The influence of magnetic fields in planetary dynamo models
The influence of magnetic fields in planetary dynamo models
复制标题
行星发电机模型中磁场的影响
DOI：
10.1016/j.epsl.2012.03.038
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发表时间：
2012
影响因子：
5.3
通讯作者：
J. Aurnou
中科院分区：
文献类型：
--
作者：
K. Soderlund;E. King;J. Aurnou
行星和恒星的磁场被认为在导致它们产生磁场的流体运动中发挥着重要作用，因为磁能最终来自动能。我们研究了磁场对对流发电机模型的影响，将它们与非磁性的、但在其他方面相同的模拟进行了对比。这项调查考虑了普朗特数Pr=1；磁性普朗特数直到Pm=5；埃克曼数在10−3≥E≥10−5范围内；以及瑞利数从近起到超过1,000倍的临界模型。这封信涉及两个主要问题。首先，我们发现在我们的大多数模型中，对流的特性，包括对流的流动结构和速度以及换热效率，不受磁场的存在的强烈影响。虽然洛伦兹力必须改变流动以限制磁场增长的幅度，但我们发现发电机作用并不需要对整个流场进行重大改变。通过直接计算每个模拟中的力，我们发现传统定义的艾尔萨瑟数Λi高估了发电机中洛伦兹力的作用。即使在ΛI≃100中，科里奥利力仍然大于洛伦兹力，这解释了ΛI>；1发电机模拟中柱状流的持久性。我们认为，动态埃尔萨瑟数Λd更好地代表了洛伦兹与科里奥利力之比。通过将Λd参数化应用于行星环境，我们预测行星内部的对流动力学(不包括纬向流)仅受其大尺度磁场的微弱影响。这里讨论的第二个要点是观察到的具有偶极和多极磁场的发电机之间的转变。我们发现，偶极场产生的破裂是由于流动中螺旋度的退化。这种螺旋度的变化与柱状对流的破坏并不一致，也不受磁场的强烈影响。力计算表明，这种转变可能与惯性力和粘性力之间的竞争有关。如果粘性对大规模磁场的产生确实很重要，这种中等的埃克曼数模型可能不能充分模拟行星发电机的动力学，那里的粘性效应预计可以忽略不计。
The magnetic fields of planets and stars are thought to play an important role in the fluid motions responsible for their field generation, as magnetic energy is ultimately derived from kinetic energy. We investigate the influence of magnetic fields on convective dynamo models by contrasting them with non-magnetic, but otherwise identical, simulations. This survey considers models with Prandtl number Pr=1; magnetic Prandtl numbers up to Pm=5; Ekman numbers in the range 10−3≥E≥10−5; and Rayleigh numbers from near onset to more than 1000 times critical. Two major points are addressed in this letter. First, we find that the characteristics of convection, including convective flow structures and speeds as well as heat transfer efficiency, are not strongly affected by the presence of magnetic fields in most of our models. While Lorentz forces must alter the flow to limit the amplitude of magnetic field growth, we find that dynamo action does not necessitate a significant change to the overall flow field. By directly calculating the forces in each of our simulations, we show that the traditionally defined Elsasser number, Λi, overestimates the role of the Lorentz force in dynamos. The Coriolis force remains greater than the Lorentz force even in cases with Λi≃100, explaining the persistence of columnar flows in Λi>1 dynamo simulations. We argue that a dynamic Elsasser number, Λd, better represents the Lorentz to Coriolis force ratio. By applying the Λdparametrization to planetary settings, we predict that the convective dynamics (excluding zonal flows) in planetary interiors are only weakly influenced by their large-scale magnetic fields. The second major point addressed here is the observed transition between dynamos with dipolar and multipolar magnetic fields. We find that the breakdown of dipolar field generation is due to the degradation of helicity in the flow. This helicity change does not coincide with the destruction of columnar convection and is not strongly influenced by the presence of magnetic fields. Force calculations suggest that this transition may be related to a competition between inertial and viscous forces. If viscosity is indeed important for large-scale field generation, such moderate Ekman number models may not adequately simulate the dynamics of planetary dynamos, where viscous effects are expected to be negligible.