Theory of stellar magnetic activity: A review

The unified sight of solar and stellar activity has revealed a worthwhile concept under several aspects, gaining in the last decade the increasing favour of observers and theorists, and the term solar‐stellar connection has recently been introduced to point out the complementarity of solar and stellar observations in the background of the basic role of the magnetic field. The great development of ground and space stellar activity observations suggests a much wider scenario than it were possible to imagine even a few years ago, and stimulates theoretical work, most of which is in the framework of the α–ω dynamo theory. Dynamo models of stellar activity in the main sequence, although subjected to different assumptions, do converge in predicting that activity should increase with the increasing angular velocity of rotation ω and colour index ( B–V ), in agreement with almost all existing observational evidence. However, even if linear dynamo theory seems to have captured the essentials of the mechanism of stellar activity, the complexity of stellar activity phenomenology and operation modes suggests that the future is in the non‐linear approach. Finally, observational and theoretical uncertainties and difficulties, which affect the present status of dynamo theory, are briefly discussed.