Development dynamics of intense solar proton flares

We consider the relationship of electromagnetic radiation in the three most intense flares of solar cycle 23, more specifically, those of October 28, 2003, January 20, 2005, and September 7, 2005, to the acceleration and release of protons into interplanetary space. The impulsive phase of these flares lasted ～ 20 min and consisted of at least three energy release episodes, which differed by their manifestation in the soft (1–8 Å, GOES) and hard (>150 keV, INTEGRAL) X-ray ranges as well as at radio frequencies of 245 MHz and 8.8 GHz. The protons and electrons were accelerated in each episode, but with a different efficiency; the relativistic protons were accelerated only after 5–6min of impulsive-phase development after the onset of a coronal mass ejection. It is at this time that maximum hard X-ray fluxes were observed in the September 7, 2005 event, which exceeded severalfold those for the other two flares considered. We associate the record fluxes of protons with energies > 200MeV observed in the heliosphere in the September 7, 2005 event with the dynamics of the impulsive phase. The extreme intensities of the microwave emission in the October 28, 2003 and January 20, 2005 events were probably attributable to the high-energy electron trapping conditions and did not reflect the acceleration process.     

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.     

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.     

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.     

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.     

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.     

A survey of the peculiar radio emission of the solar behind-limb event on 16th February 1984

Basing on radio measurements from different stations the paper presents a compilation of observations and resultinng questions concerning the interpretation of some remarkable features of the solar behind-limb event on 16th February 1984. The event was related to a very strong relativistic particle emission. Attention is paid to the related microwave radiation stimulating a discussion of the discrimination between plasma and gyromagnetic radiation being important for plasma diagnostics. Another outstanding feature is the occurrence of a spectacular multi-band U-shaped type II burst pattern challenging a discussion of possible higher harmonic plasma radiation.     

Parameters of the intense X-ray and gamma-ray radiation from the solar flare of May 20, 2002, as observed from the Coronas-F spacecraft

We consider temporal, spectral, and polarization parameters of the hard X-ray and gamma-ray radiation observed during the solar flare of May 20, 2002, in the course of experiments with the SONG and SPR-N instruments onboard the Coronas-F spacecraft. This flare is one of the most intense gamma-ray events among all of the bursts of solar hard electromagnetic radiation detected since the beginning of the Coronas-F operation (since July 31, 2001) and one of the few gamma-ray events observed during solar cycle 23. A simultaneous analysis of the Coronas-F and GOES data on solar thermal X-ray radiation suggests that, apart from heating due to currents of matter in the the flare region, impulsive heating due to the injection of energetic electrons took place during the near-limb flare S21E65 of May 20, 2002. These electrons produced intense hard X-ray and gamma-ray radiation. The spectrum of this radiation extends up to energies ≥7 MeV. Intense gamma-ray lines are virtually unobservable against the background of the nonthermal continuum. The polarization of the hard X-ray (20–100 keV) radiation was estimated to be ≤15–20%. No significant increase in the flux of energetic protons from the flare under consideration was found. At the same time, according to ACE data, the fluxes of energetic electrons in interplanetary space increased shortly (～25 min) after the flare.     

Comparison of thermal and nonthermal hard X-ray emission in electron-heated solar flares

Using the results of numerical simulations of the solar atmospheric response to heating by nonthermal electron beams during solar flares, we have calculated the spatial and temporal evolution of both (i) the direct (beam-target) nonthermal bremsstrahlung and (ii) the thermal bremsstrahlung arising from the hot plasma energized by the electron beam. Typically, we find that below a certain cross-over energy E ^*, the emission is dominated by the thermal component, while at higher energies the direct bremsstrahlung component becomes more important. This cross-over energy is dependent on the position within the loop, generally increasing with height.We have also investigated the dependence of the cross-over energy E ^* on the parameters of the electron energy input. At the time of peak electron flux injection the cross-over energy E ^* can, for plausible parameters, be as high as 52 keV at the top 1 pixel, and as low as 16 keV at the bottom 1 pixel. We conclude that a possible reassessment of SMM HXIS data as an indicator of the thermal or nonthermal character of the primary energy release (based primarily on the geometric properties of the hard X-ray source) is required. Our results also point to the minimum photon energy that future instruments should observe (where practical, giving due consideration to detector sensitivity) in order to be sure that, in the context of the thick-target interpretation, the nonthermal component is not swamped by the self-consistent thermal counterpart created by the beam heating.     

Hard X-ray emission in electron-heated solar flares — A comparison of nonthermal and thermal contributions

We calculate the spatial structure of hard X-ray emission during the impulsive phase of electron-heated solar flares. Both direct non-thermal bremsstrahlung and the thermal bremsstrahlung arising from the heated plasma are considered. Our results indicate that the spread of non-thermal emission into the upper parts of the loop, through evaporation of the chromospheric target, may be more important than the appearance of a hot thermal source in the corona. The effects of varying the viewing angle to the flare loop, and of finite-size resolution element, are also considered, and we compare our results with observations from the Solar Maximum Mission Hard X-Ray Imaging Spectrometer. We also contrast the predicted structures with those predicted by other models of flare energy release, and it is found that the electron-heated model provides the most satisfactory agreement with the observations.On leave from: Department of Physics and Astronomy, The University, Glasgow G12 8QQ, Scotland, U.K.Presidential Young Investigator.     

Microwave emission from solar flares and comparison with observations

We have studied the evolution of electron energy and angular distributions using Monte Carlo technique for electron beams directed vertically downwards towards chromosphere for incident energies 30 keV, and 300 keV at different incidence angles. Using these distributions we have calculated microwave flux for different frequencies at a fixed column density as well as for a fixed frequency at different column densities. We have also calculated the total microwave flux coming out of solar atmosphere and have compared it with observations. Our results agree well with observational results and produce the observed nature of flux.     

Charge transfer X‐rays in solar and stellar flares

Most of the energy in a solar flare, and presumably a stellar flare as well, takes the form of a power law of energetic particles. The energetic electrons produce a bremsstrahlung continuum, while the most energetic nuclei produce gamma‐rays. Nuclei around 1 MeV/AMU can produce X‐rays during and after charge transfer with neutrals. This paper predicts the fluxes for some prominent X‐ray lines and compares them to existing spectra (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the nonthermal excitation and polarization of X-ray lines during solar flares

The energy distributions of nonthermal electrons are derived from hard X-ray spectra taken during the impulsive phase of two 2B flares in February 1969. They are used to calculate the fluxes of nonthermally excited X-ray lines of hydrogen-like and helium-like ions. These fluxes are compared to the total line fluxes observed at the same time with crystal spectrometers. The nonthermal excitation is found to give only small contributions to the total line intensities. This implies that the impact polarization which is to be expected for anisotropic velocity distributions of the energetic electrons, will be low. Nevertheless it should be feasible to detect line polarization during the impulsive phase of strong X-ray flares.NAS/NRC Research Associate.     

The betatron effect in collapsing magnetic traps

We consider the question of how the betatron effect affects the particle acceleration in a magnetic trap with a rapidly decreasing length. We show that the additional increase in energy caused by the betatron acceleration as the trap contracts is exactly offset by the decrease in the time of particle confinement in the trap, because the loss cone becomes larger during the contraction. As a result, the particle energy at the time of escape from the trap remains the same as that in a collapsing trap without contraction. We estimate the Alfvén-pumping efficiency in a collapsing trap in connection with the problem of particle acceleration in solar flares. The additional energy acquired by particles from magnetic-field oscillations is shown to be negligible. We discuss the possible observational manifestations of the betatron effect in solar flares.     

Synchronous microwave brightenings of solar active regions from RATAN-600 spectral observations

The observations of the solar radio emission on September 11, 2001, with the RATAN-600 radio telescope (southern sector) at four centimeter wavelengths (1.92, 2.24, 2.74, and 3.21 cm) revealed synchronous brightenings in solar radio sources. These were identified on the solar photosphere with active regions that were spaced up to ～10⁶ km apart (AR 9608 and AR 9616). We discuss manifestations of the possible mechanisms of synchronous brightenings in solar sources in a narrow microwave spectral band. The significant linear correlation (ρ_c = 0.84–0.92) between the relative fluxes of AR 9610 and AR 9608 at 1.92 and 2.24 cm and the significant linear correlation (ρ_c = 0.65–0.84) between the relative fluxes of AR 9606 and AR 9608 at 3.21 cm in a two-hour interval of observations are indicative of the interconnection between these active regions not only during flares and bursts, but also in the periods of their absence. This confirms the existence of a large-scale temporal component in the dynamics of the radio flux variations for these active regions. We found a difference between the temporal variations of the radio emission from the halo and the solar radio sources under consideration. The times of increase in the total solar soft X-ray (0.5–4.0 Å, 1.0–8.0 Å; GOES 8, GOES 10) flux are shown to coincide with the times of increase in the fluxes from the solar radio sources at short centimeter wavelengths.     

Correction of SOHO CELIAS/SEM EUV measurements saturated by extreme solar flare events

The solar irradiance in the Extreme Ultraviolet (EUV) spectral bands has been observed with a 15 s cadence by the SOHO Solar EUV Monitor (SEM) since 1995. During remarkably intense solar flares the SEM EUV measurements are saturated in the central (zero) order channel (0.1–50.0 nm) by the flare soft X‐ray and EUV flux. The first order EUV channel (26–34 nm) is not saturated by the flare flux because of its limited bandwidth, but it is sensitive to the arrival of Solar Energetic Particles (SEP). While both channels detect nearly equal SEP fluxes, their contributions to the count rate is sensibly negligible in the zero order channel but must be accounted for and removed from the first channel count rate. SEP contribution to the measured SEM signals usually follows the EUV peak for the gradual solar flare events. Correcting the extreme solar flare SEMEUV measurements may reveal currently unclear relations between the flare magnitude, dynamics observed in different EUV spectral bands, and the measured Earth atmosphere response. A simple and effective correction technique based on analysis of SEM count‐rate profiles, GOES X‐ray, and GOES proton data has been developed and used for correcting EUV measurements for the five extreme solar flare events of July 14, 2000, October 28, November 2, November 4, 2003, and January 20, 2005. Although none of the 2000 and 2003 flare peaks were contaminated by the presence of SEPs, the January 20, 2005 SEPs were unusually prompt and contaminated the peak. The estimated accuracy of the correction is about ±7.5% for large X‐class events. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Silicon X-ray line emission from solar flares and active regions

New observations of solar flare and active region X-ray spectra obtained with the Columbia University instrument on OSO-8 are presented and discussed. The high sensitivity of the graphite crystal panel has allowed both line and continuum spectra to be observed with moderate spectral resolution. Observations with higher spectral resolution have been made with a panel of pentaerythritol crystals. Twenty-nine lines between 1.5 and 7.0 Å have been resolved and identified, including several dielectronic recombination satellite lines to Si xiv and Si xiii lines which have been observed for the first time. It has been found that thermal continuum models specified by single values of temperature and emission measure have fitted the data adequately, there being good agreement with the values of these parameters derived from line intensity ratios.     

On the superfine structure of solar microwave bursts

Solar microwave bursts with a zebra pattern commonly exhibit a superfine time structure: the zebra stripes consist of separate spike-like pulses. We investigate the superfine structure in the April 21, 2002 event. The emission pulses are shown to exhibit a high periodicity (with a period of about 30 ms); there is a clear correlation between the individual zebra stripes. This structure of the dynamic spectra most likely reflects a periodic injection of electron beams, which generate emission at the double plasma resonance levels.