Spectral and spatial observations of microwave spikes and zebra structure in the short radio burst of May 29, 2003

Context. The unusual radio burst of May 29, 2003 connected with the M1.5 flare in AR 10368 has been analyzed. It was observed by the Solar Broadband Radio Spectrometer (SBRS/Huairou station, Beijing) in the 5.2–7.6 GHz range. It proved to be only the third case of a neat zebra structure appearing among all observations at such high frequencies. Despite the short duration of the burst (25 s), it provided a wealth of data for studying the superfine structure with millisecond resolution (5 ms). Aims. We localize the site of emission sources in the flare region, estimate plasma parameters in the generation sites, and suggest applicable mechanisms for interpretating spikes and zebra-structure generation. Methods. We analyze of flare area structures and spectral parameters of millisecond spikes and their radio sources. We then interpret the superfine structure in the framework of known models. Results. Positions of radio bursts were obtained by the Siberian Solar Radio Telescope (SSRT) (5.7 GHz) and Nobeyama radioheliograph (NoRH) (17 GHz). The flare configuration includes two systems of loops with the common base near the N-spot. The loop bases coincide with polarized emission sources at 17 GHz. The sources in intensity gravitated to tops of short loops at 17 GHz, and to long loops at 5.7 GHz. Short pulses at 17 GHz (with a temporal resolution of 100 ms) are registered in the R-polarized source over the N-magnetic polarity (extraordinary mode). The positions of the subsecond pulse sources at 5.7 GHz change from pulse to pulse and are level with the tops of some loops over the magnetic field’s neutral line. Dynamic spectra show that all the emission comprised millisecond pulses (spikes) of 5–10 ms duration in the instantaneous band of 70 to 100 MHz, forming the superfine structure of different bursts, essentially in the form of fast or slow-drift fibers and various zebra-structure stripes. Five scales of zebra structures have been singled out. The occurrence of the spikes is associated with the formation of two new radio sources with different polarities, which appeared simultaneously on SSRT and NoRH maps. This took place after new magnetic fluxes of opposite polarity had emerged in the leading spot and a new magnetic “delta” configuration had been formed. Conclusions. As the main mechanism for generating spikes (as the initial emission) we suggest the coalescence of plasma waves with whistlers in the pulse regime of interaction between whistlers and ion-sound waves. In this case one can explain the appearance of fibers and sporadic zebra-structure stripes exhibiting the frequency splitting