Conservation of Magnetic Helicity and Its Constraint on $\alpha$-Effect of Dynamo Theory
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 $\alpha$ 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 $\alpha$-effect set by the magnetic helicity is time-dependent. A time-scale $t_c$ is introduced such that for $t<t_c$, the $\alpha$-coefficient calculated from the simulation is close to the result of Pouquet et al and Field et al, $-\frac{\tau_{cor}}{3}(< \v \cdot \nabla \times \v > - < \b \cdot \nabla \times \b >)$; for $t>t_c$, the classical result of the $\alpha$-coefficient given by the Mean-Field Electrodynamics is reduced by a factor of $1/({R_m |\OB|^2/v_{rms}^2})$, as argued by Gruzinov & Diamond, Seehafer and Cattaneo & Hughes. Here, $R_m$ is the magnetic Reynolds number, $v_{rms}$ the rms velocity of the turbulence, $\tau_{cor}$ the correlation time of the turbulence, and $\overline B$ is in velocity unit. The applicability of and connection between different models of dynamo theory are also discussed.