Stellar activity cycles: observing the dynamo?

The enormous complexity of the atmospheric structure observed on the Sun makes it very difficult to compare the Sun with "solar-type stars". Clearly, we need to identify parameters that can be observed on the Sun as well as on other stars which can be interpreted unambiguously. The most widely accepted dynamo signature is the presence of an activity cycle, well documented for the Sun and for main-sequence stars due to the Mount Wilson Ca II H&K project. Only recently have we detected spatial information, differential rotation and possibly meridional flows on other stars and thereby adding another constraint for its interpretation within a dynamo theory. Again, the picture is not complete yet, despite that there is just a single main ingredient that acts as the driving mechanism for activity in all atmospheric layers and the convective envelope of a solar-type star: the dynamo-related magnetic field. I stress the importance of mapping stellar surfaces as fingerprints of the underlying dynamo action over long periods of time. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.