Asymptotic methods for spherically symmetric MHD α2‐dynamos

Abstract

AbstractWe consider two models of spherically‐symmetric MHD α2–dynamos; one with idealized boundary conditions (BCs); and one with physically realistic BCs. As it has been shown in our previous work, the eigenvalues λ of a model with idealized BCs and constant α–profile α0 are linear functions of α0 and form a mesh in the (α0, λ)–plane. The nodes of the spectral mesh correspond to double‐degenerate eigenvalues of algebraic and geometric multiplicity 2 (diabolical points). It was found that perturbations of the constant α –profile lead to a resonant unfolding of the diabolical points with selection rules of the resonant unfolding defined by the Fourier coefficients of the perturbations. In the present contribution we present new exact results on the spectrum of the model with physically realistic BCs and constant α. For non‐degenerate (simple) eigenvalues perturbation gradients are found at any particular α0. We briefly discuss the spectral behavior of the α2–dynamo operator over a family of homotopic deformations of the BCs between idealized ones and physically realistic ones. Furthermore, we demonstrate that although the spectral singularities are lifted, a memory about their locations remains deeply imprinted in the homotopic family of spectral deformations due to a hidden underlying invariance. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)