Solar Tachocline Confinement by the Nonaxisymmetric Modes of a Dynamo Magnetic Field

L. Matilsky, N. Brummell, B. Hindman, J. Toomre
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Abstract

We recently presented the first 3D numerical simulation of the solar interior for which tachocline confinement was achieved by a dynamo-generated magnetic field. In this follow-up study, we analyze the degree of confinement as the magnetic field strength changes (controlled by varying the magnetic Prandtl number) in a coupled radiative zone (RZ) and convection zone (CZ) system. We broadly find three solution regimes, corresponding to weak, medium, and strong dynamo magnetic field strengths. In the weak-field regime, the large-scale magnetic field is mostly axisymmetric with regular, periodic polarity reversals (reminiscent of the observed solar cycle) but fails to create a confined tachocline. In the strong-field regime, the large-scale field is mostly nonaxisymmetric with irregular, quasi-periodic polarity reversals and creates a confined tachocline. In the medium-field regime, the large-scale field resembles a strong-field dynamo for extended intervals but intermittently weakens to allow temporary epochs of strong differential rotation. In all regimes, the amplitude of poloidal field strength in the RZ is very well explained by skin-depth arguments, wherein the oscillating field that gives rise to the skin depth (in the medium- and strong-field cases) is a nonaxisymmetric field structure at the base of the CZ that rotates with respect to the RZ. These simulations suggest a new picture of solar tachocline confinement by the dynamo, in which nonaxisymmetric, very long-lived (effectively permanent) field structures rotating with respect to the RZ play the primary role, instead of the regularly reversing axisymmetric field associated with the 22 yr cycle.
动力磁场的非轴对称模式对太阳塔克线的限制
最近,我们首次对太阳内部进行了三维数值模拟,通过动力学产生的磁场实现了塔克线约束。在这项后续研究中,我们分析了辐射区(RZ)和对流区(CZ)耦合系统中随着磁场强度变化(通过改变磁普朗特数来控制)的束缚程度。我们大致发现了三种解机制,分别对应于弱、中和强动力磁场强度。在弱磁场体制中,大尺度磁场大多是轴对称的,具有有规律的周期性极性反转(让人联想到观测到的太阳周期),但无法形成封闭的塔克线。在强磁场状态下,大尺度磁场大多是非轴对称的,具有不规则的准周期性极性反转,并形成了一个封闭的塔克线。在中场状态下,大尺度场在较长的时间间隔内类似于强场动力机,但间歇性地减弱,从而出现短暂的强差转。在所有情况下,RZ 中极性场强度的振幅都能很好地用集肤深度(skin-depth)参数来解释,其中产生集肤深度的振荡场(在中场和强场情况下)是位于 CZ 基底的非轴对称场结构,它相对于 RZ 旋转。这些模拟提出了动力机对太阳测速线限制的新图景,在这一图景中,相对于RZ旋转的非轴对称、寿命非常长(实际上是永久的)的场结构发挥了主要作用,而不是与22年周期相关的有规律逆转的轴对称场。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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