MICROWAVE EMISSION FROM CORONAL HEIGHTS - STUDY OF A NONTHERMAL RADIOFLARE

Two small radio Bares following the great gamma-ray burst on 11 June 1 991 are studied. We analyse the different association of emission feat ures at microwaves, decimeter waves, and soft and hard X-rays for the events. The first flare has well-defined emission features in microwav es and soft and hard X-rays, and a faint decimetric signature well aft er the hard X-ray burst. It is not certain if the decimetric event is connected to the burst features. The second event is characterized by an almost simultaneous appearance of hard X-ray burst maxima and decim etric narrowband drift bursts, but soft X-ray emission is missing from the event. With the exception of the possibility that the soft X-ray emission is absorbed along the way, the following models can explain t he reported differences in the second event: (1) Microwave emission in the second event is produced by less than or equal to 150 keV electro ns spiraling in the magnetic field relatively low in the corona, while the hard X-ray emission is produced at the beginning of the burst nea r the loop top as thick-target emission. If the bulk of electrons ente red the loop, the low-energy electrons would not be effectively mirror ed and would eventually hit the footpoints and cause soft X-ray emissi on by evaporation, which was not observed. The collisions at the loop top would not produce observable plasma heating. The observed decimetr ic type III bursts could be created by plasma oscillations caused by e lectron beams traveling along the magnetic field lines at low coronal heights. (2) Microwave emission is caused by electrons with MeV energi es trapped in the large magnetic loops, and the electrons are effectiv ely mirrored from the loop footpoints, The hard X-ray emission can com e both from the loop top and the loop footpoints as the accelerated lo wer energy electrons are not mirrored, The low-energy electrons are no t, however, sufficient to create observable soft X-ray emission. The t ype III emission in this case could be formed either at low coronal he ights or in local thick regions in the large loops, high in the corona .