We study the statistical relationship between optical flares and type III radio bursts, using modern and extensive computer files. Results emerge along two main lines, concerning the physical mechanism of ejection of energetic particles, and the magnetic field geometry respectively.

First, we find that type III probability of occurrence increases strongly with the brightness of a flare and its proximity to a sunspot, and with accompanying prominence activity. This suggests that Bornmann's class I and III events correspond to distinct physical phenomena, particle acceleration and magnetic expansion respectively, both working simultaneously in class II events, which are the most favorable to the ejection of energetic particles out of flaring sites.