地磁偶极稳定性与纬向流受地幔热流非均质性控制
Geomagnetic dipole stability and zonal flows controlled by mantle heat flux heterogeneities
AI总结：
本文通过不同参数的地球发电机模拟，研究核幔边界热流非均质性对地磁偶极稳定性与纬向流的影响，提出无量纲数 $M_{Za}^*$ 可表征偶极占比与反转倾向，并表明地幔对流驱动的热流变化能解释古地磁反转频率的显著波动。
摘要：
本研究旨在更全面地理解核幔边界（CMB）热流的横向非均质性如何影响地球发电机，同时将其他相关参数推向更接近真实值的范围。为此，我们运行了在核幔边界具有1阶和2阶球谐热流型的地球发电机模拟。使用了多种地球发电机模型，从标准数值发电机到更极端的参数，包括强场情形和湍流情形。我们表明，幅度与我们对地幔对流历史认识相符的热流非均质性可以促进多极发电机。多极转变与向西流的破坏有关，这种破坏要么通过向东热风，要么通过赤道对称性的丧失。发现强场发电机模型具有更大的向西流，并且对热流非均质性较不敏感。此外，我们发现磁场的偶极占比与 $M_{Za}^*=\dfrac{Λ_{Za}}{Rm_{Za}^2}$ 相关，其中 $Λ_{Za}$ 是纬向反对称埃尔萨瑟数，$Rm_{Za}$ 是纬向反对称磁雷诺数。重要的是，对地球地核估计的 $M_{Za}^*$ 与反转的偶极磁场一致。在 $M_{Za}^*$ 易发生反转的范围内，破坏赤道对称性或通过地核赤道冷却强迫向东纬向流，在我们的模拟中一致地触发反转或向多极发电机的转变。我们的结果支持，地幔对流在地球历史中驱动的热流非均质性的时间变化能够引起古地磁记录中观测到的反转频率的显著变化。
英文摘要：
This work aims at acquiring a more complete understanding of how lateral heterogeneities of the CMB heat flux affect the geodynamo while other relevant parameters are pushed towards realistic values. For this purpose, we ran geodynamo simulations with degree 1 and 2 spherical harmonic patterns of heat flux at the CMB. Several geodynamo models are used, ranging from standard numerical dynamos to more extreme parameters, including strong field cases and turbulent cases. We show that heat flux heterogeneities with amplitudes compatible with our knowledge of mantle convection history can favour multipolar dynamos. The multipolar transition is associated with a disruption of westward flows either through eastward thermal winds or through a loss of equatorial symmetry. Strong field dynamo models are found to have larger westward flows and are less sensitive to heat flux heterogeneities. Furthermore, we find that the dipolar fraction of the magnetic field correlates with $M_{Za}^*=\dfrac{Λ_{Za}}{Rm_{Za}^2}$ where $Λ_{Za}$ is the zonal antisymmetric Elsasser number and $Rm_{Za}$ is the zonal antisymmetric magnetic Reynolds number. Importantly, $M_{Za}^*$ estimated for the Earth's core is consistent with a reversing dipolar magnetic field. Within the range of $M_{Za}^*$ susceptible to reversals, breaking the equatorial symmetry or forcing eastward zonal flows through an equatorial cooling of the core consistently triggers reversals or a transition towards multipolar dynamos in our simulations. Our results support that time variations of heat-flux heterogeneities driven by mantle convection through Earth's history are capable of inducing the significant variations in the reversal frequency observed in the palaeomagnetic record.
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