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首页> 外文期刊>Geophysical and Astrophysical Fluid Dynamics >Dynamos with weakly convecting outer layers: implications for core-mantle boundary interaction
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Dynamos with weakly convecting outer layers: implications for core-mantle boundary interaction

机译：对流弱对流的动力层：对地幔边界相互作用的影响

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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 Y2 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.
机译：地球核心的对流在底部比顶部更难被驱动。这部分是因为绝热梯度朝着顶部变陡，部分是因为球形几何形状意味着所涉及的区域朝着顶部增大，部分是因为成分对流是由下部边界处释放的轻质材料驱动，并在整个外核中均匀地重新混合，从而提供大量的浮力。因此，我们研究了在底部和内部加热或冷却的组合驱动下的旋转球形壳中所含的Boussinesq流体中的热对流的发电机作用。首先，我们在外边界上施加均匀的温度，以研究散热器对发电机作用的影响。然后，我们施加与单个球谐函数Y2成正比的非均匀温度，以研究核幔相互作用。在边界条件均匀且瑞利数适中的情况下，散热器可显着降低所产生的磁场。磁雷诺数仍然很高，因为流动的主要环面分量并未显着减少。正如其他作者所发现的，该场的偶极结构变得更加明显。瑞利数的增加产生了一种状态，其中切线圆柱体内的对流受到磁场的强烈影响。在边界条件不均匀的情况下，散热片会促进边界效应并将磁场锁定到边界异常。我们表明边界锁定被外部区域的热对流抑制。在均匀加热的情况下，只有在人为地降低磁扩散率的情况下才能产生发电机作用时，边界效应仅在低瑞利数下才有意义。使用散热器时，如果对流仍然很弱或流体在顶部稳定地分层，则在较高的瑞利数下边界效应仍然很明显。深度对流强烈的对流驱动发电机动作，而边界热异常在上部区域占主导地位。这可能是地球核心的一种制度。

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