On the spectrum of solar impulsive hard X-ray bursts

This article puts forward a new method for the theoretical analysis of the X-radiation spectrum of impulsive hard X-ray bursts. It points out that the electron density energy state function must obey the fundamental kinetic equation. In the case of several model source functions, the electron density energy spectra are deduced. This can serve as a basis for an analysis of the spectrum of X-radiaiton in impulsive hard X-ray bursts. The article also makes a preliminary discussion of these energy state functions which help to explain the phenomena of softening of the X-radiation spectrum.     

Third plasma harmonic radiation in type II bursts

We present observational evidence for simultaneous fundamental, second and third harmonic radio emission during an excessively strong type II burst on February 16, 1984. This burst was emitted from an active region behind the limb allowing for fair resolution of the wave bands. If interpreted as a triple harmonic system, three different, nearly equally probable mechanisms for higher harmonic emission are qualitatively discussed. These are a four-wave process which involves very strong Langmuir waves, a decay process first proposed by Cairns (1987) for higher harmonic emission near the Earth's bow shock, and time evolution of the emitted frequency during Langmuir wave collapse. In sufficiently strong coronal shock waves, both of the former mechanisms may be more efficient than under solar wind conditions. In the third mechanism, Langmuir wave collapse may be driven by strong electron beams as are expected to exist in quasiparallel shocks where electron reflection may be strongest. We discuss the differences between the signatures of these mechanisms.     

Fast discrete emissions at hard X-rays and sub-mm-IR waves in the impulsive phase of solar bursts

The time profiles of electromagnetic fluxes at hard X-rays and short microwaves are signatures of the energy conversion mechanisms at the origin of solar flares. The distinction between continuum and discrete energy production brings drastic conceptual consequences for the interpretation of the energy conversion processes. As more sensitive detectors were used on measurements with higher time resolution, the notion of continuum energy release in the impulsive phase is being replaced by the concept of repetitive energy production or Elementary Flare Bursts manifested at hard X-rays and by rapid time structures in microwave emissions. These discrete time structures are now known to be as short as tens of milliseconds, and part of their emissions are possibly produced by the same populations of accelerated electrons. Fast spikes, with mm-wave emission fluxes increasing for shorter wavelengths, simultaneous with hard X-rays, bring severe constraints for interpretation. This problem is reviewed, with the suggestion of a possible significant burst emission component in the sub-mm-IR range, due to primeval short-lived explosive compact sources, for which there are still no diagnostics.Dedicated to Cornelis de Jager     

Hard X-ray images of impulsive bursts

A morphological study is made for the hard X-ray images (25–50 keV) of nine impulsive bursts observed by Hinotori. Most of them revealed single sources, either extended or compact, during the whole duration of the bursts. The sources of all of four spike bursts in the present sample are compact. After the main phase of the impulsive bursts, generally the source size becomes smaller accompanying a shift of position. The X-ray source size is much greater than that of the Hα kernel in two events out of three. Four possible explanations for the X-ray source to be single are suggested. One of these is the strong electric field along the magnetic field as demonstrated to be produced at the decay of force-free current.     

A new concept for detecting electromagnetic and electrostatic waves in space plasmas

The proposed system measures the conduction current associated with electromagnetic and electrostatic waves propagated in space plasmas. The current collected by a probe flows through a measuring device, and is subsequently returned to the medium by means of an electron emitter. By way of illustration, the response of this system to the electron acoustic mode is investigated.This technique not only opens the way to the measurement of an additional plasma wave parameter, but it also offers inherent advantages over usual antennas, especially in magnetospheric and planetary environments.     

Self-focusing of electromagnetic radiation in electron-positron ion plasma

We consider the self-focusing of electromagnetic (EM) radiation in electron-positron plasmas with a small fraction of ions. It is shown that in such a plasma EM pulses may form and propagate like oscillatory waveguides.     

The characteristics of impulsive solar EUV bursts

We examine a number of high time resolution intensity-time profiles of EUV impulsive bursts as observed by the Harvard College Observatory EUV Spectroheliometer carried aboard the Skylab Apollo Telescope Mount. These bursts are found to be synchronous (to within the instrumental time resolution of 5.5 s) in all wavelengths observed, corresponding to emissions from temperatures ranging from upper chromospheric to coronal. The distribution with temperature of a suitably defined emission measure parameter is also examined as a function of time throughout the bursts and a marked similarity in the shape of this distribution, both between different events and throughout the time history of any particular event, is noted. The significance of these observations for physical processes associated with EUV bursts is briefly discussed.On leave from Dept. of Astronomy, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K.     

An index of impulsiveness for 2800 MHz impulsive solar noise bursts

The initial growth of intensity, I, of the impulsive solar noise burst observed at 2800 MHz has been fitted with a parabolic curve of the form I=ct ²and the quantity c taken as the index of impulsiveness. Two groups of bursts comprising 85% of all impulsive bursts observed in 1962–63 and 1966 were selected for study. A good fit has been obtained for bursts having peak flux density up to 20 flux units, while for more intense bursts, the average observed growth is more rapid than the parabolic rate. The distribution of the index in the range 0.1 to more than 100 shows two peaks, one for c values 1–10 and another apparent peak for those with c greater than 100. The index is independent of the peak intensity of the burst and its position on the solar disc, while there is a small trend, indicating that shorter bursts are more impulsive than longer duration events. The more easily derived linear rate of rise, b = Peak Flux/Interval from start to peak is related to the parabolic impulsive index by b = 1.86 c ^0.57. The non-linear rate of expansion of a flaring volume suggested by Pneuman when applied to explain the parabolic rise of microwave bursts indicates that the impulsiveness of bursts is inversely related to the contained magnetic field.     

A coherent radiation mechanism for type IV dm radio bursts

An interpretation is presented of the decimetric type IV continuum with fine structure on March 6, 1972 and of the corresponding source region, in terms of ?erenkov plasma radiation and alternatively of synchrotron radiation, both in case of coherent and incoherent generation. If the magnetic field strength in the source region is a few gauss, in a stationary situation a loss cone instability develops which generates electron plasma waves coherently. The amount of energetic electrons required for consecutive induced scattering of the plasma waves at the thermal ions into electromagnetic waves is less than in case of synchrotron radiation. It is concluded that the former mechanism provides the explanation of type IV continua with fine structure such as intermediate drift bursts and sudden reductions of the continuum level.     

Annihilation radiation in cosmic gamma-ray bursts

The emission features observed in the energy spectra of cosmic gamma-ray bursts imply the existence of two radiation components of comparable intensity. The softer component is similar to the continua of featureless bursts. The fast decrease in the intensity of this radiation with increasing photon energy is apparently due to the neutron star's magnetosphere being opaque to hard photons because of the formation of electron-positron pairs in single- (,B) and two-photon (,), processes. The hard component originates from the annihilation of electron-positron pairs, its spectrum representing a broad line with an extended power-law wing. Such a shape of the spectrum is apparently due to either thermal broadening in a source with a spatially inhomogeneous and rapidly time-varying plasma temperature, or nonthermal energy distribution of particles in their motion along the magnetic field lines. It is assumed that the sources of these components are spatially separated, the annihilation radiation escaping from the polar regions of a strongly magnetized neutron star in a collimated beam without appreciable attenuation.     

Differences of observed characteristics between impulsive bursts and post-burst increases

The change of source characteristics during the transition from the impulsive phase to the post-burst phase is investigated for cm bursts on a statistical basis. The results are the following: (1) The sudden decrease of the circular polarization degree is found almost invariably at the transition; typically from 20–30% down to a few percent. (2) Some bursts show remarkable source expansions in the post-burst phase. There are no cases in which impulsive bursts have larger source size than the associated post-burst increases. (3) Type III bursts which are indicative of non-thermal phenomena are associated with the impulsive phase but not with the post-burst phase. Implications of these observed results are discussed.     

Dynamic spectral characteristics of thermal models for solar hard X-ray bursts

The dynamic spectral characteristics of the thermal model for solar hard X-ray bursts recently proposed by Brown et al. (1979) (BMS) are investigated. It is pointed out that this model, in which a single source is heated impulsively and cooled by anomalous conduction across an ion-acoustic turbulent thermal front, predicts that the total source emission measure should rise as the temperature falls. This prediction, which is common to all conductively cooled single-source models, is contrary to observations of many simple spike bursts. It is proposed, therefore, that the hard X-ray source may consist of a distribution of many small impulsively-heated kernels, each cooled by anomalous conduction, with lifetimes shorter than current burst data temporal resolution. In this case the dynamic spectra of bursts are governed by the dynamic evolution of the kernel production process, such as magnetic-field dissipation in the tearing mode. An integral equation is formulated, the solution of which yields information on this kernel production process, from dynamic burst spectra, for any kernel model.With a BMS-type kernel model in one-dimensional form, the derived instantaneous spectra are limited in hardness to spectral indices 4 for any kernel production process, due to the nature of the conductive cooling. Ion-acoustic conductive cooling in three dimensions, however, increases the limiting spectral hardness to 3. Other forms of anomalous conduction yield similar results but could permit bursts as hard as 2, consistent with the hardest observed.The contribution to the X-ray spectrum from the escaping tail of high-energy kernel electrons in the BMS model is calculated in various limits. If this tail dissipates purely collisionally, for example, its thick-target bremsstrahlung can significantly modify the kernel spectrum at the high-energy end. The energetics of this dynamic dissipation model for thermal hard X-ray bursts also are briefly discussed.Now at: Department of Mathematics, University of Waikato, Hamilton, New Zealand.     

Constraints on the bulk Lorentz factor in the internal shock scenario for gamma-ray bursts

We investigate, independently of specific emission models, the constraints on the value of the bulk Lorentz factor Γ of a fireball. We assume that the burst emission comes from internal shocks in a region transparent to Thomson scattering, and before deceleration caused by the swept-up external matter is effective. We consider the role of Compton drag in decelerating fast-moving shells before they interact with slower ones, thus limiting the possible differences in the bulk Lorentz factor of shells. Tighter constraints on the possible range of Γ are derived by requiring that the internal shocks transform more than a few per cent of the bulk energy into radiation. Efficient bursts may require a hierarchical scenario, where a shell undergoes multiple interactions with other shells. We conclude that fireballs with average Lorentz factors larger than 1000 are unlikely to give rise to the observed bursts.     

Some center-limb statistical trends of impulsive solar bursts at 7 GHz

The more impulsive 7 GHz bursts seem to prefer the solar limb regions, while the degree of polarization decreases with increasing impulsivity.     

Correlation of hard X-ray and type III bursts in solar flares

The observed correlations between X-ray and type III radio emissions from solar bursts are described by means of a bivariate distribution function. Procedures for determining the form of this distribution are described using a sample of data analyzed by Kane (1981). With the help of this distribution a model is constructed to explain the correlation between the X-ray spectral index and the ratio of X-ray to radio intensities. Implications of the model are discussed.     

A study of hard X-ray associated meter-decameter bursts observed on December 19, 1979

We present the results of a study of the relationship of a complex meter-decameter wavelength radio burst observed with the Clark Lake E-W and N-S interferometers, with a hard X-ray burst observed with the X-ray spectrometer aboard ISEE-3. The radio burst consisted of several type III's, reverse drift type III's, a U burst, and type II and type IV bursts. The X-ray emission was also complex. The radio as well as hard X-ray emissions were observed before the flash phase of the flare; they were not always associated and we conjecture that this may constitute evidence for acceleration of electrons high in the corona. On the other hand, all components of the reverse drift burst were associated with hard X-ray subpeaks, indicating multiple injection of electron beams along field lines with different density gradients. While the type II burst appeared to be related to the hard X-ray burst, a detailed correspondence between individual features of the radio and hard X-ray burst emissions could not be found. The type IV burst started after all hard X-ray emissions ceased. Its source appeared to be a magnetic arch, presumably containing energetic electrons responsible for the gyrosynchrotron radiation of type IV.Presently at INPE/CRAAM, São Paulo, Brazil.