Magnetohydrodynamics of stably stratified regions in planets and stars
arXiv:1910.04092 · doi:10.1080/03091929.2019.1670827
Abstract
Stably stratified layers are present in stellar interiors (radiative zones) as well as planetary interiors - recent observations and theoretical studies of the Earth's magnetic field seem to indicate the presence of a thin, stably stratified layer at the top of the liquid outer core. We present direct numerical simulations of this region, which is modelled as an axisymmetric spherical Couette flow for a stably stratified fluid embedded in a dipolar magnetic field. For strong magnetic fields, a super-rotating shear layer, rotating nearly 30% faster than the imposed rotation rate difference between the inner convective dynamo region and the outer boundary, is generated in the stably stratified region. In the Earth context, and contrary to what was previously believed, we show that this super-rotation may extend towards the Earth magnetostrophic regime if the density stratification is sufficiently large. The corresponding differential rotation triggers magnetohydrodynamic instabilities and waves in the stratified region, which feature growth rates comparable to the observed timescale for geomagnetic secular variations and jerks. In the stellar context, we perform a linear analysis which shows that similar instabilities are likely to arise, and we argue that it may play a role in explaining the observed magnetic dichotomy among intermediate-mass stars.
arXiv comment18 pages, 6 figures. Accepted for publication in GAFD
References in corpus (14)
- Magnetic fields of non-degenerate stars
- Thermal and electrical conductivity of iron at Earth's core conditions
- Seismic constraints on the radial dependence of the internal rotation profiles of six Kepler subgiants and young red giants
- Weak magnetic fields in Ap/Bp stars: Evidence for a dipole field lower limit and a tentative interpretation of the magnetic dichotomy
- Turbulent geodynamo simulations: a leap towards Earth's core
- Local Axisymmetric Diffusive Stability of Weakly-Magnetized, Differentially-Rotating, Stratified Fluids
- Experimental analysis of the Strato-rotational Instability in a cylindrical Couette flow
- Experimental study of super-rotation in a magnetostrophic spherical Couette flow
- 3D evolution of magnetic fields in a differentially rotating stellar radiative zone
- Magnetostrophic MRI in the Earth's Outer Core
- Spherical Couette flow in a dipolar magnetic field
- On rotationally driven meridional flows in stars
- Evolution of a magnetic field in a differentially rotating radiative zone
- Systematic parameter study of dynamo bifurcations in geodynamo simulations