20150515

05月15日 (金) 16:00-17:00 大セミナー室（国立天文台三鷹）

16:00-17:00, Friday, May 15, Large Seminar Room (NAOJ, Mitaka)

Speaker: Karl-Heinz Raedler (Astrophysical Institute Potsdam)

Title: "Mean-field dynamo theory - some recent developments"

Abstract:

The talk elucidates the basic ideas of electrodynamics and magnetohydro-dynamics of mean fields in turbulently moving conducting fluids. It is stressed that the connection of the mean electromotive force with the mean magnetic field and its first spatial derivatives is in general neither local nor instantaneous. Quite a few claims concerning pretended failures of the mean-field concept result from ignoring this aspect. In addition to the mean-field dynamo mechanisms of $\alpha^2$ and $\alpha$-$\Omega$ type several others are considered. Much progress in mean-field electrodynamics and magnetohydrodynamics results from the test-field method for calculating the coefficients that determine the connection of the mean electromotive force with the mean magnetic field. As an important example the memory effect in homogeneous isotropic turbulence is explained. In magnetohydrodynamic turbulence there is the possibility of a mean electromotive force that is primarily independent of the mean magnetic field and labeled as Yoshizawa effect. Despite of many efforts there is so far no convincing comprehensive theory of $\alpha$ quenching, that is, the reduction of the $\alpha$ effect with growing mean magnetic field, and of the saturation of mean-field dynamos. Steps toward such a theory are explained.