109 , 2015-03-20 , 東海大学理学部 , Tokai University. School of Science , 東海大学出版部
In order to understand the conditions for triggering the particle acceleration in solar flares, we investigated the activities before the impulsive phase of solar flares using hard X-ray, soft X-ray and microwave data. First, we classified that into `preflare' or `precursor' on the basis of the X-ray time profiles, and then judged using the hard X-ray spectrums whether the component of the emission from accelerated electrons (non-thermal component) exists. As a result, in the `precursor' events, we found no correlation between the existence of the non-thermal component, and the soft X-ray flux, which is a good indicator of total released energy. Since the feature is shown in normal flares, a `precursor' event might be just a tiny flare. On the other hand, we confirmed that most of the `preflare' events don't contain the non-thermal component. The observing fact indicates that the particle acceleration is suppressed during a `preflare' event.