Numerical modeling of the geodynamo: Mechanisms of field generation and equilibration
Numerical modeling of the geodynamo: Mechanisms of field generation and equilibration
复制标题
地球发电机的数值模拟：场产生和平衡的机制
DOI：
10.1029/1999jb900013
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发表时间：
1999
影响因子：
--
通讯作者：
G. Glatzmaier
中科院分区：
文献类型：
--
作者：
P. Olson;U. Christensen;G. Glatzmaier
对旋转导电球壳中热对流驱动的流体发电机进行了数值计算分析。我们发现两个制度的不可逆，强场发电机在埃克曼数10 - 4和瑞利数高达11倍的临界。在强烈的柱状政权，对流只发生在流体外部的内核切线圆柱体，在细长的柱状涡的形式平行于旋转轴。柱状对流在北方半球包含大量的负螺旋度，在南半球包含大量的正螺旋度，并通过宏观α 2机制在一定瑞利数以上产生发电机作用。这些发电机通过产生集中的磁通束来平衡，这些磁通束限制了对流柱的动能。在中高纬地区，偶极主导的外场是由多个通量束叠加而成。在低纬度地区，反向通量斑块的模式在逆行方向传播，导致在赤道地区的字段明显向西漂移。在较高的瑞利数，我们发现一个充分发展的制度与对流内的切圆柱极上升流和方位热风流。这些运动通过从两极排出极向通量并在内核附近的极区产生强烈的环向场来修改发电机。对流发电机在充分发展的制度表现出的特点，可以与地磁场相比，包括集中通量束的核幔边界，极低的磁场强度，并向西漂移的情节。
Numerical calculations of fluid dynamos powered by thermal convection in a rotating, electrically conducting spherical shell are analyzed. We find two regimes of nonreversing, strong field dynamos at Ekman number 10 -4 and Rayleigh numbers up to 11 times critical. In the strongly columnar regime, convection occurs only in the fluid exterior to the inner core tangent cylinder, in the form of narrow columnar vortices elongated parallel to the spin axis. Columnar convection contains large amounts of negative helicity in the northern hemisphere and positive helicity in the southern hemisphere and results in dynamo action above a certain Rayleigh number, through a macroscopic α 2 mechanism. These dynamos equilibrate by generating concentrated magnetic flux bundles that limit the kinetic energy of the convection columns. The dipole-dominated external field is formed by superposition of several flux bundles at middle and high latitudes. At low latitudes a pattern of reversed flux patches propagates in the retrograde direction, resulting in an apparent westward drift of the field in the equatorial region. At higher Rayleigh number we find a fully developed regime with convection inside the tangent cylinder consisting of polar upwelling and azimuthal thermal wind flows. These motions modify the dynamo by expelling poloidal flux from the poles and generating intense toroidal fields in the polar regions near the inner core. Convective dynamos in the fully developed regime exhibit characteristics that can be compared with the geomagnetic field, including concentrated flux bundles on the core-mantle boundary, polar minima in field intensity, and episodes of westward drift.