The role of magnetic waves in tangent cylinder convection

IF 2.4 3区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
Debarshi Majumder, Binod Sreenivasan
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引用次数: 0

Abstract

The secular variation of the geomagnetic field suggests that there are anticyclonic polar vortices in the Earth's core. Under the influence of a magnetic field, the polar azimuthal flow is thought to be produced by one or more coherent upwellings within the tangent cylinder, offset from the rotation axis. In this study, convection within the tangent cylinder in rapidly rotating dynamos is investigated through the analysis of forced magnetic waves. The first part of the study investigates the evolution of an isolated buoyancy disturbance in an unstably stratified rotating fluid subjected to an axial magnetic field. It is shown that the axial flow intensity of the slow Magnetic-Archimedean-Coriolis (MAC) waves becomes comparable to that of the fast MAC waves when |ωM/ωC|0.1, where ωM and ωC are the Alfvén wave and inertial wave frequencies respectively. In spherical shell dynamo simulations, the isolated upwellings within the tangent cylinder are shown to originate from the localized excitation of slow MAC waves in the dipole-dominated regime. Axial flow measurements in turn reveal the approximate parity between the slow and fast wave intensities in this regime, which corresponds to the existence of strong polar vortices in the Earth's core. To obtain the observed peak azimuthal motions of 0.60.9 yr1, the Rayleigh number in the low-inertia geodynamo must be 103 times the Rayleigh number for the onset of nonmagnetic convection. However, if the forcing is so strong as to cause polarity reversals, the field within the tangent cylinder decays away, and the convection takes the form of an ensemble of plumes supported entirely by the fast waves of frequency ωωC. The resulting weak polar circulation is comparable to that obtained in nonmagnetic convection.

电磁波在切线圆柱对流中的作用
地磁场的长期变化表明,地核中存在反气旋极地涡旋。在磁场的影响下,极方位角流被认为是由切线圆柱体内偏离旋转轴的一个或多个相干上升流产生的。本文通过对强迫磁波的分析,研究了快速旋转发电机切向圆筒内的对流问题。研究的第一部分研究了在轴向磁场作用下,不稳定分层旋转流体中孤立浮力扰动的演变。结果表明,当|ωM/ωC|~0.1时,慢磁阿基米德科里奥利(MAC)波的轴向流动强度与快磁阿基米德科里奥利波的轴向流强度相当,其中ωM和ωC分别是Alfvén波和惯性波的频率。在球壳发电机模拟中,切线圆柱体内的孤立上升流源于偶极子主导区域中慢MAC波的局部激发。轴流测量反过来揭示了这种情况下慢波和快波强度之间的近似平衡,这与地核中存在强极涡相对应。为了获得0.6–0.9∘yr−1的观测峰值方位角运动,低惯性地球动力学中的瑞利数必须是非磁性对流开始时瑞利数的~103倍。然而,如果强迫如此强烈,以至于导致极性反转,切向圆柱体内的场就会衰减,对流就会形成完全由频率为ω~ωC的快波支撑的羽流集合。由此产生的弱极性环流与在非磁性对流中获得的环流相当。
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来源期刊
Physics of the Earth and Planetary Interiors
Physics of the Earth and Planetary Interiors 地学天文-地球化学与地球物理
CiteScore
5.00
自引率
4.30%
发文量
78
审稿时长
18.5 weeks
期刊介绍: Launched in 1968 to fill the need for an international journal in the field of planetary physics, geodesy and geophysics, Physics of the Earth and Planetary Interiors has now grown to become important reading matter for all geophysicists. It is the only journal to be entirely devoted to the physical and chemical processes of planetary interiors. Original research papers, review articles, short communications and book reviews are all published on a regular basis; and from time to time special issues of the journal are devoted to the publication of the proceedings of symposia and congresses which the editors feel will be of particular interest to the reader.
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