The outer layers of X-ray loops in solar flares are known to be more active: they are hotter than the lower loops and above the loop-top hard X-ray sources are formed there. These phenomena are interpreted as the result of the reconnection above the loop, which converts magnetic energy into thermal and nonthermal energy of plasma. However, little direct evidence for the reconnection has been presented so far. This paper interprets the activity in the outer layer of flaring loops on a different scenario. Coronal loops filled with hot and dense plasma (high beta) or with fast plasma flow, surrounded by the low-beta corona, are unstable at their outer boundary, where the curvature is convex outward and the density gradient is inward. The centrifugal force acting upward on the plasma in the loop can exceed that of gravity. This condition is favorable for localized interchange instability called "ballooning instability," and the plasma in the loop is ejected when the instability has developed into a nonlinear phase ("high-beta disruption"). This is a natural consequence of the high-beta (and/or the high-velocity) plasma confined in a curved coronal loop. The high-beta disruption has many elements common to solar flares. In this paper, importance of the high-beta plasma is stressed, which is a neglected part of the solar activity so far.
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