Current helicity and electromotive force of magnetoconvection influenced by helical background fields

TL;DR

This paper investigates how large-scale helical magnetic fields influence small-scale current helicity and electromotive forces in nonrotating magnetoconvection, revealing new insights into their relationships and dependencies on magnetic Reynolds number.

Contribution

It demonstrates through analytic theory and simulations that large-scale helical fields induce small-scale current helicity with the same sign, challenging previous assumptions about alpha effects in nonrotating magnetoconvection.

Findings

Small-scale current helicity shares the sign of large-scale helicity.

No finite kinetic helicity is produced in the models.

Current helicity increases with magnetic Reynolds number.

Abstract

Motivated by the empirical finding that the known hemispheric rules for the current helicity at the solar surface are not strict, the excitation of small-scale current helicity by the influence of a large-scale helical magnetic background fields on nonrotating magnetoconvection is demonstrated. It is shown within a quasilinear analytic theory of driven turbulence and by nonlinear simulations of magnetoconvection that the resulting small-scale current helicity has the same sign as the large-scale current helicity while the ratio of both pseudo-scalars is of the order of the magnetic Reynolds number of the convection. The same models do not provide finite values of the small-scale kinetic helicity. On the other hand, a turbulence-induced electromotive force is produced including the diamagnetic pumping term as well as the eddy diffusivity but no alpha effect. It is thus argued that the relations by Pouquet & Patterson (1978) and Keinigs (1983) for the simultaneous existence of small-scale current helicity and alpha effect do not hold for the considered model of nonrotating magnetoconvection. Calculations for various values of the magnetic Prandtl number demonstrate that for the considered diffusivities the current helicity grows for growing magnetic Reynolds number which is not true for the velocity of the diamagnetic pumping -- in agreement with the results of the quasilinear analytical approximation.