Reconnection Diffusion Region Research Articles

Scaling of the inner electron diffusion region in collisionless magnetic reconnection

The Sweet‐Parker analysis of the inner electron diffusion region of collisionless magnetic reconnection is presented. The study includes charged particles motion near the X‐line and an appropriate approximation of the off‐diagonal term for the electron pressure tensor. The obtained scaling shows that the width of the inner electron diffusion region is equal to the electron inertial length, and that electrons are accelerated up to the electron Alfvén velocity in X‐line direction. The estimated effective plasma conductivity is based on the electron gyrofrequency rather than the binary collision frequency, and gives the extreme (minimal) value of the plasma conductivity similar to Bohm diffusion. The scaling properties are verified by means of Particle‐in‐Cell simulations. An ad hoc parameter needs to be introduced to the scaling relations in order to better match the theory and simulations.

Electron acceleration in the reconnection diffusion region: Cluster observations

We present one case study of magnetic islands and energetic electrons in the reconnection diffusion region observed by the Cluster spacecraft. The cores of the islands are characterized by strong core magnetic fields and density depletion. Intense currents, with the dominant component parallel to the ambient magnetic field, are detected inside the magnetic islands. A thin current sheet is observed in the close vicinity of one magnetic island. Energetic electron fluxes increase at the location of the thin current sheet, and further increase inside the magnetic island, with the highest fluxes located at the core region of the island. We suggest that these energetic electrons are firstly accelerated in the thin current sheet, and then trapped and further accelerated in the magnetic island by betatron and Fermi acceleration.

Electric field structure inside the secondary island in the reconnection diffusion region

Secondary islands have recently been intensively studied because of their essential role in dissipating energy during reconnection. Secondary islands generally form by tearing instability in a stretched current sheet, with or without guide field. In this article, we study the electric field structure inside a secondary island in the diffusion region using large-scale two-and-half dimensional particle-in-cell (PIC) simulation. Intense in-plane electric fields, which point toward the center of the island, form inside the secondary island. The magnitudes of the in-plane electric fields Ex and Ez inside the island are much larger than those outside the island in the surrounding diffusion region. The maximum magnitudes of the fields are about three times the B0VA, where B0 is the asymptotic magnetic field strength and VA is the Alfvén speed based on B0 and the initial current sheet density. Our results could explain the intense electric field (~100 mV/m) inside the secondary island observed in the Earth’s magnetosphere. The electric field Ex inside the secondary island is primarily balanced by the Hall term (j × B)/ne, while Ez is balanced by a combination of (j × B)/ne, −(vi × B), and the divergence of electron pressure tensor, with (j × B)/ne term being dominant. This large Hall electric field is due to the large out-of-plane current density jy inside the island, which consists mainly of accelerated electrons forming a strong bulk flow in the –y direction. The electric field Ey shows a bipolar structure across the island, with negative Ey corresponding to negative Bz and positive Ey corresponding to positive Bz. It is balanced by (j × B)/ne and the convective electric field. There are significant parallel electric fields, forming a quadrupolar structure inside the island, with maximum amplitude of about 0.3B0VA.

Magnetotail dipolarization front and associated ion reflection: Particle‐in‐cell simulations

For the Earth magnetotail, the Svenes et al. (2008) statistical study infers the lobe density to be highly variable, in the range of 0.007–0.092 cm−3. Such lobe density variation modifies reconnection diffusion region physical processes and reconnection rate drastically. This letter addresses observable reconnection signatures in the vicinity of the X‐line that are to be affected by the dynamic changes of reconnection. Using a 2.5D particle‐in‐cell (PIC) code, we find the dipolarization front (DF) a moving ram that pushes the initial equilibrium plasma sheet in front of it. The DF propagation velocity scales with the upstream Alfvén speed, leading to faster, steeper, and more compressed DFs with strongerBz at low lobe densities. These DFs reflect plasma sheet ions in a streaming manner. The streaming ions create a bipolar magnetic field straddling the central plasma sheet and can excite various instabilities.

Electron pitch angle distribution during magnetic reconnection diffusion region observations in the Earth's magnetotail

Abstract. The Earth's magnetosphere provides an excellent laboratory for magnetic reconnection research. In particular, the magnetotail current sheet that is formed between the interface of the similar Northern and Southern Hemispheres of the magnetotail provides a relatively stable symmetric reconnection configuration that can be used to study basic aspects of the reconnection process. Of particular importance is the manner in which electrons are processed by the reconnection. Simulations and satellite data analyses of the ion diffusion region have suggested that the fluxes of electrons in the inflow regions of reconnection are greater in the directions parallel and anti-parallel to the magnetic field (field-aligned) whereas the electron flux in the outflow region is distributed more isotropically. However, this has only been studied experimentally on a case-by-case basis. In this paper, we investigate this claim by analyzing the degree of bulk electron field alignment in the outflow and inflow regions during encounters of the magnetic reconnection ion diffusion region by the Cluster spacecraft in the years 2001–2006. We demonstrate that while the median electron flux in the inflow region is indeed more field aligned than in the outflow region during some ion diffusion region encounters, the variation of the signature across events is so large that it cannot be said to be a general feature of magnetic reconnection in the Earth's magnetotail.

Characteristics of Reconnection Diffusion Region in the Solar Wind

利用ACE和WIND卫星2007年1月6日的联合探测, 在1AU附近发现了一个等离子体密度极低的Petschek-like重联喷流区. 该喷流区内部出现了非常明显的Hall双极磁场、等离子体密度下降区以及与Hall电流相符的低能段电子投掷角分布. 这些特征与重联离子扩散区的Hall效应非常吻合, 说明很可能在太阳风中观测到了一个离子扩散区. 分析表明, 与之相关的磁场重联为准稳态快速完全反向重联, 其扩散区以一对慢模波为边界, 空间尺度达到80个离子惯性长度, 表现出了大尺度重联的特征.

Effect of inflow density on ion diffusion region of magnetic reconnection: Particle-in-cell simulations

We perform a systematic study of the effect of inflow density on reconnection diffusion regions using a 2.5-D particle-in-cell code. The diffusion region structures are analyzed at times when all simulations have reconnected the same amount of magnetic flux. We find that reducing the inflow density from 1 to 1/100th of the current sheet density dramatically increases the diffusion region physical size and the reconnection rate. The width of the diffusion region scales with the upstream ion inertial length systematically. Consistent with the presence of counter-streaming inflowing ion beams near the x-line, the ion meandering width in the diffusion region also scales with the ion inertial length. The aspect ratio of the ion diffusion region remains a constant, independent of the inflow density. The quadrupole Hall magnetic field is reduced. The upstream magnetic field deviates from its asymptotic value by ∼50% at the lowest simulated inflow density. The downstream ion outflow velocity scales linearly with the upstream Alfvén speed with a multiplication factor ∼0.4 < 1. When applied to magnetic reconnection in the Earth’s magnetotail, this factor of 0.4 is a possible explanation as to why bulk flow velocities in the magnetotail are typically on the order of 500 km/s, while the Alfvén speeds of inflowing plasmas can exceed 2000 km/s.

Properties of Near-Earth Magnetic Reconnection from In-Situ Observations

Many properties of magnetic reconnection have been determined from in-situ spacecraft observations in the Earth’s magnetosphere. Recent studies have focused on ion scale lengths and have largely confirmed theoretical predictions. In addition, some interesting features of reconnection regions on electron scale lengths have been identified. These recent studies have demonstrated the need for combined plasma and field measurements on electron scale lengths in the reconnection diffusion regions at the magnetopause and in the magnetotail. They have also indicated that measurements, such as those that will be made by the Magnetospheric Multiscale mission in the near future, will have a significant impact on understanding magnetic reconnection as a fundamental plasma process.

Density cavity in magnetic reconnection diffusion region in the presence of guide field

[1] Understanding the structure of the diffusion region of magnetic reconnection is crucial to pinpoint the mechanism of energy conversion from magnetic field to plasma. Characteristics of a diffusion region with guide field (i.e., component reconnection) may be significantly different from those of a diffusion region without guide field (i.e., antiparallel reconnection). In this study, we attempt to understand the structure of a diffusion region with guide field by studying the density cavity along separatrix. We present an event in which a density cavity was detected by the Cluster spacecraft in a diffusion region in the presence of guide field. The cavity was located around the separatrix region on the southern hemisphere of the neutral sheet and earthward of the X-line and was coincident with strong magnetic field compression. The width of the cavity was on the ion inertial scale. This cavity contained a relatively strong antiparallel current, which was mainly contributed by parallel streaming electrons with energy of 1–10 keV. Enhancements of lower hybrid wave and electromagnetic whistler wave were observed inside the cavity. These waves are probably excited by parallel streaming electrons along separatrix via electron beam instability. Two-dimensional electromagnetic particle-in-cell simulation was employed to study the structure of the density cavity. The location and scale of the cavity and the signature of electric current and electron velocity are consistent with our observations. It is found that there was displacement between the position of electron density minimum and out-of-plane magnetic field maximum in reconnection with guide field. However, this displacement is much less than that in reconnection without guide field. There was no significant acceleration for electrons to reach energy larger than 30 keV at the cavity.

ESTIMATION OF THE RECONNECTION ELECTRIC FIELD IN THE 2003 OCTOBER 29 X10 FLARE

The electric field in the reconnecting current sheet of the 2003 October 29 X10 flare is estimated to be a few kV m–1 in this study, based on the rate of change in the photospheric magnetic flux in the newly brightened areas of Transition Region and Coronal Explorer (TRACE) UV ribbons. For comparison, the motion speed of Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) hard X-ray (HXR) footpoints and the photospheric magnetic field strength are also used for the electric field calculation. This X10 flare event is selected due to its distinct two-phase HXR kernel motion, two arcade systems with different magnetic shear, and the high cadence and complete coverage of the TRACE 1600 A Michelson Doppler Imager (MDI) magnetogram and RHESSI HXR observations. We pay particular attention to the electric field characteristics in different flare phases, as well as the temporal correlation with the HXR emission and its power-law spectral index and the photospheric magnetic field strength. We found that in the early impulsive phase, the reconnection electric field peaks just before the HXR emission peaks and the energy spectrum hardens. The result is consistent with the scenario that more particles are accelerated to higher energies by larger reconnection electric fields and then precipitate into the lower chromosphere to produce stronger HXR emissions. Such a particle acceleration mechanism plays its most significant role in the impulsive phase of this flare. In addition, our results provide evidence that the highly sheared magnetic field lines are mapped to the magnetic reconnection diffusion region to produce a large reconnection electric field.

Wave properties in the magnetic reconnection diffusion region with high β: Application of the k‐filtering method to Cluster multispacecraft data

Magnetic reconnection is a crucial physical process in laboratory and astrophysical plasmas. Plasma waves are believed to provide the dissipation mechanism in magnetic reconnection. In this paper we analyze the properties of low‐frequency waves in a magnetotail reconnection diffusion region with a small guide field and high β. Using the k‐filtering method on the magnetic field data measured by Cluster spacecraft, we found that low‐frequency waves in the diffusion region were highly oblique propagating mode. We compared the measured dispersion relation with theoretical ones calculated using the linear (hot) two‐fluid and Vlasov‐Maxwell theory. It is found that the observed waves in the diffusion region (with high plasma β) follow the dispersion relation of the Alfvén‐Whistler wave mode. Comparisons with previous simulations and observational results are also discussed.

“Island surfing” mechanism of electron acceleration during magnetic reconnection

One of the key unresolved problems in the study of space plasmas is to explain the production of energetic electrons as magnetic field lines “reconnect” and release energy in an explosive manner. Recent observations suggest possible roles played by small‐scale magnetic islands in the reconnection region, but their precise roles and the exact mechanism of electron energization have remained unclear. Here we show from two‐dimensional particle‐in‐cell simulations that secondary islands generated in the reconnection region indeed produce energetic electrons. We found that when electrons are trapped inside the islands, they are energized continuously by the reconnection electric field prevalent in the reconnection diffusion region. Applications to observations in the Earth's magnetotail are briefly discussed.

Asymmetry of the Ion Diffusion Region Hall Electric and Magnetic Fields during Guide Field Reconnection: Observations and Comparison with Simulations

In situ measurements of magnetic reconnection in the Earth's magnetotail are presented showing that even a moderate guide field (20% of the reconnecting field) considerably distorts ion diffusion region structure. The Hall magnetic and electric fields are asymmetric and shunted away from the current sheet; an appropriately scaled particle-in-cell simulation is found to be in excellent agreement with the data. The results show the importance of correctly accounting for the effects of the magnetic shear when attempting to identify and study magnetic reconnection diffusion regions in nature.

Magnitude of the Hall fields during magnetic reconnection

In situ observation of the Earth's magnetosphere has identified Hall magnetic fields as a key signature of collisionless magnetic reconnection. The inflow portion of the reconnection diffusion region is further characterized by strong electron pressure anisotropy. These two features are tightly linked in a quantitative model, which is verified using fully kinetic simulations. The model predicts the Hall field strength and the maximum electron pressure anisotropy as functions of the upstream ratio of electron fluid and magnetic pressures.

In situ observations of reconnection Hall magnetic fields at Mars: Evidence for ion diffusion region encounters

We present Mars Global Surveyor measurements of bipolar out‐of‐plane magnetic fields at current sheets in Mars' magnetosphere. These signatures match predictions from simulations and terrestrial observations of collisionless magnetic reconnection, and could similarly indicate differential ion and electron motion and the resulting Hall current systems near magnetic X lines. Thus, these observations may represent passages through or very near reconnection diffusion regions at Mars. Out of 28 events found at 400 km altitude with well‐defined current sheet orientations, 26 have magnetic fields consistent with the expected polarities of Hall fields near diffusion regions. For these events, we find an average ratio of Hall field to main field of 0.51 ± 0.13, and an average ratio of normal to main field (reconnection rate) of 0.16 ± 0.09, consistent with terrestrial observations of reconnection. These events do not consistently correlate with the location of crustal fields or with IMF reversals, indicating that magnetic field draping alone (perhaps enhanced by high solar wind dynamic pressure) may generate current sheets capable of reconnection. For some events, we observe field‐aligned electrons that may carry parallel currents that close the Hall current loop. Electron distributions around current sheets often indicate magnetic connection to the collisional exosphere. For crossings sunward of the X line, we usually observe an electron flux minimum at the current sheet, consistent with the resulting closed magnetic structure. For crossings antisunward of the X line, we do not observe flux minima, consistent with field lines open downstream. Collisionless reconnection, if common at Mars, could represent a significant atmospheric loss process.

A simple, analytical model of collisionless magnetic reconnection in a pair plasma

A set of conservation equations is utilized to derive balance equations in the reconnection diffusion region of a symmetric pair plasma. The reconnection electric field is assumed to have the function to maintain the current density in the diffusion region and to impart thermal energy to the plasma by means of quasiviscous dissipation. Using these assumptions it is possible to derive a simple set of equations for diffusion region parameters in dependence on inflow conditions and on plasma compressibility. These equations are solved by means of a simple, iterative procedure. The solutions show expected features such as dominance of enthalpy flux in the reconnection outflow, as well as combination of adiabatic and quasiviscous heating. Furthermore, the model predicts a maximum reconnection electric field of E∗=0.4, normalized to the parameters at the inflow edge of the diffusion region.

Dynamics and waves near multiple magnetic null points in reconnection diffusion region

Identifying the magnetic structure in the region where the magnetic field lines break and how reconnection happens is crucial to improving our understanding of three‐dimensional reconnection. Here we show the in situ observation of magnetic null structures in the diffusion region, the dynamics, and the associated waves. Possible spiral null pair has been identified near the diffusion region. There is a close relation among the null points, the bipolar signature of the Z component of the magnetic field, and enhancement of the flux of energetic electrons up to 100 keV. Near the null structures, whistler‐mode waves were identified by both the polarity and the power law of the spectrum of electric and magnetic fields. It is found that the angle between the fans of the nulls is quite close to the theoretically estimated maximum value of the group‐velocity cone angle for the whistler wave regime of reconnection.

Reconnection Hall current system observed in the magnetotail and in the ionosphere

On 07 Sep 2001 around 21:40 UT, Cluster was located 19 RE downtail, and observed earthward fast flows and strong perturbations in the dawn‐dusk component of the magnetic field (BY). The perturbations corresponded to a sheet of tailward field aligned current close to the neutral sheet and another sheet of earthward field aligned current closer to the lobe. At Cluster footpoint, the ionospheric equivalent current density was monitored by the IMAGE magnetometer network. The observed equivalent current pattern is found to be consistent with a longitudinally extended current out of the ionosphere at 67° magnetic latitude and another sheet into the ionosphere 10 km further north. These observations provide evidence that the Hall current system generated in the ion diffusion region of a tail reconnection X‐line, can close in the ionosphere.

Characteristics and Roles of ESW Observed in the Separatrix of Reconnection in the Magnetotail

AbstractMagnetic reconnection plays a key role in the energy release and transformation in geospace. Though Electrostatic Solitary Wave (ESW) is widely observed in space, direct observations in the vicinity of reconnection site are rare, which makes it ambiguous of the characteristics of ESW associated with reconnection. Here we present a reconnection event for which ESW is ample in the separatrix of the reconnection diffusion region. Characteristics of the ESW associated with reconnection are studied and the effect of ESW to reconnection is discussed. The result shows that the ESW observed in the separatrix of reconnection which takes place during a substorm is more solitary and more nonlinear than that in other regions. The ESW observed associated with reconnection can propel the reconnection process via presenting a parallel electric field and accelerate electron.

Kinetic Alfvén Wave Turbulence and Transport through a Reconnection Diffusion Region

We demonstrate from observations that kinetic Alfvén waves may play an important role in facilitating magnetic reconnection. These waves radiate outwards from the diffusion region oblique to the magnetic field in a conelike pattern delimited by the X line separatrices with outward energy fluxes equivalent to that contained in the outstreaming ions. It is shown that the wave vectors reverse across the X and symmetry lines and have a large out of plane component. We estimate that these waves drive significant transport through the diffusion region.