**AAS 197, January 2001**

*Session 59. The Interstellar Medium*

Oral, Tuesday, January 9, 2001, 10:30am-12:00noon, Royal Palm 3/4
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## [59.06] MHD Turbulent Dynamo in Astrophysics: Theory and Numerical Simulation

*Hongsong Chou (Harvard)*

Astrophysical dynamo theory studies the generation of
large-scale magnetic fields from small-scale turbulence in
many celestial objects. Mean-Field Electrodynamics, a
kinematic dynamo theory, has been widely applied to solar
dynamo, geodynamo contexts. But criticisms on different
models of dynamo theory have existed ever since the early
work by Parker(1955). We identify the recent questions on
dynamo theory into the following three categories: (1) the
back reaction of magnetic field on velocity field; (2)
magnetic helicity constraints on dynamo \alpha- and
\beta-effects; and (3) small-scale magnetic field in large
magnetic Prandtl number systems. In this dissertation
presentation, I will discuss all these three questions, and
give our answers to each of them. To tackle the back
reaction problem, we have to treat velocity field and
magnetic field on equal footing. By assuming small
de-correlation times of both the velocity field and the
magnetic field, we developed a model that solves the
momentum equation and induction equation altogether. By
taking the back reaction of magnetic field into account, we
provide a new derivation of the dynamo \alpha-effect. We
also clarified the confusions over the constraints of the
dynamics of magnetic helicity on dynamo effect. By
considering both the time-dependent term and the boundary
effect term in the equation of magnetic helicity, we are
able to show, both analytically and numerically, that dynamo
effect may not be quenched by large magnetic Reynolds
numbers in astrphysical systems. Another open question on
dynamo process is the concern that fast growing small-scale
magnetic field would swamp velocity field entirely so that
dynamo effect will not happen in systems of large magnetic
Prandtl number, for example, the Galaxy. We have studied
this problem numerically. We show that in driven turbulence,
for at least moderate magnetic Prandtl number, magnetic
field near the outer scale of the turbulence will grow even
though the velocity field near the viscous cut-off scale is
strongly suppressed by magnetic field at small scales.

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