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Magnetohydrodynamics of stably stratified regions in planets and stars

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 modeled as an ax-isymmetric 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 40% faster than the inner region, 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 toward the Earth magnetostrophic regime if the density stratification is sufficiently large. The corresponding differential rotation triggers mag-netohydrodynamic instabilities and waves in the stratified region, which feature growth rates comparable to the observed timescale for geomag-netic 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 explain the observed magnetic desert among massive and intermediate mass stars.
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Submitted on : Friday, November 8, 2019 - 1:05:14 PM
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J. Philidet, C. Gissinger, F. Lignières, L. Petitdemange. Magnetohydrodynamics of stably stratified regions in planets and stars. Geophysical and Astrophysical Fluid Dynamics, Taylor & Francis, In press, pp.1-20. ⟨10.1080/03091929.2019.1670827⟩. ⟨hal-02344508⟩



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