Effects of Magnetic Helicity and its Transport on Nonlinear Dynamos

by Igor Rogachevskii (Ben-Gurion University of Negev)

Tuesday 4 Apr 2023, 15:00 → 16:00 Europe/Stockholm

Albano 3: 5230 - Xenon (12 seats) (Albano Building 3)

https://stockholmuniversity.zoom.us/j/940229961

The combined action of helical motions of plasma (the kinetic alpha effect) and non-uniform (differential) rotation is a key dynamo mechanism of solar and galactic large-scale magnetic fields. The dynamics of magnetic helicity of small-scale fields is a crucial mechanism in nonlinear dynamo saturation where turbulent magnetic helicity fluxes allow to avoid catastrophic quenching of the alpha effect. The convective zone of the Sun and solar-like stars as well as galactic discs are the source for production of turbulent magnetic helicity fluxes. In the framework of the mean-field approach and the spectral tau approach, turbulent magnetic helicity fluxes are derived using the Coulomb gauge in density-stratified turbulence. Turbulent magnetic helicity fluxes include non-gradient and gradient contributions. The non-gradient magnetic helicity flux is proportional to a nonlinear effective velocity (which vanishes in the absence of the density stratification) multiplied by small-scale magnetic helicity, while the gradient contribution describes turbulent magnetic diffusion of the small-scale magnetic helicity. In addition, turbulent magnetic helicity fluxes contain source terms proportional to the kinetic alpha effect or its gradients, and also contributions caused by the large-scale shear (differential rotation). Various DNS related to turbulent magnetic helicity fluxes and their effect on the value of the saturated mean magnetic field are discussed.

Rogachevskii_2023-04-04.mp4