Dynamos with weakly convecting outer layers: implications for core-mantle boundary interaction
Dynamos with weakly convecting outer layers: implications for core-mantle boundary interaction
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具有弱对流外层的发电机：对核幔边界相互作用的影响
DOI：
10.1080/03091920801900047
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发表时间：
2008
影响因子：
1.3
通讯作者：
D. Gubbins
中科院分区：
文献类型：
--
作者：
B. Sreenivasan;D. Gubbins
地球核心的对流在底部比在顶部更难驱动。这部分是因为绝热梯度向顶部变陡，部分是因为球形几何形状意味着所涉及的面积向顶部增加，部分是因为成分对流是由下边界释放的轻物质驱动的，并在整个外核均匀地重新混合，提供了浮力的体积汇。因此，我们研究了发电机行动的热对流Boussinesq流体包含在一个旋转的球壳驱动的底部和内部加热或冷却的组合。我们首先在外边界上施加均匀的温度，以探索热汇对发电机作用的影响;然后，我们施加与单个球谐函数Y 2²成比例的非均匀温度，以探索核幔相互作用。在均匀边界条件和中等瑞利数下，散热器可明显降低所产生的磁场;磁雷诺数仍然很高，因为流动的主导环形分量没有显着减少。场的偶极结构变得更加明显，正如其他作者所发现的那样。增加瑞利数产生一个政权，在该政权中的切线圆柱体内的对流强烈的磁场的影响。在非均匀边界条件下，热沉促进边界效应和磁场锁定到边界异常。我们发现，边界锁定被抑制的热量在外部区域的平流。在均匀加热的情况下，边界效应仅在低瑞利数时才有意义，此时发电机作用仅可能用于人工低磁扩散率。对于散热器，边界效应在较高的瑞利数下仍然很显著，前提是对流仍然很弱或流体在顶部稳定分层。发电机行动是由强烈的对流在深度，而边界热异常占主导地位的上部地区。这是地球核心的一种可能状态。
Convection in the Earth's core is driven much harder at the bottom than the top. This is partly because the adiabatic gradient steepens towards the top, partly because the spherical geometry means the area involved increases towards the top, and partly because compositional convection is driven by light material released at the lower boundary and remixed uniformly throughout the outer core, providing a volumetric sink of buoyancy. We have therefore investigated dynamo action of thermal convection in a Boussinesq fluid contained within a rotating spherical shell driven by a combination of bottom and internal heating or cooling. We first apply a homogeneous temperature on the outer boundary in order to explore the effects of heat sinks on dynamo action; we then impose an inhomogeneous temperature proportional to a single spherical harmonic Y 2² in order to explore core-mantle interactions. With homogeneous boundary conditions and moderate Rayleigh numbers, a heat sink reduces the generated magnetic field appreciably; the magnetic Reynolds number remains high because the dominant toroidal component of flow is not reduced significantly. The dipolar structure of the field becomes more pronounced as found by other authors. Increasing the Rayleigh number yields a regime in which convection inside the tangent cylinder is strongly affected by the magnetic field. With inhomogeneous boundary conditions, a heat sink promotes boundary effects and locking of the magnetic field to boundary anomalies. We show that boundary locking is inhibited by advection of heat in the outer regions. With uniform heating, the boundary effects are only significant at low Rayleigh numbers, when dynamo action is only possible for artificially low magnetic diffusivity. With heat sinks, the boundary effects remain significant at higher Rayleigh numbers provided the convection remains weak or the fluid is stably stratified at the top. Dynamo action is driven by vigorous convection at depth while boundary thermal anomalies dominate in the upper regions. This is a likely regime for the Earth's core.