Licentiate thesis: Mean-field theory in magneto-hydrodynamics and the generation of magnetic flux concentrations
by
Koen Kemel
→
Europe/Stockholm
FB53
FB53
Description
The generally accepted view that a sunspot originates from a flux tube, emerging through the solar photosphere,
after travelling coherently across the underlying convection zone, is being challenged.
While the properties of an emerging flux tube appear to reproduce the observations, it is not clear if other mechanisms are possible as well.
Furthermore, questions remain about the formation and coherent rise of such structures,which are currently strongly debated.
One of the objectives of the present work is to verify that it is possible to create coherent magnetic structures starting from a homogeneous background.
The driving idea is to understand if and under what set of conditions, sunspots can form as near surface phenomena, as opposed to rising tachoclinic magnetic flux tubes.
In direct numerical simulations we found a reduction of the effective turbulent pressure in the presence of magnetic fields and stratification, as anticipated by earlier analytical calculations.
A subsequent mean-field analysis incorporating this reduction suggests that this setup can become unstable and form magnetic flux concentrations under certain conditions.
Mean-field models are however degenerate in the sense that different closure models can be used, possibly giving different results.
In the case above, the effect of turbulence was captured by modelling the Reynolds stress tensor.
Rather than looking for higher order closure, one can also determine coefficients in a closure model from experiments or simulations.
The test-field method is an example of the latter and has been successfully used for modelling the electromotive force in dynamo problems.
Closure models are approximations within a certain parameter regime (for example only for low magnetic Reynolds number) and as such suffer from a loss in generality. In this context we also investigate the validity of the astrophysical dynamo description for conditions more relevant to laboratory experiments, such as the reversed field pinch in plasma confinement experiments.
