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 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. 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