Speaker
Simon Candelaresi
Description
The roles of magnetic helicity and magnetic helicity fluxes
in astrophysical objects are investigated using various
models and field configurations. Their roles in dynamo
theory are confirmed through magnetohydrodynamic simulations
both within the framework of mean-field theory and in direct
numerical simulations.
The constraint of magnetic helicity conservation in a
periodic system at high magnetic Reynolds numbers is
analyzed for setups of three magnetic flux rings which can
be interlocked. The linking is able to hinder the magnetic
field to decay only if the linking implies magnetic
helicity. If the magnetic field is not helical the decay
shows the same behavior irrespective of the actual linking
of the rings which supports the assumption that only the
magnetic helicity is the decisive topological quantity in
magnetic relaxation.
The regime of high magnetic Reynolds numbers is analyzed by
using a one-dimensional mean-field model for a helically
forced dynamo. A wind with linear profile is imposed such
that magnetic helicity can be advected to one of the domain
boundaries. It is shown that with vacuum boundary conditions
helicity can be shed from of the domain, which alleviates
the quenching at high magnetic Reynolds numbers.
Additionally the same boundary is closed for a different
setup where a diffusive flux is allowed at the midplane of
the system. This is shown to also reduce the quenching
mechanism and to allow for dynamo action at large magnetic
Reynolds numbers.
The influence of the gauge on magnetic helicity transport
and fluxes is explored in the Weyl gauge, the resistive
gauge and the pseudo-Lorenz gauge as well as a newly
introduced advecto-resistive gauge. In the first three
gauges spatially averaged fluxes are analyzed and compared
with the one-dimensional mean-field model. The alleviation
of the quenching is independent of the gauge as it was
expected since it is a physical effect. In the
advecto-resistive gauge magnetic helicity density evolves
like a passive scalar in the kinematic regime owing it to
the advective nature of the gauge. In the dynamical regime
magnetic helicity is advected into length scales of the
turbulent eddies.