Nonlinear magnetic diffusion and magnetic helicity transport in galactic dynamos

Abstract

Abstract.We have extended our previous mean-field galactic dynamo model which included algebraic and dynamic alpha nonlinearities (Kleeorin et al. 2002), to include also a quenching of the turbulent diffusivity. We readily obtain equilibrium states for the large-scale magnetic field in the local disc dynamo model, and these fields have strengths that are comparable to the equipartition field strength. We find that the algebraic nonlinearity alone (i.e. quenching of both the α effect and turbulent magnetic diffusion) cannot saturate the growth of the mean magnetic field; only the combined effect of algebraic and dynamic nonlinearities can limit the growth of the mean magnetic field. However, in contrast to our earlier work without quenching of the turbulent diffusivity, we cannot now find satisfactory solutions in the no-z approximation to the axisymmetric galactic dynamo problem.

The saturation of galactic dynamos

“…The magnetic field in a model with dynamical α-quenching and helicity fluxes grows, and then smoothly saturates near to equipartition between magnetic and turbulent energies. The evolution is quite similar to that Results showing the evolution of the midplane field without helicity fluxes (long-dashed); with the fluxes driven by anisotropies in turbulence (solid) as Kleeorin et al (2002Kleeorin et al ( , 2003; and with the addition of an explicit advection of flux in the zdirection (short-dashed).…”

“…Sur et al 2007) show that the equilibrium field strength can be enhanced in such a situation -see, for example, Fig. 1, which is derived from the model of Kleeorin et al (2002Kleeorin et al ( , 2003.…”

“…This approach is known generically as "dynamical alpha" and includes work by Vishniac & Cho (2001), Kleeorin et al (2000Kleeorin et al ( , 2002Kleeorin et al ( , 2003, and references therein), and Sur, Shukurov & Subramanian (2007). Although these studies differ in detail, the general thrust is similar, and here we will examine the formulation and results of the work by Kleeorin et al (2002Kleeorin et al ( , 2003, in which they specifically address the galactic dynamo problem.…”

“…Ruzmaikin et al 1988) as well as in the "no-z" model (Subramanian & Mestel 1993; Moss 1995) which considers fields to be averaged vertically across the disc, i.e. in the z-direction (Kleeorin et al 2002(Kleeorin et al , 2003.…”

Strong mean field dynamos require supercritical helicity fluxes

“…This could also indicate that another perhaps more important component still needs to be included in the present mean field models. It is possible that the helicity fluxes discussed by Kleeorin et al (2000Kleeorin et al ( , 2002Kleeorin et al ( , 2003a may capture the missing components of the helicity flux, but this remains at this point only speculation.…”