Alpha effect due to buoyancy instability of a magnetic layer

¹ NORDITA, AlbaNova University Center, Roslagstullsbacken 23, 10691 Stockholm, Sweden
e-mail: piyalic@nordita.org
² Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden

Received: 5 November 2010
Accepted: 13 May 2011

Abstract

Context. A strong toroidal field can exist in form of a magnetic layer in the overshoot region below the solar convection zone. This motivates a more detailed study of the magnetic buoyancy instability with rotation.

Aims. We calculate the α effect due to helical motions caused by an unstable magnetic layer in a rotating density-stratified system with angular velocity Ω making an angle θ with the vertical. We also study the dependence of the α effect on θ and the strength of the initial magnetic field.

Methods. We carry out three-dimensional hydromagnetic simulations in Cartesian geometry. A turbulent electromotive force (EMF) due to the correlations of the small scale velocity and magnetic field is generated. We use the test-field method to calculate the transport coefficients of the inhomogeneous turbulence produced by the layer.

Results. We show that the growth rate of the instability and the twist of the magnetic field vary monotonically with the ratio of thermal conductivity to magnetic diffusivity. The resulting α effect is non-uniform and increases with the strength of the initial magnetic field. It is thus an example of an “anti-quenched” α effect. The α effect is also nonlocal, i.e. scale dependent, requiring around 8–16 Fourier modes to reconstruct the actual mean EMF based on the actual mean field.