地球流体核中对流驱动的十年纬向加速度
Convectively driven decadal zonal accelerations in Earth's fluid core
摘要：
根据磁场观测和日长的变化，推测地球流体核中存在与自转轴同轴的柱面方位加速度。这些加速有一个典型的几十年的时间尺度。然而，引起加速度的物理机制还没有很好地理解。标度参数表明，在圆柱表面上平均的首阶扭矩应该来自洛伦兹力。在对流的驱动下，地核内部磁场的十年波动可能会迫使洛伦兹力矩发生十年变化，并产生纬向加速度。我们通过构建一个磁对流的准地转模式准地转模式来检验这一假设，热驱动流扰动一个稳定的，强加的背景磁场。我们表明，当我们的模型中的阿尔文数是类似的，在地球的流体核心，在扭矩平衡的时间波动占主导地位的洛伦兹扭矩，后者产生的平均纬向加速度。我们的模型再现了快速的自由阿尔文波和缓慢的强迫加速度，相对强度和相对时间尺度的比值与地核相似。驱动随时间变化的洛伦兹扭矩的磁场的时间变化由下面的对流产生，在与对流涡流相关的时间尺度上剪切和平流输送磁场。我们的研究结果支持的假设，即在地球的流体核心深处的磁场的时间变化驱动器所观察到的十年纬向加速度的圆柱表面通过洛伦兹力矩。
英文摘要：
Azimuthal accelerations of cylindrical surfaces co-axial with the rotation axis have been inferred to exist in Earth's fluid core on the basis of magnetic field observations and changes in the length-of-day. These accelerations have a typical timescale of decades. However, the physical mechanism causing the accelerations is not well understood. Scaling arguments suggest that the leading order torque averaged over cylindrical surfaces should arise from the Lorentz force. Decadal fluctuations in the magnetic field inside the core, driven by convective flows, could then force decadal changes in the Lorentz torque and generate zonal accelerations. We test this hypothesis by constructing a quasi-geostrophic model of magnetoconvection, with thermally-driven flows perturbing a steady, imposed background magnetic field. We show that when the Alfvén number in our model is similar to that in Earth's fluid core, temporal fluctuations in the torque balance are dominated by the Lorentz torque, with the latter generating mean zonal accelerations. Our model reproduces both fast, free Alfvén waves and slow, forced accelerations, with ratios of relative strength and relative timescale similar to those inferred for the Earth's core. The temporal changes in the magnetic field which drive the time-varying Lorentz torque are produced by the underlying convective flows, shearing and advecting the magnetic field on a timescale associated with convective eddies. Our results support the hypothesis that temporal changes in the magnetic field deep inside Earth's fluid core drive the observed decadal zonal accelerations of cylindrical surfaces through the Lorentz torque.
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