Observations of High Brightness Temperatures in Moving Type IV Solar Radio Bursts

Abstract

It has generally been accepted that moving type IV bursts are generated as synchrotron radiation from energetic electrons high in the solar corona (Boischot and Denisse 1957). At 80 MHz the peak brightness temperature is usually ~ 108 K and the radiation becomes highly circularly polarized as the burst decays. This has led several authors (Kai 1969; Dulk 1970, 1973; Schmahl 1972; Robinson 1974, 1977; Nelson 1977) to the conclusion that the radiation comes from mildly relativistic (~ 100 keV) electrons and occurs at low harmonics of the gyro-frequency (gyro-synchrotron radiation). We present evidence of moving type IV bursts at 43, 80 and 160 MHz with brightness temperatures of ~ 109 K, and one at 43 MHz as high as 1010 K. The number (~ 1033) of energetic (≥ 1 MeV) electrons which is required in order to explain such high brightness temperatures by incoherent gyro-synchrotron emission is very large and near the upper limit for the number of fast electrons accelerated in the second phase of a solar flare. If amplification takes place a smaller number of electrons with energies ~ 100 keV would be required.