Complexes of activity ("active longitudes") and their role in dynamo

Mantere Maarit, maarit.mantere@helsinki.fi, University of Helsinki, Finland

Abstract
The spectroscopic time series obtained for rapidly rotating late-type stars, analyzed with Doppler imaging methods, have revealed huge high-latitude temperature anomalies unevenly distributed over the stellar longitudes. The analysis of photometric time series have in many cases confirmed the spectroscopic results, and allowed the follow-up of the evolution of the light curve minima for longer time extents and with better time sampling. From these investigations it seems clear that in many stars the magnetic activity manifests itself as active longitude(s) that undergo apparently quite irregular phase jumps, sometimes called flip-flops, and drifts in the rotational frame of reference. In terms of dynamo theory, a change from the solar-type mostly axisymmetric and quite regular oscillatory solution to a non-axisymmetric non-stationary one occurs as the rotation rate is increasing. Some of the phenomena related to these active longitudes can rather straightforwardly be understood from mean-field dynamo theory. The change from axi- to nonaxisymmetric solutions with increasing rotation rate, for instance, is a prediction dating back to simple linear dynamo solutions, that has been later confirmed by more complex non-linear modelling. Oscillatory solutions emerge if e.g. anisotropic turbulent transport coefficients are allowed for. Severe challenges, however, remain. The spectropolarimetric observations, yielding also the surface magnetic field strength and configuration, very seldom show clear anticorrelation between the magnetic field strength and temperature, expected from the sunspot analogy. From theoretical point of view, it is, indeed, quite poorly understood how the dynamo-generated sub-surface fields transform themselves into sun- or starspots, and whether the same mechanisms are actually at play in all the objects under study. Here we review different scenarios, ranging from the rising flux tubes models, turbulent effects causing negative magnetic pressure leading into instability (NEMPI), and purely hydrodynamic instability leading into the formation of large vortices with significant temperature anomalies.