Abstract

Radio imaging observation of a solar flare on January 2, 1993, showed the formation isolated radio sources over the loop top of a flaring loop at several times during the flare development. In this paper, it is suggested that these isolated loop-top radio sources are magnetic islands or balloons produced by nonlinear ballooning instabilities due to the high β plasma in the loop. The plasma β is estimated by combining the total flux soft X-ray data, and radio brightness and circular polarization data. The upper boundaries of magnetic loops with high β plasma are unstable against the ballooning instability because of unfavorable curvature (convex outward). A new energy transfer scenario in flares is proposed based on this ballooning instability. It is also pointed out that the centrifugal acceleration caused by thermal motion in the curved magnetic line of force far exceeds the surface gravity and can sustain hot and dense plasma at the top of the elongated loop.