Microwave preflare enhancement and depletion in long duration events Original Research Article

Combination of weak microwave enhancement and depletion has been found in the pre-flare phases of three long-duration solar flares observed with the Nobeyama radioheliograph (NRH) at 17 GHz. This starts about two hours prior to subsequent main flare. Emission measure and temperature indicate that the enhancement and the depletion are caused by a relatively cool plasma motion.