共查询到20条相似文献，搜索用时 31 毫秒
1.
We analyze the changes that projection effects produce in the evaluation of magnetic shear in off-disk center active regions by comparing angular shear calculated in image plane and heliographic coordinates. We describe the procedure for properly evaluating magnetic shear by transforming the observed vector magnetic field into the heliographic system and then apply this procedure to evaluate magnetic shear along the magnetic neutral line in an active region that was observed on 1984 April 24 at a longitude offset of -45°. In particular, we show that the number of critically sheared pixels along an east-west directed segment of the neutral line in the leader sunspot group changes from 16 in the image plane magnetogram to 14 in the heliographic magnetogram. We also show that the critical shear as calculated in the image plane served as a good predictor for the location of flaring activity since the flare ribbons of the great flare of April 24 bracketed the inversion line where the critical shear was located. These results indicate that for this particular region, projection effects did not significantly affect the evaluation of critical shear. 相似文献
2.
The magnetic shear at a point within an active region field configuration can be defined (Hagyard et al., 1984b) as the difference in angle between the observed photospheric transverse field and that of a reference potential field calculated using the observed line-of-sight field as a boundary condition. Using analytic models for non-potential (but force-free) fields representative of preflaring active regions, we calculate the degree of magnetic shear along the magnetic neutral line that such fields would exhibit, as a function of the location and orientation of the active region on the solar disk. We find that, except for regions close to disk center, the position of the inferred neutral line (zero line-of-sight field) is significantly different from the actual neutral line (zero radial field), and that the calculated reference potential field also varies significantly with the position of the region. Thus the inferred degree of shear can vary significantly with the position and orientation of the region, due to (a) straightforward geometric projection effects, (b) the shift of the inferred neutral line relative to its true position, and (c) variations in the reference potential field. The significance of these results for flare prediction is considered.Presidential Young Investigator. 相似文献
3.
We study the magnetic field evolution and topology of the active region NOAA 10486 before the 3B/X1.2 flare of October 26, 2003, using observational data from the French–Italian THEMIS telescope, the Michelson Doppler Imager (MDI) onboard Solar and Heliospheric Observatory (SOHO), the Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observation Station (HSOS), and the Transition Region and Coronal Explorer (TRACE). Three dimensional (3D) extrapolation of photospheric magnetic field, assuming a potential field configuration, reveals the existence of two magnetic null points in the corona above the active region. We look at their role in the triggering of the main flare, by using the bright patches observed in TRACE 1600 Å images as tracers at the solar surface of energy release associated with magnetic reconnection at the null points. All the bright patches observed before the flare correspond to the low-altitude null point. They have no direct relationship with the X1.2 flare because the related separatrix is located far from the eruptive site. No bright patch corresponds to the high-altitude null point before the flare. We conclude that eruptions can be triggered without pre-eruptive coronal null point reconnection, and the presence of null points is not a sufficient condition for the occurrence of flares. We propose that this eruptive flare results from the loss of equilibrium due to persistent flux emergence, continuous photospheric motion and strong shear along the magnetic neutral line. The opening of the coronal field lines above the active region should be a byproduct of the large 3B/X1.2 flare rather than its trigger. 相似文献
4.
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
5.
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
6.
We demonstrate a simple method of transforming vector magnetograms to heliographic coordinates. The merits of this transformation are illustrated using a vector magnetogram obtained with the MSFC vector magnetograph 80 minutes prior to a white light flare in active region AR 4474 on 25 April, 1984. The original magnetogram shows strong magnetic shear along the neutral line at both the flare site and a non-flaring site. The transformation of the magnetogram to heliographic coordinates shows that the elimination of projection effects results in a much shorter length of the sheared region at the non-flaring site than what is inferred from the image plane vector magnetogram. The length of the sheared region at the flare site is relatively less affected by the transformation.National Research Council Resident Research Associate on leave from Indian Institute of Astrophysics, Bangalore 560034, India. 相似文献
7.
We have evaluated the shear angle of the neutral line of the non-potential magnetic field for one or two days prior to and after the flare event for 10 cases. We have used the H filament positions to evaluate the shear in the neutral line. We find from the samples we have studied that it is the change in the shear that occurs a day prior to the flare that can lead to the event. This change can be in either direction, i.e., it can be a large increase from a small value or a decrease from a large initial value. Thus it is the change in the shear angle that seems to be a deciding criterion for a flare to occur and not a large value for the shear angle itself. We have one instance where there was no significant change in the shear angle over a period of a few days and this region, although similar to other active regions studied, did not produce any flare activity. 相似文献
8.
Tuan-Hui Zhou Hai-Sheng Ji 《中国天文和天体物理学报》2009,9(3)
We give an extensive multi-wavelength analysis of an eruptive M1.0/1N class solar flare, which occurred in the active region NOAA 10044 on 2002 July 26. Our empha-sis is on the relationship between magnetic shear and flare shear. Flare shear is defined as the angle formed between the line connecting the centroids of the two ribbons of the flare and the line perpendicular to the magnetic neutral line. The magnetic shear is computed from vector magnetograms observed at Big Bear Solar Observatory (BBSO), while the flare shear is computed from Transition Region and Coronal Explorer (TRACE) 1700A images. By a detailed comparison, we find that: 1) The magnetic shear and the flare shear of this event are basically consistent, as judged from the directions of the transverse mag-netic field and the line connecting the two ribbons' centroids. 2) During the period of the enhancement of magnetic shear, flare shear had a fast increase followed by a fluctuated decrease. 3) When the magnetic shear stopped its enhancement, the fluctuated decreasing behavior of the flare shear became very smooth. 4) Hard X-ray (HXR) spikes are well correlated with the unshearing peaks on the time profile of the rate of change of the flare shear. We give a discussion of the above phenomena. 相似文献
9.
K. Jockers 《Solar physics》1976,47(1):221-221
The two-dimensional force-free field equations are studied. The solar photosphere is considered as flat and infinitely extended and the magnetic field component perpendicular to the photosphere is prescribed as the field of a submerged line dipole, i.e. with two magnetic polarities divided by a straight infinitely long neutral line. In addition the shear of the field lines along the neutral line, i.e. the difference of the coordinate parallel to the neutral line of the two foot-points of a field line, is prescribed as a function f of the distance to the neutral line times a nonnegative constant . The function f is zero at the neutral line, goes through a maximum and drops to zero at large distances from the neutral line. The case = 0 corresponds to the current-free field. An approximate solution is obtained by a test function method. It is shown that for certain choices of the function f there exists a maximum value of beyond which a steady continuation of the solution is impossible. This forces the field to jump to a state of lower energy. The potential field, for instance, is such a lower energy state. Since the shear was prescribed as a boundary condition, the jump of the magnetic field will always be accompanied by a field line reconnection. Even though the field calculated does not closely resemble the flare geometry it is speculated that discontinuities like this one may also occur in more realistic field configurations and may actually trigger the flare.An extended version of this paper is to be published elsewhere. 相似文献
10.
We study the magnetic structure of five well-known active regions that produced great flares (X5 or larger). The six flares under investigation are the X12 flare on 1991 June 9 in AR 6659, the X5.7 flare on 2000 July 14 in AR 9077, the X5.6 flare on 2001 April 6 in AR 9415, the X5.3 flare on 2001 August 25 in AR 9591, the X17 flare on 2003 October 28 and the X10 flare on 2003 October 29, both in AR 10486. The last five events had corresponding LASCO observations and were all associated with Halo CMEs. We analyzed vector magne-tograms from Big Bear Solar Observatory, Huairou Solar Observing Station, Marshall Space Right Center and Mees Solar Observatory. In particular, we studied the magnetic gradient derived from line-of-sight magnetograms and magnetic shear derived from vector magne-tograms, and found an apparent correlation between these two parameters at a level of about 90%. We found that the magnetic gradient could be a better proxy than the shear for predicting where a major flare might occur: all six flares occurred in neutral lines with maximum gradient. The mean gradient of the flaring neutral lines ranges from 0.14 to 0.50 G km-1, 2.3 to 8 times the average value for all the neutral lines in the active regions. If we use magnetic shear as the proxy, the flaring neutral line in at least one, possibly two, of the six events would be mis-identified. 相似文献
11.
A. M. Uralov V. V. Grechnev G. V. Rudenko I. G. Rudenko H. Nakajima 《Solar physics》2008,249(2):315-335
Neutral Line associated Sources (NLSs) are quasi-stationary microwave sources projected onto vicinities of the neutral line
of the photospheric magnetic field. NLSs are often precursors of powerful flares, but their nature is unclear. We endeavor
to reveal the structure of an NLS and to analyze a physical connection between such a source with a site of energy release
in the corona above NOAA 10488 (October/November 2003). Evolution of this AR includes emergence and collision of two bipolar
magnetic structures, rise of the main magnetic separator, and the appearance of an NLS underneath. The NLS appears at a contact
site of colliding sunspots, whose relative motion goes on, resulting in a large shear along a tangent. Then the nascent NLS
becomes the main source of microwave fluctuations in the AR. The NLS emission at 17 GHz is dominated by either footpoints
or the top of a loop-like structure, an NLS loop, which connects two colliding sunspots. During a considerable amount of time,
the emission dominates over that footpoint of the NLS loop, where the magnetic field is stronger. At that time, the NLS resembles
a usual sunspot–associated radio source, whose brightness center is displaced towards the periphery of a sunspot. Microwave
emission of an X2.7 flare is mainly concentrated in an ascending flare loop, initially coinciding with the NLS loop. The top
of this loop is located at the base of a non-uniform bar-like structure visible in soft X-rays and at 34 GHz at the flare
onset. We reveal i) upward lengthening of this bar before the flare onset, ii) the motion of the top of an apparently ascending flare loop along the axis of this bar, and iii) a non-thermal microwave source, whose descent along the bar was associated with the launching of a coronal ejection. We
connect the bar with a probable position of a nearly vertical diffusion region, a site of maximal energy release inside an
extended pre-flare current sheet. The top of the NLS loop is located at the bottom of this region. A combination of the NLS
loop and diffusion region constitutes the skeleton of a quasi-stationary microwave NLS. 相似文献
12.
Haimin Wang 《Solar physics》1992,140(1):85-98
This paper studies the evolution of vector magnetic fields in the active region Boulder No. 6233 during an 11-hour observing period and its relationship to an X-3 flare on August 27, 1990.We observed the evolution of magnetic fields, which includes magnetic shear build-up, directly in high-resolution vector magnetograph movies. The magnetic shear is observed to be built up in two ways: (1) shear motion between two poles of opposite magnetic polarities and (2) direct collision of two poles of opposite polarities. When two magnetic elements of opposite polarities are canceling, the field lines are observed to turn from direct connection (potential) to a sheared configuration during the process.An X-3 flare occurred at 2100 UT. The vector magnetic structure showed an unexpected pattern of changes during and after the flare. The shear (defined as the angle between the measured transverse field and the calculated potential field) in the area covering two major footpoints increased rapidly coinciding with the burst of GOES X-ray flux. While the flare faded away in about one hour, the high shear status dropped slowly for the remainder of the observing period. Immediately after the flare, new flux emerged more rapidly and the flow speed of several magnetic elements increased near the flare footpoints.In this active region and a few other flare-productive regions we have studied recently, we always find rapid and complicated flow motions near the sites where flares occur. Photospheric flows appear to be another important factor for the production of flares. 相似文献
13.
We have analyzed the H filtergrams and vector magnetograms of the active region NOAA 7070, in which a 3B/X3.3 flare occurred on February 27, 1992. The average area per sunspot of this active region was in declining phase at the time of the flare. The vector magnetograms indicate that the magnetic field was non-potential at the flaring site. Besides non-potentiality, the longitudinal field gradient was found to be the highest at the region showing initial H brightening. Further, in H filtergrams no appreciable change in the morphology of the filament tracing the magnetic neutral line was noticed in the post-flare stage. Also, the photospheric vector magnetograms show considerable shear in post-flare magnetic field of the active region. In this paper we present the observations and discuss the possible mechanism responsible for the 3B/X3.3 flare. 相似文献
14.
On September 14–18, 2000, a medium-small solar active region was observed at Ganyu Station of Purple Mountain Observatory. Its spots were not large, but it had a peculiar active filament. On Sep.16, a flare of importance IIIb with rather intense geophysical effects was produced. Our computation of the magnetic structure of the active region reveals that the rope-shaped filament was concerned with a low magnetic arc close to magnetic neutral line. An intense shear of magnetic field occurred near magnetic rope. The QSL analysis shows that a 3-D magnetic reconnection might appear in the vicinity of filament, and this can be used to interpret the formation of a large flare. 相似文献
15.
We study the changes of the CaI λ6102.7 Å line profile and the magnetic field structure during the 1B/M2.2 while-light flare of August 12, 1981. The two brightest flare knots located in the penumbra of a sunspot with a δ configuration are investigated. The 1 ± V line profiles are analyzed. The reduction and analysis of our observations have yielded the following results. (1) The line profiles changed significantly during the flare, especially at the time of optical continuum emission observed near the flare maximum. In addition to the significant decrease in the depth, a narrow polarized emission whose Zeeman splitting corresponded to a longitudinal magnetic field strength of 3600 Gs was observed. This is much larger than the magnetic field strength in the underlying sunspot determined from the Zeeman splitting of absorption lines. (2) The largest changes of the CaI λ6102.7 Å line profile observed during the flare can lead to an underestimation of the longitudinal magnetic field strength measured with a video magnetograph by a factor of 4.5, but they cannot be responsible for the polarity reversal. (3) A sharp short-term displacement of the neutral line occurred at a time close to the flare maximum, which gave rise to a reversed-polarity magnetic field on a small area of the active region, i.e., a magnetic transient. This can be interpreted as a change in the inclination of the magnetic field lines to the line of sight during the flare. The short-term depolarization of the CaI λ6102.7 Å line emission observed at the other flare knot can also be the result of a change in the magnetic field structure. (4) These fast dynamic changes of the magnetic field lines occurred after the maximum of the impulsive flare phase and were close in time to the appearance of type II radio emission. 相似文献
16.
Marcos E. Machado Boris V. Somov Marta G. Rovira Cornelis De Jager 《Solar physics》1983,85(1):157-184
We discuss the spatial and temporal characteristics of X-ray flares occurring in the active region NOAA2372 from April 6 to 13, 1980. The flares are seen to extend in most cases across the whole active complex, involving several magnetic features. They originate in an intermediate bipole, between the two main sunspots of the active region, where high magnetic shear was detected. A rapid expansion is seen in some cases, in conjunction with the start of the impulsive hard X-ray bursts. We also detect, in the late phases of some of the events, a large soft X-ray structure overlying the whole active region, which also shows up as a noise storm region at metric wavelengths. These large loops cool by heat conduction but, in some cases, Hα condensations seem to appear, probably as a result of magnetic compression and a condensation mode of the thermal instability. The topological aspects of the field configuration are discussed, in the context of flare models invoking magnetic reconnection at the site of the primary energy release. In such a model, the intermediate bipole is the natural site of initial magnetic reconnection, particle acceleration and heating. In one particular case of a flare observed at the limb, we find possible evidence of particle acceleration in a neutral sheet at the boundary between two clearly defined magnetic structures. 相似文献
17.
Hongqi Zhang 《Monthly notices of the Royal Astronomical Society》2002,332(2):500-512
In this paper we analyse the non-potential magnetic field and the relationship with current (helicity) in the active region NOAA 9077 in 2000 July, using photospheric vector magnetograms obtained at different solar observatories and also coronal extreme-ultraviolet 171-Å images from the TRACE satellite.
We note that the shear and squeeze of magnetic field are two important indices for some flare-producing regions and can be confirmed by a sequence of photospheric vector magnetograms and EUV 171-Å features in the solar active region NOAA 9077. Evidence on the release of magnetic field near the photospheric magnetic neutral line is provided by the change of magnetic shear, electric current and current helicity in the lower solar atmosphere. It is found that the 'Bastille Day' 3B/5.7X flare on 2000 July 14 was triggered by the interaction of the different magnetic loop systems, which is relevant to the ejection of helical magnetic field from the lower solar atmosphere. The eruption of the large-scale coronal magnetic field occurs later than the decay of the highly sheared photospheric magnetic field and also current in the active region. 相似文献
We note that the shear and squeeze of magnetic field are two important indices for some flare-producing regions and can be confirmed by a sequence of photospheric vector magnetograms and EUV 171-Å features in the solar active region NOAA 9077. Evidence on the release of magnetic field near the photospheric magnetic neutral line is provided by the change of magnetic shear, electric current and current helicity in the lower solar atmosphere. It is found that the 'Bastille Day' 3B/5.7X flare on 2000 July 14 was triggered by the interaction of the different magnetic loop systems, which is relevant to the ejection of helical magnetic field from the lower solar atmosphere. The eruption of the large-scale coronal magnetic field occurs later than the decay of the highly sheared photospheric magnetic field and also current in the active region. 相似文献
18.
The high-resolution H images observed during the decay phase of a long-duration flare on 23 March 1991 are used to study the three-dimensional magnetic field configuration of the active region NOAA 6555. Whereas all the large flares in NOAA 6555 occurred at the location of high magnetic shear and flux emergence, this long-duration flare was observed in the region of low magnetic shear at the photosphere. The H loops activity started soon after the maximum phase of the flare. There were a few long loop at the initial phase of the activity. Some of these were sheared in the chromosphere at an angle of about 45° to the east-west axis. Gradually, an increasing number of shorter loops, oriented along the east-west axis, started appearing. The chromospheric Dopplergrams show blue shifts at the end points of the loops. By using different magnetic field models, we have extrapolated the photospheric magnetograms to chromospheric heights. The magnetic field lines computed by using the potential field model correspond to most of the observed H loops. The height of the H loops were derived by comparing them with the computed field lines. From the temporal evolution of the H loop activity, we derive the negative rate of appearance of H features as a function of height. It is found that the field lines oriented along one of the neutral lines were sheared and low lying. The higher field lines were mostly potential. The paper also outlines a possible scenario for describing the post-flare stage of the observed long-duration flare. 相似文献
19.
The present MSFC Vector Magnetograph has sufficient spatial resolution (2.7 arcsec pixels) and sensitivity to the transverse field (the noise level is about 100 gauss) to map the transverse field in active regions accurately enough to reveal key aspects of the sheared magnetic fields commonly found at flare sites. From the measured shear angle along the polarity inversion line in sites that flared and in other shear sites that didn't flare, we find evidence that a sufficient condition for a flare to occur in 1000 gauss fields in and near sunspots is that both (1) the maximum shear angle exceed 85 degrees and (2) the extent of strong shear (shear angle > 80 degrees) exceed 10,000 km. 相似文献
20.
In this paper, we reconstruct the finite energy force-free magnetic field of the active region NOAA 8100 on 4 November 1997
above the photosphere. In particular, the 3-D magnetic field structures before and after a 2B/X2 flare at 05:58 UT in this
region are analyzed. The magnetic field lines were extrapolated in close coincidence with the Yohkoh soft X-ray (SXR) loops accordingly. It is found that the active region is composed of an emerging flux loop, a complex loop
system with differential magnetic field shear, and large-scale, or open field lines. Similar magnetic connectivity has been
obtained for both instants but apparent changes of the twisting situations of the calculated magnetic field lines can be observed
that properly align with the corresponding SXR coronal loops. We conclude that this flare was triggered by the interaction
of an emerging flux loop and a large loop system with differential magnetic field shear, as well as large-scale, or open field
lines. The onset of the flare was at the common footpoints of several interacting magnetic loops and confined near the footpoints
of the emerging flux loop. The sheared configuration remained even after the energetic flare, as demonstrated by calculated
values of the twist for the loop system, which means that the active region was relaxed to a lower energy state but not completely
to the minimum energy state (two days later another X-class flare occurred in this region). 相似文献