Solar Magnetism and Dynamo
Jan Stenflo
Abstract
Solar and stellar activity have their origin in the complex interaction between magnetic fields and turbulent motions in a highly conducting and rotating medium. Dynamo action is a consequence of the properties of magnetoconvection. Here we present an overview of magnetoconvection as manifested at the solar surface. The scale spectrum extends over a dynamic range that spans approximately 8 orders of magntidue. At one end of the spectrum we have the global scales that are strongly affected by differential rotation, Hale's polarity law, and Joy's law for the tilt of bipolar regions. At the other end, at scales around 10 m, where the magnetic Reynold's number becomes unity, diffusion and magnetic energy dissipation takes over. It is the scale of reconnection processes. The spatial resolution frontier of solar instruments lies right in the middle of this scale spectrum, at around 100 km, the stratification scale height, where we have a transition between optically thick and thin structures. This is a critical scale for the convective collapse mechanism.
The flux emergence rate increases dramatically when we go to smaller scales, replenishing the surface flux on a time scale much less than the solar rotation period. How this happens remains mysterious. New magnetic flux is supposed to emerge in bipolar form, as pairs of opposite polarities, but the observation of the emergence of small-scale bipoles turns out to be elusive. Instead one frequently observes unipolar flux fragments appear or fade away. Another fundamental issue is how all the flux is removed from the photosphere. Statistically it has to occur on the time scale of the emergence rate. While submergence (reprocessing in the convection zone) and uprooting (removal as CMEs) may play a role, it seems that the underlying process has to be reconnection. However, this process is only viable if it occurs on spatial scales that are much smaller than what can be resolved in the foreseeable future. The unresolved scales can only be diagnosed by indirect methods. For this type of magnetic diagnostic the Hanle effect has now become our main tool.