We have evaluated the shear angle of the neutral line of the non-potential magnetic field for one or two days prior to and after the flare event for 10 cases. We have used the He filament positions to evaluate the shear in the neutral line. We find from the samples we have studied that it is the change in the shear that occurs a day prior to the flare that can lead to the event. This change can be in either direction, i.e., it can be a large increase from a small value or a decrease from a large initial value. Thus it is the change in the shear angle that seems to be a deciding criterion for a flare to occur and not a large value for the shear angle itself. We have one instance where there was no significant change in the shear angle over a period of a few days and this region, although similar to other active regions studied, did not produce any flare activity.
We have studied the evolution of the double-ribbon, spotless flare of 21 February, 1992, using Kodaikanal He and Kfl observations. The analysis of the data shows that the He filament underwent a large change in shear prior to the day of the onset of the flare. We find considerable rotation of the plage region before the emergence of a small magnetic pore. It is concluded that shear plays an important role in the triggering of a spotless flare.
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