Towards a new way to understand sunspot formation: the negative effective magnetic pressure instability
by
Koen Kemel
→
Europe/Stockholm
FA32
FA32
Description
The main goal of this thesis is to develop a new approach to understanding the origin of active regions in the sun. This approach is based on mean-field theory, which is a well-developed concept used to explain the origin of the sun's magnetic field on scales much larger than the energy-carrying scales of the turbulence. This approach has been applied with great success to the induction equation, and is still being developed further in this context, but here we focus mainly on magnetic effects in the momentum equation. On the one hand, these effects determine the magnetic feedback on turbulent transport coefficients such as the alpha effect, but they can also lead to new effects, including a large-scale instability operating on scales much larger than that of the turbulent eddies. Although this instability was predicted in the early work of Kleeorin and collaborators over 20 years ago, its validity could be challenged on the grounds that the mean-field approach might not apply to the parameter regime of interest. The main breakthrough presented in this thesis is the numerical demonstration that this instability really does exist and that its mean-field theory yields predictions that are well reproduced by numerical simulations. This instability is distinct from the dynamo instability, because it merely concentrates flux rather than amplifying it. Furthermore, it requires strong density stratification and large scale separation, which are all reasons why this instability has been overlooked in previous work. Estimates suggest that this instability should also operate in the sun. It might then be responsible for the development of active regions in the surface layers of the sun and would thereby lend support to an alternative scenario in which solar activity in mainly a surface phenomenon and not tied to the bottom of the solar convection zone, as is generally believed.
