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.

中文翻译：

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行星和恒星中稳定分层区域的磁流体动力学

摘要稳定分层层存在于恒星内部（辐射区）以及行星内部——最近对地球磁场的观察和理论研究似乎表明，在液态外核的顶部存在一个薄的、稳定分层的层。我们提出了该区域的直接数值模拟，该区域被建模为嵌入偶极磁场中的稳定分层流体的轴对称球形库埃特流。对于强磁场，在稳定分层区域中会产生超旋转剪切层，其旋转速度比内对流发电机区域和外边界之间施加的旋转速率差快近 30%。在地球的背景下，与先前所相信的相反，我们表明，如果密度分层足够大，这种超级旋转可能会扩展到地球磁转区。相应的微分旋转在分层区域触发磁流体动力学不稳定性和波，其特征是增长率与观察到的地磁长期变化和抽搐的时间尺度相当。在恒星环境中，我们进行了线性分析，表明可能会出现类似的不稳定性，我们认为它可能在解释观察到的中等质量恒星之间的磁二分法方面发挥作用。其特点是增长率与观察到的地磁长期变化和颠簸的时间尺度相当。在恒星环境中，我们进行了线性分析，表明可能会出现类似的不稳定性，我们认为它可能在解释观察到的中等质量恒星之间的磁二分法方面发挥作用。其特点是增长率与观察到的地磁长期变化和颠簸的时间尺度相当。在恒星环境中，我们进行了线性分析，表明可能会出现类似的不稳定性，我们认为它可能在解释观察到的中等质量恒星之间的磁二分法方面发挥作用。