On hydromagnetic instabilities and the mean electromotive force in a non-uniformly stratified Earth’s core affected by viscosity

Abstract

Linear magnetoconvection in a model of a non-uniformly stratified horizontal rotating fluid layer with a toroidal magnetic field is investigated for no-slip and finitely electrically conductive boundaries and with very thin stably stratified upper sublayer. The basic parabolic temperature profile is determined by the temperature difference between the boundaries and by the homogeneous heat source distribution in the layer. This results in a density pattern, in which a stably stratified upper sublayer is present. The developed diffusive perturbations (modes) are strongly affected by the complicated coupling of viscous, thermal and magnetic diffusive processes. The calculations were performed for various values of Roberts number (q ≪ 1 and q = O(1)). The mean electromotive force produced by the developed hydromagnetic instabilities is investigated to find the modes, which can be appropriate for creating the α-effect. It was found that the azimuthal part of the EMF is dominant for westward modes when the Elsasser number Λ ∼ < O(1).K e y w o r d s : non-uniform stratification of the Earth's core, linear magnetoconvection, diffusive hydromagnetic instabilities, mean electromotive force Stud. Geophys. Geod., 50 (2006), 645-661 645 c 2006 StudiaGeo s.r.o., Prague J.Šimkanin et al.

Convection in rotating non-uniformly stratified spherical fluid shells in dependence on Ekman and Prandtl numbers

“…(2-3)). This behaviour is known from previous analyses [24,16,17,3]. Increasing R a it is possible to observe a transition from the large-scale convection to the small-scale convection (Figs.…”

“…The case, in which the thickness of both sublayers is the same, was not investigated in previous studies (e.g., [24,16,17,18]). Consequently, we located the change of the sign to the middle of the convective shell.…”

“…Models of a non-uniformly stratified fluid shell are an acceptable simplification of the real Earth-like conditions. In the Earth's core the stably stratified sublayer is probably very thin (the outer Earth's core is almost unstably stratified, see [1,8,24,16,17,21]). Such a stratification is probably typical for the terrestrial planets [24,20,21].…”

“…Non-uniform stratification can be simulated thermodynamically also in the Boussinesq models by means of internal heat sources [24,16,17]. If the stably stratified sublayer is very thin (for a geometric configuration stable/unstable), then behaviour is close to the case of uniform stratification when the whole layer is unstably stratified.…”

“…Likewise, if the unstably stratified sublayer is very thin, then behaviour is close to the case of uniform stratification when the whole layer is stably stratified. Thus, the effects of non-uniform stratification are noticeable if the thicknesses of the stably and unstably stratified sublayers are comparable [24,16,17,18].…”