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Effect of Fractional Kinetic Helicity on Turbulent Magnetic Dynamo Spectra
Magnetic field amplification in astrophysics ultimately requires an
understanding of magnetohydrodynamic turbulence. Kinetic helicity has long been
known to be important for large scale field growth in forced MHD turbulence,
and has been recently demonstrated numerically to be asymptotically consistent
with slow mean field dynamo action in a periodic box. Here we show numerically
that the magnetic spectrum at and below the forcing scale is also strongly
influenced by kinetic helicity. We identify a critical value,
above which the magnetic spectrum develops maxima at wavenumber scale
{\it and} at the forcing scale, For the field peaks only at the
resistive scale. Kinetic helicity may thus be important not only for generating
a large scale field, but also for establishing observed peaks in magnetic
spectra at the forcing scale. The turbulent Galactic disk provides an example
where both large scale ( supernova forcing scale) fields and small scale
( forcing scale, with peak at forcing scale) fields are observed. We
discuss this, and the potential application to the protogalaxy, but also
emphasize the limitations in applying our results to these systems.Comment: version accepted to ApJL, 10 pages, 3 fig
Implications of mean field accretion disc theory for vorticity and magnetic field growth
In addition to the scalar Shakura-Sunyaev turbulent viscosity
transport term used in simple analytic accretion disc modeling, a pseudoscalar
transport term also arises. The essence of this term can be captured even in
simple models for which vertical averaging is interpreted as integration over a
half-thickness and one separately studies each hemisphere. The additional term
highlights a complementarity between mean field magnetic dynamo theory and
accretion disc theory treated as a mean field theory. Such pseudoscalar terms
have been studied, and can lead to large scale magnetic field and vorticity
growth. Here it is shown that vorticity can grow even in the simplest azimuthal
and half-height integrated disc model, for which mean quantities depend only on
radius. The simplest vorticity growth solutions seem to have scales and vortex
survival times consistent those required for facilitating planet formation.
Also it is shown that when the magnetic back-reaction is included to lowest
order, the pseudoscalar driving the magnetic field growth and that driving the
vorticity growth will behave differently with respect to shearing and
non-shearing flows: the former can reverse sign in the two cases, while the
latter will have the same sign.Comment: 17 Pages LaTex, revised versio
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