Abstract

An impulsive burst which occurred on 1992 October 27 was observed simultaneously with the hard X-ray telescope on board the Yohkoh satellite and the Nobeyama Radioheliograph at 17 GHz. The hard X-ray images show a double-source structure during the main phase. One footpoint, A, shows a nonthermal spectrum while another, B, shows a better fit to the X-rays from extremely hot thermal electrons with about 80 million degrees, flowing into the chromosphere. The loop top also shows a better fit to a quasi-thermal spectrum of 75 million degrees at the main peak. On the other hand, the radio images are a larger single source covering the X-ray source, and are highly polarized in the L-sense. The larger single image is mainly ascribed to the beamwidth, and the single polarization may be due to 'limiting polarization'. Consequently, the 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 extremely 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 thermal gyro emission at the outer layers with small 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 nonthermal electrons in footpoint A.