Type Of Reconnection Research Articles

Three‐Dimensional Particle‐In‐Cell Simulations of Electron‐Only Magnetic Reconnection Between Laser‐Produced Plasma Bubbles

AbstractElectron‐only magnetic reconnection, a novel type of reconnection where only electron dynamics is involved, has recently been observed in turbulent plasmas in the Earth's magnetosphere. In this letter, using particle‐in‐cell simulations, we demonstrate that electron‐only reconnection can be created via laser‐plasma interactions. The reconnection current sheet is carried by electrons, and its width is at the electron inertial scale. Moreover, only electron outflow is observed, while the ion outflow is negligible. The duration of the reconnection is found to be within the electron scale, thus there is no sufficient time for ions to respond. The energy conversion during electron‐only reconnection mainly occurs in the vicinity of the X‐line, and is dominated along the perpendicular direction. By analyzing the Poynting flux patterns, we find that the reconnection electric field dominates the whole energy conversion. This study advances our knowledge of the energy dissipation mechanism in the electron‐only reconnection regime.

Electron-only reconnection and associated electron heating and acceleration in PHASMA

Using incoherent Thomson scattering, electron heating and acceleration at the electron velocity distribution function (EVDF) level are investigated during electron-only reconnection in the PHAse Space MApping (PHASMA) facility. Reconnection arises during the merger of two kink-free flux ropes. Both push and pull type reconnection occur in a single discharge. Electron heating is localized around the separatrix, and the electron temperature increases continuously along the separatrix with distance from the X-line. The local measured gain in enthalpy flux is up to 70% of the incoming Poynting flux. Notably, non-Maxwellian EVDFs comprised of a warm bulk population and a cold beam are directly measured during the electron-only reconnection. The electron beam velocity is comparable to, and scales with, electron Alfvén speed, revealing the signature of electron acceleration caused by electron-only reconnection. The observation of oppositely directed electron beams on either side of the X-point provides “smoking-gun” evidence of the occurrence of electron-only reconnection in PHASMA. 2D particle-in-cell simulations agree well with the laboratory measurements. The measured conversion of Poynting flux into electron enthalpy is consistent with recent observations of electron-only reconnection in the magnetosheath [Phan et al., Nature 557, 202 (2018)] at similar dimensionless parameters as in the experiments. The laboratory measurements go beyond the magnetosheath observations by directly resolving the electron temperature gain.

Turbulence-driven magnetic reconnection and the magnetic correlation length: Observations from Magnetospheric Multiscale in Earth's magnetosheath

Turbulent plasmas generate a multitude of thin current structures that can be sites for magnetic reconnection. The Magnetospheric Multiscale (MMS) mission has recently enabled the detailed examination of such turbulent current structures in Earth's magnetosheath and revealed that a novel type of reconnection, known as electron-only reconnection, can occur. In electron-only reconnection, ions do not have enough space to couple to the newly reconnected magnetic fields, suppressing ion jet formation and resulting in thinner sub-proton-scale current structures with faster super-Alfvénic electron jets. In this study, MMS observations are used to examine how the magnetic correlation length (λC) of the turbulence, which characterizes the size of the large-scale magnetic structures and constrains the length of the current sheets formed, influences the nature of turbulence-driven reconnection. We systematically identify 256 reconnection events across 60 intervals of magnetosheath turbulence. Most events do not appear to have ion jets; however, 18 events are identified with ion jets that are at least partially coupled to the reconnected magnetic field. The current sheet thickness and electron jet speed have a weak anti-correlation, with faster electron jets at thinner current sheets. When λC≲20 ion inertial lengths, as is typical near the sub-solar magnetosheath, a tendency for thinner current sheets and potentially faster electron jets is present. The results are consistent with electron-only reconnection being more prevalent for turbulent plasmas with relatively short λC and may be relevant to the nonlinear dynamics and energy dissipation in turbulent plasmas.

Three-dimensional magnetic reconnection in astrophysical plasmas

Magnetic reconnection is a fundamental process in laboratory, magnetospheric, solar and astrophysical plasmas, whereby magnetic energy is converted into heat, bulk kinetic energy and fast particle energy. Its nature in two dimensions is much better understood than that in three dimensions, where its character is completely different and has many diverse aspects that are currently being explored. Here, we focus on the magnetohydrodynamics of three-dimensional reconnection in the plasma environment of the Solar System, especially solar flares. The theory of reconnection at null points, separators and quasi-separators is described, together with accounts of numerical simulations and observations of these three types of reconnection. The distinction between separator and quasi-separator reconnection is a theoretical one that is unimportant for the observations of energy release. A new paradigm for solar flares, in which three-dimensional reconnection plays a central role, is proposed.

Bifurcated outflow jet in a solar wind reconnection exhaust

We present new observations of the reconnection outflow jet obtained from the crossing of a solar wind reconnection exhaust by the Wind spacecraft on 5 March 2001. The outflow jet is characterized by a bifurcated configuration, which is different from typical reconnection exhausts. The two outflow jets are mainly distributed in the region close to the exhaust boundaries. Between the two jets, the bulging of the magnetic field and the slight depression of the plasma density are observed. The possible mechanism for the bifurcation of the outflow jet has also been investigated by using a magnetohydrodynamics (MHD) simulation of Petschek-type reconnection. The dynamic patterns of the reconnection exhaust are analyzed in detail. We find that the bifurcated jet is one of the transient configurations produced in the magnetic reconnection. Other observed features of the exhaust show similar agreement with the predictions from the simulation. The study of the reconnection exhaust, over multiple spatial scales, provides an insight into the dynamic evolution of solar wind reconnection exhausts.

Particle-in-cell simulations of asymmetric reconnection driven by laser-powered capacitor coils

During magnetic reconnection, such as in the magnetopause of magnetized planets, the upstream plasma conditions between the two inflow regions are usually different. In this paper, we demonstrate that such a kind of asymmetric reconnection can be studied in the laboratory using the recently proposed experimental scheme where reconnection is driven by laser-powered capacitor coils. Two-dimensional particle-in-cell simulations on the plane in a cylindrical coordinate are conducted to study magnetic reconnection with the inflow along the direction. Magnetic reconnection is found to be asymmetric with a stronger magnetic field in the inner (small ) inflow region and a weaker magnetic field in the outer (large ) inflow region due to the cylindrical symmetric geometry. Electron crescent velocity distributions are observed near the flow stagnation point while ion crescent velocity distributions are observed in the region with Larmor electric field. The out-of-plane Hall magnetic field is asymmetric between the two inflow regions with a larger spatial scale in the outer inflow region. This asymmetric Hall magnetic field configuration is different from that in previous studies. The typical reconnection rate increases with a stronger driver and the highest rate is around 0.2, where is the typical value of the magnetic field and is the Alfven speed. This study provides a new method to experimentally study asymmetric reconnection in the laboratory and has potential applications regarding magnetic reconnection in the magnetopause of magnetized planets.

Rapid Evolution of Type II Spicules Observed in Goode Solar Telescope On-disk Hα Images

Abstract We analyze ground-based chromospheric data acquired at a high temporal cadence of 2 s in wings of the H α spectral line using the Goode Solar Telescope operating at the Big Bear Solar Observatory. We inspected a 30 minute long H α −0.08 nm data set to find that rapid blueshifted H α excursions (RBEs), which are a cool component of type II spicules, experience very rapid morphological changes on timescales of the order of 1 s. Unlike typical reconnection jets, RBEs very frequently appear in situ without any clear evidence of H α material being injected from below. Their evolution includes inverted “Y,” “V,” “N,” and parallel splitting (doubling) patterns as well as sudden formation of a diffuse region followed by branching. We also find that the same feature may undergo several splitting episodes within about a 1 minute time interval.

Mechanism of non-steady Petschek-type reconnection with uniform resistivity

The Sweet-Parker and Petschek models are well-established magnetohydrodynamics (MHD) models of steady magnetic reconnection. Recent findings on magnetic reconnection in high-Lundquist-number plasmas indicate that Sweet-Parker-type reconnection in marginally stable thin current sheets connecting plasmoids can produce fast reconnection. By contrast, it has proven difficult to achieve Petschek-type reconnection in plasmas with uniform resistivity because sustaining it requires localization of the diffusion region. However, Shibayama et al. [Phys. Plasmas 22, 100706 (2015)] recently noted that Petschek-type reconnection can be achieved spontaneously in a dynamical manner even under uniform resistivity through what they called dynamical Petschek reconnection. In this new type of reconnection, Petschek-type diffusion regions can be formed in connection with plasmoids. In this paper, we report the results of two-dimensional resistive MHD simulation with uniform resistivity, undertaken to determine the diffusion region localization mechanism under dynamical Petschek reconnection. Through this modeling, we found that the separation of the X-point from the flow stagnation point (S-point) plays a crucial role in the localization of the diffusion region because the motion of the X-point is restricted by the strong flow emanating from the flow stagnation point. This mechanism suggests that dynamical Petschek reconnection is possible even in large systems such as the solar corona.

Proton acceleration at tearing coronal null-point current sheets

Context. Non-thermal particle acceleration in the solar corona is thought to constitute a substantial part of the energy budget of explosive events such as solar flares. One well-established mechanism of non-thermal acceleration is directly via fields in current sheets.Aims. In this paper we study proton acceleration during “spine-fan reconnection” at a 3D magnetic null point. This type of reconnection has recently been implicated in some flares known as circular-ribbon flares. It has also recently been discovered that the reconnecting current sheet may undergo a non-linear tearing-type instability. This tearing leads to the formation of flux ropes and quasi-turbulent dynamics.Methods. A predictor-corrector test particle code is used to model the trajectories of protons at different stages of sheet tearing: when the sheet is intact, just after the formation of the first major flux rope, and once the non-linear phase of the instability has become more fully developed. The fields for these proton trajectories were taken from snapshots of a 3D magnetohydrodynamics simulation treated as three static field geometries represented by interpolated grids. Acceleration in the intact current sheet is compared to earlier simulations of infinite static current sheets and then used as a control case with which to compare the later snapshots.Results. Protons are found to be predominantly accelerated along the fan surface, especially in the absence of current sheet tearing. Most of the highest energy protons are accelerated in the main body of the current sheet, along the direction of strongest parallel electric field. A high energy tail is present in the kinetic energy distribution. After tearing commences, this direct acceleration no longer dominates and acceleration in the outflow regions makes a proportionally greater contribution. Sheet tearing appears overall to hinder the acceleration of protons in the fan plane, at least in the absence of time-dependent acceleration mechanisms. Some correlation is found between high energy protons and locations of flux ropes formed by the instability, but the nature of the link remains at present unclear.

Flows and magnetic field structures in reconnection regions of simulations of the solar atmosphere: Do flux pile-up models work?

Aims. We study the process of magnetic field annihilation and reconnection in simulations of magnetised solar photosphere and chromosphere with magnetic fields of opposite polarities and constant numerical resistivity. Methods. Exact analytical solutions for reconnective annihilations were used to interpret the features of magnetic reconnection in simulations of flux cancellation in the solar atmosphere. We used MURaM high-resolution photospheric radiative magneto-convection simulations to demonstrate the presence of magnetic field reconnection consistent with the magnetic flux pile-up models. Also, a simulated data-driven chromospheric magneto-hydrodynamic simulation is used to demonstrate magnetic field and flow structures, which are similar to the theoretically predicted ones. Results. Both simulations demonstrate flow and magnetic field structures roughly consistent with accelerated reconnection with magnetic flux pile-up. The presence of standard Sweet–Parker type reconnection is also demonstrated in stronger photospheric magnetic fields.

Fast Ion Heating in Transient Collisionless Magnetic Reconnection via an Intrinsic Stochastic Mechanism

Abstract Stochastic heating has been known to be a powerful ion heating mechanism in the solar wind, atmosphere, and flares. In this Letter, we show that stochastic ion heating is inherent to transient collisionless magnetic reconnection. The explanation exploits the connected nature of electron canonical vorticity to show analytically that the in-plane electric and magnetic fields in a typical reconnection geometry satisfy the condition for stochastic heating of ions. Electron fluid simulations, test ion simulations, and comparisons to experiments all support the existence of this mechanism.

Energetic Particles of keV–MeV Energies Observed near Reconnecting Current Sheets at 1 au

Abstract We provide evidence for particle acceleration up to ∼5 MeV at reconnecting current sheets in the solar wind based on both case studies and a statistical analysis of the energetic ion and electron flux data from the five Advanced Composition Explorer Electron, Proton, and Alpha Monitor (EPAM) detectors. The case study of a typical reconnection exhaust event reveals (i) a small-scale peak of the energetic ion flux observed in the vicinity of the reconnection exhaust and (ii) a long-timescale atypical energetic particle event (AEPE) encompassing the reconnection exhaust. AEPEs associated with reconnecting strong current sheets last for many hours, even days, as confirmed by statistical studies. The case study shows that time-intensity profiles of the ion flux may vary significantly from one EPAM detector to another partially because of the local topology of magnetic fields, but mainly because of the impact of upstream magnetospheric events; therefore, the occurrence of particle acceleration can be hidden. The finding of significant particle energization within a time interval of ±30 hr around reconnection exhausts is supported by a superposed epoch analysis of 126 reconnection exhaust events. We suggest that energetic particles initially accelerated via prolonged magnetic reconnection are trapped and reaccelerated in small- or medium-scale magnetic islands surrounding the reconnecting current sheet, as predicted by the transport theory of Zank et al. Other mechanisms of initial particle acceleration can contribute also.

Occurrence frequency and location of magnetic islands at the dayside magnetopause

AbstractSimulations of magnetic reconnection often predict the formation of magnetic islands forming and propagating out through the reconnection exhaust region. However, to date, only a few observations of magnetic islands at the Earth's magnetopause have been identified and analyzed. Crossings near reconnection sites at the dayside magnetopause by Cluster from 2001 to 2009 are examined in a systematic effort to determine the frequency and location of magnetic islands. Using the maximum magnetic shear model as a guide for the distance to the reconnection site, 47 magnetopause crossings within 3 RE of the predicted reconnection site are examined. The occurrence of island structures in the reconnection exhaust is investigated in the context of how the type of reconnection, antiparallel or component, affects the frequency of formation of the islands. For Cluster magnetopause crossings confirmed to be near reconnection sites, island signatures, primarily bidirectional streaming electrons in the magnetosheath boundary layer, are seen for approximately 85% of the crossings and are common features for both antiparallel and component reconnection.

Large-volume flux closure during plasmoid-mediated reconnection in coaxial helicity injection

A large-volume flux closure during transient coaxial helicity injection (CHI) in NSTX-U is demonstrated through resistive magnetohydrodynamics (MHD) simulations. Several major improvements, including the improved positioning of the divertor poloidal field coils, are projected to improve the CHI start-up phase in NSTX-U. Simulations in the NSTX-U configuration with constant in time coil currents show that with strong flux shaping the injected open field lines (injector flux) rapidly reconnect and form large volume of closed flux surfaces. This is achieved by driving parallel current in the injector flux coil and oppositely directed currents in the flux shaping coils to form a narrow injector flux footprint and push the injector flux into the vessel. As the helicity and plasma are injected into the device, the oppositely directed field lines in the injector region are forced to reconnect through a local Sweet–Parker type reconnection, or to spontaneously reconnect when the elongated current sheet becomes MHD unstable to form plasmoids. In these simulations for the first time, it is found that the closed flux is over 70% of the initial injector flux used to initiate the discharge. These results could work well for the application of transient CHI in devices that employ super conducting coils to generate and sustain the plasma equilibrium.

Relation between cusp ion structures and dayside reconnection for four IMF clock angles: OpenGGCM‐LTPT results

AbstractWhen, where, and which type of reconnection (antiparallel or component) happens on the dayside magnetopause are long‐standing unsolved questions due to insufficient in situ observation of reconnection sites. Previous studies showed that the dispersed ion signatures observed in the magnetospheric cusps depend on the reconnection mechanism, suggesting that cusp ion signatures can be a good tool to investigate the locations and properties of dayside reconnection. We investigate this close relation between cusp signatures and magnetopause reconnection for four different interplanetary magnetic field (IMF) clock angles (CA) using the Open Global Geospace Circulation Model (OpenGGCM) and the Liouville Theorem Particle Tracer(LTPT). OpenGGCM produces dayside reconnection under the resistive MHD theory, and LTPT calculates cusp ion signatures caused by the simulated reconnection. Our model results show that for CA = 0∘, antiparallel reconnection at both the northern and southern lobes causes a reverse dispersion in which ion energies increase with increasing latitude. For CA = 60∘, unsteady antiparallel reconnection at both the northern and southern lobes causes double reverse dispersions. For CA = 120∘, component reconnection near the subsolar point produces a dispersionless signature in the low‐latitude cusp, and antiparallel reconnection on the duskside northern magnetopause produces a normal dispersion in the high‐latitude cusp in which ion energies decrease with increasing latitude. For CA = 180∘, antiparallel reconnection near the subsolar point causes a normal dispersion.

Distinguishing between pulsed and continuous reconnection at the dayside magnetopause.

Magnetic reconnection has been established as the dominant mechanism by which magnetic fields in different regions change topology to create open magnetic field lines that allow energy and momentum to flow into the magnetosphere. One of the persistent problems of magnetic reconnection is the question of whether the process is continuous or intermittent and what input condition(s) might favor one type of reconnection over the other. Observations from imagers that record FUV emissions caused by precipitating cusp ions demonstrate the global nature of magnetic reconnection. Those images show continuous ionospheric emissions even during changing interplanetary magnetic field conditions. On the other hand, in situ observations from polar‐orbiting satellites show distinctive cusp structures in flux distributions of precipitating ions, which are interpreted as the telltale signature of intermittent reconnection. This study uses a modification of the low‐velocity cutoff method, which was previously successfully used to determine the location of the reconnection site, to calculate for the cusp ion distributions the “time since reconnection occurred.” The “time since reconnection” is used to determine the “reconnection time” for the cusp magnetic field lines where these distributions have been observed. The profile of the reconnection time, either continuous or stepped, is a direct measurement of the nature of magnetic reconnection at the reconnection site. This paper will discuss a continuous and pulsed reconnection event from the Polar spacecraft to illustrate the methodology.

Magnetic reconnection at Uranus' magnetopause

AbstractThe magnetosphere of Uranus has barely been explored by spacecraft but is distinct from other solar system magnetospheres in many respects. Determining how this magnetosphere is coupled to the solar wind is central to understanding energy flow through the system. Here we assess how the solar wind interacts with the Uranian magnetosphere via magnetic reconnection. Analytical models of conditions at the magnetopause are combined with current understanding of reconnection onset to predict where reconnection may occur on the boundary. The results suggest that conditions at Uranus' magnetopause are generally less favorable for reconnection than those at the magnetopause of any planet closer to the Sun, as a result of how typical solar wind parameters vary with heliocentric distance. The location of reconnection sites on the Uranian magnetopause is likely to be highly dependent on not only the interplanetary magnetic field orientation but also planetary longitude and season. Solar wind–magnetosphere coupling via magnetic reconnection may be stronger under near‐solstice conditions than under near‐equinox conditions. We discuss the typical reconnection electric field strength at Uranus' magnetopause and suggest that the typical reconnection voltage is considerably less than 40 kV. Complimentary assessments of other means of coupling to the solar wind (e.g., via a “viscous‐like” interaction) are needed to establish the overall nature of solar wind–magnetosphere coupling at Uranus.

Physics of forced magnetic reconnection in coaxial helicity injection experiments in National Spherical Torus Experiment

We numerically examine the physics of fast flux closure in transient coaxial helicity injection (CHI) experiments in National Spherical Torus Experiment (NSTX). By performing resistive Magnetohydrodynamics (MHD) simulations with poloidal injector coil currents held constant in time, we find that closed flux surfaces are formed through forced magnetic reconnection. Through a local Sweet-Parker type reconnection with an elongated current sheet in the injector region, closed flux surfaces expand in the NSTX global domain. Simulations demonstrate outflows approaching poloidally Alfvénic flows and reconnection times consistent with the Sweet-Parker model. Critical requirements for magnetic reconnection and flux closure are studied in detail. These primary effects, which are magnetic diffusivity, injector flux, injector flux footprint width, and rate of injector voltage reduction, are simulated for transient CHI experiments. The relevant time scales for effective reconnection are τV<τrec≈τAS(1+Pm)1/4<τR, where τV is the time for the injector voltage reduction, τA is the poloidal Alfvén transit time, τR is the global resistive diffusion time, and Pm and S are Prandtl and Lundquist numbers.

Can magnetopause reconnection drive Saturn's magnetosphere?

While solar wind-driven compression of Saturn's magnetosphere is an important trigger of magnetospheric dynamics, the importance of magnetopause reconnection has been the subject of debate. Here we use Cassini observations at Saturn's magnetopause to address this open issue. Measured conditions at the boundary suggest a typical reconnection electric field strength of order 0.01 mV m−1. Although the strongest reconnection electric fields correspond to a compressed magnetosphere, compressed conditions do not guarantee a relatively strong reconnection electric field at the magnetopause location sampled by the spacecraft. By considering northward Interplanetary Magnetic Field (IMF), we predict reconnection voltages (open magnetic flux production rates) for the strongest driving of Saturn's magnetosphere by magnetopause reconnection. The typical northward IMF reconnection voltage is less than 100 kV for a wide range of reconnection X line lengths. This suggests that magnetopause reconnection is not a major driver of dynamics in Saturn's magnetosphere, except possibly on rare occasions.

Magnetohydrodynamic study for three-dimensional instability of the Petschek type magnetic reconnection

The 3D instability of the spontaneous fast magnetic reconnection process is studied with magnetohydrodynamics (MHD) simulations, where the 2D model of the spontaneous fast magnetic reconnection is destabilized in three dimension. As well known in many 2D numerical MHD studies, when a 1D current sheet is destabilized with the current-driven anomalous resistivity, the 2D Petschek type fast magnetic reconnection is established. This paper shows that the 2D Petschek type fast magnetic reconnection can be destabilized in three dimension by an initial resistive disturbance which includes a weak fluctuation in the sheet current direction, i.e., along the magnetic neutral line. The resulting 3D fast magnetic reconnection finally becomes intermittent and random through a 3D instability. In addition, it is also shown that the 3D instability is suppressed by the uniform resistivity. It suggests that the 3D instability is caused in the Petschek-type reconnection process which is characterized by a strongly localized magnetic diffusion region and the slow shock acceleration of the plasma jets and is suppressed in the Sweet-Parker type reconnection process.