An efficient method for computing the eigenfunctions of the dynamo equation

¹ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany e-mail: schmitt@mps.mpg.de
² MAG (ENS/IPGP), LRA, École Normale Supérieure, 24 rue Lhomond, 75252 Paris Cedex 05, France
³ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

Received: 19 November 2009
Accepted: 19 May 2010

Abstract

Aims. We present an elegant method of determining the eigensolutions of the induction and dynamo equations in a fluid embedded in a vacuum.

Methods. The magnetic field is expanded in a complete set of functions. The new method is based on the biorthogonality of the adjoint electric current and the vector potential with an inner product defined by a volume integral over the fluid domain. The advantage of this method is that the velocity and the dynamo coefficients of the induction and the dynamo equation do not have to be differentiated and thus even numerically determined tabulated values of the coefficients produce reasonable results.

Results. We provide test calculations and compare with published results obtained by the classical treatment based on the biorthogonality of the magnetic field and its adjoint. We especially consider dynamos with mean-field coefficients determined from direct numerical simulations of the geodynamo and compare with initial value calculations and the full MHD simulations.