Evolution of vector magnetic fields and the August 27 1990 X-3 flare

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.     

On the polarization and anisotropy of solar X-radiation during flares

Starting from the idea that the electrons accelerated during a solar flare have originally a preferred direction, the angular distribution and the polarization of bremsstrahlung below 10 Å is calculated taking into account the influence of the magnetic field. The energy distribution of the nonthermal electrons is based on X-ray spectra measured by the Leicester group during flares in 1962 and 1967. In addition to the case of a fixed angle between the electron velocity and the magnetic field, an angular distribution of the form sin ⁿ is considered. The results may be used to test flare models. Recent measurements of the polarization of solar X-radiation yield the expected order of magnitude.Paper presented to the Int. Symp. on Solar-Terr. Phys., Leningrad, May 1970.     

3-D Magnetic Field Configuration Late in a Large Two-Ribbon Flare

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.     

Flares on opposite sides of the star: Observational aspects

The problem of the flare taking place on opposite sides of a star is considered. Such a screened flare, diffused through the star's atmosphere (chromosphere), may also be registered. The theoretical light curve for diffused flare event is derived, which differs strongly from the usual flare light curves. The light curve of diffused flare is characterized first of all by its very slow rise of brightness. This result opens quite a new direction to understand the nature of the so-called slow flares, observed often among the UV Cet-type stars as well as flare stars in aggregates. All slow flares can be interpreted as quite ordinary flares of quite ordinary flare stars — taking place, however, on the opposite sides of the star. The results of interpretation of some slow flare events of YY Gem and three flare stars in Orion are presented. An attempt is made for the determination of actual amplitudes of screened flares taking place on the opposite sides of a star.     

The solar particle events of May 23 and May 28, 1967

McMath plage region 8818 passed over the visible solar disk on May 17–31, 1967. It was very active from its first appearance on the Eastern limb, several times producing bright optical flares and hard X-ray emission, accompanied by intense type II, type IV and centimeter radio bursts. Nevertheless, no solar particles could be detected near the earth until the evening of May 23, when three bright flares were observed in close succession at 25°–28° E. During the following build-up of the solar particle flux over 36 hours, the galactic cosmic ray flux > 1 GeV decreased gradually by about 5%. The flux of solar particles decreased in two steps on May 25, both accompanied by decreases in the equatorial geomagnetic field. These field depressions are attributed to storm plasma ejected from the parent flare of the May 23 particle event. The propagation of solar particles from May 23 on thus appears to be strongly affected by storm plasma from the parent flare of the May 23 event, without any indications of solar particles being trapped in that plasma.A later particle event early on May 28 was also associated with a bright flare in McMath region 8818, at 33° W. This event displayed a rapid build-up, with electrons arriving first, and an exponential decay. A smooth proton peak, 20 min wide, was detected on May 30 closely associated with an SSC attributed to plasma ejection from the parent flare of the May 28 event.Between the geomagnetic storms beginning on May 25 and May 30 an anomalous daily variation was observed in the cosmic ray flux >1 GeV, the time of maximum falling 7–10 hours earlier than normal. Storm time increases in the flux of galactic cosmic rays were seen on May 26 when the equatorial geomagnetic field was depressed by more than 400 . Low latitude auroras were also observed during that time.On leave from the University of Uppsala, Sweden.     

Analysis of EUV limb brightening observations from ATM

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.     

Flares From the Giant ar 9433 on 24 April 2001

We present H CCD observations of three small-to-medium-size two-ribbon flares observed in the giant AR 9433 on 24 April 2001. Flare observations at other associated wavelengths (e.g., soft X-rays (SXR), hard X-rays (HXR), microwaves (MW)) obtained from archives are also presented and compared. We have tested the Neupert effect for the most energetic third flare. The flare observations are in agreement with the thick-target model. In the case of this flare the HXR emitting electrons appears to be the heating source of SXR and H emissions. The flares are also studied in EUV and UV emissions using TRACE data. We discuss the complexity of the magnetic field using SOHO/MDI magnetograms. The flares are observed to occur in both (f/p) polarity regions in highly sheared magnetic field with emerging flux regions and MMFs.     

Solar flares through electric current interaction

The fundamental hypothesis by Alfvén and Carlqvist (1967) that solar flares are related to electrical currents in the solar chromosphere and low corona is investigated in the light of modern observations. We confirm the important role of currents in solar flares. There must be tens of such current loops (flux threads) in any flare, and this explains the hierarchy of bursts in flares. We summarize quantitative data on energies, numbers of particles involved and characteristic times. A special case is the high-energy flare: this one may originate in the same way as less energetic ones, but it occurs in regions with higher magnetic field strength. Because of the high particle energies involved their emission seats live only very briefly; hence the area of emission coincides virtually with the seat of the instability. These flares are therefore the best examples for studying the primary instability leading to the flare. Finally, we compare the merits of the original Alfvén-Carlqvist idea (that flares originate by current interruption) with the one that they are due to interaction (reconnection) between two or more fluxthreads. We conclude that a final decision cannot yet be made, although the observed extremely short time constants of flare bursts seem to demand a reconnection-type instability rather than interruption of a circuit.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988.     

Pre-flare association of magnetic fields and millimeter-wave radio emission

Observations of radio emission at 3.3 mm wavelength associated with magnetic fields in active regions are reported. Results of more than 200 regions during the years 1967–1968 show a strong correlation between peak enhanced millimeter emission, total flux of the longitudinal component of photospheric magnetic fields and the number of flares produced during transit of active regions. For magnetic flux greater than 10²¹ maxwells flares will occur and for flux of 10²³ maxwells the sum of the H flare importance numbers is about 40. The peak millimeter enhancement increases with magnetic flux for regions which subsequently flared. Estimates of the magnetic energy available and the correlation with flare production indicate that the photospheric fields and probably chromospheric currents are responsible for the observed pre-flare heating and provide the energy of flares.This work was supported in part by NASA Contract No. NAS2-7868 and in part by Company funds of The Aerospace Corporation.     

Flares,magnetic configurations,and magnetic energy release

Using a newly developed Aerospace digital videomagnetograph, three solar active regions are studied as to their magnetic configurations and their flare productivity. These three regions have very different types of magnetic configurations and different types of flare productivity. We review previous theoretical and experimental research on flares and magnetic energy storage, and discuss various ways to observe magnetic energy release due to flares. Results for six subflares are presented. Five showed no measurable magnetic energy change and one result is questionable.We show three counterexamples to Zirin's (1972) contention that as a rule H plage brightness is proportional to magnetic field strength. Each of these three cases involved two plage regions of the same polarity and equal field strengths with one of the plages adjacent to a neutral line. In all three cases the plage region nearer the neutral line was much brighter.     

Flare morphologies and coronal field configurations

Chromospheric flares are the footpoints of closed coronal field lines. In this paper we present different flare morphologies from observations and examine the implied coronal field configurations above the flaring region. Flares are grouped according to the number of ribbons, from unresolved compact point-like flare to four-ribbon flares. Quiet region flares having characteristics all their own are also presented here.We find that compact, unresolved point-like flares have two distinct footpoints when viewed in offband H. The footpoints of some of the compact flares also show increased separation as a function of time.Unlike large two-ribbon flares, the ribbons of many small and/or short-lived two-ribbon flares usually have no measurable separation of ribbons.Multiple-ribbon (three or more ribbon) flares consist of two or more pairs of two-ribbons, or two or more sets of field lines. Parity of the ribbons in multiple-ribbon flares, or the lack of it, depends on the magnetic makeup of the locale of the ribbons.Flares in old quiet regions resulting from sudden filament eruptions show discrete small patches of emissions reflecting the spottiness of decayed and dispersed field of quiet region.     

Flare positions relative to photospheric magnetic fields

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.     

The role of the magnetic field intensity and geometry in the type III burst generation

We study the association of type III bursts related to H flares in different magnetic environments in the period 1970–1981. Special attention is paid to flares which partly cover a major spot umbra (Z-flares). In particular we consider the location of the spots in the active regions and the magnetic field intensities of spots covered by a ribbon. The association rate with type III bursts decreases to 17% when the flare is located inside the bipolar pattern of a large active region, compared with an association rate of 54% when the flare is situated outside it. The association rate increases with the magnetic field intensity of the spot covered by H emission; this is most clearly revealed for the flares occurring outside the bipolar pattern of active regions. Ninety-three percent of the flare-associated type III burst were accompanied by 10 cm radio bursts. For the most general case in which a flare is developing anywhere in an active region, the association with type III bursts generation increases with the increasing magnetic field intensity of the main spot of the group.     

Emerging Flux and X-class Flares in NOAA 6555

The active region NOAA 6555 had several locations of highly sheared magnetic field structure, yet, only one of them was the site for all the five X-class flares during its disk passage in March 1991. The pre-flare observations of high-resolution H filtergrams, vector magnetograms and H Dopplergrams of the 2B/X5.3 flare on 25 March 1991 show that the flaring site was characterized by a new rising emerging flux region (EFR) near the highly sheared magnetic field configuration. The polarity axis of the emerging flux was nearly perpendicular to the pre-existing magnetic neutral line. The location of the EFR was the site of initial brightening in H. The post-flare magnetograms show higher magnetic shear at the flare location compared to the post-flare magnetograms, which might indicate that the EFR was sheared at the time of its emergence. As the new EFR coincided with the occurrence of the flare, we suggest that it might have triggered the observed flare. Observations from Big Bear Solar Observatory and Marshall Space Flight Center also show that there was emergence of new flux at the same location prior to two other X-class flares. We find that out of five observed X-class flares in NOAA 6555, at least in three cases there are clear signatures of flare-related flux emergence. Therefore, it is concluded that EFRs might play an important role in destabilizing the observed sheared magnetic structures leading to large X-class flares of NOAA 6555.     

X-ray observations of filament eruption in the 1980 May 21 flare

X-ray and H observations of an erupting filament, discussed herein, and other observations of the associated flare on 1980 May 21, suggest that an erupting filament played a major role in the X-ray flare. While Antonucci et al. (1985) analyzed the May 21 flare as one of the best cases of chromospheric evaporation, the possible contribution from X-ray emitting erupting plasma has been ignored. We show that pre-heated plasma existed and may have contributed part of the blue-shifted X-ray emission observed in the Caxix line, which was formerly attributed solely to chromospheric evaporation. Thus it remains an open question - in two-ribbon flares in particular - just how important chromospheric evaporation is in flare dynamics.     

Energetics of two-ribbon solar flares

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.     

Hβ Doppler-velocity fields within sites of flares in a solar active region

In this paper, we analyze the relationship between photospheric magnetic fields and chromospheric velocity fields in a solar active region, especially evolving features of the chromospheric velocity field at preflare sites. It seems that flares are related to unusually distributed velocity field structures, and initial bright kernels and ribbons of the flares appear in the red-shifted areas (i.e., downward flow areas) close to the inversion line of H Dopplergrams with steep gradients of the velocity fields, no matter whether the areas have simple magnetic structure or a weak magnetic field, or strong magnetic shear and complex structure of the magnetic fields. The data show that during several hours prior to the flares, while the velocity field evolves, the sites of the flare kernels (or ribbons) with red-shifted features come close to the inversion line of the velocity field. This result holds regardless of whether or not the flare sites are wholly located in blue-shifted areas (i.e., upward flow areas), or are far from the inversion line of the Doppler velocity field (V = 0 line), or are partly within red-shifted areas. There are two cases favourable for the occurrence of flares, one is that the gulf-like neutral lines of the magnetic field (B = 0 line) occur in the H red-shifted areas, the other is that the gulf-like inversion lines of the H Doppler velocity field (V = 0 line) occur in the unipolar magnetic areas. These observational facts indicate that the velocity field and magnetic field have the same effect on the process of flare energy accumulation and release.     

Studies of Microflares and C5.2 flare of 27 September 1998

On 27 September 1998, Big Bear Solar Observatory (BBSO) and Transition Region and Coronal Explorer (TRACE) coordinated observations from 16:00 to 19:00 UT to study properties of microflares in AR NOAA No. 8340. Fortuitously, a C5.2 flare occurred at 16:30 UT in this active region. H and magnetograph movies were obtained at BBSO; Civ 1550 Å, Feix 171 Å, and Fexii 195 Å movies were obtained by TRACE; both with a cadence about 1 min. In this paper, we concentrate on the study of magnetic properties of 70 Civ microflares, as well as their relationship to the C5.2 flare. We obtained the following results: (1) We found two kinds of microflares: microflares of transient brightenings with a time scale of 1 to 5 min (impulsive events) and microflares lasting half an hour or longer (persistent events). Ninety percent of the microflares are impulsive events. Most of the event in this category are associated with well defined magnetic neutral lines, but some are found in non-neutral line areas. All of seven persistent events are found at parasitic magnetic configurations with inclusions of small magnetic flux within dominant magnetic flux of opposite polarity. (2) More than a third of the impulsive microflares occurred near the C5.2 flare site indicating that a local instability is responsible for both the C5.2 flare and microflares. This indirectly supports the avalanche theory of flare energy release, which implies that a big flare may be spatially associated with many small flares.     

Gamma-ray and microwave evidence for two phases of acceleration in solar flares

Relativistic electrons in large solar flares produce gamma-ray continuum by bremsstrahlung and microwave emission by gyrosynchrotron radiation. Using observations of the 1972, August 4 flare, we evaluate in detail the electron spectrum and the physical properties (density, magnetic field, size, and temperature) of the common emitting region of these radiations. We also obtain information on energetic protons in this flare by using gamma-ray lines. From the electron spectrum, the proton-to-electron ratio, and the time dependences of the microwave emission, the 2.2 MeV line and the gamma-ray continuum, we conclude that in large solar flares relativistic electrons and energetic nuclei are accelerated by a mechanism which is different from the mechanism which accelerates 100 keV electrons in flares.Research supported by NASA Grant 21-002-316 at the University of Maryland, College Park.