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1.
Using high-cadence magnetograms from the SOHO/MDI we have investigated variations of the photospheric magnetic field during
solar flares and CMEs. In the case of a strong X-class flare of May 2, 1998, we have detected variations of magnetic field
in a form of a rapidly propagating magnetic wave. During the impulsive phase of the flare we have observed a sudden decrease
of the magnetic energy in the flare region. This provides direct evidence of magnetic energy release in solar flares. We discuss
the physics of the magnetic field variations, and their relations to the Moreton Hα waves and the coronal waves observed by
the EIT. 相似文献
2.
B. C. Low 《Solar physics》1996,167(1-2):217-265
This review puts together what we have learned about coronal structures and phenomenology to synthesize a physical picture of the corona as a voluminous, thermally and electrically highly-conducting atmosphere responding dynamically to the injection of magnetic flux from below. The synthesis describes complementary roles played by the magnetic heating of the corona, the different types of flares, and the coronal mass ejections as physical processes by which magnetic flux and helicity make their way from below the photosphere into the corona, and, ultimately, into interplanetary space. In these processes, a physically meaningful interplay among dissipative magnetohydrodynamic turbulence, ideal ordered flows, and magnetic helicity determines how and when the rich variety of relatively long-lived coronal structures, spawned by the emerged magnetic flux, will evolve quasi-steadily or erupt with the impressive energies characteristic of flares and coronal mass ejections. Central to this picture is the suggestion, based on recent theoretical and observational works, that the the emerged flux may take the form of a twisted flux rope residing principally in the corona. Such a flux rope is identified with the low-density cavity at the base of a coronal helmet, often but not always encasing a quiescent prominence. The flux rope may either be bodily transported into the corona from below the photosphere, or reform out of a state of flaring turbulence under some suitable constraint of magnetic-helicity conservation. The appeal of this synthesis is its physical simplicity and the manner it relates a large set of diverse phenomena into a self-consistent whole. The implications of this view point are discussed.The topics covered are: the large-scale corona; helmet streamers; quiescent prominences; coronal mass ejections; flares and heating; magnetic reconnection and magnetic helicity; and, the hydromagnetics of magnetic flux emergence.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
3.
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
4.
Louise K. Harra Nancy U. Crooker Cristina H. Mandrini Lidia van Driel-Gesztelyi Sergio Dasso Jingxiu Wang Heather Elliott Gemma Attrill Bernard V. Jackson Mario M. Bisi 《Solar physics》2007,244(1-2):95-114
We describe the interplanetary coronal mass ejections (ICMEs) that occurred as a result of a series of solar flares and eruptions
from 4 to 8 November 2004. Two ICMEs/magnetic clouds occurring from these events had opposite magnetic orientations. This
was despite the fact that the major flares related to these events occurred within the same active region that maintained
the same magnetic configuration. The solar events include a wide array of activities: flares, trans-equatorial coronal loop
disappearance and reformation, trans-equatorial filament eruption, and coronal hole interaction. The first major ICME/magnetic
cloud was predominantly related to the active region 10696 eruption. The second major ICME/magnetic cloud was found to be
consistent with the magnetic orientation of an erupting trans-equatorial filament or else a rotation of 160° of a flux rope
in the active region. We discuss these possibilities and emphasize the importance of understanding the magnetic evolution
of the solar source region before we can begin to predict geoeffective events with any accuracy. 相似文献
5.
We present H and coronal X-ray images of the large two-ribbon flare of 25–26 June, 1992 during its long-lasting gradual decay phase. From these observations we deduce that the 3-D magnetic field configuration late in this flare was similar to that at and before the onset of such large eruptive bipolar flares: the sheared core field running under and out of the flare arcade was S-shaped, and at least one elbow of the S looped into the low corona. From previous observations of filament-eruption flares, we infer that such core-field coronal elbows, though rarely observed, are probably a common feature of the 3-D magnetic field configuration late in large two-ribbon flares. The rare circumstance that apparently resulted in a coronal elbow of the core field being visible in H in our flare was the occurrence of a series of subflares low in the core field under the late-phase arcade of the large flare; these subflares probably produced flaring arches in the northern coronal elbow, thereby rendering this elbow visible in H. The observed late-phase 3-D field configuration presented here, together with the recent sheared-core bipolar magnetic field model of Antiochos, Dahlburg, and Klimchuk (1994) and recent Yohkoh SXT observations of the coronal magnetic field configuration at and before the onset of large eruptive bipolar flares, supports the seminal 3-D model for eruptive two-ribbon flares proposed by Hirayama (1974), with three modifications: (1) the preflare magnetic field is closed over the filament-holding core field; (2) the preflare core field has the shape of an S (or backward S) with coronal elbows; (3) a lower part of the core field does not erupt and open, but remains closed throughout flare, and can have prominent coronal elbows. In this picture, the rest of the core field, the upper part, does erupt and open along with the preflare arcade envelope field in which it rides; the flare arcade is formed by reconnection that begins in the middle of the core field at the start of the eruption and progresses from reconnecting closed core field early in the flare to reconnecting opened envelope field late in the flare. 相似文献
6.
Studies of solar flares indicate that the mechanism of flares is magnetic in character and that the coronal magnetic field is a key to understanding solar high-energy phenomena. In our ongoing research we are conducting a systematic study of a large database of observations which includes both coronal structure (from the Soft X-ray Telescope on the Yohkoh spacecraft) and photospheric vector magnetic fields (from the Haleakala Stokes Polarimeter at Mees Solar Observatory). We compare the three-dimensional nonlinear force-free coronal magnetic field, computed from photospheric boundary data, to images of coronal structure. In this paper we outline our techniques and present results for active region AR 7220/7222. We show that the computed force-free coronal magnetic field agrees well with Yohkoh X-ray coronal loops, and we discuss the properties of the coronal magnetic field and the soft X-ray loops. 相似文献
7.
D. J. Mullan 《Solar physics》1977,54(1):183-206
Short-lived increases in the brightness of many red dwarfs have been observed for the last 30 yr, and a variety of more or less exotic models have been proposed to account for such flares. Information about flares in the Sun has progressed greatly in recent years as a result of spacecraft experiments, and properties of coronal flare plasma are becoming increasingly better known. In this paper, after briefly reviewing optical, radio and X-ray observations of stellar flares, we show how a simplified model which describes conductive plus radiative cooling of the coronal flare plasma in solar flares has been modified to apply to optical and X-ray stellar flare phenomena. This model reproduces many characteristic features of stellar flares, including the mean UBV colors of flare light, the direction of flare decay in the two-color diagram, precursors, Stillstands, secondary maxima, lack of sensitivity of flare color to flare amplitude, low flux of flare X-rays, distinction between so-called spike flares and slow flares, Balmer jumps of as much as 6–8, and emission line redshifts up to 3000 km s–1. In all probability, therefore, stellar flares involve physical processes which are no more exotic (and no less!) than those in solar flares. Advantages of observing stellar flares include the possibilities of (i) applying optical diagnostics to coronal flare plasma, whereas this is almost impossible in the Sun, and (ii) testing solar flare models in environments which are not generally accessible in the solar atmosphere. 相似文献
8.
R. Chandra B. Schmieder C. H. Mandrini P. Démoulin E. Pariat T. Török W. Uddin 《Solar physics》2011,269(1):83-104
We present and interpret observations of two morphologically homologous flares that occurred in active region (AR) NOAA 10501
on 20 November 2003. Both flares displayed four homologous Hα ribbons and were both accompanied by coronal mass ejections
(CMEs). The central flare ribbons were located at the site of an emerging bipole in the centre of the active region. The negative
polarity of this bipole fragmented in two main pieces, one rotating around the positive polarity by ≈ 110° within 32 hours.
We model the coronal magnetic field and compute its topology, using as boundary condition the magnetogram closest in time
to each flare. In particular, we calculate the location of quasi-separatrix layers (QSLs) in order to understand the connectivity between the flare ribbons. Though several polarities were present in AR 10501,
the global magnetic field topology corresponds to a quadrupolar magnetic field distribution without magnetic null points.
For both flares, the photospheric traces of QSLs are similar and match well the locations of the four Hα ribbons. This globally
unchanged topology and the continuous shearing by the rotating bipole are two key factors responsible for the flare homology.
However, our analyses also indicate that different magnetic connectivity domains of the quadrupolar configuration become unstable
during each flare, so that magnetic reconnection proceeds differently in both events. 相似文献
9.
10.
In this study we analyse the positions of major flares from 1978 and 1979, with respect to the magnetic structure of the solar corona, as described by a potential field model. We find that major flares exhibit no strong association with the neutral line at the chromospheric level. However, when we calculate the neutral line's position at higher and higher altitudes in the corona, we find that major flares show an increasing tendency to be found close to these high-altitude coronal neutral lines. The correlation between flares and higher-altitude coronal neutral lines reaches a maximum at an altitude of 0.35R
, and thereafter decreases as the neutral line is moved out to the source surface at an altitude of 1.50R
. This indicates that major flares are strongly associated with coronal structure at the 0.35R
level ( 250 000 km) - an altitude surprisingly high in the corona. This reinforces the idea that flares are associated with large-scale coronal magnetic fields and also indicates that the region of coronal magnetic topology important to solar flare processes may be larger than previously thought. 相似文献
11.
Two classes of coronal expansion phenomena have been studied in Sacramento Peak coronal movies: Slow, slightly decelerated
expansion phenomena (v=∼10− =∼ 2 km/sec) and fast, accelerated, quasi-exploding arches (v =∼ 10 - > 100 km/sec). The various phenomena were found to be associated with flares in different ways: The slow expansions
were long lived post-flare phenomena initiated by the flare; the accelerated expanding arches were either (a) arches expanding
prior to and apparently exploding at flare onset, or (b) arches apparently emerging from the flare (probably in its initial
phase) and rapidly expanding and exploding, or (c) the expansion and disruption of (originally stable) coronal arches during
occurrence of a distant flare. These expansions may be considered as evidence for corresponding flare associated changes in
the coronal magnetic field.
Mitteilung aus dem Fraunhofer Institut Nr. 96. 相似文献
12.
Data assimilation techniques, developed in the past two decades mainly for weather prediction, produce better forecasts by
taking advantage of both theoretical/numerical models and real-time observations. In this paper, we explore the possibility
of applying the four-dimensional variational data assimilation (4D-VAR) technique to the prediction of solar flares. We do
so in the context of a continuous version of the classical cellular-automaton-based self-organized critical avalanche models
of solar flares introduced by Lu and Hamilton (Astrophys. J.
380, L89, 1991). Such models, although a priori far removed from the physics of magnetic reconnection and magnetohydrodynamical evolution of coronal structures, nonetheless
reproduce quite well the observed statistical distribution of flare characteristics. We report here on a large set of data
assimilation runs on synthetic energy release time series. Our results indicate that, despite the unpredictable (and unobservable)
stochastic nature of the driving/triggering mechanism within the avalanche model, 4D-VAR succeeds in producing optimal initial
conditions that reproduce adequately the time series of energy released by avalanches and flares. This is an essential first
step toward forecasting real flares. 相似文献
13.
Tilaye Tadesse T. Wiegelmann P. J. MacNeice B. Inhester K. Olson A. Pevtsov 《Solar physics》2014,289(3):831-845
Measurements of magnetic fields and electric currents in the pre-eruptive corona are crucial to the study of solar eruptive phenomena, like flares and coronal mass ejections (CMEs). However, spectro-polarimetric measurements of certain photospheric lines permit a determination of the vector magnetic field only at the photosphere. Therefore, there is considerable interest in accurate modeling of the solar coronal magnetic field using photospheric vector magnetograms as boundary data. In this work, we model the coronal magnetic field above multiple active regions with the help of a potential field and a nonlinear force-free field (NLFFF) extrapolation code over the full solar disk using Helioseismic and Magnetic Imager (SDO/HMI) data as boundary conditions. We compare projections of the resulting magnetic field lines with full-disk coronal images from the Atmospheric Imaging Assembly (SDO/AIA) for both models. This study has found that the NLFFF model reconstructs the magnetic configuration closer to observation than the potential field model for full-disk magnetic field extrapolation. We conclude that many of the trans-equatorial loops connecting the two solar hemispheres are current-free. 相似文献
14.
Magnetic fields in the low corona are the only plausible source of energy for solar flares. Other energy sources appear inadequate or uncorrelated with flares. Low coronal magnetic fields cannot be measured accurately, so most attention has been directed toward measurements of the photospheric magnetic fields from which coronal developments may be inferred. Observations of these magnetic fields are reviewed. It is concluded that, except possibly for the largest flares, changes in the photospheric magnetic fields in flaring centers are confined to evolutionary changes associated with emergence of new magnetic flux. Flare observations with the 10830 Å line of helium, in particular, are discussed. It is concluded that the brightest flare knots appear near points of emergent magnetic flux. Pre-flare activation and eruptions of H filaments are discussed. It is concluded that the rapid motions in filaments indicate unambiguously that the magnetic fields in the low corona are severely disrupted prior to most flares. The coronal signature of H filament eruptions is illustrated with soft X-ray photographs from the S-054 experiment of the NASA Skylab mission. An attempt is made, by studying X-ray flare morphology, to determine whether flares grow by reconnections between adjacent or intertwined magnetic elements or by triggering, in which each flaring loop drives adjacent loops to unstable states. It is concluded that successive loop brightenings are most easily interpreted as the result of magnetic field reconnections, although better time resolution is required to settle the question. A model of magnetic field reconnections for flares associated with filament activation and emerging magnetic flux is presented. 相似文献
15.
The well-known Chandrasekhar-Prendergastmagnetostatic solution for a sphericalmagnetic vortex with axial symmetry squeezed
externally by a potential magnetic field is of considerable interest both for describing the magnetic field of a star as a
whole and for modeling solar active phenomena (flares, coronal spiders, etc.). This solution is generalized to the case of
a uniform gravity field. In contrast to the Chandrasekhar-Prendergast model, the dependence of the plasma density in the spherical
vortex on magnetic flux appears in the new solution. This expands considerably the class of magnetoplasma equilibria being
analyzed and allows new scenarios for energy release in solar flares and coronal spiders to be proposed. 相似文献
16.
A. Hewish 《Solar physics》1988,116(1):195-198
A recent study of associations between geomagnetic storms and solar phenomena has found more associations with solar flares than with coronal holes (Garcia and Dryer, 1987). This disagrees with observations of earthbound transients obtained from IPS imaging which showed that nearly all geomagnetically effective disturbances originated from coronal holes at low latitudes. The discrepancy has arisen because the former study failed to take into account the large angular extent of transient eruptions from coronal holes. It is highly probable that the intense geomagnetic storm of February 1986, discussed by Garcia and Dryer, was caused by a low-latitude coronal hole which was present at that time. This answers their question concerning moderately strong flares that apparently cause major storms, while much larger flares often do not; flares may sometimes be associated with eruptions from coronal holes, but only as peripheral events. 相似文献
17.
M. R. Kundu 《Solar physics》1996,169(2):389-402
We present a review of selected studies based upon simultaneous radio and X-ray observations of solar flares and coronal transients. We use primarily the observations made with large radio imaging instruments (VLA, BIMA, Nobeyama, and Nançay) along with Yohkoh/SXT and HXT and CGRO experiments. We review the recent work on millimeter imaging of solar flares, microwave and hard X-ray observations of footpoint emission from flaring loops, metric type IV continuum bursts, and coronal X-ray structures. We discuss the recent studies on thermal and nonthermal processes in coronal transients such as XBP flares, coronal X-ray jets, and active region transient brightenings.Dedicated to Cornelis de Jager 相似文献
18.
Kenneth H. Schatten 《Solar physics》1970,12(3):484-491
The Faraday rotation of a radio source (Pioneer 6) occulted by the solar corona has been measured by Levy et al. (1969).
During the course of these measurements, three large-scale transient phenomena were observed. These events were preceded by
subflares and class 1 flares. These transient events are interpreted as evidence for a coronal magnetic bottle at 10 R
⊙. The velocity of propagation for the disturbance is set at 200 km/sec; the dimension of the region, 10 R
⊙; field strength at 10 R
⊙, 0.02 G; particle density, 2.0 × 104/cm3; Alfvén speed, 320 km/sec. From the nature of the observations and the lack of related effects from similar flares on the
interplanetary sector pattern observed at 1 AU, it is suggested that such coronal magnetic bottles expand to perhaps 10–30
R
⊙ and then contract to a few solar radii. Such a phenomena is evidence for an expansion of the corona with a sub-Alfvénic velocity.
It is further suggested that such magnetic bottles may be important in the storage and diffusion of solar generated cosmic
ray particles.
NAS-NRC Postdoctoral Resident Research Associate. 相似文献
19.
In our recent paper (Jakimiec and Tomczak, Solar Physics
261, 233, 2010) we investigated quasi-periodic oscillations of hard X-rays during the impulsive phase of solar flares. We have come to the
conclusion that they are caused by magnetosonic oscillations of magnetic traps within the volume of hard-X-ray (HXR) loop-top
sources. In the present paper we investigate four flares that show clear quasi-periodic sequences of the HXR pulses. We also
describe our phenomenological model of oscillating magnetic traps to show that it can explain the observed properties of the
HXR oscillations. The main results are the following: i) Low-amplitude quasi-periodic oscillations occur before the impulsive
phase of some flares. ii) The quasi-periodicity of the oscillations can change in some flares. We interpret this as being
due to changes of the length of oscillating magnetic traps. iii) During the impulsive phase a significant part of the energy
of accelerated (non-thermal) electrons is deposited within a HXR loop-top source. iv) The quick development of the impulsive
phase is due to feedback between the pressure pulses by accelerated electrons and the amplitude of the magnetic-trap oscillation.
v) The electron number density and magnetic field strength values obtained for the HXR loop-top sources in several flares
fall within the limits of N≈(2 – 15)×1010 cm−3, B≈(45 – 130) gauss. These results show that the HXR quasi-periodic oscillations contain important information about the energy
release in solar flares. 相似文献
20.
The `ribbons' of two-ribbon flares show complicated patterns reflecting the linkages of coronal magnetic field lines through
the lower solar atmosphere. We describe the morphology of the EUV ribbons of the July 14, 2000 flare, as seen in SOHO, TRACE,
and Yohkoh data, from this point of view. A successful co-alignment of the TRACE, SOHO/MDI and Yohkoh/HXT data has allowed us to locate the EUV ribbon positions on the underlying field to within ∼ 2′′, and thus to investigate
the relationship between the ribbons and the field, and also the sites of electron precipitation. We have also made a determination
of the longitudinal magnetic flux involved in the flare reconnection event, an important parameter in flare energetic considerations.
There are several respects in which the observations differ from what would be expected in the commonly-adopted models for
flares. Firstly, the flare ribbons differ in fine structure from the (line-of-sight) magnetic field patterns underlying them,
apparently propagating through regions of very weak and probably mixed polarity. Secondly, the ribbons split or bifurcate.
Thirdly, the amount of line-of-sight flux passed over by the ribbons in the negative and positive fields is not equal. Fourthly,
the strongest hard X-ray sources are observed to originate in stronger field regions. Based on a comparison between HXT and
EUV time-profiles we suggest that emission in the EUV ribbons is caused by electron bombardment of the lower atmosphere, supporting
the hypothesis that flare ribbons map out the chromospheric footpoints of magnetic field lines newly linked by reconnection.
We describe the interpretation of our observations within the standard model, and the implications for the distribution of
magnetic fields in this active region. 相似文献