Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...295..245h&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 295, no. 1, p. 245-248
Astronomy and Astrophysics
Astrophysics
5
Approximation, Electromagnetic Fields, Pressure Distribution, Stress-Strain Relationships, Turbulent Diffusion, Convolution Integrals, Dirac Equation, Dynamo Theory, Kernel Functions, Tensors
Scientific paper
The diffusion approximation works with simple `stress-strain' relations with eddy diffusivities as scaling coefficients. Numerical simulations of the Parker instability are analyzed with respect to the non-local mean-field relation between magnetic field and turbulent EMF. The kernel in the convolution integral is expanded in a series of derivatives of Dirac's delta functions. The diffusion approximation holds if the coefficient of the first derivative (the traditional `eddy diffusivity') dominates. In fact, all the considered simulations contain a dominating eddy diffusivity but also subsequent coefficient appears. The traditional diffusion approximation, therefore, only works for mean magnetic fields with scales clearly exceeding the pressure-scale height. The turbulent-advection effect (the zero-order coefficient) proves to be very small.
Hasler K. H.
Kaisig Michael
Ruediger Guenther
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