Abstract:
We have taken the case of a circular H-alpha filament observed on May 9, 1979 erupting into a double-ribbon flare associated with a nonspot region. The plage motions are responsible for the filament reorientation and, here as a special case, wherein the filament attains a clear circular shape before the onset of a flare. We conclude that the change in the orientation of the H-alpha filament marks the instability giving rise to the flare.
