Theory of solar radio pulsation

We show that the observed modulation of some coronal microwave, X-ray and Type III emission into pulses of 10 sec intervals is a consequence of the stimulation of electron cyclotron waves propagated in the whistler mode in dipole-like bipolar regions of dimension 0.2 R . Assuming that a power law spectrum of 10 keV electrons with a slope similar to solar flare protons can be trapped in a bipolar region, we show that whistlers can be generated by pitch angle instability. The resultant 10 sec bounce motion of whistler wave trains leads to enhanced, modulated emission in microwave and X-ray frequencies by pitch angle scattering of MeV electrons, and to modulated Type III emission by scattering with coherent plasma waves. A direct prediction of the theory is the existence of sympathetic pulsations at two sources a fraction of a solar radius apart. A second test of the theory is that modulated Type III emission should show strong polarization.This work was conducted under U.S. Air Force Space and Missile Systems Organization (SAMSO) Contract No. F04701-69-C-0066.     

Coronal X-ray activity preceding solar flares

A comprehensive survey of Skylab S-054 soft X-ray images was performed to investigate the characteristics of coronal enhancements preceding solar flares. A search interval of 30 min before flare onset was used. A control sample was developed and tests of the statistical results performed. X-ray images with preflare enhancements were compared with high resolution H images and photospheric magnetograms.The results are as follows: preflare X-ray enhancements were found in a statistically significant number of the preflare intervals, and consisted of one to three loops, kernels or sinuous features per interval. Typically, the preflare feature was not at the flare site and did not reach flare brightness. There was no systematically observed time within the preflare interval for the preflare events to appear and no correlation of preflare event characteristics with the subsequent flare energy. Gas pressures of several preflare features were calculated to be on the order of several dyne cm^–2, typical of active region loops, not flares. These results suggest that observations with both high spatial resolution and low coronal temperature sensitivity are required to detect these small, low pressure enhancements that preceded the smaller flares typical of the Skylab epoch. H brightenings were associated with nearly all of the preflare X-ray enhancements. Changing H absorption features in the form of surges or filament activations were observed in about half of the cases. These results do not provide observational support for models which involve preheating of the flare loop, but they are consistent with some current sheet models which invoke the brightening of structures displaced from the flare site tens of min before onset.     

Satellite information system malfunctions caused by broadband solar radio emission during solar flares

The paper briefly discusses the impairment of normal functioning of the global positioning system (GPS) and the global navigation satellite system (GLONASS) caused by broadband solar radio emission during large solar flares on October 28, 2003, and December 6 and 13, 2006.     

Hard X-ray and radio emission at the onset of great solar flares

A study of the onset phase often great hard X-ray bursts is presented. It is shown from hard X-ray and radio observations in different wavelength ranges that the energization of the electrons proceeds on a global time-scale of some tens of seconds. In nine of the bursts two phases of emission can be distinguished during the onset phase: the pre-flash phase, during which emission up to an energy limit ranging from some tens of keV to 200 keV is observed, followed ten to some tens of seconds later by the flash phase, where the count rate in all detector channels rises simultaneously to within some seconds. For two of the events strong -ray line emission is observed and is shown to start close to the onset of the flash phase.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.     

High-energy gamma-ray emission from solar flares: Constraining the accelerated proton spectrum

Using a multi-component model to describe the γ-ray emission, we investigate the flares of December 16, 1988 and March 6, 1989 which exhibited unambiguous evidence of neutral pion decay. The observations are then combined with theoretical calculations of pion production to constrain the accelerated proton spectra. The detection of π⁰ emisson alone can indicate much about the energy distribution and spectral variation of the protons accelerated to pion producing energies. Here both the intensity and detailed spectral shape of the Doppler-broadened π⁰ decay feature are used to determine the spectral form of the accelerated proton energy distribution. The Doppler width of this γ-ray emission provides a unique diagnostic of the spectral shape at high energies, independent of any normalisation. To our knowledge, this is the first time that this diagnostic has been used to constrain the proton spectra. The form of the energetic proton distribution is found to be severely limited by the observed intensity and Doppler width of the π⁰ decay emission, demonstrating effectively the diagnostic capabilities of the π⁰ decay γ-rays. The spectral index derived from the γ-ray intensity is found to be much harder than that derived from the Doppler width. To reconcile this apparent discrepancy we investigate the effects of introducing a high-energy cut-off in the accelerated proton distribution. With cut-off energies of around 0.5–0.8 GeV and relatively hard spectra, the observed intensities and broadening can be reproduced with a single energetic proton distribution above the pion production threshold.     

Gyrosynchrotron emission of solar flares

The current status of our knowledge on the theory of radio emission from mildly relativistic electrons and its application in the interpretation of solar radio bursts are reviewed. The recent high spatial resolution microwave observations have given important information about the geometry of the emitting region and have helped in the computation of better inhomogeneous models that reproduce qualitatively several observational characteristics of the emission. The limitations of the observations and the theory (particularly the effect of mode coupling on the observed polarisation) are pointed out and the potential of the gyrosynchrotron process as a diagnostic of the physical conditions is discussed. This will help us to obtain quantitative information about the changes of the magnetic field and the acceleration of particles in solar flares.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.     

X-ray emission from solar flares

Solar X-ray Spectrometer (SOXS), the first space-borne solar astronomy experiment of India was designed to improve our current understanding of X-ray emission from the Sun in general and solar flares in particular. SOXS mission is composed of two solid state detectors, viz., Si and CZT semiconductors capable of observing the full disk Sun in X-ray energy range of 4–56 keV. The X-ray spectra of solar flares obtained by the Si detector in the 4–25 keV range show evidence of Fe and Fe/Ni line emission and multi-thermal plasma. The evolution of the break energy point that separates the thermal and non-thermal processes reveals increase with increasing flare plasma temperature. Small scale flare activities observed by both the detectors are found to be suitable to heat the active region corona; however their location appears to be in the transition region.     

Hard X-rays,ejecta, and their associated decimetric radio emission in solar flares

We investigate temporal and spatial correlations in solar flares of hard X-rays (HXR) and decimetric continuum emissions, ejecta, and CMEs. The focus is on three M-class flares, supported by observations from other flares. The main conclusions of our observations are that (1) major hard X-ray flares are often associated with ejecta seen in soft X-rays or EUV. (2) Those ejecta seem to start before HXR or related decimetric radio continua (DCIM emission). (3) DCIM occurring nearly simultaneously with the first HXR peak are located very close to the HXR source. Later in the flare, DCIM generally becomes stronger, drifts to lower frequency and occurs far from the HXR source. Thus the positions at high frequency are generally closer to the HXR source. DCIM emission consists of pulses that drift in frequency. The very high and sometimes positive drift rate suggests spatially extended sources or type III like beams in an inhomogeneous source. Movies of selected flares used in this study can be found on the CD-ROM accompanying this volume. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1026194227110     

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.     

Hard X-ray emission processes in solar flares

Solar hard X-ray emission is one of the most direct diagnostics of accelerated particles during solar flares. In this review, the current understanding of hard X-ray emission processes is discussed: first the different emission mechanisms (in particular inverse Compton radiation, energetic ion or electron bremsstrahlung) are presented and the plausibility of each of these mechanisms is discussed. Then, different types of hard X-ray models (thermal or non-thermal, homogeneous or inhomogeneous emission regions) are presented together with the comparison of their predictions with X-ray observations (spectral, spatial and temporal informations - directivity and polarization).Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.     

High-energy continuum emission from solar flares

The properties of solar flare continuum emission at energies >300 keV are discussed. Emphasis is placed on observations made during the 21st Solar Maximum by -ray detectors aboard the Solar Maximum Mission and Hinotori satellites. The statistical properties of high-energy flares are presented, including their size-frequency distribution, spectral-index distribution, position distribution, and associated soft X-ray size. The temporal structure of the high-energy continuum is reviewed as well as attempts to model the structure by two-step acceleration and particle trapping. Evidence for the directivity of flare radiation is presented and statistical and stereoscopic analysis techniques are compared and contrasted. The first observations of flare -rays at energies > 10 MeV are examined. We show that the very high-energy emission must be a mixture of pion-decay radiation and primary electron bremsstrahlung. Finally, we present high-energy observations from the extended phase of the giant 3 June, 1982 flare which seem to require a new acceleration component.     

Type III radio burst productivity of solar flares

We study the statistical relationship between type III radio bursts and optical flares, using the comprehensive flare data base at the NOAA National Geophysical Data Center (Boulder, Colorado), and the radio observations obtained with the ARTEMIS multichannel spectrograph in Nancay (France), operating at 500–100 MHz.At variance with previous results, we find that type III probability of occurrence depends only weakly upon the spatial extension of the flare observed in H, but strongly upon its brightness. We also confirm that type III probability increases with proximity to sunspots and with mass motions (surges and prominence activity); in addition, our statistical data are consistent with both relations holding at fixed flare brightness. Thus, some of the conditions favorable to type III occurrence are characteristic of compact flares, while others are characteristic of large and long-duration flares, which are often related to mass ejections. This apparent paradox suggests that particle acceleration and magnetic expansion are at work simultaneously in the ejection of electron streams out of flaring sites.     

Type III radio burst productivity of solar flares

We study the occurrence probability of type III radio bursts during flares as a function of the flare position on the Sun. We find that this probability peaks around 30° east of the central meridian, which points to a reciprocal tilt of the average radiation pattern of type IIIs. We argue that anisotropic scattering of the radiation by overdense coronal fibers parallel to the magnetic field is the dominant factor determining the orientation of radiation patterns. It follows that the average magnetic field appears to be tilted 30° west from the vertical. We also find that within a given active region, the average type III production rate of flares peaks 1° west of the center of gravity of all the flares of this active region.We infer that the coronal magnetic field above active regions presents a strong east-west asymmetry, resulting from the well known asymmetry at the photospheric level. As the west side of an active region covers a smaller area with stronger magnetic field than the east side, western flares are generally closer to open field lines than eastern flares. As a consequence, accelerated particles on the trailing (east) side of active regions usually stay trapped in magnetic loops, while on the leading (west) side they are more likely to escape along open lines into interplanetary space. As a result of the initial westward tilt of these open lines, we estimate that the corresponding Archimedean spiral is on average (apparently) rooted 15° west of the flare.     

Center-to-limb variation of the peak flux spectra of type IV radio bursts associated with solar proton flares

Type IV radio bursts with wide band from microwave to metric-wave frequency are generally associated with solar proton flares. Recently, Castelli et al. (1967, 1968) have shown that the type IV radio bursts associated with solar proton flares show the U-shaped peak flux spectra with the minimum flux at decimetric frequencies. In this paper, the center-to-limb variation of such peak flux spectra is investigated in order to examine the effect of decrease of the peak flux at metric frequencies with increase of the angular distance from the central meridian of the Sun. It is shown that the U-shaped spectra are obtained independent of the position of proton flares, although the spectral form changes significantly in the case of the flares near the limb. It is further suggested that the U-shaped spectra consist of the two essentially independent components for microwave and metric-wave frequencies, respectively.     

Spotless flares and the associated radio continuum emission

We studied 24 spotless flares of Ha importance 1 which occurred during the 21st cycle of solar activity. The spotless flares could be grouped in three categories according to their location and time history of the associated active region. Our association of the flares with radio events was based on relative timing and on the flare importances. Weak microwave gradual rise and fall events were frequently recorded during the occurrence of the spotless flares. A few flares from our sample could be associated with impulsive and complex microwave bursts. Only in one case an association of a spotless flare with a significant metric type II/IV event seems to be justified.Proceedings of the Second CESRA Workshop on Particle Acceleration and Trapping in Solar Flares, held at Aubigny-sur-Nère (France), 23–26 June, 1986.     

Correlation of solar decimetric radio bursts with X-ray flares

Several hundred radio bursts in the decimetric wavelength range (300–1000 MHz) have been compared with simultaneous soft and hard X-ray emission. Long lasting (type IV) radio events have been excluded. The association of decimetric emission with hard X-rays has been found to be surprisingly high (48%). The association rate increases with bandwidth, duration, number of structural elements, and maximum frequency. Type III-like bursts are observed up to the upper limit of the observed band. This demonstrates that the corona is transparent up to densities of about 10¹⁰ cm^–3, contrary to previous assumptions. This can only be explained in an inhomogeneous corona with the radio source being located in a dense structure. The short decimetric bursts generally occur during the impulsive phase, i.e. simultaneously with hard X-rays. The times of maximum flux are well correlated (within 2 s). The HXR emission lasts 4 times longer then the radio emission in the average. This work finds a close relationship between decimetric and HXR emission with sufficient statistics offering additional information on the flare process.     

Electron cyclotron maser emission from solar flares

Energetic electrons, with energies 10–100 keV, accelerated during the impulsive phase of solar flares, sometimes encounter increasing magnetic fields as they stream towards the chromosphere. A consequence of the conservation of their magnetic moment is that the electrons with large initial pitch angle will be reflected at different heights from the atmosphere. Energetic electrons reflected below the transition zone will lose most of their energy to collisions and will never return to the corona. Thus, electrons reflected above the transition zone form a loss-cone velocity distribution which can be unstable to Electron Cyclotron Maser (ECM). The interaction of quasi-perpendicular shocks with the ambient coronal plasma will form a ‘ring’ or ‘hollow beam’ velocity distribution upstream of the shock. ‘Ring’ velocity distributions are also unstable to the ECM instability. A review of the recent results on the theory of ECM will be presented. We will focus our discussion on the questions: (a) What are the characteristics of the linear growth rate of the ECM during solar flares? (b) How does the ECM saturate and what is its efficiency? (c) How does the ECM generated radiation modify the flare environment? Finally we will review the outstanding questions in the theory of ECM and we will relate the theoretical predictions to current observations.     

Thermal instability of the reconnecting current layer in solar flares

To interpret the present-day satellite observations of the sequential brightening of coronal loops in solar flares, we have solved the problem of the stability of small longitudinal perturbations of a homogeneous reconnecting current layer (CL). Within the magnetohydrodynamic approximation we show that an efficient suppression of plasma heat conduction by amagnetic field perturbation inside the CL serves as an instability condition. The instability in the linear phase grows in the characteristic radiative plasma cooling time. A periodic structure of cold and hot filaments located across the direction of the electric current can be formed as a result of the instability in the CL. The proposed mechanism of the thermal instability of a reconnecting CL can be useful for explaining the sequential brightening (“ignition”) of flare loops in solar flares.     

Directivity of non-thermal X-ray emission from solar flares

An attempt has been made in the present work to reveal the directivity of solar non-thermal X-ray emission using the data obtained from the Prognoz and Explorer satellites. The frequency of occurrence of X-ray bursts and the mean intensities of the emission are studied as a function of distance from the central meridian. The most complete statistics have been obtained for the 4–24 keV X-ray bursts for the period 1970–1973. The X-ray burst frequency of occurrence normalized to the corresponding H flare frequency increases towards the solar limb. During the studied period this trend is more pronounced to the east than to the west. Distributions of the mean intensities of X-ray bursts are very similar to those of the frequency of occurrence of X-ray bursts; the effect is more noticeable for the low intensity bursts. The effect of the east-west asymmetry for H flares has been found to vary in magnitude and direction during the 20th solar activity cycle.     

On the threshold effect of proton acceleraton in solar flares

Based on the reconnection theory of a flare and on recent observational and statistical findings, the problem of the initial acceleration of solar cosmic rays (SCR) is discussed. Simple estimates of the electric fields required to start the electron acceleration are obtained and the problem of proton ionization losses for overcoming the Coulomb barrier is considered. We take into account also the possible differences between proton and electron spectra from the very beginning of the acceleration process. Special attention is paid to the distribution functions of solar flare events in various parameters (peak fluxes and/or energy fluences in X-ray and radio wave bursts, in proton and electron emissions, etc.). It is shown that the distribution functions allow the interpretation of some scale and time flare parameters in terms of expected threshold effects. However, these functions are still insufficient to evaluate the relative share of different emissions in the global energy budget of a flare. In this context, a more promising approach is to derive the direct ratio between the number of accelerated protons,N _p, and total flare energy,W _f, within the frame of a certain acceleration model. It is argued that an absolute threshold for proton production (in Hudson's formulation) does not exist. Meanwhile, the flux and threshold energy of accelerated protons overcoming the Coulomb loss maximum, in fact, may depend heavily on the global output of flare energy.