Determining the acceleration height of the electrons responsible for bremsstrahlung gamma-ray bursts with energies above 10 MeV during the solar flares of April 2, 1982, and March 26, 1991

We determined the acceleration height of the electrons that produced short high-energy gamma-ray bursts with E _γ > 10 MeV during the development of the solar flares on March 26, 1991, and April 2, 1982. In both cases, the height was found to be h _acc<4×10⁹ cm. This result suggests that the low acceleration location is a typical and, possibly, necessary condition for electron acceleration up to tens and hundreds of MeV. We describe two different methods of height determination that are based on a multi-wave analysis of flares.     

Localization of the solar flare SF900610 in X-rays with the WATCH instrument of the GRANAT observatory

During the solar flare of June 10, 1990, the WATCH instrument of the GRANAT space observatory obtained 110 localizations of the X-ray source in the X-ray range 8–20 keV. Its coordinates were measured with an accuracy of ～2 arcmin at a 3σ confidence level. The coordinates of the X-ray source do not coincide with the coordinates of the Hα-line flare. The X-ray source moved over the solar disk during the flare. This probably implies that, as the X-ray emission was generated, different parts of one loop or a system of magnetic loops dominated at different flare times.     

Puzzles and potential for gamma-ray line observations of solar flare ion acceleration

The best tool for understanding ion acceleration in solar flares is gamma-ray line emission from nuclear de-excitation, positron annihilation, and neutron capture. These techniques have not yet come close to reaching their potential due to limited counting statistics in the lines. Instruments with focusing optics and large effective areas promise real breakthroughs in understanding high-energy solar processes. I discuss what can be learned from the various lines and the instrumental requirements for future focusing observations.“Mama always told me not to look into the sights of the Sun; oh, but mama, that’s where the fun is...” [1]     

Particle distributions and X-ray spectra in single or multiple solar current sheets

The acceleration of charged particles in a site of magnetic reconnection is analysed by detailed numerical simulations. Single or multiple encounters of the particles with Harris-type reconnecting current sheets (RCSs) are modelled as an overall stochastic process taking place within an active region. RCS physical parameters are selected in a parameter space relevant to solar flares. Initially, the charged particles form a thermal (Maxwellian) distribution corresponding to coronal temperature  ≃2 × 10⁶ K  . Our main goal is to investigate how the acceleration process changes the shape of the particles' kinetic energy distribution. The evolution of the kinetic energy distribution, calculated numerically after one encounter of the particles with a single RCS, is found to be in good agreement with our previously published analytical formulae. In the case of consecutive encounters, we find that the kinetic distribution tends to converge to a practically invariant form after a relatively small number of encounters. We construct a discrete stochastic process that reproduces the numerical distributions and we provide a theoretical interpretation of the asymptotic convergence of the energy distribution. We finally compute the theoretical X-ray spectra that would be emitted by the simulated particles in a thick target model of radiation.     

The hemispheric variation of the flare index during solar cycles 20–23

In this paper, the monthly counts of flare index in the northern and southern hemispheres are used to investigate the hemispheric variation of the flare index in each of solar cycles 20–23. It is found that, (1) the flare index is asymmetrically distributed in each solar cycle and its asymmetry is a real phenomenon; (2) the flare index in the northern hemisphere begins earlier than that in the southern hemisphere in each of solar cycles 20–23, and the phase shifts between the two hemispheres show an odd‐even pattern; (3) although the flare index dominating in a hemisphere does not mean that it leads in phase in this hemisphere in individual solar cycle, these two features have an intrinsic relationship. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic field, helicity and the 2000 July 14 flare in solar active region 9077

In this paper we analyse the non-potential magnetic field and the relationship with current (helicity) in the active region NOAA 9077 in 2000 July, using photospheric vector magnetograms obtained at different solar observatories and also coronal extreme-ultraviolet 171-Å images from the TRACE satellite.
We note that the shear and squeeze of magnetic field are two important indices for some flare-producing regions and can be confirmed by a sequence of photospheric vector magnetograms and EUV 171-Å features in the solar active region NOAA 9077. Evidence on the release of magnetic field near the photospheric magnetic neutral line is provided by the change of magnetic shear, electric current and current helicity in the lower solar atmosphere. It is found that the 'Bastille Day' 3B/5.7X flare on 2000 July 14 was triggered by the interaction of the different magnetic loop systems, which is relevant to the ejection of helical magnetic field from the lower solar atmosphere. The eruption of the large-scale coronal magnetic field occurs later than the decay of the highly sheared photospheric magnetic field and also current in the active region.     

On the topological trigger of large eruptive solar flares

The possibility of studying the topological properties of the magnetic fields in solar active regions is considered in terms of simple models. Analysis of the field topology shows that the topological trigger effect should be taken into account when large eruptive flares are modeled.     

Peculiarities of the microwave emission from active regions generating intense solar flares

RATAN-600 multiwavelength observations of the Sun reveal sharp spectral inhomogeneities in the polarized radiation from active regions that produce intense flares. These events occur in a wide range of radio fluxes (0.05–10 s.f.u.) in a relatively narrow wavelength range (2–5 cm). They are detected on time scales from several hours to several days before and during an intense flare. We analyze the detected events and their relationship to the preliminary phase of intense flares. Significant statistical material was obtained in 2001. The new flare-plasma properties can be used to test existing solar-flare models and to develop new criteria of flaring activity.     

Evolution of the photospheric magnetic field and coronal null points before solar flares

Based on a topological model for the magnetic field of a solar active region (AR), we suggest a criterion for the existence of magnetic null points on the separators in the corona. With the problem of predicting solar flares in mind, we have revealed a model parameter whose decrease means that the AR evolves toward a major eruptive flare. We analyze the magnetic field evolution for AR 9077 within two days before the Bastille Day flare on July 14, 2000. The coronal conditions are shown to have become more favorable for magnetic reconnection, which led to a 3B/X5.7 eruptive flare.     

Derivation of flare magnetic field and density from two simultaneous frequency observations

We developed a simple model for a flare loop, which was used to fit the emission in the microwave (17 GHz) and millimetre-wave (80 GHz) ranges for the giant flare of 1991 June 4. The simplicity of the model enabled the exploration of a wide range of parameters in a reasonable time. It was possible, using the simple model, to derive from the 17- and 80-GHz data the magnetic field and the number density for every measurement point in the time range we chose to fit.     

Magnetic reconnection,electric field,and particle acceleration in the July 14, 2000 solar flare

A topological model with magnetic reconnection at two separators in the corona is used to account for the recently discovered changes of the photospheric magnetic field in the active region NOAA 9077 during the July 14, 2000 flare. The model self-consistently explains the following observed effects: (1) the magnetic field strength decreases on the periphery of the active region but increases in its inner part near the neutral line of the photospheric magnetic field; (2) the center-of-mass positions of the fields of opposite (northern and southern) polarities converge; and (3) the magnetic flux of the active region decreases after the flare. The topological model gives not only a qualitative interpretation of the flare phenomena (the structure of the interacting magnetic fluxes in the corona, the location of the energy sources, the shape of the flare ribbons and kernels in the chromosphere and photosphere), but also correct quantitative estimates of the large-scale processes that form the basis for solar flares. The electric field emerging in the flare during large-scale reconnection is calculated. The electric field strength correlates with the observed intensity of the hard X-ray bremsstrahlung, suggesting an electron acceleration as a result of reconnection.     

Prolonged nonthermal emission from solar flares and the Neupert effect

We analyze the observations of the hard (ACS SPI, > 150 keV) and soft (GOES, 1–8 Å) X-ray emissions and the microwave (15.5 GHz) emission in the solar flares on September 7, 2005 and December 6 and 13, 2006. The time profiles of the nonthermal emission from these flares had a complex structure, suggesting that active processes in the flare region continued for a long time (more than an hour). We have verified the linear relationship between the nonthermal flux and the time derivative of the soft X-ray flux (the Neupert effect) in the events under consideration. In the first two cases, the Neupert effect held at the time of the most intense nonthermal emission peak, but not at the decay phase of the soft X-ray emission, when the intensity of the nonthermal emission was much higher than the background values. At the same time, the hard X-ray emission was suppressed compared to the main peak, while the microwave emission remained approximately at the same level. In the December 13, 2006 event, the prolonged hard X-ray emission was difficult to observe due to the fast arrival of solar protons, but the Neupert effect did not hold for its main peak either. At comparable intensities of the microwave emission on December 6 and 13, the intensity of the hard X-ray emission on December 13 at the time of the main peak was suppressed approximately by an order of magnitude. These observational facts are indicative of several particle acceleration and interaction episodes under various physical conditions during one flare. When the Neupert effect did not hold, the interaction of electrons took place mainly in a low-density medium. An effective escape of accelerated particles into interplanetary space rather than their precipitation into dense layers of the solar atmosphere may take place precisely at this time.     

Study of electron energy and angular distributions and calculations of X-ray,EUV line flux and rise times

Evolution of energy and angular distributions of electrons has been studied by combining small-angle analytical treatment with large-angle Monte Carlo calculations as a function of column density for initially monoenergetic and monodirectional electrons. The incident electron energies considered are 20, 30 and 60 keV at 0°, 30° and 60° angles of incidence. Using these distributions, time evolution of extreme ultraviolet (EUV) spectrum has been studied. The slopes of the curves calculated compare well with the experimentally observed curve.     

Statistics of faint gamma-ray bursts from the GRIF experiment on the Mir orbiting station

During the GRIF experiment onboard the Mir orbiting station, cosmic gamma-ray bursts (GRBs) were observed in the photon energy range 10–300 keV. We developed a technique for selecting events, cosmic GRB candidates, based on output readings from the PX-2 scintillation spectrometer, the main astrophysical instrument. Six events interpreted as cosmic GRBs were identified at a threshold sensitivity level of ≥10^?7 erg cm^?2. The GRIF burst detection rate recalculated to all the sky is ～10³ yr^?1 (fluence ≥10^?7 erg cm^?2). This rate matches the BATSE/CGRO estimate and significantly differs from the value predicted by the S^?3/2 dependence, which holds for a spatially uniform source distribution. The GRB detection rate at low peak fluxes is compared with the results of analysis for BATSE/CGRO “nontriggered” events and with predictions of major cosmological models. We conclude that the PX-2 observational data on faint cosmic GRBs are consistent with predictions of models with the highest frequency of GRB occurrence at z ≥1.5–2.     

Comparison of the Fermi and betatron acceleration efficiencies in collapsing magnetic traps

The acceleration of charged particles in the solar corona during flares is investigated in terms of a model in which the electrons and ions preaccelerated in the magnetic reconnection region are injected into a collapsing magnetic trap. Here, the particle energy increases rapidly simultaneously through the Fermi and betatron mechanisms. Comparison of the efficiencies of the two mechanisms shows that the accelerated electrons in such a trap produce more intense hard X-ray (HXR) bursts than those in a trap where only the Fermi acceleration mechanism would be at work. This effect explains the Yohkoh and RHESSI satellite observations in which HXR sources more intense than the HXR emission from the chromosphere were detected in the corona.     

Extreme ultraviolet and X-ray emission of solar flares as observed from the CORONAS-F spacecraft in 2001–2003

The results of measuring UV radiation onboard the CORONAS-F spacecraft during solar flares in 2001–2003 are considered. Some conclusions from the analysis of variations of solar-flare emission in several spectral intervals, namely, in soft X-rays, in the 10-to 130-nm range, and in the band near 120 nm, are discussed. The data were obtained by the VUSS-L and SUFR instruments. Time and energy characteristics of flares recorded onboard the CORONAS-F spacecraft are compared to the GOES measurements in the interval 0.1–0.8 nm and to the SOHO measurements of UV radiation in the 26-to 34-nm band. In particular, it is demonstrated that UV radiation is generated several (1–10) minutes before X-ray emission for most flares considered in the study. It is shown that the energy of flare emission in the extreme ultraviolet is usually not greater than ～10% of its preflare level and that energy fluxes in different wavelength ranges are related by a power law. Such an analysis makes it possible to better understand the mechanism of flare development.     

on the role of nonthermal electrons in EUV and X-ray line emissions from solar flares

The energy and angular distribution of electrons as a function of column densities initially for monoenergetic and monodirectional electron beams and incidence angles of 0‡, 30‡ and 60‡ have been studied by combining small angle scattering using analytical treatment with large angle collisions using Monte Carlo calculations. Using these distributions, X-ray and EUV-line flux have been studied as a function of column density. It is observed that the line flux increases with the increase in column density, becoming significant at intermediate column densities where the electron energies and angular distributions have a non-Maxwellian nature.     

Radial oscillations of coronal loops and microwave radiation from solar flares

We consider the damping mechanisms for the radial oscillations of solar coronal loops in the approximation of a thin magnetic flux tube. We show that the free tube oscillations can have a high Q if the plasma density inside the magnetic flux tube is much higher than the density outside. We analyze the effect of radial coronal-loop magnetic-field oscillations on the modulation of the microwave radiation from solar flares. In the case of a nonthermal gyrosynchrotron mechanism, the fluxes from optically thin and optically thick sources are modulated in antiphase. Based on our model, we diagnose the flare plasma. For the event of May 23, 1990, we estimate the spectral index for accelerated electrons, α≈4.4, and the magnetic-field strength in the region of energy release, B≈190 G.     

Spectral analysis of the 3B flare on September 19, 1979: asymmetries and shifts of metallic lines

The asymmetries and Doppler shifts of metallic lines of the 3B flare on September 19, 1979 are investigated in this paper. The results show that: (1) red asymmetries dominate in strong metallic lines, but blue asymmetries also exist in some weak lines; (2) the maximum of the line asymmetry always precedes the maximum of the line intensity; (3) the blue asymmetry occurs generally in the early phase, and can occasionally turn to a red one in the later phase; and (4) the line center has no obvious shift, regardless of the line asymmetry. It is proposed that, the mass motions around the temperature minimum region caused by the impulsive heating or the propagation of the chromospheric condensation can explain these observational results. The study on metallic lines has an auxiliary help for understanding the dynamic process in the lower atmosphere of solar flares.