Solar Surface and Atmospheric Dynamics

Abstract

Various aspects of the magnetism of the quiet sun are reviewed. The suggestion that a small scale dynamo acting at granular scales generates what we call the quiet sun fields is studied in some detail. Although dynamo action has been proved numerically, it is argued that current simulations are still far from achieving the complexity that might be present on the Sun. We based this statement not so much on the low magnetic Reynolds numbers used in the simulations but, above all, in the smallness of the kinetic Reynolds numbers employed by them. It is argued that the low magnetic Prandtl number at the solar surface may pose unexpected problems for the identification of the observed internetwork fields with dynamo action at granular scales. Some form of turbulent dynamo at bigger (and deeper) scales is favored. The comparison between the internetwork fields observed by Hinode and the magnetism inferred from Hanle measurements are converging towards a similar description. They are both described as randomly oriented, largely transverse fields in the several hecto-Gauss range. These similarities are ever making more natural to assume that they are the same. However, and because of the large voids of magnetic flux observed in the spatial distribution of the internetwork fields, it is argued that they are not likely to be generated by dynamo action in the intergranular lanes. It is concluded that if a dynamo is acting at granular scales, the end product might have not been observed yet at current spatial resolutions and sensitivities with the Zeeman effect. Thus an effort to increase these resolutions and polarimetric sensitivities must be made. New ground- and space-based telescopes are needed. The opportunity offered by the Solar Orbiter mission to observe the Quiet Sun dynamics at the poles is seen as one of the most important tests for confirming the existence, or otherwise, of a granularly driven surface dynamo.