共查询到20条相似文献，搜索用时 312 毫秒
1.
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
2.
Su Qing-Rui 《Solar physics》1982,75(1-2):229-236
In this paper, we extend B. C. Low's study on nonlinear force-free magnetic fields. Based on Low's mathematical method, a revised boundary-value problem of the two-dimensional nonlinear force-free magnetic field is solved analytically. The solution shows that higher magnetic loops evolve towards preflare loops when the gradient of longitudinal magnetic field at the photospheric level and the angle (shear) included between the magnetic field line and magnetic neutral line increase with time. The density, temperature and the current density are higher in the preflare loops than in the high-lying magnetic loops. We believe it is the loops that provide conditions for the eruption of the flare.The original has been published in the Acta Astronomica Sinica
21 (1980), 152, in Chinese. The present paper completes the discussion and revises some of the preliminary results. 相似文献
3.
We analyzed simultaneous EUV images from the Transition Region And Coronal Explorer (TRACE) and H and H filtergrams from Huairou Solar Observing Station (HSOS). In active region NOAA 8307, an H C5.5 flare occurred near 06:10 UT on 23 August 1998. In this paper, we concentrated on loop–loop interaction, as well as their relationship to the C5.5 flare. We find that while opposite polarity magnetic fields cancelled each other, H bright points appeared, and then the flare occurred. Looking at EUV images, we noticed that a TRACE flare, associated with the C5.5 flare in H and H filtergrams, first appeared as patch-shaped structures, then the flare patches expanded to form bright loops. We used a new numerical technique to extrapolate the chromospheric and coronal magnetic field. Magnetic field loops, which linked flare ribbons, were found. It was suggested that loop interaction in the active region was the cause of the TRACE and H flare; the magnetic topological structures were clearly demonstrated and the TRACE flare was probably due to the interaction among energetic low-lying and other longer (higher) magnetic loops. Each primary flare kernel, seen from H, H filtergrams, and EUV images, was located near the footpoints of several interacting loops. 相似文献
4.
A three-dimensional coronal magnetic field is reconstructed for the NOAA active region 11158 on 14 February 2011. A GPU-accelerated direct boundary integral equation (DBIE) method is implemented which is approximately 1000 times faster than the original DBIE used on solar non-linear force-free field modeling. Using the SDO/HMI vector magnetogram as the bottom boundary condition, the reconstructed magnetic field lines are compared with the projected EUV loop structures as observed in the front-view (SDO/AIA) and the side-view (STEREO-A/B) images for the first time; they show very good agreement three-dimensionally. A quantitative comparison with some stereoscopically reconstructed coronal loops shows that the average misalignment angles in our model are at the same order as the state-of-the-art results obtained from reconstructed coronal loops. It is found that the observed coronal loop structures can be grouped into a number of closed and open field structures with some central bright coronal loop features around the polarity inversion line. The reconstructed highly sheared magnetic field lines agree very well with the low-lying sigmoidal filament along the polarity inversion line. This central low-lying magnetic field loop system must have played a key role in powering the flare. It should be noted that while a strand-like coronal feature along the polarity inversion line may be related to the filament, one cannot simply interpret all the coronal bright features along the polarity inversion line as manifestation of the filament without any stereoscopic information. 相似文献
5.
Mordvinov A.V. Salakhutdinova I.I. Plyusnina L.A. Makarenko N.G. Karimova L.M. 《Solar physics》2002,211(1-2):241-253
We investigate the topological properties and evolution of background magnetic fields on synoptic maps from Wilcox Solar Observatory using mathematical morphology methods in terms of the Minkowski functionals. The total length of the neutral line, the total areas occupied by positive and negative polarities, and the Euler characteristics of background magnetic fields vary over an eleven-year cycle. Changes in the length of the neutral line that separates the polarities of the background magnetic field correlate well with flare activity. A time–longitude analysis of solar flare activity revealed a complicated organization and rotation of the entire flare ensemble. On the time–longitude diagram, flare activity is organized into the patterns which follow the rearrangements in background magnetic field and exhibit coexisting and alternating modes of rigid rotation. The character of rotation of the entire flare ensemble is similar to the rotation of background magnetic fields. The emergence of background magnetic fields and changes in their topology and rotation are often accompanied by enhancements in flare activity. A comparative analysis of the topological changes in background magnetic fields and flare activity reveals their causal relation. 相似文献
6.
G. W. Pneuman 《Solar physics》1982,78(2):229-241
A theory of two-ribbon solar flares is presented which identifies the primary energy release site with the tops of the flare loops. The flare loops are formed by magnetic reconnection of a locally opened field configuration produced by the eruption of a pre-flare filament. Such eruptions are commonly observed about 15 min prior to the flare itself. It is proposed that the flare loops represent the primary energy release site even during the earliest phase of the flare, i.e., the flare loops are in fact the flare itself.Based upon the supposition that the energy release at the loop tops is in the form of Joulean dissipation of magnetic energy at the rising reconnection site, a quantitative model of the energy release process is developed based upon an analytic reconnecting magnetic field geometry believed to represent the basic process. Predicted curves of energy density vs time are compared with X-ray observations taken aboard Skylab for the events of 29 July, 13 August, and 21 August in 1973. Considering the crudity of the model, the comparisons appear reasonable. The predicted field strengths necessary to produce the observed energy density curves are also reasonable, being in the range 100–1000 G.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
7.
Magnetic field configurations associated with polarity intrusion in a solar active region 总被引：1，自引：0，他引：1
B. C. Low 《Solar physics》1982,77(1-2):43-61
This paper presents a new class of exact solutions describing the non-linear force-free field above a spatially localized photospheric bipolar magnetic region. An essential feature is the variation in all three Cartesian directions and this could not be modelled adequately with previously known symmetric force-free fields. Sequences of force-free fields are constructed and analyzed to simulate the slow growth of a pair of spots on the photosphere. The axis connecting the spots executes rotational motion, distorting the photospheric neutral line separating fluxes of opposite signs. We show directly from the analytic solutions that the resulting reversal of the positions of the spots relative to the background field is associated with (i) the creation of magnetic free energy, (ii) the severe shearing of localized low-lying loops in the vicinity where the photospheric transverse field aligns with the photospheric neutral line, and (iii) the emergence and disappearance of flux from the photosphere at these highly stressed regions. The model relates theoretically for the first time these different magnetic field features that have been suggested by observation and theoretical considerations to be flare precursors. A general formula, based on the virial theorem, is also given for the free energy of a force-free field, strictly in terms of the field value at the photosphere. This formula has obvious practical application.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
8.
We study a flare which occurred on 3 November 1997 at 10:31 UT in the vicinity of a parasitic polarity of AR 8100. Using SOHO/EIT 195 Å observations, we identify the brightening of thin transequatorial loops connecting AR 8100 and AR 8102, and dimmings located between the two active regions. Difference images highlight the presence of a loop-like structure rooted near the flare location usually called an EIT wave. The coronal magnetic field derived from potential extrapolations from a SOHO/MDI magnetogram shows that the topology is complex near the parasitic polarity. There, a `bald patch' (defined as the locations where the magnetic field is tangent to the photosphere) is present. We conclude that the flare was a `bald patch flare'. Moreover, the extrapolation confirms that there is a large coronal volume filled with transequatorial field lines interconnecting AR 8100 and AR 8102, and overlaying the bald patch. We show that the dimmings are located at the footpoints of these large field lines, which can be also related to the thin bright loops observed during the flare. As this event was related to a coronal mass ejection (CME) observed by SOHO/LASCO, we propose that the observed dimmings are due to a decrease in plasma density during the opening of the transequatorial loops connecting both ARs. We propose a scenario where these large field lines are in fact pushed up by the opening of low-lying sheared field lines forming the bald patch. We finally discuss how the fast opening of these field lines can produce the brightening near the footpoints of the separatrix, observed as an `EIT wave'. 相似文献
9.
By using a topological model for the potential magnetic field above the photosphere, the appearance and development of the separator as a result of vortex plasma flows in the locality of the photospheric neutral line is considered. The possible relation of such vortex flows with a flare activity is revealed. The arrangement and shape of the flare ribbons in the chromosphere, the formation of X-ray intersecting loops, the early appearance of bright knots on flare ribbon edges are naturally explained by the model provided a reconnecting current sheet arises along the separator in the coronal magnetic field of active regions as a result of the evolution of the magnetic field sources in the photosphere. 相似文献
10.
Of 21 flares of importance 1 or greater, observed on 15 days, all were found to lie adjacent to a neutral line in the longitudinal component of photospheric magnetic fields. In most of these cases, the flare consisted of two or more segments separated by the neutral line and located in areas of strong field and high-longitudinal field gradient. In some cases, the flare segments extended into areas of weak-magnetic field and low-field gradient, but maintained an orientation adjacent to a neutral line.Optical and magnetic field records of higher resolution were obtained on 6 July 1965. These observations reveal an excellent correlation between the size, shape, and intensity of the H fine structures and the longitudinal component of the photospheric magnetic fields, except in the vicinity of the neutral line. Sections of the neutral line are marked by long fibrils lying perpendicular to the neutral line or by small filaments lying along the neutral line.The development of a flare of importance 1 in this region appeared to be more precisely related to the neutral line than was found for the flares and magnetic fields observed with lower resolution. The two major segments of this flare lengthened in directions approximately parallel to the neutral line, while simultaneously drifting perpendicularly away from the neutral line. The initial rate of drift systematically varied from 1 to 12 km/sec at a series of positions approximately parallel to the neutral line and corresponding to increasing distance from strong fields. The rate of drift was also observed to decelerate throughout the life of the flare. 相似文献
11.
Models for the motions of flare loops and ribbons 总被引：1，自引：0，他引：1
We have found a conformal mapping which is valid for any magnetic boundary condition at the photosphere and which can be used to determine the evolution of an open, two-dimensional magnetic field configuration as it relaxes to a closed one. Solutions obtained with this mapping are in quasi-static equilibrium, and they contain a vertical current sheet and have line-tied boundary conditions. As a specific example, we determine the solution for a boundary condition corresponding to a submerged, two-dimensional dipole below the photosphere. We assume that the outer edges of the hottest X-ray loops correspond to field lines mapping from the outer edges of the H ribbon to the lower tip of the current sheet where field lines reconnect at aY-type neutral line which rises with time. The cooler H loops are assumed to lie along the field lines mapping to the inner edges of the flare ribbons. With this correspondence between the plasma structures and the magnetic field we determine the shrinkage that field lines are observed to undergo as they are disconnected from the neutral line. During the early phase of the flare, we predict that shrinkage inferred from the height of the H and X-ray loops is close to 100% of the loop height. However, the shrinkage should rapidly decrease with time to values on the order of 20% by the late phase. We also predict that the shrinkage in very large loops obeys a universal scaling law which is independent of the boundary condition, provided that the field becomes self-similar (i.e., all field lines have the same shape) at large distances. Specifically, for any self-similar field containing aY-type neutral line, the observed shrinkage at large distances should decrease as (X/X
R)–2/3, where X is the ribbon width andX
Ris the ribbon separation. Finally, we discuss the relation between the electric field at the neutral line and the motions of the flare loops and ribbons. 相似文献
12.
A solar flare on June 15, 1973 has been observed with high spatial and temporal resolution by the S-054 grazing-incidence X-ray telescope on Skylab. Both morphological and quantitative analyses are presented. Some of the main results are: (a) the overall configuration of the flare is that of a compact region with a characteristic size of the order of 30 at the intensity peak, (b) this region appears highly structured inside with complex systems of loops which change during the event, (c) a brightening over an extended portion of the active region precedes the flare onset, (d) the impulsive phase indicated by the non-thermal radio emission is a period during which a rapid brightening occurs in loop structures, (e) the X-ray emission is centered over the neutral line of longitudinal magnetic field, and the brightest structures at the flare onset bridge the neutral line, (f) loop systems at successively increasing heights form during the decay phase, finally leading to the large loops observed in the postflare phase, (g) different parts of the flare show distinctly different light curves, and the temporal development given by full disk detectors is the result of integrating the different intensity vs time profiles.The implications of these observations for mechanisms of solar flares are discussed. In particular, the flux profiles of different regions of the flare give strong evidence for continued heating during the decay phase, and a multiplicity of flare volumes appears to be present, in all cases consisting of loops of varying lengths.On leave from Arcetri Astrophysical Observatory, Florence, Italy. 相似文献
13.
M. J. Hagyard 《Solar physics》1988,115(1):107-124
We have analyzed the vector magnetic field of an active region at a location of repeated flaring to determine the nature of the currents flowing in the areas where the flares initiated. The component of electric current density crossing the photosphere along the line-of-sight was derived from the observed transverse component of the magnetic field. The maximum concentrations of these currents occurred exactly at the sites of flare initiation and where the photospheric field was sheared the most. The calculated distribution of current density at the flare sites suggested that currents were flowing out of an area of positive magnetic polarity and across the magnetic inversion line into two areas of negative polarity. This interpretation was reinforced by a calculation of the source field, the magnetic field produced in the photosphere by the electric currents above the photosphere. In the vicinity of the flare sites, the calculated source field exhibited three particular characteristics: (1) maximum magnitudes at the sites of flare initiation, (2) a rotational direction where the vertical current density was concentrated, and (3) a fairly constant angular orientation with the magnetic inversion line. The source field was thus very similar to the field produced by two arcades of currents crossing the inversion line at the locations of greatest magnetic shear with orientations of about 60° to the inversion line. With this orientation, the inferred arcades would be aligned with the observed chromospheric fibrils seen in the H data so that the currents were field-aligned above the photosphere. The field thus exhibited a vertical gradient of magnetic shear with the shear decreasing upward from the photosphere. We estimated the currents in the two arcades by matching the source field derived from observations with that produced by a model of parallel loops of currents. We found that the loops of the model would each have a radius of 4500 km, a separation of 1830 km, and carry a current of 0.15 × 1012 A. Values of vertical current densities and source fields appearing in the umbrae of the two large sunspots away from the flare sites were shown to lie at or below the level of uncertainty in the data. The main source of this uncertainty lay in the method by which the 180° ambiguity in the azimuth of the transverse field is resolved in umbral areas. We thus concluded that these quantities in large umbrae should be treated with a healthy skepticism. Finally, we found that the source field at the flare sites was produced almost entirely by the angular difference between the observed and potential field and not by the difference in field intensity. 相似文献
14.
From magnetograms of the active region 15266 obtained at Yunnan observatory we found that 1) an inverted field configuration and a twisted neutral line are both closely correlated with high-energy flare eruptions. After the flares, the field configuration and the neutral line revert to more stable states. 2) Most of the flares of the present region occurred away from the neutral line; those near the line occurred either in regions of high field gradient or in the vicinity of Severny's “neutral points”. 相似文献
15.
G. J. D. Petrie 《Solar physics》2013,287(1-2):415-440
The active region NOAA 11158 produced the first X-class flare of Solar Cycle 24, an X2.2 flare at 01:44 UT on 15 February 2011. The Helioseismic and Magnetic Imager (HMI) instrument on the Solar Dynamics Observatory (SDO) satellite produces 12-minute, 0.5′′ pixel?1 vector magnetograms. Here we analyze a series of these data covering a 12-hour interval centered at the time of this flare. We describe the spatial distributions of the photospheric magnetic changes associated with the flare, including the abrupt changes in the field vector, vertical electric current and Lorentz-force vector acting on the solar interior. We also describe these parameters’ temporal evolution. The abrupt magnetic changes were concentrated near the neutral line and in two neighboring sunspots. Near the neutral line, the field vectors became stronger and more horizontal during the flare and the shear increased. This was due to an increase in strength of the horizontal field components near the neutral line, most significant in the horizontal component parallel to the neutral line but the perpendicular component also increased in strength. The vertical component did not show a significant, permanent overall change at the neutral line. The increase in field strength at the neutral line was accompanied by a compensating decrease in field strength in the surrounding volume. In the two sunspots near the neutral line the integrated azimuthal field abruptly decreased during the flare but this change was permanent in only one of the spots. There was a large, abrupt, downward vertical Lorentz-force change acting on the solar interior during the flare, consistent with results of past analyses and recent theoretical work. The horizontal Lorentz force acted in opposite directions along each side of neutral line, with the two sunspots at each end subject to abrupt torsional forces relaxing their magnetic twist. These shearing forces were consistent with a contraction of field and decrease of shear near the neutral line, whereas the field itself became more sheared as a result of the field collapsing towards the neutral line from the surrounding volume. The Lorentz forces acting on the atmospheric volume above the photosphere were equal and opposite. 相似文献
16.
Chung-Chieh Cheng 《Solar physics》1977,55(2):413-419
The analysis of the high temperature plasma in Fe xxiii–xxiv in the 15 June 1973 flare is presented. The observations were obtained with the NRLXUV spectroheliograph on Skylab. The results are: (1) There was preheating of the active region in which the flare occurred. In particular, a large loop in the vicinity of the flaring region showed enhanced brightness for many hours before the flare. The loop disappeared when the flare occurred, and returned in the postflare phase, as if the energy flux which had been heating the large loop was blocked during the flare and restored after the flare was gone. The large magnetic fields did not change significantly. (2) The flare occurred in low-lying loop or loops. The spatial distribution of flare emission shows that there was a temperature gradient along the loop. (3) The high temperature plasma emitting Fe xxiii and xxiv had an initial upward motion with a velocity of about 80 km s–1. (4) There was large turbulent mass motion in the high temperature plasma with a random velocity of 100 to 160 km s–1. (5) The peak temperature of the hot plasma, determined from the Fe xxiii and xxiv intensity ratio, was 14 × 106 K. It decreased slightly and then, for a period of 4 min, remained at 12.6 × 106 K before dropping sharply to below 10 × 106 K. The density of the central core of the hot plasma, determined from absolute intensity of Fe xxiv 255 Å line, was of the order of 1011 cm–3.The persistence of the high level of turbulence and of the high temperature plateau in the decaying phase of the flare indicates the presence of secondary energy release. From the energy balance equation the required energy source is calculated to be about 3 to 7 ergs cm–3 s–1.Ball Brothers Research Corporation. 相似文献
17.
Every two-ribbon flare observed during the Skylab period produced an observable coronal transient, provided the flare occurred close enough to the limb. The model presented here treats these two events as a combined process. Transients that occur without flares are believed to involve magnetic fields that are too weak to produce significant chromospheric emission. Adopting the hypothesis that the rising flare loop systems observed during two-ribbon flares are exhibiting magnetic reconnection, a model of a coronal transient is proposed which incorporates this reconnection process as the driving force. When two oppositely directed field lines reconnect a lower loop is created rooted to the solar surface (the flare loop) and an upper disconnected loop is produced which is free to rise. The magnetic flux of these upper loops is proposed as the driver for the transient. The force is produced by the increase in magnetic pressure under the filament and transient.A quantitative model is developed which treats the transient configuration in terms of four distinct parts- the transient itself with its magnetic field and material, the region just below the transient but above the filament, the filament with its magnetic field, and the reconnected flux beneath the filament. Two cases are considered - one in which all the prominence material rises with the transient and one in which the material is allowed to fall out of the transient. The rate of rise of the neutral line during the reconnection process is taken from the observations of the rising X-ray flare loop system during the 29 July, 1973 flare. The MHD equations for the system are reduced to four non-linear ordinary coupled differential equations which are solved using parameters believed to be realistic for solar conditions. The calculated velocity profiles, widths, etc., agree quite well with the observed properties of coronal transients as seen in white light. Since major flares are usually associated with a filament eruption about 10–15 min before the flare and since this model associates the transient with the filament eruption, we suspect that the transient is actually initiated some time before the actual flare itself.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
18.
The spatial and temporal evolution of the high temperature plasma in the flare of 1973 June 15 has been studied using the flare images photographed by the NRL XUV spectroheliograph on Skylab.The overall event involves the successive activations of a number of different loops and arches bridging the magnetic neutral line. The spatial shifts and brightenings observed in the Fe xxiii–xxiv lines are interpreted as the activation of new structures. These continued for four or five minutes after the end of the microwave burst phase, implying additional energy-release unrelated to the nonthermal phase of the flare. A shear component observed in the coronal magnetic field may be a factor in the storage and release of the flare energy.The observed Fe xxiii–xxiv intensities define a post-burst heating phase during which the temperature remained approximately constant at 13 × 106 K while the Fe xxiv intensity and 0–3 Å flux rose to peak values. This phase coincided with the activation of the densest structure (N
e
= 2 × 1011 cm–3). Heating of higher loops continued into the decay phase, even as the overall temperature and flux declined with the fading of the lower Fe xxiv arches.The observed morphology of individual flaring arches is consistent with the idea of energy release at altitude in the arch (coincident with a bright, energetic core in the Fe xxiv image) and energy flow downward into the ribbons. The Doppler velocity of the Fe xxi 1354 Å line is less than 5 km s–1, indicating that the hot plasma region is stationary.The relation of this flare to the larger class of flares associated with filament eruptions and emerging magnetic flux is discussed. 相似文献
19.
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
20.
A quantitative study relating observed shear in photospheric magnetic fields to repeated flaring 总被引：3，自引：0，他引：3
In this paper we present a quantitative evaluation of the shear in the magnetic field along the neutral line in an active region during an epoch of flare activity. We define shear as the angular difference in the photosphere between the potential magnetic field, which fits the boundary conditions imposed by the observed line-of-sight field, and the observed magnetic field. For the active region studied, this angular difference (shear) is non-uniform along the neutral line with maxima occurring at the locations of repeated flare onsets. We suggest that continued magnetic evolution causes the field's maximum shear to exceed a critical value of shear, resulting in a flare around the site of maximum shear. Evidently, the field at the site of the flare must relax to a state of shear somewhat below the critical value (but still far from potential), with subsequent evolution returning the field to the critical threshold. We draw this inference because several flares occurred at sites of maximum photospheric shear which were persistent in location.NOAA, Boulder, Colorado. 相似文献