Sunspot formation: theory, simulations and observations

Scientific recording of sunspots started with Galileo in 1609. Since Hale (1908) we know that sunspots are strong concentrations of magnetic field of up to 4000 gauss. They could be formed by subsurface magnetic flux tubes piercing the surface. Meanwhile, numerical simulations by many different groups suggest that strong magnetic fields could be generated in the bulk of the convection zone. This would mean that sufficiently strong magnetic fields may be generated not far from the surface. However, at the surface the magnetic field appears to be strongly concentrated into only a few isolated spots – in stark contrast to the more diffuse magnetic field beneath the surface. This is still a mystery.

To set the scene, the meeting will start with a session on global properties of the Sun’s magnetic field. Next, we need to discuss observational constraints and results from global helioseismology. This will lead to the consideration of what causes flux emergence from an observational and theoretical point of view. There are conflicting ideas about deep seated versus distributed magnetic field generation in the Sun that need to be highlighted. Next, we focus on local magnetic flux concentrations. There are new theoretical results that can potentially be checked using local helioseismology. One of the particular characteristics of sunspots are surface effects relevant to spot formation. There are new ideas about enhanced cooling mechanisms relevant to spot formation. The region around sunspots, i.e., the so-called penumbra, is another important example that demonstrates the interaction of magnetic fields with convection. The importance of magnetic effects is also evidenced by the occurrence of umbral dots and light bridges. Finally, we open the forum to discuss opposing views on sunspot formation and try a reach a coherent picture of solar activity.