The magnetic nature of solar flares

The main challenge for the theory of solar eruptions has been to understand two basic aspects of large flares. These are the cause of the flare itself and the nature of the morphological features which form during its evolution. Such features include separating ribbons of H emission joined by a rising arcade of soft x-ray loops, with hard x-ray emission at their summits and at their feet. Two major advances in our understanding of the theory of solar flares have recently occurred. The first is the realisation that a magnetohydrodynamic (MHD) catastrophe is probably responsible for the basic eruption and the second is that the eruption is likely to drive a reconnection process in the field lines stretched out by the eruption. The reconnection is responsible for the ribbons and the set of rising soft x-ray loops, and such a process is well supported by numerical experiments and detailed observations from the Japanese satellite Yohkoh. Magnetic energy conversion by reconnection in two dimensions is relatively well understood, but in three dimensions we are only starting to understand the complexity of the magnetic topology and the MHD dynamics which are involved. How the dynamics lead to particle acceleration is even less well understood. Particle acceleration in flares may in principle occur in a variety of ways, such as stochastic acceleration by MHD turbulence, acceleration by direct electric fields at the reconnection site, or diffusive shock acceleration at the different kinds of MHD shock waves that are produced during the flare. However, which of these processes is most important for producing the energetic particles that strike the solar surface remains a mystery. Received 2 January 2001 / Published online 17 July 2001     

A numerical experiment relevant to line-tied reconnection in two-ribbon flares

The nonlinear evolution of a reconnecting magnetic field configuration similar to that occurring just before the onset of ‘post’-flare loops in two-ribbon flares is determined. The evolution, which is obtained by numerically solving the resistive MHD equations, shows two new features that have not yet been incorporated into contemporary models of ‘post’-flare loops. The first of these new features is the formation of a nearly stationary fast-mode shock above the region corresponding to the top of the loops. This fast-mode shock occurs just below the magnetic neutral line and between the slow-mode shocks associated with fast magnetic reconnection at the neutral line. The second new feature is the creation and annihilation of large-scale magnetic islands in the current sheet above the loops. The annihilation of the islands occurs very rapidly and appears to be a manifestation of the coalescence instability. The creation and annihilation of magnetic islands could be important in understanding the energetics of ‘post’-flare loops since the coalescence instability can produce an intermittent energy release more than an order of magnitude faster than that predicted by steady-state reconnection theories.     

Forced reconnection by nonlinear magnetohydrodynamic waves

Forced magnetic reconnection induced by magnetohydrodynamic (MHD) waves may account for the triggering of explosive solar activities such as flares. Reconnection in a neutral sheet plasma can be driven by the ponderomotive force associated with nonlinear MHD waves accompanying plasma vortex motion. The nonlinear stage of forced reconnection by MHD waves is simulated with a MHD particle-code: Some conditions for fast reconnection are discussed with applications to solar flares.     

Magnetic reconnection: flares and coronal heating in active galactic nuclei

A magnetically structured accretion disc corona, generated by buoyancy instability in the disc, can account for observations of flare-like events in active galactic nuclei. We examine how Petschek magnetic reconnection, associated with MHD turbulence, can result in a violent release of energy and heat the magnetically closed regions of the corona up to canonical X-ray emitting temperatures. X-ray magnetic flares, the after effect of the energy released in slow shocks, can account for the bulk of the X-ray luminosity from Seyfert galaxies and consistently explain the observed short-time-scale variability.     

Mass upflows in ‘post’-flare loops

A self-consistent numerical model of a reconnecting magnetic field configuration similar to that occurring during the main-phase of two-ribbon flares is used to estimate the upflow caused by the fast-mode expansion of the magnetic field moving into the reconnection region. Such an expansion creates a field-aligned pressure gradient which accelerates plasma upward from the chromospheric base of magnetic field lines in the region external to the loops. The numerical results imply that the amount of mass sucked up in this way is even smaller than was previously estimated by Kopp and Pneuman who used a kinematic model. Therefore, some indirect mechanism (such as evaporation), which would probably derive its motive power from the thermal energy generated by the reconnection, is required to explain the large mass upflows inferred from observations.     

Soft X-ray emission from a hot turbulent current sheet and the precursor phase of solar flares

The properties and development of a high-temperature current sheet characterized by increasing merging velocity are studied and related to the early phases of solar flares. It is shown that the system can be described by the Petschek-type geometry for a wide range of merging velocities. In the diffusion region and the standing MHD shocks a certain low-frequency plasma microturbulence is generated from the very beginning of the reconnection process. We present qualitative solutions for the case of ion-acoustic turbulence in marginally stable state, which provide a comparison with observations. The increasing merging velocity leads to the appearance of the soft X-ray precursor. The precursor temperature maximum should appear during the current sheet formation, before the Petschek regime is established. In the Petschek regime the temperature of the hot plasma decreases due to the decrease of the magnetic field strength at the diffusion region boundary, while the soft X-ray radiation still increases, reaching precursor maximum for merging velocities about 1% of the external Alfvén velocity. The precursor phase ends when the value of the merging velocity surpasses the upper limit for the Petschek regime and the system enters into the pile-up regime, causing a new increase of plasma temperature and soft X-ray radiation.It is shown that Alfvén velocities in the range 800–1200 km s ^–1 are sufficient to explain typical soft X-ray precursors. Cases of low merging velocities and low Alfvén velocities are discussed and can be applied to describe the properties of spotless flares.     

Numerical simulation of reconnection in an emerging magnetic flux region

The resistive MHD equations are numerically solved in two dimensions for an initial-boundary-value problem which simulates reconnection between an emerging magnetic flux region and an overlying coronal magnetic field. The emerging region is modelled by a cylindrical flux tube with a poloidal magnetic field lying in the same plane as the external, coronal field. The plasma betas of the emerging and coronal regions are 1.0 and 0.1, respectively, and the magnetic Reynolds number for the system is 2 × 10³. At the beginning of the simulation the tube starts to emerge through the base of the rectangular computational domain, and, when the tube is halfway into the computational domain, its position is held fixed so that no more flux of plasma enters through the base. Because the time-scale of the emergence is slower than the Alfvén time-scale, but faster than the reconnection time-scale, a region of closed loops forms at the base. These loops are gradually opened and reconnected with the overlying, external magnetic field as time proceeds.The evolution of the plasma can be divided into four phases as follows: First, an initial, quasi-steady phase during which most of the emergence is completed. During this phase, reconnection initially occurs at the slow rate predicted by the Sweet model of diffusive reconnection, but increases steadily until the fast rate predicted by the Petschek model of slow-shock reconnection is approached. Second, an impulsive phase with large-scale, super-magnetosonic flows. This phase appears to be triggered when the internal mechanical equilibrium inside the emerging flux tube is upset by reconnection acting on the outer layers of the flux tube. During the impulsive phase most of the flux tube pinches off from the base to form a cylindrical magnetic island, and temporarily the reconnection rate exceeds the steady-state Petschek rate. (At the time of the peak reconnection rate, the diffusion region at the X-line is not fully resolved, and so this may be a numerical artifact.) Third, a second quasi-steady phase during which the magnetic island created in the impulsive phase is slowly dissipated by continuing, but low-level, reconnection. And fourth, a static, non-evolving phase containing a potential, current-free field and virtually no flow.During the short time in the impulsive phase when the reconnection rate exceeds the steady-state Petschek rate, a pile-up of magnetic flux at the neutral line occurs. At the same time the existing Petschek-slow-mode shocks are shed and replaced by new ones; and, for a while, both new and old sets of slow shocks coexist.     

On the relativistic magnetic reconnection

Reconnection of the magnetic lines of force is considered in case the magnetic energy exceeds the rest energy of the matter. It is shown that the classical Sweet–Parker and Petschek models are generalized straightforwardly to this case and the reconnection rate may be estimated by substituting the Alfven velocity in the classical formulae with the speed of light. The outflow velocity in the Sweet–Parker configuration is mildly relativistic. In the Petschek configuration, the outflow velocity is ultrarelativistic whereas the angle between the slow shocks is very small. As a result of the strong compression, the plasma outflow in the Petschek configuration may become strongly magnetized if the reconnecting fields are not exactly antiparallel.     

Observation of Two Slow Shocks Associated with Magnetic Reconnection Exhausts in the Interplanetary Space

In the Petschek magnetic reconnection model, two groups of slow shocks play an important role in the energy release. In the past half century, a large number of slow shocks were observed in the geomagnetic tail, and many slow shocks were associated with magnetic reconnection events in the geomagnetic tail. Slow shocks in the interplanetary space are rarer than in the geomagnetic tail. We investigated whether slow shocks associated with interplanetary reconnection exhausts are rare. We examined the boundaries of 50 reconnection exhausts reported by Phan, Gosling, and Davis (Geophys. Res. Lett. 36:L09108, 2009) in interplanetary space to identify slow shocks by fitting the Rankine–Hugoniot relations. Two slow shocks associated with magnetic reconnection exhausts were found and evaluated using observations from Wind and the Advanced Composition Explorer. The observed slow shocks associated with interplanetary reconnection exhausts are rarer than the observed slow shocks associated with geomagnetic tail reconnection exhausts.     

Generalized analytical models of Syrovatskii’s current sheet

Two-dimensional stationary magnetic reconnection models that include a thin Syrovatskii-type current sheet and four discontinuous magnetohydrodynamic flows of finite length attached to its endpoints are considered. The flow pattern is not specified but is determined from a self-consistent solution of the problem in the approximation of a strong magnetic field. Generalized analytical solutions that take into account the possibility of a current sheet discontinuity in the region of anomalous plasma resistivity have been found. The global structure of the magnetic field in the reconnection region and its local properties near the current sheet and attached discontinuities are studied. In the reconnection regime in which reverse currents are present in the current sheet, the attached discontinuities are trans-Alfvénic shock waves near the current sheet endpoints. Two types of transitions from nonevolutionary shocks to evolutionary ones along discontinuous flows are shown to be possible, depending on the geometrical model parameters. The relationship between the results obtained and numerical magnetic reconnection experiments is discussed.     

Understanding the Physical Nature of Coronal “EIT Waves”

For almost 20 years the physical nature of globally propagating waves in the solar corona (commonly called “EIT waves”) has been controversial and subject to debate. Additional theories have been proposed over the years to explain observations that did not agree with the originally proposed fast-mode wave interpretation. However, the incompatibility of observations made using the Extreme-ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliospheric Observatory with the fast-mode wave interpretation was challenged by differing viewpoints from the twin Solar Terrestrial Relations Observatory spacecraft and data with higher spatial and temporal resolution from the Solar Dynamics Observatory. In this article, we reexamine the theories proposed to explain EIT waves to identify measurable properties and behaviours that can be compared to current and future observations. Most of us conclude that the so-called EIT waves are best described as fast-mode large-amplitude waves or shocks that are initially driven by the impulsive expansion of an erupting coronal mass ejection in the low corona.     

Energy balance in current sheets: From Petschek to gravity driven reconnection?

It has been shown earlier that energy balance processes play a very important role in the determination of the reconnection regime in the central diffusive region of a steady Petschek flow (usually considered elsewhere as isothermal and incompressible): as a consequence of the plasma thermal properties, abrupt transitions in the reconnection regime may occur for special external conditions. The regime becomes then a dynamical one, and it was suggested that onset of plasma microturbulence may result and act as a primary triggering mechanism in solar flares.In this paper we will reexamine the problem of onset of such dynamical transition and conclude that plasma microturbulence does not appear in a straightforward way. However it is possible that the canonical Petschek regime may evolute into a new one in which the dissipative sheet is no longer infinitesimal with respect to the dimensions of the structure, and in which gravity plays an important role. Flare triggering, if related to the reconnection regime, must then proceed by more complex processes, possibly related to tearing mode dynamics, or to more global properties of the magnetic structure of the active region.     

耦合热传导的磁重联的数值模拟

磁重联被认为是太阳耀斑的产生机制,本文数值模拟在日冕中发生在磁重联过程,结果表明耀斑环的表观运动是磁重联的自洽结果;由重联点发出的慢激波对耀斑环的加热有贡献;耀斑环的上升并不意味着重联点的上升。     

耀斑环动力学的数值研究

数值模拟了太阳耀斑环动力学演化的二维磁重联过程。结果表明,在重联Ｘ 点比较高时,演化过程能再现双带耀斑中的耀斑环活动;而重联Ｘ 点比较低时,演化情况能解释致密耀斑的观测特征     

The flare-associated magnetic changes in an active region

The associations of flares to flux emergence and cancellation have been further examined and clarified with the aid of complete time sequences of vector magnetograms of an active region for a 4-day period around the central meridian passage.It is found that the emergence of new flux and its driven flux cancellation with existing flux is a wholly inseparable, elementary process in the active region, favorable for flare occurence. The early discovery ofstructures magnetique evolutive (Martreset al., 1968) is confirmed and identified to be the net result of this process.All events of flux cancellation appear in the interface of two topologically separated magnetic loops. Direct indications of magnetic reconnection between two cancelling components in the photospheric layer are identified. The cancellation is most likely a slow reconnection in the lower atmosphere of the Sun. The quite popular view of interpreting flux cancellation as a pure flux submergence could not fit the magnetic topology learned from alignments of the transverse magnetic field. In this sense, the association of flares to flux cancellation seems to represent a coupling of the slow reconnection in the lower atmosphere to the fast reconnection higher in the corona.This slow reconnection can even take place below the photosphere. In one case, an inferred sub-photospheric reconnection eventually prevents one pole of an emerging flux region with the polarity opposite to the background from showing up at the photospheric level.Six of all eight flares which appeared in this period are spatially and temporally associated with the emergence of new flux and its driven cancellation. They might be divided into two groups. The first group of flares appears at the early phase of flux emergence and in close proximity to the cancelling site between new and old flux; the second ones appear after several hours of flux cancellation, centering around the cancelling site.     

Solar flares: The gradual phase

One has to distinguish between two kinds of the gradual phase of flares: (1) a gradual phase during which no energy is released so that we see only cooling after the impulsive phase (a confined flare), and (2) a gradual phase during which energy release continues (a dynamic flare).The simplest case of (1) is a single-loop flare which might provide an excellent opportunity for the study of cooling processes in coronal loops. But most confined flares are far more complicated: they may consist of sets of unresolved elementary loops, of conglomerates of loops, or they form arcades the components of which may be excited sequentially. Accelerated particles as well as hot and cold plasma can be ejected from the flare site (coronal tongues, flaring arches, sprays, bright and dark surges) and these ejecta may cool more slowly than the source flare itself.However, the most important flares on the Sun are flares of type (2) in which a magnetic field opening is followed by subsequent reconnection of fieldlines that may continue for many hours after the impulsive phase. Therefore, the main attention in this review is paid to the gradual phase of this category of long-decay flares. The following items are discussed in particular: The wide energy range of dynamic flares: from eruptions of quiescent filaments to most powerful cosmic-ray flares. Energy release at the reconnection site and modelling of the reconnection process. The post-flare loops: evidence for reconnection; observations at different wavelengths; energy deposit in the chromosphere, chromospheric ablation, and velocity fields; loops in emission; shrinking loops; magnetic modelling. The gradual phase in X-rays and on radio waves. Post-flare X-ray arches: observations, interpretation, and modelling; relation to metric radio events and mass ejections, multiple-ribbon flares and anomalous events, hybrid events, possible relations between confined and dynamic flares.     

The gradual and impulsive reconnection and the preheating of solar flares

We discuss the preheating phase of solar flares triggered by emerging magnetic flux. We consider the development of microinstabilities in the diffusion region during the emergence process and we propose four different types of reconnection, by which we explain the preheating, as well as the impulsive phase of flares. We find that during the emergence of new magnetic flux the current sheet will not jump from the initial classical state to a fully turbulent one, but will remain in a marginally turbulent state which may develop either gradually or impulsively depending on the conditions of emergence. As a consequence of this, we find that four cases of reconnection are indeed possible: a week gradual heating, a weak impulsive process, a gradual preheating followed by an impulsive phase, and violent bursty reconnection.The expansion rate of the diffusion region, the duration of the gradual phase, the magnetic energy release, and the energy deposition rate in coronal loops during the gradual phase are derived under simplifying assumptions and applied to X-ray and UV observations of flares from the Solar Maximum Mission.On leave from the Department of Astronomy, Nanjing University, Nanjing, The People's Republic of China.     

激光驱动亥姆霍兹电容线圈靶磁重联实验的数值模拟

激光驱动亥姆霍兹电容线圈靶的磁重联实验已经提出并进行了多年.当实验中的金属板被强激光照射时产生自由电子,这些自由电子的运动在连接两金属板的两个平行线圈中产生电流,由两个平行线圈内部电流产生的磁场之间随即发生重联.该实验不同于其他直接由Biermann电池效应所产生高β(等离子体热压与磁压的比值)环境下的磁重联实验.对该类实验进行了3维磁流体动力学数值模拟,首次展示了亥姆霍兹电容器线圈靶如何驱动磁重联的过程.数值模拟结果清楚地表明,磁重联的出流等离子体在线圈周围发生与实验结果相一致的堆积现象.线圈电流产生的磁场可高达100 T,使得磁重联区域周围的等离子体β值达到10^-2.与实验室结果进行比较,数值模拟重复了实验展示的大多数特征,可有助于深入认识和理解实验结果背后的物理学原理.     

Particle Acceleration by a Time-Varying Electric Field in Merging Magnetic Fields

Traditional models for particle acceleration by magnetic reconnection in solar flares assumed a constant electric field in a steady reconnecting magnetic field. Although this assumption may be justified during the gradual phase of flares, the situation is different during the impulsive phase. Observed rapid variations in flare emissions imply that reconnection is non-steady and a time-varying electric field is present in a reconnecting current sheet. This paper describes exploratory calculations of charged particle orbits in an oscillating electric field present either at a neutral plane or a neutral line of two-dimensional magnetic field. A simple analytical model makes it possible to explain the effects of particle trapping and resonant acceleration previously noted by Petkaki and MacKinnon in a numerical simulation. As an application, electron acceleration to X-ray generating energies in impulsive solar flares is discussed within the context of the model.     

Implications of solar flare dynamics for reconnection in magnetospheric substorms

From observations of two-ribbon solar flares, we present a new line of evidence that magnetic reconnection is of key importance in magnetospheric substorms. We infer that in substorms reconnection of closed field lines in the near-Earth thinned plasma sheet both initiates and is driven by the overall MHD instability that drives the tailward expulsion of the reconnected closed field (0 loops). The general basis for this inference is the longstanding notion that two-ribbon flares and substorms are essentially similar phenomena, driven by similar processes. We give an array of observed similarities that substantiate this view. More specifically, our inference for substorms is drawn from observations of filament eruptions in two-ribbon flares, from which we conclude that the heart of the overall instability consists of reconnection and eruption of the closed magnetic field in and around the filament. We propose that essentially the same overall instability operates in substorms. Our point is not that the magnetic field configuration or the microphysics in substorms is identical to that in two-ribbon flares, but that the overall instability results from essentially the same combination of reconnection and eruption of closed magnetic field.