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Magnetic Fluctuations in Fast Dynamos

Published online by Cambridge University Press:  11 May 2010

R. Kulsrud
Affiliation:
Princeton Plasma Physics Lab Princeton, NJ 08543 USA
S. Anderson
Affiliation:
Princeton Plasma Physics Lab Princeton, NJ 08543 USA
M. R. E. Proctor
Affiliation:
University of Cambridge
P. C. Matthews
Affiliation:
University of Cambridge
A. M. Rucklidge
Affiliation:
University of Cambridge
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Summary

One theory for the origin of the galactic field is that it grows from a very weak seed field by fast dynamo action associated with turbulent motions. However, the dynamo also amplifies small scale fields faster than the large scale. In this paper we calculate the time evolution of the spectrum of small scale fields. We show that the magnetic turbulence reaches the resistive scale in a reasonably short time where some damping occurs. We also show that the damping is not strong enough to stop the exponential growth of the random turbulence which grows to equipartition with the turbulent power in a time short compared to the dynamo growth time for the large scale fields. Our conclusion is that a dynamo origin from a weak seed field is not plausible.

A hotly debated topic is the origin of the large scale galactic magnetic field. Originally, it was supposed by Fermi and others that the field had a primordial origin and was maintained against Ohmic decay by the large inductance of the galactic disk. (The time scale for Ohmic decay by ordinary Spitzer resistivity is extremely long, of order 1026 years.) However, there have been several objections to the primordial theory (Parker 1979). One objection is that turbulent resistivity is sufficiently large to destroy the field in a Hubble time. A second objection is that if it is not destroyed by turbulent resistivity, it can escape from the galactic disc by ambipolar diffusion. Probably the strongest objection has been that there seems no known way to produce a magnetic field in the early universe on a large enough scale and of sufficient strength to provide a primordial origin.

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Publisher: Cambridge University Press
Print publication year: 1994

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