On the Origin of Multiply-Impulsive Emission from Solar Flares.

Over the past twenty years, our understanding of solar flares has been augmented greatly by the advent of rocket-, balloon-, and satellite-borne instrumentation dedicated to observations of the Sun. In particular, the use of spacecraft-borne detectors has permitted coverage of the shorter-wavelength regions of the electromagnetic spectrum, inaccessible to ground-based facilities. Hard X-ray emission from solar flares provides direct evidence of the role of energetic electrons in these powerful explosions. Analyses of flare-associated hard X-rays, in conjunction with coincident coverage at other wavelengths, have contributed much of our current understanding of the basic energizing processes and resultant particle acceleration which characterize the flare phenomenon. During the previous solar maximum, the hard X -ray burst spectrometer on board the OSO-5 satellite observed hundreds of hard X-ray events on the Sun, in the energy range 14 to 254 keV. The 1.8-second temporal resolution of the detector enabled detailed studies of the evolution of burst intensity with time, as well as the spectral evolution, and was comparable to the resolution of most solar radio observatories operating at that time. The analysis and interpretation of a set of complex X-ray bursts, selected from the OSO-5 data, are presented in this work. The multiply-impulsive events were chosen on the basis of morphological characteristics: each event appears to consist of a number of overlapping spikes, with no apparent gradual component of significance. The two -stage events were selected on the basis of both morphological characteristics and association with the appropriate phenomena at other wavelengths. Concident microwave and meter-wave radio, soft X-ray, H-alpha, interplanetary particle, and magnetographic data were obtained from several observatories, to aid in developing comprehensive and self-consistent pictures of the physical processes underlying the complex bursts. The present research is focussed on two specific aspects of the multiple-spike and two-stage bursts: (1)To look for the causes of multiplicity in complex impulsive events; and (2)To compare the impulsive emissions with associated gradual emissions, to pinpoint the basic processes which are applicable to each phase alone. The investigation is concentrated on the hard X-ray and microwave emissions, with reference to associated meter-wave phenomena were appropriate. The X-ray and microwave radiations are bremsstrahlung and gyrosynchrotron, respectively, from the electrons accelerated in the relevant regions of the solar atmosphere. Together, they are used to deduce the characteristics of the source: electron density, temperature or spectral index of the electron distribution, magnetic -field strength, and area. The hard X-ray emissions alone are used to determine the parent electron spectrum and its evolution throughout an event, to search for correlated variations in spectral parameters which may indicate the underlying acceleration mechanism. The main conclusions of this work are: (1)The multiplicity of the impulsive events studied requires at least two distinct causes. On the basis of derived source parameters, the bursts fall into two categories: events whose component spikes apparently originate in one location, and events in which groups of spikes appear to come from separate regions which flare sequentially. (2)The origin of multiplicity in the case of a single source region remains unidentified. Progress was made, however, in critical evaluation of postulated explanations. One hypothesis, which attributes intensity variations to betatron acceleration of electrons in a magnetic trap, was tested. It was found that the purely impulsive emissions show no signs of betatron acceleration, thus ruling out this mechanism as a candidate for inducing multiply-spiked structure. The second-stage emissions of several complex bursts also were tested, with results differing markedly from the analysis of the impulsive bursts. The majority of the two-stage bursts exhibited spectral behaviour consistent with the betatron model, for the first few minutes of the second stage. Therefore, it appears that betatron acceleration may be an integral feature of the early stages of the second-stage emission, for many two-stage bursts.