Flux tubes, surface magnetism, and the solar dynamo: constraints and open problems

The flux-tube paradigm has proven to be a remarkably useful tool to understand the connection between the dynamo process in the solar interior and its observable manifestations at the surface. After a brief review of the justification of this approach and of its successes, we discuss in some detail its loose ends and the remaining open questions - and attempt to provide some tentative answers. This includes the origin of fields much stronger than the dynamical equipartition value, the structure of the stored magnetic flux (flux tubes versus a magnetic layer) and the importance of convective pumping, as well as processes connected with flux emergence and the subsequent development of the magnetic flux. It is argued that the observations of the surface field indicate a dynamical disconnection of the emerged flux from its roots in the deep convection zone. Based on the 'explosion' of magnetic flux tubes, a scenario and an illustrative model for the disconnection process are suggested. In the last part of the paper, the significance of observed properties of the surface magnetism for constraining solar dynamo models a critically discussed. It is shown that some properties of the butterfly diagram do neither confirm nor refute specific dynamo models. Furthermore, the observed phase relationship between the average toroidal and poloidal magnetic field components is shown to result from the tilt angle of bipolar magnetic regions, so that it imposes no constraint for models of the deep-seated solar dynamo.