Solar and stellar activity - Overview
Sunspots, plages, flares - these phenomena are collectively known as solar activity. Along the solar-stellar analogy, stellar activity means phenomena that are very similar to those that we see on the Sun. The "moving spirit" of any kind of activity is the magnetic field. Sunspots occur, where the inner magnetic field lines break through the surface and locally slow down the emission of energy. Thus, formation, evolution, and motion of starspots, on the analogy of sunspots, give us information on the state of the stellar photosphere and on the distribution of the surface magnetic field, finally, the global magnetic field structure.
Solar magnetic cycle - the paradigm of stellar activity
Solar magnetic field shows a cyclic behaviour throughout the 11 year-long sunspot cycle. The changing magnetic field is described by dynamo models: the rotation takes effect on the convective envelope and the result is the surface differential rotation, when the angular rotation has an equator-to-pole gradient. Differential rotation acts on the primordial poloidal field, and starts to build up a toroidal one (see the figure). Under the surface, somewhere at the bottom of the convective zone, this spin up effect streches, thus reinforces the toroidal field until it becomes an unstable state, when the buoyant force begins to act on them. Thus, the toroidal magnetic field arranged in vast flux tubes begins to emerge and reaches the solar surface. At this time, a typical magnetic feature appears: a magnetic loop, with a cool bipolar spot pair at the footpoints. The large scale meridional convection works just the opposite: somehow (that we do not understand clearly) it regenerates the poloidal field, transporting the emerging flux towards the poles. Finally, this process results in a poloidal field, similar to that we had at the beginning, but the polarity is now the opposite in sign.