ion velocity distributions created by magnetic reconnection in the presence of a guide field

By means of theory and particle simulations, crescent-shaped ion velocity distributions in the outflow region of symmetric magnetic reconnection in the presence of a guide magnetic field are investigated. Assuming a spatial one-dimensional electromagnetic field, a theoretical model accounting for the shape of crescents is derived. First, following the earlier theoretical models suggested by Bessho et al. [Geophys. Res. Lett. 43, 1828–1836 (2016)] and Zenitani et al. [J. Geophys. Res.: Space Phys. 122, 7396–7413 (2017)], we derive a theoretical model for 2D velocity distributions based on the conservation of the canonical momentum and the energy. The 2D theoretical model exhibits a two-dimensional structure of crescent-shaped velocity distributions and further demonstrates that no magnetic field reversal is required in the formation of crescents, although many researchers have considered that magnetic field reversal plays an essential role. Next, we construct a theoretical model for 3D velocity distributions based not only on the conservation of the canonical momentum in two directions and the energy but also on the conservation of the magnetic moment and the kinetic energy in the moving frame with the reconnection outflow speed, which are applied when the guide field ratio is high. The 3D theoretical model derives a three-dimensional structure of crescents by combination of an antiparallel magnetic field and a guide magnetic field comparable to or greater than the reconnection field. Both of the 2D and 3D theoretical models are consistent with ion velocity distributions found in our particle simulations.

Efficient Electron Acceleration Driven by Flux Rope Evolution during Turbulent Reconnection

Magnetic flux ropes or magnetic islands are important structures responsible for electron acceleration and energy conversion during turbulent reconnection. However, the evolution of flux ropes and the corresponding electron acceleration process still remain open questions. In this paper, we present a comparative study of flux ropes observed by the Magnetospheric Multiscale mission in the outflow region during an example of turbulent reconnection in Earth's magnetotail. Interestingly, we find the farther the flux rope is away from the X-line, the bigger the size of the flux rope and the slower it moves. We estimate the power density converted at the observed flux ropes via the three fundamental electron acceleration mechanisms: Fermi, betatron, and parallel electric field. The dominant acceleration mechanism at all three flux ropes is the betatron mechanism. The flux rope that is closest to the X-line, having the smallest size and the fastest moving velocity, is the most efficient in accelerating electrons. Significant energy also returns from particles to fields around the flux ropes, which may facilitate the turbulence in the reconnection outflow region.

Statistical Properties of Lower Hybrid Waves in the Magnetopause Reconnection Exhaust Region

AbstractLower hybrid waves (LHWs) are frequently observed associated with magnetic reconnection. A general picture of the statistical properties of LHWs in magnetopause reconnection, however, has not yet been established. Here we report such a statistical study of LHWs using magnetospheric multiscale mission data in the magnetopause reconnection exhaust region. LHWs are identified in reconnection outflow regions over a broad range of magnetic local time. Most of the LHWs are near strong density gradients located on the low‐density magnetospheric side of the reconnection layer. The wave amplitude is typically several mV/m but can occasionally reach a few tens of mV/m. Compared with intervals that do not exhibit LHWs, intervals with LHWs show a ∼50% increase in parallel electron temperature, providing statistical evidence that LHWs contribute to parallel electron heating in reconnection exhausts and the dayside low‐latitude boundary layer.

Formation of the bow shock indentation: MHD simulation results

Simulation results from a global magnetohydrodynamic (MHD) model are used to examine whether the bow shock has an indentation and characterize its formation conditions, as well as its physical mechanism. The bow shock is identified by an increase in plasma density of the solar wind, and the indentation of the bow shock is determined by the shock flaring angle. It is shown that when the interplanetary magnetic field (IMF) is southward and the Alfven Mach number (Mα) of solar wind is high (> 5), the bow shock indentation can be clearly determined. The reason is that the outflow region of magnetic reconnection (MR) that occurs in the low latitude area under southward IMF blocks the original flow in the magnetosheath around the magnetopause, forming a high-speed zone and a low-speed zone that are upstream and downstream of each other. This structure hinders the surrounding flow in the magnetosheath, and the bow shock behind the structure widens and forms an indentation. When Mα is low, the magnetosheath is thicker and the disturbing effect of the MR outflow region is less obvious. Under northward IMF, MR occurs at high latitudes, and the outflow region formed by reconnection does not block the flow inside the magnetosheath, thus the indentation is harder to form. The study of the conditions and formation process of the bow shock indentation will help to improve the accuracy of bow shock models.

Fluid and kinetic aspects of magnetic reconnection and some related magnetospheric phenomena

Magnetic field reconnection plays a key role in determining the magnetic field topology and in the conversion of magnetic energy into kinetic energy in cosmic plasmas. Magnetic reconnection was proposed by solar physicists in an attempt to explain solar flares. The concept of magnetic reconnection was extended to Earth’s magnetosphere to explain auroral substorms. In this review, we first briefly review the classical fluid steady models with small separatrix angle. We then report magnetic reconnection with a large separatrix angle, and the time-dependent multiple X line reconnection (MXR) associated with flux transfer events. The transition from a single X line reconnection to MXR is examined. Global three-dimensional (3-D) magnetic reconnection is reviewed. The particle pressure gradient associated mainly with the electron (ion) off-diagonal pressure tensor terms, $$P_{xy}$$ and $$P_{zy}$$ , is shown to play an important role for electron (ion) dynamics to balance the reconnection electric field ( $$E_{y}$$ ) near the X line. In addition, various plasma waves and instabilities may occur near the ion diffusion and electron diffusion regions, including the upper hybrid drift waves, lower hybrid drift waves, ion-ion beam instability, electron beam instability, whistler waves and kinetic Alfven waves. The layered structure of outflow region of magnetic reconnection is examined. These layered structures include slow shocks, rotation discontinuities, expansion waves and plasma jets. Recent satellite observations of magnetic reconnection in the Earth and planetary magnetospheres are also discussed. The polar cap electric field associated with magnetic reconnection at the Earth's magnetopause and the energy flow in the polar cap region are examined.

Electron Acceleration in a Magnetotail Reconnection Outflow Region Using Magnetospheric MultiScale Data

AbstractWe study Magnetospheric MultiScale observations in the outflow region of magnetotail reconnection. We estimate the power density converted via the three fundamental electron acceleration mechanisms: Fermi, betatron, and parallel electric fields. The dominant mechanism, both on average and the peak values, is Fermi acceleration with a peak power density of about +200 pW/m3. The magnetic field curvature during the most intense Fermi acceleration is comparable to the electron gyroradius, consistent with efficient electron scattering. The peak power densities due to the betatron acceleration are a factor of 3 lower than that for the Fermi acceleration, the average betatron acceleration is close to zero and slightly negative. The contribution from parallel electric fields is significantly smaller than those from the Fermi and betatron acceleration. However, the observational uncertainties in the parallel electric field measurement prevent further conclusions. There is a strong variation in the power density on a characteristic ion time scale.

A three-dimensional model of spiral null pair to form ion-scale flux ropes in magnetic reconnection region observed by Cluster

The magnetic structure and topology of the three-dimensional magnetic reconnection region are significantly dynamic and complex. Small-scale flux ropes and magnetic null points are frequently detected in the reconnection outflow region and diffusion region due to the increased in situ measurements at high temporal cadences. Previous studies have demonstrated that X-line and small-scale flux ropes are both related to null points. In this study, by applying a fitting-reconstruction method with the input of the Cluster dataset, we reveal three types of spiral null pairs that serve as the skeleton of the flux ropes. Two spiral nulls can be connected by a spine, or by a separator, or by both a spine and a separator. A theoretical model is proposed to explain these spiral null pairs. The observational results and the model indicate that the number of magnetic loops of the flux rope is restricted by the linkage pattern of two nulls, while the flux rope is confined by the two nulls and their fan surfaces. The model predicts that the magnetic perturbations in the reconnection region can transform the linkage types of the nulls and eventually lead to the evolution of flux ropes.

Erratum: “On the role of separatrix instabilities in heating the reconnection outflow region” [Phys. Plasmas 25, 122902 (2018)

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Electron Distribution and Energy Release in Magnetic Reconnection Outflow Regions during the Pre-impulsive Phase of a Solar Flare

Abstract We present observations of electron energization in magnetic reconnection outflows during the pre-impulsive phase of solar flare SOL2012-07-19T05:58. During a time-interval of about 20 minutes, starting 40 minutes before the onset of the impulsive phase, two X-ray sources were observed in the corona, one above the presumed reconnection region and one below. For both of these sources, the mean electron distribution function as a function of time is determined over an energy range from 0.1 keV up to several tens of keV, for the first time. This is done by simultaneous forward fitting of X-ray and extreme ultraviolet (EUV) data. Imaging spectroscopy with RHESSI provides information on the high-energy tail of the electron distribution in these sources while EUV images from SDO/Atmospheric Imaging Assembly are used to constrain the low specific electron energies. The measured electron distribution spectrum in the magnetic reconnection outflows is consistent with a time-evolving kappa-distribution with κ = 3.5–5.5. The spectral evolution suggests that electrons are accelerated to progressively higher energies in the source above the reconnection region, while in the source below, the spectral shape does not change but an overall increase of the emission measure is observed, suggesting density increase due to evaporation. The main mechanisms by which energy is transported away from the source regions are conduction and free-streaming electrons. The latter dominates by more than one order of magnitude and is comparable to typical nonthermal energies during the hard X-ray peak of solar flares, suggesting efficient acceleration even during this early phase of the event.

On the role of separatrix instabilities in heating the reconnection outflow region

A study of the role microinstabilities at the reconnection separatrix can play in heating the electrons during the transition from inflow to outflow is being presented. We find that very strong flow shears at the separatrix layer lead to counterstreaming electron distributions in the region around the separatrix, which become unstable to a beam-type instability. Similar to what has been seen in earlier research, the ensuing instability leads to the formation of propagating electrostatic solitons. We show here that this region of strong electrostatic turbulence is the predominant electron heating site when transiting from inflow to outflow. The heating is the result of heating generated by electrostatic turbulence driven by overlapping beams, and its macroscopic effect is a quasi-viscous contribution to the overall electron energy balance. We suggest that instabilities at the separatrix can play a key role in the overall electron energy balance in magnetic reconnection.

Observations of the Electron Jet Generated by Secondary Reconnection in the Terrestrial Magnetotail

Abstract We report in situ observations of an electron jet generated by secondary reconnection within the outflow region of primary reconnection in the terrestrial magnetotail by the Magnetospheric Multiscale (MMS) mission. The MMS spacecraft first passed through the primary X-line and then crossed the electron jet in the outflow of primary reconnection. There are a series of small-scale flux ropes in the secondary reconnection region. Decoupling from the magnetic field for both ions and electrons, an intense out-of-plane current, unambiguous Hall currents, and a Hall electromagnetic field appear in the electron jet. Strong electron dissipation ( ), a nonzero electric field in the electron frame ( ), and electron crescent-like shaped distributions are detected in the center of the electron jet, implying that MMS spacecraft were likely passing through the electron diffusion region. The significant electron dissipation indicates that the electrons can be accelerated in the electron jet and the electron jet may be another important electron acceleration channel along with the electron diffusion region.

MMS Observation of Magnetic Reconnection in the Turbulent Magnetosheath

AbstractIn this paper we use the full armament of the MMS (Magnetospheric Multiscale) spacecraft to study magnetic reconnection in the turbulent magnetosheath downstream of a quasi‐parallel bow shock. Contrarily to the magnetopause and magnetotail cases, only a few observations of reconnection in the magnetosheath have been reported. The case study in this paper presents, for the first time, both fluid‐scale and kinetic‐scale signatures of an ongoing reconnection in the turbulent magnetosheath. The spacecraft are crossing the reconnection inflow and outflow regions and the ion diffusion region (IDR). Inside the reconnection outflows D shape ion distributions are observed. Inside the IDR mixing of ion populations, crescent‐like velocity distributions and ion accelerations are observed. One of the spacecraft skims the outer region of the electron diffusion region, where parallel electric fields, energy dissipation/conversion, electron pressure tensor agyrotropy, electron temperature anisotropy, and electron accelerations are observed. Some of the difficulties of the observations of magnetic reconnection in turbulent plasma are also outlined.

Cold Ionospheric Ions in the Magnetic Reconnection Outflow Region

AbstractMagnetosheath plasma usually determines properties of asymmetric magnetic reconnection at the subsolar region of Earth's magnetopause. However, cold plasma that originated from the ionosphere can also reach the magnetopause and modify the kinetic physics of asymmetric reconnection. We present a magnetopause crossing with high‐density (10–60 cm−3) cold ions and ongoing reconnection from the observation of the Magnetospheric Multiscale (MMS) spacecraft. The magnetopause crossing is estimated to be 300 ion inertial lengths south of the X line. Two distinct ion populations are observed on the magnetosheath edge of the ion jet. One population with high parallel velocities (200–300 km/s) is identified to be cold ion beams, and the other population is the magnetosheath ions. In the deHoffman‐Teller frame, the field‐aligned magnetosheath ions are Alfvénic and move toward the jet region, while the field‐aligned cold ion beams move toward the magnetosheath boundary layer, with much lower speeds. These cold ion beams are suggested to be from the cold ions entering the jet close to the X line. This is the first observation of the cold ionospheric ions in the reconnection outflow region, including the reconnection jet and the magnetosheath boundary layer.

Particle acceleration with anomalous pitch angle scattering in 2D magnetohydrodynamic reconnection simulations

The conversion of magnetic energy into other forms (such as plasma heating, bulk plasma flows, and non-thermal particles) during solar flares is one of the outstanding open problems in solar physics. It is generally accepted that magnetic reconnection plays a crucial role in these conversion processes. In order to achieve the rapid energy release required in solar flares, an anomalous resistivity, which is orders of magnitude higher than the Spitzer resistivity, is often used in magnetohydrodynamic (MHD) simulations of reconnection in the corona. The origin of Spitzer resistivity is based on Coulomb scattering, which becomes negligible at the high energies achieved by accelerated particles. As a result, simulations of particle acceleration in reconnection events are often performed in the absence of any interaction between accelerated particles and any background plasma. This need not be the case for scattering associated with anomalous resistivity caused by turbulence within solar flares, as the higher resistivity implies an elevated scattering rate. We present results of test particle calculations, with and without pitch angle scattering, subject to fields derived from MHD simulations of two-dimensional (2D) X-point reconnection. Scattering rates proportional to the ratio of the anomalous resistivity to the local Spitzer resistivity, as well as at fixed values, are considered. Pitch angle scattering, which is independent of the anomalous resistivity, causes higher maximum energies in comparison to those obtained without scattering. Scattering rates which are dependent on the local anomalous resistivity tend to produce fewer highly energised particles due to weaker scattering in the separatrices, even though scattering in the current sheet may be stronger when compared to resistivity-independent scattering. Strong scattering also causes an increase in the number of particles exiting the computational box in the reconnection outflow region, as opposed to along the separatrices as is the case in the absence of scattering.

Non-Maxwellian electron distribution functions due to self-generated turbulence in collisionless guide-field reconnection

Non-Maxwellian electron velocity space distribution functions (EVDFs) are useful signatures of plasma conditions and non-local consequences of collisionless magnetic reconnection. In the past, EVDFs were obtained mainly for antiparallel reconnection and under the influence of weak guide-fields in the direction perpendicular to the reconnection plane. EVDFs are, however, not well known, yet, for oblique (or component-) reconnection in case and in dependence on stronger guide-magnetic fields and for the exhaust (outflow) region of reconnection away from the diffusion region. In view of the multi-spacecraft Magnetospheric Multiscale Mission (MMS), we derived the non-Maxwellian EVDFs of collisionless magnetic reconnection in dependence on the guide-field strength bg from small (bg≈0) to very strong (bg = 8) guide-fields, taking into account the feedback of the self-generated turbulence. For this sake, we carried out 2.5D fully kinetic Particle-in-Cell simulations using the ACRONYM code. We obtained anisotropic EVDFs and electron beams propagating along the separatrices as well as in the exhaust region of reconnection. The beams are anisotropic with a higher temperature in the direction perpendicular rather than parallel to the local magnetic field. The beams propagate in the direction opposite to the background electrons and cause instabilities. We also obtained the guide-field dependence of the relative electron-beam drift speed, threshold, and properties of the resulting streaming instabilities including the strongly non-linear saturation of the self-generated plasma turbulence. This turbulence and its non-linear feedback cause non-adiabatic parallel electron acceleration. We further obtained the resulting EVDFs due to the non-linear feedback of the saturated self-generated turbulence near the separatrices and in the exhaust region of reconnection in dependence on the guide field strength. We found that the influence of the self-generated plasma turbulence leads well beyond the limits of the quasi-linear approximation to the creation of phase space holes and an isotropizing pitch-angle scattering. EVDFs obtained by this way can be used for diagnosing collisionless reconnection by using the multi-spacecraft observations carried out by the MMS mission.

Understanding the ion distributions near the boundaries of reconnection outflow region

AbstractAn interesting signature observed shortly after the onset of magnetotail reconnection is the gradual appearance of a local peak of ion phase space density (PSD) in the duskward and downstream direction separated from the colder, nearly isotropic ion population. Such a characteristic ion distribution, served as a diagnostic signature of magnetotail reconnection and well reproduced by a particle‐tracing Liouville simulation, are found to appear only near the off‐equatorial boundaries of the reconnection outflow region. Further analysis on ion trajectories suggests that the ions within the local peak and within the neighboring PSD cleft both belong to the outflowing population; on top of their outflowing motion, they both meander across the neutral sheet to exhibit duskward velocities near the off‐equatorial edges of their trajectories. The difference between them is that the local peak originates from ions previously constituting the preonset plasma sheet, whereas the cleft corresponds to the inflowing lobe ions before they are repelled in the downstream direction. As reconnection proceeds, the local PSD peak gradually attenuates and then disappears, which is a signature of reconnection flushing effect that depletes the ions in the preonset plasma sheet and eventually replaces them by lobe ions.

In-situ observations of flux ropes formed in association with a pair of spiral nulls in magnetotail plasmas

Signatures of secondary islands are frequently observed in the magnetic reconnection regions of magnetotail plasmas. In this paper, magnetic structures with the secondary-island signatures observed by Cluster are reassembled by a fitting-reconstruction method. The results show three-dimensionally that a secondary island event can manifest the flux rope formed with an As-type null and a Bs-type null paired via their spines. We call this As-spine-Bs-like configuration the helically wrapped spine model. The reconstructed field lines wrap around the spine to form the flux rope, and an O-type topology is therefore seen on the plane perpendicular to the spine. Magnetized electrons are found to rotate on and cross the fan surface, suggesting that both the torsional-spine and the spine-fan reconnection take place in the configuration. Furthermore, detailed analysis implies that the spiral nulls and flux ropes were locally generated nearby the spacecraft in the reconnection outflow region, indicating that secondary reconnection may occur in the exhaust away from the primary reconnection site.