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1.
2.
Simultaneous visible, EUV, and X-ray observations of magnetic structures before and during the onset of the flare of 5 September 1973 are co-registered and interpreted. Ninety minutes before the flare, intense EUV knots fluctuate near the loops which subsequently flare. The pre-flare loop is observed in O IV 554, but not in X-rays, which show instead a parallel structure which is related either to a darkening filament or the subsequent flare kernels. As the full disk X-ray emission increases, first the EUV flare loop appears, then X-ray kernels form at the feet of two EUV loops, one of which overlies the activated filament. The flaring, at any given time, is confined to a single loop (or bundle of loops) whose long axis (barely) crosses the neutral line. As time progresses, the flaring moves to other (probably higher) loops sharing the off-band H footpoints but whose axes are rotated relative to the earlier loops by angles of about 30°. Previous interpretations of single-telescope observations are revised in this joint investigation. 相似文献
3.
We analyze multiple-wavelength observations of a two-ribbon flare exhibiting apparent expansion motion of the flare ribbons
in the lower atmosphere and rising motion of X-ray emission at the top of newly-formed flare loops. We evaluate magnetic reconnection
rate in terms of V
r
B
r by measuring the ribbon-expansion velocity (V
r) and the chromospheric magnetic field (B
r) swept by the ribbons. We also measure the velocity (V
t) of the apparent rising motion of the loop-top X-ray source, and estimate the mean magnetic field (B
t) at the top of newly-formed flare loops using the relation 〈V
t
B
t〉≈〈V
r
B
r〉, namely, conservation of reconnection flux along flare loops. For this flare, B
t is found to be 120 and 60 G, respectively, during two emission peaks five minutes apart in the impulsive phase. An estimate
of the magnetic field in flare loops is also achieved by analyzing the microwave and hard X-ray spectral observations, yielding
B=250 and 120 G at the two emission peaks, respectively. The measured B from the microwave spectrum is an appropriately-weighted value of magnetic field from the loop top to the loop leg. Therefore,
the two methods to evaluate coronal magnetic field in flaring loops produce fully-consistent results in this event. 相似文献
4.
A model is presented to explain the observed frequency distribution of flare energies, based on independent flaring at a number of distinct topological structures (separators) within active-region magnetic fields. The model is a modification and generalization of a recent model due to Craig (2001), and reconciles that model with the observed flare waiting-time distribution, and the observed absence of a flare waiting-time versus energy relationship. The basic assumptions of the model are that flares of energy E
2 occur at separators of length
, and that the frequency of flaring at a separator is defined by the Alfvén transit time of the structure. To reproduce the observed distribution of flare energies the model requires a probability distribution P(
)
–1 of separator lengths
within active regions. This prediction of the model is in principle testable. A theoretical origin for this distribution is also discussed. 相似文献
5.
The multi-wavelength analysis is performed on a flare on September 9, 2002 with data of Owens Valley Solar Arrays (OVSA),
Big Bear Solar Observatory (BBSO), Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and Extreme UV Imager Telescope
(EIT), and The Michelson Doppler Imager (MDI) on board of the Solar and Heliospheric Observatory (SOHO). The radio sources
at 4.8 and 6.2 GHz located in the intersection of two flaring loops at 195 of SOHO/EIT respectively with two dipole magnetic
fields of SOHO/MDI, in which one EIT loop was coincident with an X-ray loop of RHESSI at 12–25 keV, and two Hαbright kernels a1 and a2 of BBSO, respectively at the two footpoints of this loop; the second EIT loop connected another two
Hαkernels b1 and b2 and radio sources at 7.8 and 8.2 GHz of OVSA. The maximum phase of microwave bursts was evidently later
than that of hard X-ray bursts and Hαkernels a1 and a2, but consistent with that of Hαkernels b1 and b2. Moreover, the flare may be triggered by the interaction of the two flaring loops, which is suggested by
the cross-correlation of radio, optical, and X-ray light curves of a common quasi-periodic oscillation in the rising phase,
as well as two peaks at about 7 and 9 GHz of the microwave spectra at the peak times of the oscillation, while the bi-directional
time delays at two reversal frequencies respectively at 7.8 and 9.4 GHz (similar to the peak frequencies of the microwave
spectra) may indicate two reconnection sites at different coronal levels. The microwave and hard X-ray footpoint sources located
in different EUV and optical loops may be explained by different magnetic field strength and the pitch angle distribution
of nonthermal electrons in these two loops. 相似文献
6.
Rates of flaring in individual active regions on the Sun during the period 1981–1999 are examined using United States Air Force/Mount Wilson (USAF/MWL) active-region observations together with the Geostationary Operational Environmental Satellite (GOES) soft X-ray flare catalog. Of the flares in the catalog above C1 class, 61.5% are identified with an active region. Evidence is presented for obscuration, i.e. that the increase in soft X-ray flux during a large flare decreases the likelihood of detection of soft X-ray events immediately following the large flare. This effect means that many events are missing from the GOES catalog. It is estimated that in the absence of obscuration the number of flares above C1 class would be higher by (75±23)%. A second observational selection effect – an increased tendency for larger flares to be identified with an active region – is also identified. The distributions of numbers of flares produced by individual active regions and of mean flaring rate among active regions are shown to be approximately exponential, although there are excess numbers of active regions with low flare numbers and low flaring rates. A Bayesian procedure is used to analyze the time history of the flaring rate in the individual active regions. A substantial number of active regions appear to exhibit variation in flaring rate during their transit of the solar disk. Examples are shown of regions with and without rate variation, illustrating the different distributions of times between events (waiting-time distributions) that are observed. A piecewise constant Poisson process is found to provide a good model for the observed waiting-time distributions. Finally, applications of analysis of the rate of flaring to understanding the flare mechanism and to flare prediction are discussed. 相似文献
7.
In this paper, we suggest that a solar flare may be triggered by a lack of thermal equilibrium rather than by a magnetic instability. The possibility of such a thermal nonequilibrium (or catastrophe) is demonstrated by solving approximately the energy equation for a loop under a balance between thermal conduction, optically thin radiation and a heating source. It is found that, if one starts with a cool equilibrium at a few times 104 K and gradually increases the heating or decreases the loop pressure (or decreases the loop length), then, ultimately, critical metastable conditions are reached beyond which no cool equilibrium exists. The plasma heats up explosively to a new quasi-equilibrium at typically 107 K. During such a thermal flaring, any magnetic disruption or particle acceleration are secondary in nature. For a simple-loop (or compact) flare, the cool core of an active-region loop heats up and the magnetic tube of plasma maintains its position. For a two-ribbon flare, the material of an active-region (or plage) filament heats up and expands along the filament; it slowly rises until, at a critical height, the magnetic configuration becomes magnetohydrodynamically unstable and erupts violently outwards. In this case thermal nonequilibrium acts as a trigger for the magnetic eruption and subsequent magnetic energy release as the field closes back down. 相似文献
8.
Solar flares occur due to the sudden release of energy stored in active-region magnetic fields. To date, the precursors to
flaring are still not fully understood, although there is evidence that flaring is related to changes in the topology or complexity
of an active-region’s magnetic field. Here, the evolution of the magnetic field in active region NOAA 10953 was examined using
Hinode/SOT-SP data over a period of 12 hours leading up to and after a GOES B1.0 flare. A number of magnetic-field properties and
low-order aspects of magnetic-field topology were extracted from two flux regions that exhibited increased Ca ii H emission during the flare. Pre-flare increases in vertical field strength, vertical current density, and inclination angle
of ≈ 8° toward the vertical were observed in flux elements surrounding the primary sunspot. The vertical field strength and
current density subsequently decreased in the post-flare state, with the inclination becoming more horizontal by ≈ 7°. This
behavior of the field vector may provide a physical basis for future flare-forecasting efforts. 相似文献
9.
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
10.
J. Ireland C. A. Young R. T. J. McAteer C. Whelan R. J. Hewett P. T. Gallagher 《Solar physics》2008,252(1):121-137
Two different multiresolution analyses are used to decompose the structure of active-region magnetic flux into concentrations
of different size scales. Lines separating these opposite polarity regions of flux at each size scale are found. These lines
are used as a mask on a map of the magnetic field gradient to sample the local gradient between opposite polarity regions
of given scale sizes. It is shown that the maximum, average, and standard deviation of the magnetic flux gradient for α,β,β
γ, and β
γ
δ active-regions increase in the order listed, and that the order is maintained over all length scales. Since magnetic flux
gradient is strongly linked to active-region activity, such as flares, this study demonstrates that, on average, the Mt. Wilson
classification encodes the notion of activity over all length scales in the active-region, and not just those length scales
at which the strongest flux gradients are found. Further, it is also shown that the average gradients in the field, and the
average length-scale at which they occur, also increase in the same order. Finally, there are significant differences in the
gradient distribution, between flaring and non-flaring active regions, which are maintained over all length scales. It is
also shown that the average gradient content of active-regions that have large flares (GOES class “M” and above) is larger
than that for active regions containing flares of all flare sizes; this difference is also maintained at all length scales.
All of the reported results are independent of the multiresolution transform used. The implications for the Mt. Wilson classification
of active-regions in relation to the multiresolution gradient content and flaring activity are discussed. 相似文献
11.
Radosław Rek 《Solar physics》2010,267(2):361-375
Solar flares take place in regions of strong magnetic fields and are generally accepted to be the result of a resistive instability
leading to magnetic reconnection. When new flux emerges into a pre-existing active region it can act as a flare and coronal
mass ejection trigger. In this study we observed active region 10955 after the emergence of small-scale additional flux at
the magnetic inversion line. We found that flaring began when additional positive flux levels exceeded 1.38×1020 Mx (maxwell), approximately 7 h after the initial flux emergence. We focussed on the pre-flare activity of one B-class flare
that occurred on the following day. The earliest indication of activity was a rise in the non-thermal velocity one hour before
the flare. 40 min before flaring began, brightenings and pre-flare flows were observed along two loop systems in the corona,
involving the new flux and the pre-existing active region loops. We discuss the possibility that reconnection between the
new flux and pre-existing loops before the flare drives the flows by either generating slow mode magnetoacoustic waves or
a pressure gradient between the newly reconnected loops. The subsequent B-class flare originated from fast reconnection of
the same loop systems as the pre-flare flows. 相似文献
12.
It is well known that the oscillating MHD waves drive periodic variations in the magnetic field. But how the MHD waves can be triggered in the flaring loops is not yet well known. It seems to us that this problem should be connected with the physical processes occurring in the flare loop during a solar flare. A peculiar solar flare event at 04:00–04:51 UT on May 23, 1990 was observed simultaneously with time resolutions 1 s and 10 ms by Nanjing University Observatory and Beijing Normal University Observatory, which are about 1000 km apart, at 3.2 cm and 2 cm wavelengths, respectively. Two kinds of pulsations with quasi-periods 1.5 s and 40 s were found in radio bursts at the two short centimeter waves; however, the shorter quasi-periodic pulsations were superimposed upon the longer ones. From the data analysis of the above-mentioned quasi-periodic pulsations and associated phenomena in radio and soft X-ray emissions during this flare event published in Solar Geophysical Data (SGD), the authors suggest that the sudden increase in plasma pressure inside (or underlying) the flare kernel due to the upward moving chromospheric evaporated gas, which is caused by the explosive collision heating of strong non-thermal electrons injected downwards from the microwave burst source, plays the important role of triggering agents for MHD oscillations (fast magneto-acoustic mode and Alfvén mode) of the flare loop. These physical processes occurring in the flare loop during the impulsive phase of the solar flare may be used to account for the origin and observational characteristics of quasi-periodic pulsations in solar radio bursts at the two short centimeter wavelengths during the flare event of 1990 May 23. In addition, the average physical parameters N, T, B inside or underlying the flare kernel can be also evaluated. 相似文献
13.
A. A. Akopian 《Astrophysics》2010,53(4):544-553
Methods of detecting probable cycles in the flaring activity of flare stars and determining the cycle durations are examined.
A new method of detecting a cycle of flaring activity and determining its duration is proposed assuming periodic flaring activity.
This method is applied to two stars from the list of flare stars in the Pleiades cluster, Ton91 and Ton377. Variable flaring
activity is found in both stars and the periods are estimated to be P=15.2 and 17.7 years for Ton91 and Ton377, respectively. 相似文献
14.
We have made an attempt to obtain relationship of magnetic shear and vertical currents in NOAA AR7321. Intercomparison of
changes observed at several flaring and non-flaring sites associated with an M4/2B flare observed on October 26, 1992 is reported. 相似文献
15.
Observations of the flare on 7 January 1992 are interpreted using a topological model of the magnetic field. The model, developed here, applies a theory of three-dimensional reconnection to the inferred magnetic field configuration for 7 January. In the model field a new bipole ( 1021 Mx) emerges amidst pre-existing active region flux. This emergence gives rise to two current ribbons along the boundaries (separators) separating the distinct, new and old, flux systems. Sudden reconnection across these boundary curves transfers 3 ×1020 Mx of flux from the bipole into the surrounding flux. The model also predicts the simultaneous (sympathetic) flaring of the two current ribbons. This explains the complex two-loop structure noted in previous observations of this flare. We subject the model predictions to comparisons with observations of the flare. The locations of current ribbons in the model correspond closely with those of observed soft X-ray loops. In addition the footpoints and apexes of the ribbons correspond with observed sources of microwave and hard X-ray emission. The magnitude of energy stored by the current ribbons compares favorably to the inferred energy content of accelerated electrons in the flare. 相似文献
16.
We present the results of an analysis of a flare event of importance M2.8 that occurred at 00:56 UT 28 August 1999. The analysis is based upon observations made with the Nobeyama radioheliograph (NoRH) and polarimeters (NoRP), TRACE, SOHO/MDI, EIT, and Yohkoh/SXT. The images show a very complex flaring region. Pre-flare TRACE and EIT images at 00:24 UT show a small brightening in the region before the flare occurred. The active region in which the flare occurred had evolving magnetic fields, and new magnetic flux seems to have emerged. The X-ray and radio time profiles for this event show a double-peaked structure. The polarimeter data showed that the maximum radio emission (1200 s.f.u.) occurred at 9.4 GHz. At 17 GHz the NoRH images appear to show four different radio sources including the main spot and the main flare loop. Most of the microwave emission seems to originate from the main flare loop. Comparison of BATSE and microwave time profiles at 17 and 34 GHz from the main sunspot source shows that these profiles have similar structures and they coincide with the hard X-ray peaks. The maximum of the flare loop emission was delayed by 10 s relative to the second maximum of the sunspot associated flare emission. Analysis of SXT images during the post-flare phase shows a complex morphology – several intersecting loops and changes in the shape of the main flare loop. 相似文献
17.
A simple model is presented to account for theYohkoh flare observations of Feldmanet al. (1994), and Masuda (1994). Electrons accelerated by the flare are assumed to encounter the dense, small regions observed by Feldmanet al. at the tops of impulsively flaring coronal magnetic loops. The values of electron density and volume inferred by Feldmanet al. imply that these dense regions present an intermediate thick-thin target to the energised electrons. Specifically, they present a thick (thin) target to electrons with energy much less (greater) thanE
c
, where 15 keV <E
c
< 40 keV. The electrons are either stopped at the loop top or precipitate down the field lines of the loop to the footpoints. Collisional losses of the electrons at the loop top produce the heating observed by Feldmanet al. and also some hard X-rays. It is argued that this is the mechanism for the loop-top hard X-ray sources observed in limb flares by Masuda. Adopting a simple model for the energy losses of electrons traversing the dense region and the ambient loop plasma, hard X-ray spectra are derived for the loop-top source, the footpoint sources and the region between the loop top and footpoints. These spectra are compared with the observations of Masuda. The model spectra are found to qualitatively agree with the data, and in particular account for the observed steepening of the loop-top and footpoint spectra between 14 and 53 keV and the relative brightnesses of the loop-top and footpoint sources. 相似文献
18.
19.
An X17 class (GOES soft X-ray) two-ribbon solar flare on October 28, 2003 is analyzed in order to determine the relationship
between the timing of the impulsive phase of the flare and the magnetic shear change in the flaring region. EUV observations
made by the Transition Region and Coronal Explorer (TRACE) show a clear decrease in the shear of the flare footpoints during the flare. The shear change stopped in the middle
of the impulsive phase. The observations are interpreted in terms of the splitting of the sheared envelope field of the greatly
sheared core rope during the early phase of the flare. We have also investigated the temporal correlation between the EUV
emission from the brightenings observed by TRACE and the hard X-ray (HXR) emission (E > 150 keV) observed by the anticoincidence system (ACS) of the spectrometer SPI on board the ESA INTEGRAL satellite. The
correlation between these two emissions is very good, and the HXR sources (RHESSI) late in the flare are located within the
two EUV ribbons. These observations are favorable to the explanation that the EUV brightenings mainly result from direct bombardment
of the atmosphere by the energetic particles accelerated at the reconnection site, as does the HXR emission. However, if there
is a high temperature (T > 20 MK) HXR source close to the loop top, a contribution of thermal conduction to the EUV brightenings cannot be ruled out. 相似文献
20.
C. Debi Prasad 《Astrophysics and Space Science》2000,274(3):463-471
The properties of the flaring sites in the active region NOAA 6555 and 6659 are presented in this paper. Although, several
locations of these active regions displayed high magnetic shear, the X-class flares occurred only at one of them. Our investigations
show that these locations are characterized by the `crossing' of magnetic and velocity field neutral lines. Based on a new
parameter to define the stressed magnetic fields, we show that some of these flaring locations possessed highly stressed magnetic
field which disappeared after the flare.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献