EVOLUTION OF A FLARE SOURCE INFERRED FROM HARD X-RAY AND RADIO OBSERVATIONS - SOLAR-BURST ON 1992 OCTOBER 27

An impulsive burst which occurred on 1992 October 27 was observed simu ltaneously with the hard X-ray telescope on board the Yohkoh satellite and the Nobeyama Radioheliograph at 17 GHz. The hard X-ray images sho w a double-source structure during the main phase. One footpoint, A, s hows a nonthermal spectrum, while another, B, shows a better fit to th e X-rays from extremely hot thermal electrons with about 80 million de grees, flowing into the chromosphere. The loop top also shows a better fit to a quasi-thermal spectrum of 75 million degrees at the main pea k. On the other hand, the radio images are a larger single source cove ring the X-ray source, and are highly polarized in the L-sense. The la rger single image is mainly ascribed to the beamwidth, and the single polarization may be due to ''limiting polarization''. Consequently, th e radio source at 17 GHz may be cospatial with the X-ray source around the loop top. The radio emission at 17 GHz and a part of the emission at 9.4 GHz are attributed to the thermal gyro emissions from the extr emely hot thermal electrons emitting the X-rays. The rest of the radio emission at 9.4 GHz and the emission at 3.75 GHz are ascribed to ther mal gyro emission at the outer layers with smaller magnetic fields and lower electron temperatures. The intense radio emission at 35 GHz at the main peak is ascribed to gyro-synchrotron emission from the nonthe rmal electrons in footpoint A.