Solar-type activity: Epochs of cycle formation

The diagram of indices of coronal and chromospheric activity reveals stars where solar-type appears and regular cycles are forming. Using new consideration of a relation between coronal activity and the rotation rate, together with new data on the ages of open clusters, we estimate the age of the young Sun corresponding to the epoch of formation of its cycle. The properties of the activity of this young Sun, with an age slightly older than one billion years, are briefly discussed. An analysis of available data on the long-term regular variability of late-type stars leads to the conclusion that durations of cycles associated with solartype activity increase with the deceleration of the stellar rotation; i.e., with age. New data on the magnetic fields of comparatively young G stars and changes in the roles of large-scale and local magnetic fields in the formation of the activity of the young Sun are discussed. Such studies can help identify observational tests aimed at identifying the conditions for the formation of cyclic activity on stars in the lower part of the main sequence, and test some predictions of dynamo theory.