Speaker
Axel Brandenburg
(Nordita)
Description
There was a time when primordial magnetic fields posed a serious
contender to explaining the origin of magnetic fields in galaxies
and on larger scales. This has changed drastically during the past
three decades, and now the dynamo origin of galactic magnetic fields is
unchallenged. Nevertheless, primordial magnetic fields might still be an
attractive possibility to explaining magnetic fields between clusters
of galaxies, which are difficult to get magnetized by astrophysical
mechanisms such as outflows from active galactic nuclei. Primordial
magnetic fields generated during the electroweak phase transition,
for example would decay during much of their subsequent evolution,
but helicity slows down the decay by inverse cascading the field to
larger scales. Dynamo-generated magnetic fields, on the other hand,
also tend to be helical, if the dynamo operates in the presence of
rotation and stratification. In my talk, I will focus on the evolution
of primordial magnetic fields using numerical simulations.
In the presence of magnetic helicity, inverse transfer from small to
large scales is well known in magnetohydrodynamic (MHD) turbulence and has
applications in astrophysics, cosmology, and fusion plasmas.
Using high resolution direct numerical simulations of magnetically
dominated self-similarly decaying MHD turbulence, we report
a similar inverse transfer even in the absence of magnetic helicity.
We compute for the first time spectral energy transfer rates to show
that this inverse transfer is about half as strong as with helicity, but
in both cases the magnetic gain at large scales results from velocity
at similar scales interacting with smaller-scale magnetic fields.
This suggests that both inverse transfers are a consequence of
a universal mechanisms for magnetically dominated turbulence.
Possible explanations include inverse cascading of the mean squared vector
potential associated with local near two-dimensionality and the
shallower k^2 subinertial range spectrum of kinetic energy
forcing the magnetic field with a k^4 subinertial range
to attain larger-scale coherence.
The inertial range shows a clear k^-2 spectrum and is
the first example of fully isotropic magnetically dominated
MHD turbulence exhibiting weak turbulence scaling.
Primary author
Axel Brandenburg
(Nordita)
Co-authors
Dr
Alexander Tevzadze
(Tbilisi State University)
Dr
Tina Kahniashvili
(Carnegie Mellon University)
