Kiyoto Shibasaki

Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano 384-1305, Japan

Outer layers of bright flaring loops are known to be active: hotter than the bright loops below and hard x-ray sources are formed. These phenomena are interpreted as the result of the reconnection above the loop, which convert magnetic energy into thermal and non-thermal energy. However, no direct evidences of reconnection have been detected so far. In this paper, activities in the outer layer of flaring loops are interpreted based on a different scenario. Coronal loops filled with hot and dense plasma (high beta) are unstable at their outer boundary where the curvatures are convex outward (bad curvature). Centrifugal force acting upward on hot plasma in the loop far exceeds gravity force. Due to the anchoring of the magnetic field at the photosphere and the magnetic shearing, only a small-scale interchange mode (balloon instability) can develop. It can develop into nonlinear phase and the outer surface of the coronal loop is broken, then the plasma in the loop is ejected across the field. This instability will play an important role in solar flares. Observational evidences supporting this scenario will be presented.