Turbulent transport coefficients in galactic dynamo simulations using
  singular value decomposition
  Coherent magnetic fields in disc galaxies are thought to be generated by a
large-scale (or mean-field) dynamo operating in their interstellar medium. A
key driver of mean magnetic field growth is the turbulent electromotive force
(EMF), which represents the influence of correlated small-scale (or
fluctuating) velocity and magnetic fields on the mean field. The EMF is usually
expressed as a linear expansion in the mean magnetic field and its derivatives,
with the dynamo tensors as expansion coefficients. Here, we adopt the singular
value decomposition (SVD) method to directly measure these turbulent transport
coefficients in a simulation of the turbulent interstellar medium that realizes
a large-scale dynamo. Specifically, the SVD is used to least-square fit the
time series data of the EMF with that of the mean field and its derivatives, to
determine these coefficients. We demonstrate that the spatial profiles of the
EMF reconstructed from the SVD coefficients match well with that taken directly
from the simulation. Also, as a direct test, we use the coefficients to
simulate a 1-D mean-field dynamo model and find an overall similarity in the
evolution of the mean magnetic field between the dynamo model and the direct
simulation. We also compare the results with those which arise using simple
regression and the ones obtained previously using the test-field (TF) method,
to find reasonable qualitative agreement. Overall, the SVD method provides an
effective post-processing tool to determine turbulent transport coefficients
from simulations.
