2000/11/29 by Hongsong Chou, Chou, Hongsong, George B. Field +1
Physics and Astronomy · #Astrophysics (astro-ph) #FOS: Physical sciences #astro-ph
paper · pdf · doi:10.48550/arxiv.astro-ph/0011548
31 pages, 5 figures, submitted to ApJ
arxiv created 2001/04/23 · arxiv updated 2009/12/01
Dynamical studies of MHD turbulence on the one hand, and arguments based upon magnetic helicity on the other, have yielded seemingly contradictory estimates for the α parameter in turbulent dynamo theory. Here we show, with direct numerical simulation of three-dimensional magnetohydrodynamic turbulence with a mean magnetic field, \OB, that the constraint on the dynamo α-effect set by the magnetic helicity is time-dependent. A time-scale tc is introduced such that for t<tc, the α-coefficient calculated from the simulation is close to the result of Pouquet et al and Field et al, -\fracτcor3(< \v ⋅ ∇ × \v > - < \b ⋅ ∇ × \b >); for t>tc, the classical result of the α-coefficient given by the Mean-Field Electrodynamics is reduced by a factor of 1/(Rm |\OB|2/vrms2), as argued by Gruzinov & Diamond, Seehafer and Cattaneo & Hughes. Here, Rm is the magnetic Reynolds number, vrms the rms velocity of the turbulence, τcor the correlation time of the turbulence, and B is in velocity unit. The applicability of and connection between different models of dynamo theory are also discussed.