**AAS 197, January 2001**

*Session 31. Solar System and The Sun*

Oral, Monday, January 8, 2001, 1:30-3:00pm, Sunrise
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## [31.04] Magnetic Helicity Conservation and Astrophysical Dynamos

*E. Vishniac (Johns Hopkins U.), J. Cho (U. Wisconsin)*

We construct a magnetic helicity conserving dynamo theory
which incorporates a calculated magnetic helicity current.
In this model the fluid helicity plays a small role in large
scale magnetic field generation. Instead, the dynamo process
is dominated by a new quantity, derived from asymmetries in
the second derivative of the velocity correlation function,
closely related to the `twist and fold' dynamo model. The
turbulent damping term is, as expected, almost unchanged.
Numerical simulations with a spatially constant fluid
helicity and vanishing resistivity are not expected to
generate large scale fields in equipartition with the
turbulent energy density. In fact, there seems to be little
prospect for driving a fast dynamo in a closed box
containing homogeneous turbulence. On the other hand, there
is an efficient analog to the \alpha-\Omega dynamo.
Systems whose turbulence is driven by some anisotropic local
instability in shearing flow, like real stars and accretion
disks, and some computer simulations, may successfully drive
the generation of strong large scale magnetic fields,
provided that \partial_{r}\Omega\langle \partial_{\}theta
v_{z}\omega_{\}theta\rangle>0. We show that this criterion is
usually satisfied. We comment on the role of random magnetic
helicity currents in storing turbulent energy in a
disordered magnetic field, which will generate an
equipartition, disordered field in a turbulent medium, and
also a declining long wavelength tail to the power spectrum.
As a result, calculations of the galactic `seed' field are
largely irrelevant.

The author(s) of this abstract have provided an email address
for comments about the abstract:
ethan@pha.jhu.edu

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