A numerical study of dynamo action as a function of spherical shell geometry
A numerical study of dynamo action as a function of spherical shell geometry
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发电机作用作为球壳几何形状函数的数值研究
DOI：
10.1016/j.epsl.2005.04.032
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发表时间：
2005
影响因子：
5.3
通讯作者：
N. Pérez
中科院分区：
文献类型：
--
作者：
M. Heimpel;J. Aurnou;F. Al;N. Pérez
行星核心液体区域的几何形状可以影响核心对流和磁场产生过程。改变球壳半径比，χ=ri/ro，说明了不同核心半径比的行星之间的差异，以及发电机过程如何在演化的行星核心中随时间变化。在这里，我们研究的数值模型的热驱动发电机行动的旋转壳的外半径rowith导电Boussinesq流体，围绕一个同样导电的固体内球的半径ri。当埃克曼数E=3×10−4，普朗特数Pr=1，磁普朗特数Pm=5，机械刚性，等温边界条件下，在0.15≤χ≤0.65时，可以找到发电机解。在轻度超临界瑞利数Ra的情况下，对流以与旋转轴对齐的柱状羽流的形式发生。在厚壳的情况下（χ=0.15），轴向偶极发电机行动的发病发生在相对较高的Ra的单一湍流柱状羽流的行动。在较薄的壳层（χ≥0.25），多羽柱对流产生强偶极发电机场附近的对流开始。虽然我们的一些控制参数与行星的值相差甚远，但薄壳情况下的外边界Elsasser数Λ（磁场强度的无量纲度量）与地球的核幔边界以及木星和土星发电机的估计值相当。相比之下，厚壳，单羽发电机在外壳边界产生相对较低的埃尔萨瑟数，可能与水星的磁场有关。对于一个单一的不断发展的行星，这些结果意味着，在早期的内核增长从一个单一的羽流或区域对流制度的多羽流或全球对流制度的过渡可能会产生显着增加的外部全球磁场强度。我们的计算是一致的轴向偶极子占主导地位的行星磁场的观测和支持的概念，地心轴向偶极子场的大部分地球的历史。
The geometry of the liquid region of a planetary core can effect core convection and magnetic field generation processes. Varying the spherical shell radius ratio, χ=ri/ro, illustrates differences between planets with differing core radius ratios as well as how dynamo processes vary with time in an evolving planetary core. Here we study numerical models of thermally driven dynamo action in a rotating shell of outer radius rowith electrically conducting Boussinesq fluid that surrounds an equally conductive solid inner sphere of radius ri. Dynamo solutions are found for 0.15≤χ≤0.65 at Ekman number E=3×10−4, Prandtl number Pr=1, and magnetic Prandtl number Pm=5, with mechanically rigid, isothermal boundary conditions. In cases with mildly supercritical Rayleigh numbers, Ra, convection occurs in the form of columnar plumes aligned with the rotation axis. In thick-shelled cases (χ=0.15), onset of axial dipolar dynamo action occurs at relatively high Ra by the action of a single turbulent columnar plume. In thinner shells (χ≥0.25), multi-plume columnar convection generates strongly dipolar dynamo fields near the onset of convection. Although some of our control parameters are far from planetary values, the outer boundary Elsasser number, Λ, (a non-dimensional measure of magnetic field strength) for thinner shell cases is comparable to that for the Earth's core–mantle boundary and estimates for the dynamos of Jupiter and Saturn. In contrast, thick shell, single plume dynamos produce relatively low Elsasser numbers at the outer shell boundary, possibly relevant to the magnetic field of Mercury. For a single evolving planet, these results imply that a transition during early inner core growth from a single plume or regional convective regime to a multi-plume or global convective regime could produce a significant increase in the exterior global magnetic field intensity. Our calculations are consistent with observations of axial dipole dominated planetary magnetic fields and support the concept of a geocentric axial dipolar field for much of Earth's history.