A high-energy solar flare burst complex and the physical properties of its source region

We discuss a solar flare microwave burst complex, which included a major structure consisting of some 13 spikes of 60 ms FWHM each, observed 21 May, 1984 at 90 GHz (3 mm). It was associated with a simultaneous very hard X-ray burst complex. We suggest that the individual spikes of both bursts were caused by the same electron population: the X-bursts by their bremsstrahlung, and the microwave bursts by their gyrosynchrotron emission. This latter conclusion is based on the evidence that the radio turnover frequency was 150 GHz. It follows that the emission sources were characterized by an electron density of about 10¹¹ cm^–3, a temperature of 5 × 10⁸ K and a magnetic field of about 1400–2000 G. They had a size of about 350 km; if the energy release is caused by reconnection the sources of primary instability could have been smaller and in the form of thin sheets with reconnection speed at a fraction of the Alfvén velocity and burst-like energy injections of 10²⁷ erg during about 50 ms each. The energized plasma knots lost their injection energy by saturated convective flux (collisionless conduction) in about 30 ms.     

The emission and propagation of ∼ 40keV solar flare electrons

Observations of prompt 40 keV solar flare electron events by the IMP series of satellites in the period August, 1966 to December, 1967 are tabulated along with prompt energetic solar proton events in the period 1964–1967. The interrelationship of the various types of energetic particle emission by the sun, including relativistic energy electrons reported by Cline and McDonald (1968) are investigated. Relativistic energy electron emission is found to occur only during proton events. The solar optical, radio and X-ray emission associated with these various energetic particle emissions as well as the propagation characteristics of each particle species are examined in order to study the particle acceleration and emission mechanisms in a solar flare. Evidence is presented for two separate particle acceleration and/or emission mechanisms, one of which produces 40 keV electrons and the other of which produces solar proton and possibly relativistic energy electrons. It is found that solar flares can be divided into three categories depending on their energetic particle emission: (1) small flares with no accompanying energetic phenomena either in particles, radio or X-ray emission; (2) small flares which produce low energy electrons and which are accompanied by type III and microwave radio bursts and energetic ( 20 keV) X-ray bursts; and (3) major solar flare eruptions characterized by energetic solar proton production and type II and IV radio bursts and accompanied by intense microwave and X-ray emission and relativistic energy electrons.     

Observations of magnetic field evolution in a solar flare

I±V and I±Q profiles of nine spectral lines of Fei, Feii, and Hi in the 2B flare of 16 June 1989 have been analyzed. Two bright flare knots outside and inside of a spot were investigated. To measure the true magnetic field strength in the flare, two different methods were applied. In addition to these data, the magnetic field and thermodynamic conditions were determined using the non-LTE program for line profile synthesis. According to the measurements, the magnetic field in both flare knots changed in synchronism and non-monotonically, and reached its peak (nearly 1.6 kG for non-spot areas and approximately 4.0 kG for sunspot locations) at the time of flare peak. For the flare knot outside the spot, a background field component was also detected; the magnetic field in this component was found to have mixed polarity and remained practically unchanged during the flare. The non-LTE calculations show that the unique local magnetic field peak existed near the temperature minimum zone in the flare peak too. The observed perturbations do not exclude such phenomena as a magnetic field transient in flare.     

Motions in the solar atmosphere associated with the white light flare of 11 July 1978

Series of white light heliograms and oft- and on-band H filtergrams have been obtained, with an average spatial resolution of 1, to study the flare active McMath region 15403 on 11 July, 1978. A great number of accurate heliographic positions were determined for the umbrae, the white light flare patches and several bright H flare knots, as well as along the principal zero filament and an arch prominence. Using the measured heliographic coordinates of these objects their motions could be analyzed in some detail. The velocities of several different objects could be deduced from the coordinates. Since the heliocentric angle of the region was about 45°, the variation in apparent heliographic coordinates also enabled some variations in heights to be determined.It is pointed out that the flare when fully developed, consisted almost entirely of loops. The zero filament which was activated prior to the flare ran between two umbrae of common penumbra and opposite polarity, one belonging to an old, the other to a new spot group. The white light flare developed on both sides of the filament where it passed between these two umbrae; it was also the place where the flare started. Observational evidence appears to indicate that the erupted filament re-formed from below.An indication has been found that there was a link between the motion of some umbrae and the major flare occurrence.     

Solar proton flare 4B/X17.2 on October 28, 2003. Photometric results

The powerful flare 4B/X17.2 of October 28, 2003 in the NOAA 10486 active region is studied by using H_α filtergrams. This active region had a complicated βγδ magnetic configuration and a sigmoidal pattern of the polarity inversion line, it had the largest AR area in the cycle 23. Local filaments, loops, and systems of loops were also observed in the AR. The light curves obtained for all flare knots clearly reveal two stages in their evolution. The first stage is the pre-flare one, when the accumulation of the nonpotential magnetic energy (the energy of electric currents) comes to an end and the situation becomes favorable for the realization of the second period. The intensity of flare knots (except one knot) changed slightly and slowly, and some structure features (twists and connections) became more active. By the end of the first stage a new magnetic flux emerged and a system of interrelated filaments and loops (IFL) was formed at the center of the AR as well as at its periphery. New flare knots appeared about the main S-like filament. The second flare stage began at about 11:02 UT with a dramatic increase of the intensity and area of all flare knots and the formation of new knots. In a space of 8 min the major part of the AR around the main filament was covered with flare emission which fluctuated at its maximum period. The intensity of all knots was observed to drop slowly after the maximum, at the decay phase. As the IFL system extended over the entire AR, the magnetic field energy accumulated in it was released in the form of powerful electromagnetic and corpuscular emission by way of magnetic reconnection.     

Impulsive phase heating and a coronal explosion in a solar flare

The flare of 12 November 1980, 0250 UT, in Active Region 2779 (NOAA classification) was studied by using X-ray images obtained with the Hard X-Ray Imaging Spectrometer aboard NASA's Solar Maximum Mission. In a ten-minute period, between about 0244 and 0254 UT, some five short-lived impulsive bursts occurred. We found that the so-called hard bursts ( 15 keV) are also detectable in low energy images. During that 10 min period - the impulsive phase - the heat input into the flare and the total number of energetic electrons increased practically exponentially, to reach their maximum values at 0254 UT. At the end of that period, when the thermal energy content of the flare was largest, a burst was observed, for the first time, to spread in a broad southern direction from an initially small area with a speed of about 50 km s^–1. We have called this phenomenon a coronal explosion.Fokker Aircraft Industries, Schiphol, The Netherlands.     

Magnetic fields and flares in the region CMP 20 September 1963

The position of bright knots of 30 flares at their very beginning relative to the high-resolution isogauss maps of the longitudinal component (H ) and maps of the transverse component (H ) of magnetic field are considered for seven days during the passage of the active and large spot group in Sept. 1963 (see Table I and maps on Figures 1–8).The flare bright knots occur simultaneously in regions of opposite magnetic polarity, and the majority of these knots are adjacent to neutral line H = 0, although not coinciding precisely with this line (Figure 9). Lenticular form of flare knots and the motions of bright material of flares is restrained by transversal field H . Also flares are closely associated (83%) with so-called bifurcated regions, where specific crossing of transverse components takes place (Figures 4–5). There is well-expressed (80%) coincidence of flare knots with the strongest (positive or negative) electric currents as determined from the relation j = c/4 rot H. The relation of results obtained to some existing theories of flares is briefly discussed.U.S. Nat. Acad. of Science - U.S.S.R. Acad. Nauk. Exchange Scientist Program; now at CSIRO Division of Physics, Australia.     

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.     

X-radiation (E > 10keV), Hα and microwave emission during the impulsive phase of solar flares

A study has been made of the variation in hard (E 10 keV) X-radiation, H and microwave emission during the impulsive phase of solar flares. Analysis shows that the rise-time in the 20–30-keV X-ray spike depends on the electron hardness, i.e., t _rise exp (0.87 ). The impulsive phase is also marked by an abrupt, very intense increase in H emission in one or more knots of the flare. Properties of these H kernels include: (1) a luminosity several times greater than the surrounding flare, (2) an intensity rise starting about 20–30 s before, peaking about 20–25 s after, and lasting about twice as long as the hard spike, (3) an effective diameter of 3000–6000 km for class 1 flares, representing less than 1/8-1/2 of the main flare, (4) a location lower in the chromosphere than the remaining flare, (5) essentially no expansion prior to the hard spike, (6) a position within 6000 km of the boundary separating polarities, usually forming on both sides of the neutral line near both feet of the same tube of force, (7) a shape often resembling isogauss contours of the photospheric field indicated on magnetograms and (8) total radiated energy less than l/50 that of the hard electrons. Correspondingly, impulsive microwave events are characterized by: (1) the detection of a burst at 8800 MHz for every X-ray spike ifthe number of electrons above 100 keV is greater than 10³³, (2) great similarity in burst structure with 20–32 keV X-rays but only at f > 5000 MHz, (3) typical low frequency burst cutoff between 1400–3800 MHz, and (4) maximum emission at f > 7500 MHz. Finally the H, X-ray and microwave data are combined to present a picture of the impulsive phase consistent with the above observations.     

Spectral analysis of Hα, Hβ, and Hγ lines of a white-light flare (3B/X6.1) on October 27, 1991

We have analyzed time series of H, H, and H line profiles taken from a 3B/X6.1 flare which occurred on October 27, 1991 in active region NOAA 6891. Each set of the spectra was taken simultaneously for the first 10 min of the flare event with a low and non-uniform time resolution of 10–40 s. A total of 22 sets of H, H, and H were scanned by a PDS with absolute intensity calibration to derive the dynamics and energetics of material in the flare region. Our results are as follows: (1) The Balmer line emitting region is accelerated downward to about 50 km s^-1 for the first 50 s and then is decelerated to about 10 km s^-1 for the next 150 s. (2) The radial velocity peak precedes the Balmer line intensity peak by about 40 s. (3) The total energy radiated from these Balmer lines is estimated to be 4.9 × 10²⁹ erg.     

MULTISPECTRAL OBSERVATIONS OF AN ERUPTIVE FLARE

We analyze the pre-flare and impulsive phase of an eruptive (two-ribbon) flare at several wavelengths. The total energy (mechanical plus radiative) released by the flare is 8 x 10³⁰ erg, about a factor 6 higher than the free magnetic energy (1.3 10³⁰ erg) estimated from the non-potentiality of the magnetic field configuration in the flare area. During the impulsive phase, we find a very good time coincidence between the hard X-ray light curve and the light curves for 2 small areas ( 4 in size) in the red wing of the H line and in the He-D₃ line center. This temporal coincidence is compatible with the interpretation that hard X-ray emission is produced by bremsstrahlung of accelerated electron beams striking these dense areas. For the other regions of the H ribbons we find more gradual light curves, suggesting a different energy transport mechanism such as conduction.     

The pre-onset morphology of the 5 September 1973 flare

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.     

Flare event statistics on UV Ceti-type stars

A statistical study of 228 flares on the three UV Ceti-type stars, i.e., YZ CMi, AD Leo, and EV Lac, is presented. Observations were gathered by Ichimura and Shimizu over a total monitoring time of 907 hours distributed over 18 years (1971 to 1988). Period analysis of flare activity was performed, and no periodicity was detected on the three stars for either the flare number rate or the energy rate in time-scales ranging from a year up to 14 years. Average colour of flares at peak was (U-B)=–0.98±0.17 and (B-V)=0.05±0.13. Cumulative number distributions of flare event time-integrated energies were solved by a least-squares method on a log-log plot for a power-law function to get both the constant of and the gradient , which were found to be similar among the three stars. The gradient showed that rare large flare events radiate most of the energy released by all the flare events in the monitoring time. The flare number rate and energy rate are similar if the power-law distributions are extended up to a specific maximum energy. In reality, the Kolmogorov-Smirnov test showed that the observed cumulative number distributions of flare event energy were not necessarily a power-law function. The monte-Carlo simulation, however, indicates that the monitoring time and/or the patrol observation time interval may not be long enough to get the average flare number rate and energy rate, especially at the upper energy limits which are statistically unreliable.     

Studies on a very flare-active δ group: Peculiar δ spot evolution and inferred subsurface magnetic rope structure

The complex subsurface magnetic rope structure of a very flare-active isolated group (McMath 13043, July 1974) is studied by means of high-resolution evolutionary data from BBSO magnetic and velocity data. This group showed unusually fast evolution accompanied by a number of intense flares occurring on the neutral line of a spot, and provided an excellent opportunity to study the inherent relation of flare occurrence to changes of the magnetic configuration. We first examine the abnormal evolution of this group started by formation of a large, compact, reversed spot by squeezing of multipoles. The configuration was deformed by penetration into the opposite polarity umbra and its subsequent disappearance, decaying by rapid shear motions. Strong transverse fields over 4000 G were detected in the penumbrae and some umbral components.Combining these data with the August 1972 region, the evolution of these isolated groups is shown to decompose into two flare-associated elementary modes: (A) shearing produced by spot growth and (B) reduction of shear as spots disappear. We propose a model of an emerging twisted magnetic knot to explain the two modes and apply realistically to the present evolution. The inferred magnetic topological structure of this region consists of tightly twisted (sheet-like) knots and a long-winding twisted rope with an internally reversed loop and a hooked bottom struture. Their consecutive emergences are suggested to explain the abnormal evolution of this 5 group. This result indicates that the origin of the concentrated flare activity in these isolated groups may be traced to internal magnetic activity responsible for forming anomalous magnetic ropes.The author died on January 2, 1990. This paper, prepared for publication at the time of his death, was edited by Professor H. Zirin.     

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.     

Remote flare brightenings and type III reverse slope bursts

We present two large flares which were exceptional in that each produced an extensive chain of H emission patches in remote quiet regions more than 10⁵ km away from the main flare site. They were also unusual in that a large group of the rare type III reverse slope bursts accompanied each flare.The observations suggest that this is no coincidence, but that the two phenomena are directly connected. The onset of about half of the remote H emission patches were found to be nearly simultaneous with RS bursts. One of the flares (August 26, 1979) was also observed in hard X-rays; the RS bursts occurred during hard X-ray spikes. For the other flare (June 16, 1973), soft X-ray filtergrams show coronal loops connecting from the main flare site to the remote H brightenings. There were no other flares in progress during either flare; this, along with the X-ray observations, indicates that the RS burst electrons were generated in these flares and not elsewhere on the Sun. The remote H brightenings were apparently not produced by a blast wave from the main flare; no Moreton waves were observed, and the spatially disordered development of the remote H chains is further evidence against a blast wave. From geometry, time and energy considerations we propose: (1) That the remote H brightenings were initiated by direct heating of the chromosphere by RS burst electrons traveling in closed magnetic loops connecting the flare site to the remote patches; and (2) that after onset, the brightenings were heated by thermal conduction by slower thermal electrons (kT1 keV) which immediately follow the RS burst electrons along the same loops.     

Search for weak white-light flares by time-wise photographic cancellation

This study proposes as a working hypothesis that small white-light flares accompany all major (proton) flare events and suggests a new method for systematically finding these patches of white-light emission. The new technique consists of the time-wise application of the photographic cancellation method to detect small time-varying features around the time of the impulsive phase of a flare.     

Observations of limb flares with a soft X-ray telescope

The structure and evolution of 26 limb flares have been observed with a soft X-ray telescope flown on Skylab. The results are:

1. One or more well defined loops were the only structures of flare intensity observed during the rise phase and near flare maximum, except for knots which were close to the resolution of the telescope in size (≈2 arc seconds) and whose structure can therefore not be determined.
2. The flare core features were always sharply defined during the rise phase.
3. For the twenty events which contain loops, the geometry of the structure near maximum was that of a loop in ten cases, a loop with a spike at the top in four cases, a cusp or triangle in four cases, and a cusp combined with a spike in another two cases.
4. Of the fifteen cases in which sufficient data were available to allow us to follow a flare's evolution, five showed no significant geometrical deviation from a loop structure, one displayed little change except for a small scale short-lived perturbation on one side of the loop 10 seconds before a type III radio burst was observed, eight underwent a large scale deformation of the loop or loops on a time scale comparable to that of the flare itself and one double loop event changed in a complex and undetermined manner, with reconnection being one possibility.

Based on observation of the original film, it is suggested that the eight flares which underwent large scale deformations had become unstable to MHD kinks. This implies that these flares occurred in magnetic flux tubes through which significant currents were flowing. It is suggested that the high energy electrons responsible for type III bursts accompanying these flares could have been accelerated by the V x B electric field induced by a small scale short-lived perturbation of parts of a flaring flux tube, similar to the one perturbation which was observed having these characteristics.     

Spectrograph,filtergraph and magnetograph observations of the two-ribbon flare of 29 July, 1973

We present high resolution detailed observations of the class 3N two-ribbon flare of 1973, July 29 (McMath 12461), which was associated with the disappearance of a large filament (disparition brusque). This flare occurred in a diffuse bipolar magnetic region completely devoid of sunspots, and was further associated with a type IV radio burst and a soft X-ray event. Extensive H filtergraph, spectrograph and magnetograph records during the main phase of the flare suggest that downfalling and streaming material is present on both ribbons for several hours during the H emission enhancement, but only at a small number of points located both on and off the ribbons. We find a poor spatial correspondence between bright emission knots in the H ribbons and the positions of the observed downward motion. We conclude that the model of infall-impact of Hyder (1967a, b) is not consistent with our filtergraph and spectrograph observations.Presently at the University of Michigan, Ann Arbor, Michigan.     

Low-level decimetric (1.6 GHz) solar burst activity

Observations of solar bursts at 1.6 GHz were carried out in the month of July 1985 for about two weeks. Five intervals of solar burst activity, each one lasting for a couple of minutes, were observed. Predominantly, two classes of fast bursts were observed: viz: spike and blips. However, some of these bursts were two orders of magnitude less intense than those reported earlier.Low-level blips have typical duration 350 ms, excitation time 200 ± 25 ms, decay time 130 ± 25 ms and a low degree of circular polarization of about 15%. Detailed investigations of decay times of the blips have been carried out in terms of collisional damping and Landau damping. Observed decay times of the blips seem to favour the hypothesis of collisional damping. This investigation suggests that blips probably originate at second harmonic by beam plasma interaction as that of metric type III bursts. Also, low-level ms-spikes with the half power duration in the range of 5 to 20 ms suggest that source sizes be smaller than 50 km if the process of emission is electron-cyclotron maser.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.