THE COEVOLUTION OF DECIMETRIC MILLISECOND SPIKES AND HARD X-RAY-EMISSION DURING SOLAR-FLARES

A comprehensive data set of 27 solar flares with decimetric millisecond spikes between 1980 and 1989, simultaneously observed with the Zurich radio spectrometers (Ikarus, 0.1-1 GHz; Phoenix, 0.1-3 GHz) and the Hard X-Ray Burst Spectrometer (HXRBS, 25-500 keV on the Solar Maximum Mission (SMM) spacecraft, has been analyzed. A strong functional dependence was found between the radio spike flux and.the associated 25-50 keV hard X-ray emission, which can be expressed by a convolution of both flux time profiles with a delayed Gaussian kernel. From this convolution technique, we obtain the following results: (1) the delayed radio spike flux is best reproduced during the rise and decay of the impulsive hard X-ray (HXR) phase, (2) the smoothed radio spike flux is delayed by typically 2-5 s, and (3) there are correlations among the radio flux, the spike modulation depth of HXR flux, the HXR-radio delay, and the radio frequency. In order to study the evolution of the radio spike burst rate, we developed an algorithm that discriminates spike bursts against instrumental noise and irrelevant burst types. This counting algorithm yields the following results: (1) the total number of spikes per cluster varies from 10 to 10(4), (2) the maximum spike rate varies from 5 up to 100 s-1, (3) the irradiated radio energy per single spike is 10(16)-10(18) ergs, (4) the correlated photon flux per single radio spike is 0.1-10 photons (cm2 keV)-1 in the 25-50 keV range, and (5) the single radio spikes (with a duration of 20-100 ms) do not show corresponding fine structures in hard X-rays. However, the spatial dispersion of precipitating beams, the finite energy loss time, and the instrumental sensitivity can obscure possible HXR fine structures beyond a few tenths of a second. A number of scenarios are discussed which can account for the observed delay of 2-5 s between HXR and spiky radio emission: (1) propagation effects, (2) particle trapping, (3) anisotropic acceleration, (4) trigger by shock waves or conduction fronts, and (5) nonlinear threshold effects for coherent radio emission. Although the observed proportionality between the HXR flux and the delayed radio spike rate suggests a functional relationship between both emission mechanisms, no conclusive evidence can be drawn as to whether the spikiness in radio emission results from a possibly fragmented energy release or from secondary effects related to particle propagation and plasma instabilities in an inhomogeneous medium.