Active regions. II: Mount Wilson 16997: A small spot with big flares

Abstract

We describe the great activity associated with Mount Wilson 16997, a single αp spot without plage that produced a number of large flares and developed into a substantial center. The activity was signalled by the emergence of f polarity and weak plage ahead of the spot on 24 September 1968 (although the region was already seen at the limb at the 22 September 1968 eclipse as a active center). A number of explosive flares with X-rays on the 25th were climaxed by a large flare a 0036 UT 26 September; the latter was associated with the splitting of the old spot and the emergence of a new one. The polarity axis rotated around to a normal configuration by the 29th, but a large flare occurred on that day, too. Several of the flares show interesting details such as the return of spray material to the surface at the boundary of the region. The behavior of this region shows that: (1) Large flares may come from small, round spots; (2) The magnetic field rotation due to the action of the Hale-Nicholson force is a possible source of flare energy; and (3) Appearance and disappearance of spots is clearly connected with flare activity.

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