共查询到20条相似文献，搜索用时 250 毫秒
1.
Yohkoh observations strongly suggest that electron acceleration in solar flares occurs in magnetic reconnection regions in the corona above the soft X-ray flare loops. Unfortunately, models for particle acceleration in reconnecting current sheets predict electron energy gains in terms of the reconnection electric field and the thickness of the sheet, both of which are extremely difficult to measure. It can be shown, however, that application of Ohm's law in a turbulent current sheet, combined with energy and Maxwell's equations, leads to a formula for the electron energy gain in terms of the flare power output, the magnetic field strength, the plasma density and temperature in the sheet, and its area. Typical flare parameters correspond to electron energies between a few tens of keV and a few MeV. The calculation supports the viewpoint that electrons that generate the continuum gamma-ray and hard X-ray emissions in impulsive solar flares are accelerated in a large-scale turbulent current sheet above the soft X-ray flare loops. 相似文献
2.
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. 相似文献
3.
Current carrying magnetic fields which penetrate sunspots can be unstable to current convective modes caused by the large gradient of electrical conductivity. The linear growth rates and wavelengths of the unstable modes are found. The unstable modes produce fine-scale vortices perpendicular to the magnetic field, which overshoot well into the solar corona. The modes provide a turbulent vorticity source at the photospheric footpoints of the field. This can cause braiding and reconnection of the coronal magnetic field. The modes twist the coronal magnetic field into loops with a typical radius of 200 km, consistent with recent X-ray observations. 相似文献
4.
Jun Lin 《中国天文和天体物理学报》2007,7(4):457-476
Large-scale magnetic structures are the main carrier of major eruptions in the solar atmosphere. These structures are rooted in the photosphere and are driven by the unceas-ing motion of the photospheric material through a series of equilibrium configurations. The motion brings energy into the coronal magnetic field until the system ceases to be in equilib-rium. The catastrophe theory for solar eruptions indicates that loss of mechanical equilibrium constitutes the main trigger mechanism of major eruptions, usually shown up as solar flares, eruptive prominences, and coronal mass ejections (CMEs). Magnetic reconnection which takes place at the very beginning of the eruption as a result of plasma instabilities/turbulence inside the current sheet, converts magnetic energy into heating and kinetic energy that are responsible for solar flares, and for accelerating both plasma ejecta (flows and CMEs) and energetic particles. Various manifestations are thus related to one another, and the physics behind these relationships is catastrophe and magnetic reconnection. This work reports on re- cent progress in both theoretical research and observations on eruptive phenomena showing the above manifestations. We start by displaying the properties of large-scale structures in the corona and the related magnetic fields prior to an eruption, and show various morphological features of the disrupting magnetic fields. Then, in the framework of the catastrophe theory, we look into the physics behind those features investigated in a succession of previous works, and discuss the approaches they used. 相似文献
5.
Ü.D. Göker 《New Astronomy》2012,17(2):130-136
A Lagrangian Remap (LareXd) Code is employed to investigate the shock wave formation in the current sheet of a solar coronal magnetic loop and its effect on the magnetic reconnection. We constructed the slow shock structure in the presence of viscosity and heat conduction parallel and perpendicular to the magnetic field and pairs of slow shocks propagate away from the central current sheet, the so-called diffusion region. Significant jumps in plasma density, pressure, velocity and magnetic field occur across the main shock while the temperature appears in the foreshock. In the presence of dissipative effects, the distinct jumps disappear and the shock profiles show smooth transition between the downstream and the upstream regions while the plasma density and the pressure show very narrow and a sharp decrease with time. These results can be applied to the heating of the solar corona, the structure of the magnetic reconnection and the solar wind. 相似文献
6.
《Chinese Astronomy and Astrophysics》1987,11(1):83-86
A double current sheet forms when an opposite magnetic flux emerges into a background magnetic field which has a zero field-line in the corona. It consists of an upper sheet, resulting from the squeezing of field lines near the original zero field region and a lower sheet formed in the region between the new and old fields. We use a pair of linear dipoles to model the background and a pair of line charges to model the emerging field and discuss the formation and evolution of the double current sheet. Matter will condense onto the sheets during their formation. The matter in the lower sheet comes mainly from the transition region and the photosphere; it is further cooled by radiation, giving rise to a low-temperature prominence loop. The matter in the upper sheet comes from the corona and forms a high-temperature coronal loop.This scenario seems to be realized in the coronal transient of 1984 April 14. 相似文献
7.
B. V. Somov 《Astronomy Letters》2010,36(7):514-519
In connection with the RHESSI satellite observations of solar flares, which have revealed new properties of hard X-ray sources
during flares, we offer an interpretation of these properties. The observed motions of coronal and chromospheric sources are
shown to be the consequences of three-dimensional magnetic reconnection at the separator in the corona. During the first (initial)
flare phase, the reconnection process releases an excess of magnetic energy related predominantly to themagnetic tensions
produced before the flare by shear plasma flows in the photosphere. The relaxation of a magnetic shear in the corona also
explains the downward motion of the coronal source and the decrease in the separation between chromospheric sources. During
the second (main) flare phase, ordinary reconnection dominates; it describes the energy release in the terms of the “standard
model” of large eruptive flares accompanied by the rise of the coronal source and an increase in the separation between chromospheric
sources. 相似文献
8.
9.
With the SDO/AIA instrument, continuous and intermittent plasma outflows are observed on the boundaries of an active region along two distinct open coronal loops. By investigating the temporal sequence magnetograms obtained from HMI/SDO, it is found that a small-scale magnetic reconnection probably plays an important role in the generation of the plasma outflows in the coronal loops. It is found that the origin of the plasma outflows coincides with the locations of the small-scale magnetic fields with mixed polarities, which suggests that the plasma outflows along coronal loops probably results from the magnetic reconnection between the small-scale closed emerging loops and the large-scale open active region coronal loops. 相似文献
10.
The finite element method was used to solve a partial differential equation (magnetostatic equation) for multipolar magnetic regions. It is found that the height of magnetic field lines above the magnetic neutral line of a central strong bipolar magnetic field decreases as the field lines' footpoints approach the neutral line and also with increased magnetic shear. Both the electric current density and plasma pressure in the sheared low-lying loops are high. We suggest that the sheared low-lying loops may store the energies of large coronal mass ejections (CMEs) and filament eruptions. In addition, it is found that a lower pressure area exists above the low-lying loops and that it is similar in morphology to a coronal cavity. Above the lower pressure area there is a higher pressure area, which may be the source of CMEs. In this area magnetic shear leads to magnetic reconnection, which may be the cause of high coronal temperature. 相似文献
11.
A topological model with magnetic reconnection at two separators in the corona is used to account for the recently discovered changes of the photospheric magnetic field in the active region NOAA 9077 during the July 14, 2000 flare. The model self-consistently explains the following observed effects: (1) the magnetic field strength decreases on the periphery of the active region but increases in its inner part near the neutral line of the photospheric magnetic field; (2) the center-of-mass positions of the fields of opposite (northern and southern) polarities converge; and (3) the magnetic flux of the active region decreases after the flare. The topological model gives not only a qualitative interpretation of the flare phenomena (the structure of the interacting magnetic fluxes in the corona, the location of the energy sources, the shape of the flare ribbons and kernels in the chromosphere and photosphere), but also correct quantitative estimates of the large-scale processes that form the basis for solar flares. The electric field emerging in the flare during large-scale reconnection is calculated. The electric field strength correlates with the observed intensity of the hard X-ray bremsstrahlung, suggesting an electron acceleration as a result of reconnection. 相似文献
12.
从理论和观测两个方面来介绍和讨论出现在太阳爆发过程中的磁重联电流片及其物理本质和动力学特征。首先介绍在理论研究和理论模型中,磁重联电流片是如何在爆发磁结构当中形成并发展的,对观测研究有什么指导意义。然后介绍观测工作是从哪几个方面对理论模型预测的电流片进行证认和研究的。第三,将介绍观测研究给出了哪些过去所没有能够预期的结果,这些结果对深入研究耀斑一CME电流片以及其中的磁重联过程的理论工作有什么重要的、挑战性的意义。第四,讨论最新的与此有关的理论研究和数值实验。最后,对未来的研究方向和重要课题进行综述和展望。 相似文献
13.
Lucas A. Tarr Dana W. Longcope David E. McKenzie Keiji Yoshimura 《Solar physics》2014,289(9):3331-3349
When magnetic flux emerges from beneath the photosphere, it displaces the preexisting field in the corona, and a current sheet generally forms at the boundary between the old and new magnetic domains. Reconnection in the current sheet relaxes this highly stressed configuration to a lower energy state. This scenario is most familiar and most often studied in flares, where the flux transfer is rapid. We present here a study of steady, quiescent flux transfer occurring at a rate three orders of magnitude lower than that in a large flare. In particular, we quantify the reconnection rate and the related energy release that occurred as the new polarity emerged to form NOAA Active Region 11112 (SOL16 October 2010T00:00:00L205C117) within a region of preexisting flux. A bright, low-lying kernel of coronal loops above the emerging polarity, observed with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and the X-ray Telescope onboard Hinode, originally showed magnetic connectivity only between regions of newly emerged flux when overlaid on magnetograms from the Helioseismic and Magnetic Imager. Over the course of several days, this bright kernel advanced into the preexisting flux. The advancement of an easily visible boundary into the old flux regions allows measuring the rate of reconnection between old and new magnetic domains. We compare the reconnection rate with the inferred heating of the coronal plasma. To our knowledge, this is the first measurement of steady, quiescent heating related to reconnection. We determined that the newly emerged flux reconnects at a fairly steady rate of 0.38×1016 Mx?s?1 over two days, while the radiated power varies between (2?–?8)×1025 erg?s?1 over the same time. We found that as much as 40 % of the total emerged flux at any given time may have reconnected. The total amounts of transferred flux (～?1×1021 Mx) and radiated energy (～?7.2×1030 ergs) are comparable to that of a large M- or small X-class flare, but are stretched out over 45 hours. 相似文献
14.
We present a quantitative model of the magnetic energy stored and then released through magnetic reconnection for a flare on 26 February 2004. This flare, well observed by RHESSI and TRACE, shows evidence of non-thermal electrons for only a brief, early phase. Throughout the main period of energy release there is a super-hot (T?30 MK) plasma emitting thermal bremsstrahlung atop the flare loops. Our model describes the heating and compression of such a source by localized, transient magnetic reconnection. It is a three-dimensional generalization of the Petschek model, whereby Alfvén-speed retraction following reconnection drives supersonic inflows parallel to the field lines, which form shocks: heating, compressing, and confining a loop-top plasma plug. The confining inflows provide longer life than a freely expanding or conductively cooling plasma of similar size and temperature. Superposition of successive transient episodes of localized reconnection across a current sheet produces an apparently persistent, localized source of high-temperature emission. The temperature of the source decreases smoothly on a time scale consistent with observations, far longer than the cooling time of a single plug. Built from a disordered collection of small plugs, the source need not have the coherent jet-like structure predicted by steady-state reconnection models. This new model predicts temperatures and emission measure consistent with the observations of 26 February 2004. Furthermore, the total energy released by the flare is found to be roughly consistent with that predicted by the model. Only a small fraction of the energy released appears in the super-hot source at any one time, but roughly a quarter of the flare energy is thermalized by the reconnection shocks over the course of the flare. All energy is presumed to ultimately appear in the lower-temperature (T?20 MK) post-flare loops. The number, size, and early appearance of these loops in TRACE’s 171 Å band are consistent with the type of transient reconnection assumed in the model. 相似文献
15.
Jun Lin 《Solar physics》2004,222(1):115-136
Kopp–Pneuman-type magnetic configurations, which include a vertical current sheet, with various background fields are investigated. Dissipation of the current sheet as a result of magnetic reconnection produces bright flare ribbons on the solar disk and a growing flare loop system in the corona. In principle, the growth of flare loop system is governed by a reconnection process only, and the behavior of flare ribbons is also controlled by the background field. The flare ribbons may appear either separate or attached to one another at the onset of the flare depending on the background field distribution on the boundary surface. We calculate the decrease in height that magnetic field lines undergo after they have reconnected to form closed loops. Following previous practice, we refer to this decrease as field line shrinkage. Unlike the motions of flare ribbons, the shrinkage of flare loops depends weakly on the background field. Individual loops always shrink fastest at the moment it is produced by reconnection and just starts to leave the current sheet. The earlier the loop forms, the more and faster it shrinks. The relevant observations are explained on the basis of our calculations, and the aspects of the explanation that need improvement are also discussed. 相似文献
16.
The rate of two-dimensional flux pile-up magnetic reconnection is known to be severely limited by gas pressure in a low-beta plasma of the solar corona. As earlier perturbational calculations indicated, however, the pressure limitation should be less restrictive for three-dimensional flux pile-up. In this paper the maximum rate of reconnection is calculated in the approximation of reduced magnetohydrodynamics (RMHD), which is valid in the solar coronal loops. The rate is calculated for finite-magnitude reconnecting fields in the case of a strong axial field in the loop. Gas pressure effects are ignored in RMHD but a similar limitation on the rate of magnetic merging exists. Nevertheless, the magnetic energy dissipation rate and the reconnection electric field can increase by several orders of magnitude as compared with strictly two-dimensional pile-up. Though this is still not enough to explain the most powerful solar flares, slow coronal transients with energy release rates of order 1025– 1026 erg s–1and heating of quiet coronal loops are within the compass of the model. 相似文献
17.
胡友秋 《中国天文和天体物理学报》1999,(2)
采用二维三分量磁流体力学模型,对日冕三重无力场电流片的磁场重联进行了数值研究,揭示了重联过程的基本物理特征．这类重联过程将加热和加速日冕等离子体,并导致多个高温、高密度、高磁螺度的磁岛的形成和向上喷发．这表明,多重无力场电流片的重联可能在日冕磁能释放、上行等离子体团的形成和太阳磁场螺度向行星际空间的逃逸方面起重要的作用． 相似文献
18.
Very Large Array (VLA) observations at 91-cm wavelength are combined with data from the SOHO EIT, MDI and LASCO and used to study the evolving coronal magnetic environment in which Type I noise storms and large-scale coronal loops occur. On one day, we have shown the early evolution of a coronal mass ejection (CME) in projection in the disk by tracing its decimetric continuum emission. The passage of the CME and an associated EUV ejection event coincided with an increase in the 91-cm brightness temperature of an extended coronal loop located a significant distance away and with the displacement of the 91-cm source during the early stage of the CME. We suggest that the energy deposited into the corona by the CME may have caused a local increase in the thermal or nonthermal electron density or in the electron temperature in the middle corona resulting in a transient increase in the brightness of the 91-cm loop. On a second observing day, we have consolidated the known association between magnetic changes in the photosphere and low corona with noise storm enhancements in an overlying radio source well in advance of a flare event in the same region. We find anti-correlated changes in the brightness of a bipolar 91-cm Type I noise storm that appear to be associated with the cancellation and emergence of magnetic flux in the underlying photosphere. In this case, the evolving fields may have led to magnetic instabilities and reconnection in the corona and the acceleration of nonthermal particles that initiated and sustained the Type I noise storm. 相似文献
19.
Fletcher Lyndsay López Fuentes Marcelo C. Mandrini Cristina H. Schmieder Brigitte Démoulin Pascal Mason Helen E. Young Peter R. Nitta Nariaki 《Solar physics》2001,203(2):255-287
We present multi-instrument observations of active region (AR) 8048, made between 3 June and 5 June 1997, as part of the SOHO
Joint Observing Program 33. This AR has a sigmoid-like global shape and undergoes transient brightenings in both soft X-rays
and transition region (TR) lines. We compute a magneto-hydrostatic model of the AR magnetic field, using as boundary condition
the photospheric observations of SOHO/MDI. The computed large-scale magnetic field lines show that the large-scale sigmoid
is formed by two sets of coronal loops. Shorter loops, associated with the core of the SXT emission, coincide with the loops
observed in the hotter CDS lines. These loops reveal a gradient of temperature, from 2 MK at the top to 1 MK at the ends.
The field lines most closely matching these hot loops extend along the quasi-separatrix layers (QSLs) of the computed coronal
field. The TR brightenings observed with SOHO/CDS can also be associated with the magnetic field topology, both QSL intersections
with the photosphere, and places where separatrices issuing from bald patches (sites where field lines coming from the corona
are tangent to the photosphere) intersect the photosphere. There are, furthermore, suggestions that the element abundances
measured in the TR may depend on the type of topological structure present. Typically, the TR brightenings associated with
QSLs have coronal abundances, while those associated with BP separatrices have abundances closer to photospheric values. We
suggest that this difference is due to the location and manner in which magnetic reconnection occurs in two different topological
structures.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1013302317042 相似文献
20.
In this review paper we discuss several aspects of magnetic reconnection theory, focusing on the field-line motions that are
associated with reconnection. A new exact solution of the nonlinear MHD equations for reconnective annihilation is presented which represents a two-fold generalization of the previous solutions. Magnetic reconnection at null points by
several mechanisms is summarized, including spine reconnection, fan reconnection and separator reconnection, where it is pointed out that two common features of separator reconnection are the rapid flipping of magnetic field lines
and the collapse of the separator to a current sheet. In addition, a formula for the rate of reconnection between two flux
tubes is derived. The magnetic field of the corona is highly complex, since the magnetic carpet consists of a multitude of
sources in the photosphere. Progress in understanding this complexity may, however, be made by constructing the skeleton of the field and developing a theory for the local and global bifurcations between the different topologies. The eruption
of flux from the Sun may even sometimes be due to a change of topology caused by emerging flux break-out. A CD-ROM attached to this paper presents the results of a toy model of vacuum reconnection, which suggests that rapid flipping
of field lines in fan and separator reconnection is an essential ingredient also in real non-vacuum conditions. In addition,
it gives an example of binary reconnection between a pair of unbalanced sources as they move around, which may contribute significantly to coronal heating. Finally,
we present examples in TRACE movies of geometrical changes of the coronal magnetic field that are a likely result of large-scale
magnetic reconnection.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005248007615 相似文献