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arXiv 2209.11652astro-ph.SR
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分层系统中多极发电机到偶极发电机的转变

Transition from multipolar to dipolar dynamos in stratified systems

B. Zaire, L. Jouve, T. Gastine, J-F. Donati, J. Morin, N. Landin, C. P. Folsom

备注：
15 pages, 15 figures, accepted for publication in MNRAS
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摘要：

对冷恒星表面磁场的观测揭示了其结构的巨大差异。虽然现在有一个共识，这些领域是通过发电机过程中发生的对流区，物理机制驱动这样的各种领域的拓扑结构仍有争议。本文讨论了可能的起源偶极子和多极主导的形态使用三维数值模拟的分层系统中的流体运动的磁反馈是显着的。我们的主要结果是，偶极的解决方案被发现在Rossby数高达0.4强分层模拟，以前的工作表明，只有多极场应该存在。我们认为，这些模拟是让人想起的离群星观察到的Rossby数大于0.1，其大规模的磁场是由他们的轴对称极向分量。正如在以前的Boussinesq计算中所建议的，惯性力相对于洛伦兹力的相对重要性再次控制了偶极到多极的转变。或者，我们发现，动能与磁能的比率可以同样很好地捕捉场形态的转变。我们测试的能力，这个新的代理预测的磁性形态的几个M-矮星的内部结构相匹配，我们的模拟和均匀的磁场表征是可用的。最后，在我们的模拟中获得的差分旋转的幅度进行比较，在文献中报道的M-矮星的实际测量。在我们的模拟中，我们发现了一个明确的反太阳差异旋转和偶极场的出现之间的关系。

英文摘要：

Observations of surface magnetic fields of cool stars reveal a large diversity of configurations. Although there is now a consensus that these fields are generated through dynamo processes occurring within the convective zone, the physical mechanism driving such a variety of field topologies is still debated. This paper discusses the possible origins of dipole and multipole-dominated morphologies using three-dimensional numerical simulations of stratified systems where the magnetic feedback on the fluid motion is significant. Our main result is that dipolar solutions are found at Rossby numbers up to 0.4 in strongly stratified simulations, where previous works suggested that only multipolar fields should exist. We argue that these simulations are reminiscent of the outlier stars observed at Rossby numbers larger than 0.1, whose large-scale magnetic field is dominated by their axisymmetric poloidal component. As suggested in previous Boussinesq calculations, the relative importance of inertial over Lorentz forces is again controlling the dipolar to multipolar transition. Alternatively, we find that the ratio of kinetic to magnetic energies can equally well capture the transition in the field morphology. We test the ability of this new proxy to predict the magnetic morphology of a few M-dwarf stars whose internal structure matches that of our simulations and for which homogeneous magnetic field characterization is available. Finally, the magnitude of the differential rotation obtained in our simulations is compared to actual measurements reported in the literature for M-dwarfs. In our simulations, we find a clear relationship between anti-solar differential rotation and the emergence of dipolar fields.

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