Magnetic Reconnection Site Research Articles

RECONNECTION-POWERED LINEAR ACCELERATOR AND GAMMA-RAY FLARES IN THE CRAB NEBULA

The recent discovery of day-long gamma-ray flares in the Crab Nebula, presumed to be synchrotron emission by PeV (10^{15} eV) electrons in milligauss magnetic fields, presents a strong challenge to particle acceleration models. The observed photon energies exceed the upper limit (~100 MeV) obtained by balancing the acceleration rate and synchrotron radiation losses under standard conditions where the electric field is smaller than the magnetic field. We argue that a linear electric accelerator, operating at magnetic reconnection sites, is able to circumvent this difficulty. Sufficiently energetic electrons have gyroradii so large that their motion is insensitive to small-scale turbulent structures in the reconnection layer and is controlled only by large-scale fields. We show that such particles are guided into the reconnection layer by the reversing magnetic field as they are accelerated by the reconnection electric field. As these electrons become confined within the current sheet, they experience a decreasing perpendicular magnetic field that may drop below the accelerating electric field. This enables them to reach higher energies before suffering radiation losses and hence to emit synchrotron radiation in excess of the 100 MeV limit, providing a natural resolution to the Crab gamma-ray flare paradox.

SOLAR WIND AND CORONAL BRIGHT POINTS INSIDE CORONAL HOLES

Observations of 108 coronal holes (CHs) from 1998-2008 were used to investigate the correlation between fast solar wind (SW) and several parameters of CHs. Our main goal was to establish the association between coronal bright points (CBPs; as sites of magnetic reconnection) and fast SW. Using in situ measurements of the SW, we have connected streams of the fast SW at 1 AU with their source regions, CHs. We studied a correlation between the SW speed and selected parameters of CHs: total area of the CH, total intensity inside the CH, fraction of area of the CH associated with CBPs, and their integrated brightness inside each CH. In agreement with previous studies, we found that the SW speed most strongly correlates with the total area of the CHs. The correlation is stronger for the non (de)projected areas of CHs (which are measured in image plane) suggesting that the near-equatorial parts of CHs make a larger contribution to the SW measured at near Earth orbit. This correlation varies with solar activity. It peaks for periods of moderate activity, but decreases slightly for higher or lower levels of activity. A weaker correlation between the SW speed and other studied parameters was found, but it can be explained by correlating these parameters with the CH's area. We also studied the spatial distribution of CBPs inside 10 CHs. We found that the density of CBPs is higher in the inner part of CHs. As such, results suggest that although the reconnection processes occurring in CBPs may contribute to the fast SW, they do not serve as the main mechanism of wind acceleration.

Energy source for auroral electrons from two proposed substorm onset processes

[1] There are two main processes in the tail that are proposed to link electron precipitation for the initial brightening auroral arc at substorm onset. These are current disruption/dipolarization (CDD) and magnetic reconnection (MR). To provide further clues in differentiating these two processes, we examine the electron energy spectrum and the associated total field-aligned energy flux during six CDD events and compare them with five MR events. Data from the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE), Geotail, and Time History of Events and Macroscale Ineractions during Substorms (THEMIS) missions are used in this study. It is found that (1) the total field-aligned electron energy flux from CDD sites is generally high enough to account for the energy flux required to produce bright auroral arcs at substorm onsets and (2) the total field-aligned electron energy flux from MR sites is generally a factor of one to two orders of magnitude lower than that from CDD sites. These results favor a direct link between the CDD sites and the substorm onset arcs, but they do not rule out further acceleration of auroral electrons away from these sites by other energy sources (e.g., field-aligned potential drops and kinetic Alfven waves) to produce even higher-intensity auroral arcs.

Budget of energetic electrons during solar flares in the framework of magnetic reconnection

Context. Among other things, solar flares are accompanied by the production of energetic electrons as seen in the nonthermal radio and X-ray radiation of the Sun. Observations of the RHESSI satellite show that 10 32 −10 36 nonthermal electrons are produced per second during flares. They are related to an energy flux in the range 10 18 −10 22 W. These electrons play an important role, since they carry a substantial part of the energy released during a flare. Aims. In which way so many electrons are accelerated up to high energies during a fraction of a second is still an open question. By means of radio and hard X-ray data, we investigate under which conditions this acceleration happens in the corona. Methods. The flare is considered in the framework of magnetic reconnection. The conditions in the acceleration region in the corona are deduced by using the conservation of the total electron number and energy. Results. In the inflow region of the magnetic reconnection site, there are typical electron number densities of about 2.07×10 9 cm −3 and magnetic fields of about 46 G. These are regions with high Alfven speeds of about 2200 kms −1 . Then, sufficient energetic electrons (as required by the RHESSI observations) are only generated if the plasma inflow towards the reconnection site has Alfven-Mach numbers in the range 0.1−1, which can lead to a super-Alfvenic outflow with speeds up to 3100 km s −1 .

Using a global magnetohydrodynamic model to study the start of the substorm recovery phase

Magnetospheric substorms are a key manifestation of the transfer of energy from the solar wind to the Earth. Substorms consist of a loading of energy from the solar wind via magnetic reconnection and then a rapid release of that energy. One of the results of this energy release is the intense brightening of the aurora observed in the Earth's high‐latitude regions. At some point, the dissipation of energy peaks and the system recovers. This paper focuses on addressing the question of what causes the transition to the recovery phase of the substorm typically defined by the retreat of the substorm magnetic reconnection site, or neutral line, in the magnetotail. One suggested hypothesis is that the expansion of the dipolarization of the magnetic field in the inner magnetosphere causes the neutral line to move tailward. We use the Lyon‐Fedder‐Mobarry magnetohydrodynamic code to model seven substorms to examine what causes the neutral line retreat in the simulation. We conclude that while the simulation reproduces the substorms with the loading and unloading of energy, the retreat of the neutral line is a directly driven process that is controlled by the solar wind.

Observations of Slow Electron Holes at a Magnetic Reconnection Site

We report in situ observations of high-frequency electrostatic waves in the vicinity of a reconnection site in the Earth's magnetotail. Two different types of waves are observed inside an ion-scale magnetic flux rope embedded in a reconnecting current sheet. Electron holes (weak double layers) produced by the Buneman instability are observed in the density minimum in the center of the flux rope. Higher frequency broadband electrostatic waves with frequencies extending up to f(pe) are driven by the electron beam and are observed in the denser part of the rope. Our observations demonstrate multiscale coupling during the reconnection: Electron-scale physics is induced by the dynamics of an ion-scale flux rope embedded in a yet larger-scale magnetic reconnection process.

Test of methods to infer the magnetic reconnection geometry from spacecraft data

When analyzing data from a cluster of spacecraft (such as Cluster or MMS) crossing a site of magnetic reconnection, it is desirable to be able to determine the orientation of the reconnection site. If the reconnection is quasi‐two dimensional, there are three key directions, the direction of inhomogeneity (direction across the reconnection site), the direction of the reconnecting component of the magnetic field, and the direction of rough invariance (the “out of plane” direction). Using simulated spacecraft observations of an MHD simulation of magnetic reconnection in the geomagnetic tail, we test a direction‐finding method based on the gradient of the vector magnetic field and find that the directions can be well determined. The results from this method, however, can be in error if there are systematic calibration errors in the magnetic field measurements. We show that the effect of these errors can be eliminated if an average gradient is subtracted from a time series of gradient values before they are used for the analysis. We also test a method to determine the velocity of the reconnecting structure relative to the spacecraft using the time derivative and gradient of the magnetic field and show that this velocity can be well determined. Calibration errors can be eliminated in this case also if an average time derivative and gradient are subtracted from the time series of values before they are used for the analysis.

Separatrix regions of magnetic reconnection at the magnetopause

Abstract. Using data from the four Cluster spacecraft we study the separatrix regions of magnetic reconnection sites at the dayside magnetopause under conditions when reconnection is occurring in the magnetopause current layer which separates magnetosheath plasma from the hot magnetospheric plasma sheet. We define the separatrix region as the region between the separatrix – the first field line opened by reconnection – and the reconnection jet (outflow region). We analyze eight separatrix region crossings on the magnetospheric side of the magnetopause and present detailed data for two of the events. We show that characteristic widths of the separatrix regions are of the order of ten ion inertial lengths at the magnetopause. Narrow separatrix regions with widths comparable to a few ion inertial lengths are rare. We show that inside the separatrix region there is a density cavity which sometimes has complex internal structure with multiple density dips. Strong electric fields exist inside the separatrix regions and the electric potential drop across the regions can be up to several kV. On the magnetosheath side of the region there is a density gradient with strong field aligned currents. The observed strong electric fields and currents inside the separatrix region can be important for a local energization of ions and electrons, particularly of ionospheric origin, as well as for magnetosphere-ionosphere coupling.

The magnetic field reconnection site and dissipation region

Magnetic field reconnection is said to involve an ion diffusion region surrounding an electron diffusion region. Because of uncertainties in the meanings of these terms and on the physical parameters that characterize them, this paper defines the reconnection site as a region having an electron scale size and containing magnetic fields from four topologies, and the dissipation region, as having an ion scale size and surrounding the reconnection site. Two-dimensional, asymmetric, open, particle-in-cell simulations, with and without guide fields, examine these regions. It is found that significant values of (E+UI×B) and/or (E+Ue×B) are not confined to either the reconnection site or the dissipation region. The reconnection site is uninteresting because, for asymmetric reconnection, it does not contain processes that serve to locate it, such as electron acceleration, parallel electric fields, super-Alfvenic electron flow, maximum electron beta, electron nongyrotopy, or demagnetized thermal electrons. However, the surrounding dissipation region exhibits these features.

HINODE OBSERVATION OF THE MAGNETIC FIELDS IN A SUNSPOT LIGHT BRIDGE ACCOMPANIED BY LONG-LASTING CHROMOSPHERIC PLASMA EJECTIONS

We present high-resolution magnetic field measurements of a sunspot light bridge (LB) that produced chromospheric plasma ejections intermittently and recurrently for more than 1 day. The observations were carried out with the Hinode Solar Optical Telescope on 2007 April 29 and 30. The spectro-polarimeter reveals obliquely oriented magnetic fields with vertical electric current density higher than 100 mA m–2 along the LB. The observations suggest that current-carrying highly twisted magnetic flux tubes are trapped below a cusp-shaped magnetic structure along the LB. The presence of trapped current-carrying flux tubes is essential for causing long-lasting chromospheric plasma ejections at the interface with pre-existing vertically oriented umbral fields. A bidirectional jet was clearly detected, suggesting magnetic reconnections occurring at very low altitudes, slightly above the height where the vector magnetic fields are measured. Moreover, we found another strong vertical electric current on the interface between the current-carrying flux tube and pre-existing umbral field, which might be a direct detection of the currents flowing in the current sheet formed at the magnetic reconnection sites.

SLIP-SQUASHING FACTORS AS A MEASURE OF THREE-DIMENSIONAL MAGNETIC RECONNECTION

A general method for describing magnetic reconnection in arbitrary three-dimensional magnetic configurations is proposed. The method is based on the field-line mapping technique previously used only for the analysis of magnetic structure at a given time. This technique is extended here so as to analyze the evolution of magnetic structure. Such a generalization is made with the help of new dimensionless quantities called "slip-squashing factors". Their large values define the surfaces that border the reconnected or to-be-reconnected magnetic flux tubes for a given period of time during the magnetic evolution. The proposed method is universal, since it assumes only that the time sequence of evolving magnetic field and the tangential boundary flows are known. The application of the method is illustrated for simple examples, one of which was considered previously by Hesse and coworkers in the framework of the general magnetic reconnection theory. The examples help us to compare these two approaches; they reveal also that, just as for magnetic null points, hyperbolic and cusp minimum points of a magnetic field may serve as favorable sites for magnetic reconnection. The new method admits a straightforward numerical implementation and provides a powerful tool for the diagnostics of magnetic reconnection in numerical models of solar-flare-like phenomena in space and laboratory plasmas.

A state‐of‐the‐art picture of substorm‐associated evolution of the near‐Earth magnetotail obtained from superposed epoch analysis

We have obtained a state‐of‐the‐art picture of substorm‐associated evolution of the near‐Earth magnetotail and the inner magnetosphere for understanding the substorm triggering mechanism. We performed superposed epoch analysis of Geotail, Polar, and GOES data with 2‐min resolution, utilizing a total of 3787 substorms for each of which auroral breakup was determined from Polar UVI or IMAGE FUV auroral imager data. The decrease of the north‐south magnetic field associated with plasmoids and the initial total pressure decrease suggest that the magnetic reconnection first occurs in the premidnight tail, on average, at X ∼ −16 to −20 RE at least 2 min before auroral onset. The magnetic reconnection site is located near the tailward edge of a region of considerably taillike magnetic field lines and intense cross‐tail current, which extends from X ∼ −5 to −20 RE in the premidnight sector. Then the plasmoid substantially evolves tailward of X ∼ −20 RE immediately after onset. Almost simultaneously with the magnetic reconnection, the dipolarization begins first at X ∼ −7 to −10 RE 2 min before onset. The dipolarization region then expands tailward as well as in the dawn‐dusk directions and earthward. We find that the total pressure generally enhances in association with the dipolarization, with the contribution of high‐energy particles. Also, energy release is more significant between the regions of the magnetic reconnection and the initial dipolarization. The present results will be helpful as a reference guide to developing the overall picture of magnetotail evolution and studying the causal relationship between the magnetic reconnection and the dipolarization as well as detailed mechanisms of each of the two processes on the basis of multispacecraft observations.

Magnetic layers and neutral points near a rotating black hole

Magnetic layers are narrow regions where the field direction changes sharply. They often occur in the association with neutral points of the magnetic field. We show that an organized field can produce these structures near a rotating black hole (BH), and we identify them as potential sites of magnetic reconnection. To that end, we study the field lines affected by the frame-dragging effect, twisting the magnetic structure and changing the position of neutral points. We consider oblique fields in vacuum. We also include the possibility of translational motion of the black hole which may be relevant when the black hole is ejected from the system. The model settings apply to the innermost regions around black holes with the ergosphere dominated by a super-equipartition magnetic field and loaded with a negligible gas content.

Relationship between the topological skeleton, current concentrations, and 3D magnetic reconnection sites in the solar atmosphere

Aims. The aim of this work is to determine the relationship between the 3D structure of the coronal magnetic field, diagnosed by the topological skeleton, and current concentrations as potential sites of 3D reconnection. Methods. We utilised the results of 3D numerical MHD simulations of an observed EUV bright point (BP) in the solar atmosphere. The simulations are based on MDI line-of-sight magnetogram data from 13 June 1998. We analysed the results of the simulations using the method of magnetic charge topology. Three different methods of reducing the magnetogram to a set of point magnetic sources are tested. Results. Observations of the BP show a rotation of one of its main magnetic source regions. Numerical simulations of this rotational motion result in a localised build-up of parallel electric current, which is dissipated by anomalous resistivity, causing 3D magnetic reconnection and BP heating. The magnetic topological structure of the simulated BP was also calculated, and a portion of the topological separatrix surface bounding the magnetic flux of the rotating source region is found to correspond to the locations of current build-up and heating. All three magnetogram reduction methods produce similar results for the large-scale magnetic field structure. Conclusions. Magnetic topology is a useful method for predicting the locations of coronal current concentrations, insofar as the results of our simulations show that strong integrated parallel electric fields are found only along topological separatrix surfaces. However, further investigation is necessary to determine exactly which parts of the reconstructed separatrices will host the electric currents. Topological magnetic field reconstructions also cast light on the location of coronal BP heating, which occurs as a result of the dissipation of the currents by 3D reconnection. The choice of the magnetogram reduction algorithm does not greatly affect the large-scale topological features of the resulting reconstructed magnetic field. Further work is required to compare these results with data for other observed BPs.

Particle Interactions with Single or Multiple 3D Solar Reconnecting Current Sheets

The acceleration of charged particles (electrons and protons) in flaring solar active regions is analyzed using numerical experiments. The acceleration is modeled as a stochastic process taking place by the interaction of the particles with local magnetic reconnection sites via multiple steps. Two types of local reconnecting topologies are studied: the Harris-type and the X-point. A formula for the maximum kinetic energy gain in a Harris-type current sheet, found in a previous work of ours, fits well the numerical data for a single step of the process. A generalization is then given approximating the kinetic energy gain through an X-point. In the case of the multiple step process, in both topologies the kinetic energy distribution of the particles is found to acquire a practically invariant form after a small number of steps. This tendency is interpreted theoretically. Other characteristics of the acceleration process are given, such as the mean acceleration time and the pitch angle distributions of the particles.

Double Coronal Hard and Soft X‐Ray Source Observed byRHESSI: Evidence for Magnetic Reconnection and Particle Acceleration in Solar Flares

We present data analysis and interpretation of an M1.4 class flare observed with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on 2002 April 30. This event, with its footpoints occulted by the solar limb, exhibits a rarely observed, but theoretically expected, double-source structure in the corona. The two coronal sources, observed over the 6-30 keV range, appear at different altitudes and show energy-dependent structures with the higher energy emission being closer together. Spectral analysis implies that the emission at higher energies in the inner region between the two sources is mainly nonthermal, while the emission at lower energies in the outer region is primarily thermal. The two sources are both visible for about 12 minutes and have similar light curves and power-law spectra above about 20 keV. These observations suggest that the magnetic reconnection site lies between the two sources. Bidirectional outflows of the released energy in the form of turbulence and/or particles from the reconnection site could be the source of the observed radiation. The spatially resolved thermal emission below about 15 keV, on the other hand, indicates that the lower source has a larger emission measure but a lower temperature than the upper source. This is likely the result of the differences in the magnetic field and plasma density of the two sources.

Multiple magnetic reconnection sites associated with a coronal mass ejection in the solar wind

Magnetic reconnection exhausts in the solar wind are often associated with interplanetary coronal mass ejections (ICMEs). Occasionally, an exhaust is observed at the interface between ambient solar wind and the leading edge of an ICME where reconnection serves to erode away some of the magnetic flux carried by the ICME. More often the exhausts are observed at thin current sheets within the interiors of ICMEs or near their trailing edges. We have examined an unusually large set of seven reconnection exhausts, including both sunward and antisunward directed events, observed within the interior of and near the trailing edge of an ICME. Six of these exhausts occurred as pairs on the opposite sides of relatively small magnetic filaments of locally reversed fields. For three of the exhausts the local magnetic shear was considerably less than 90°, indicating that reconnection can occur at times when the so‐called guide field component considerably exceeds the antiparallel field component. One exhaust had a local width of only 1.6 × 103 km (25.4 ion inertial lengths) and was convected past the Wind spacecraft in ∼5.4 s. No Hall field rotations were observed in association with this event, probably because of the significant guide field present and because even this extremely narrow exhaust was probably sampled well downstream (∼380 ion inertial lengths) from the reconnection site. The current sheets bounding the exhaust had widths of 3.5 and 5.2 ion inertial lengths, respectively, exceeding by several times the expected scale size of the associated diffusion region.

Particle distributions and X-ray spectra in single or multiple solar current sheets

The acceleration of charged particles in a site of magnetic reconnection is analysed by detailed numerical simulations. Single or multiple encounters of the particles with Harris-type reconnecting current sheets (RCSs) are modelled as an overall stochastic process taking place within an active region. RCS physical parameters are selected in a parameter space relevant to solar flares. Initially, the charged particles form a thermal (Maxwellian) distribution corresponding to coronal temperature � 2 × 10 6 K. Our main goal is to investigate how the acceleration process changes the shape of the particles’ kinetic energy distribution. The evolution of the kinetic energy distribution, calculated numerically after one encounter of the particles with a single RCS, is found to be in good agreement with our previously published analytical formulae. In the case of consecutive encounters, we find that the kinetic distribution tends to converge to a practically invariant form after a relatively small number of encounters. We construct a discrete stochastic process that reproduces the numerical distributions and we provide a theoretical interpretation of the asymptotic convergence of the energy distribution. We finally compute the theoretical X-ray spectra that would be emitted by the simulated particles in a thick target model of radiation.

Source of whistler emissions at the dayside magnetopause

Observations of whistler emissions are common in the magnetosphere near the dayside magnetopause. We show that one of the major source regions for these emissions is magnetic field minima that form along magnetic flux tubes at high latitudes. Using multispacecraft Cluster observations we experimentally confirm for the first time the existence of the magnetic field minima at high latitudes and we show that whistler emissions propagate away from the magnetic field minima. The strongest whistler emissions are observed on the magnetospheric flux tubes that are newly opened due to the magnetic reconnection. These flux tubes still have a density of magnetospheric plasma, but part of the high energy magnetospheric electrons have already been lost from the flux tubes. The partial loss of high energy electrons most probably causes anisotropy in electron distributions at high energies which should be the source of whistler emissions. Whistler emissions on opened flux tubes disappear as soon as the plasma density of flux tubes increases due to the entering of magnetosheath ions. We speculate that whistler emissions can most probably be used to trace the dynamics of the first opened field lines and thus the dynamics of magnetic reconnection sites.

3He-Rich Solar Energetic Particle Events

He-rich solar energetic particle (SEP) events show huge enrichments of 3 He and association with kilovolt electrons and Type-III radio bursts. Observations from a new gener- ation of high resolution instruments launched on the Wind, ACE, Yohkoh, SOHO, TRACE, and RHESSI spacecraft have revealed many new properties of these events: the particle en- ergy spectra are found to be either power-law or curved in shape, with the 3 He spectrum often being distinctly different from other species. Ultra-heavy nuclei up to >200 amu are found to be routinely present at average enrichments of >200 times solar-system abun- dances. The high ionization states previously observed near ∼ 1M eV /nucleon have been found to decrease towards normal solar coronal values in these events. The source regions have been identified for many events, and are associated with X-ray jets and EUV flares that are associated with magnetic reconnection sites near active regions. This paper reviews the current experimental picture and theoretical models, with emphasis on the new insights found in the last few years.