Speaker
Alexander Hubbard
(Nordita)
Description
Magnetic helicity has risen to be a major player in dynamo
theory, with the helicity of the small-scale field being
linked to the dynamo saturation process for the large-scale
field. It is a nearly conserved quantity, which allows its
evolution equation to be written in terms of production and
flux terms. The flux term can be decomposed in a variety of
fashions. One particular contribution that has been expected
to play a significant role in dynamos in the presence of
mean shear was isolated by Vishniac & Cho. Magnetic helicity
fluxes are explicitly gauge dependent however, and the
correlations that have come to be called the Vishniac-Cho
flux were determined in the Coulomb gauge, which turns out
to be fraught with complications in shearing systems. While
the fluxes of small-scale helicity are explicitly gauge
dependent, their divergences can be gauge independent. We
use this property to investigate magnetic helicity fluxes of
the small-scale field through direct numerical simulations
in a shearing-box system and find that in a numerically
usable gauge the divergence of the small-scale helicity flux
vanishes, while the divergence of the Vishniac-Cho flux
remains finite. We attribute this seeming contradiction to
the existence of horizontal fluxes of small-scale magnetic
helicity with finite divergences.
