Long-term persistence of solar active longitudes and its implications for the solar dynamo theory

We present an overview of the observational results related to the existence of long-lived sunspot active longitudes. These are affected by the solar differential rotation. The existence of such migrating active longitudes imposes an important constraint on the dynamo theory. We review different approaches to model non-axisymmetry in solar dynamo models and find that, in principle, plausible mechanisms exist to reproduce the observed non-axisymmetry. The most favorable interpretation is suggested by the 'stroboscopic effect', where a quasi-rigidly rotating non-axisymmetric mean field can produce seemingly migrating active longitudes in sunspots. Other scenarios are less favorable but cannot yet be excluded.