Physics
Scientific paper
Apr 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aps..apr.v9003v&link_type=abstract
American Physical Society, April Meeting, April 28 - May 1, 2001 Washington, DC Bulletin of the American Physical Society, Vol.
Physics
Scientific paper
We rederive two-scale, mean-field dynamo theory, explicitly conserving magnetic helicity, and give a simple physical picture of the dynamo process. Fluid helicity is unimportant. Instead, field generation is driven by asymmetries in the second derivative of the velocity correlation function. The prospects for driving a fast dynamo without global shear, analogous to the `alpha squared' dynamo of classical theory, are uncertain, and limited to force free fields. However, there is an efficient analog to the α-ω dynamo. Systems whose turbulence is driven by an anisotropic local instability in a shearing flow, like real stars and accretion disks, will typically drive the generation of strong large scale magnetic fields. Such dynamos include a persistent, spatially coherent vertical magnetic helicity current with the same sign as the radial gradient of the rotation rate, positive for an accretion disk and negative for the Sun. We find that random magnetic helicity currents in a turbulent medium will generate an equipartition, disordered field with a declining long wavelength tail to the power spectrum. This makes calculations of the galactic `seed' field largely irrelevant.
Cho Jungyeon
Vishniac Ethan
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