Magnetic helicity fluxes from triple correlators and dynamical models of large-scale dynamos

by Kandaswamy Subramanian (IUCAA)

Tuesday 1 Nov 2022, 15:00 → 16:00 Europe/Stockholm

Albano 3: 6231 - Giga (8 seats) (Albano Building 3)

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

Fluxes of the magnetic helicity density play an important role in large-scale turbulent dynamos, allowing the growth of large-scale magnetic fields while overcoming catastrophic quenching. We show, analytically, and using a closure approximation, how several important types of magnetic helicity fluxes can arise from terms involving triple correlators of fluctuating fields in the helicity density evolution equation. First, we show how a diffusive helicity flux, till now only measured in simulations, arises from the triple correlation term.  This is accompanied by what we refer to as a `random advective flux', which predominantly transports magnetic helicity along the gradients of the random fields. We also find that a new helicity flux contribution, in some aspects similar to that first proposed by Vishniac, can arise from the triple correlator. This contribution depends on the gradients of the random magnetic and kinetic energies along the large-scale vorticity, and thus arises in any rotating, stratified system, even if the turbulence is predominantly nonhelical. It can source a large-scale dynamo by itself while spatially transporting magnetic helicity within the system. The talk ends with an example which tells us to be cautious in using the equation for magnetic helicity conservation to also model the current helicity evolution and the magnetic alpha effect.

(Based largely on https://arxiv.org/abs/2209.14810 by Kishore Gopalakrishnan and Kandaswamy Subramanian)

Subramanian_2022-11-01.mp4