Impulsive acceleration during gradual type solar radio flares

Gradual rise and fall type solar radio flares recorded at 37 GHz (8 mm wavelength) are analysed and compared with simultaneous soft and hard X-ray events. Emission measures and plasma temperatures were calculated from the GOES soft X-ray data, and optically thin thermal bremsstrahlung flux at 37 GHz was calculated assuming the same emitting volumes. The main emission mechanism behind the millimeter wave radio flares was determined to be thermal bremsstrahlung although many of the flares showed impulsive, non-thermal features. The radio flares were compared with simultaneous BATSE hard X-ray events, but significant temporal correlation was not found. It is suggested that there might be two different types of gradual radio flares, connected to gradual or more impulsive hard X-ray events. Another explanation for the observed two types would be different viewing angles to the emitting regions.