On umbral flashes in different sunspot groups

The results of a statistical investigation of the occurrence of umbral flashes for 40 sunspot groups are reported for the period 1966-1983. The following characteristics were chosen for the analysis: (a) position on the solar disk; (b) group area; (c) sunspot areas; (d) maximum magnetic field strength of a sunspot; (e) modified Zuerich class; (f) sunspot age; (g) magnetic structure; and (h) flare activity of a group. The dependence of umbral flashes on magnetic structure of a sunspot is the most essential feature. The absence of umbral flashes in the umbrae of main sunspots perhaps may be used as one of the predictors of flare activity. (orig.)