SUMMARY Convection in the liquid outer core of the Earth is driven by thermal and chemical perturbations. The main purpose of this study is to examine the impact of double-diffusive convection on magnetic field generation by means of 3-D global geodynamo models, in the so-called ‘top-heavy’ regime of double-diffusive convection, when both thermal and compositional background gradients are destabilizing. Using a linear eigensolver, we begin by confirming that, compared to the standard single-diffusive configuration, the onset of convection is facilitated by the addition of a second buoyancy source. We next carry out a systematic parameter survey by performing 79 numerical dynamo simulations. We show that a good agreement between simulated magnetic fields and the geomagnetic field can be attained for any partitioning of the convective input power between its thermal and chemical components. On the contrary, the transition between dipole-dominated and multipolar dynamos is found to strongly depend on the nature of the buoyancy forcing. Classical parameters expected to govern this transition, such as the local Rossby number—a proxy of the ratio of inertial to Coriolis forces—or the degree of equatorial symmetry of the flow, fail to capture the dipole breakdown. A scale-dependent analysis of the force balance instead reveals that the transition occurs when the ratio of inertial to Lorentz forces at the dominant length scale reaches 0.5, regardless of the partitioning of the buoyancy power. The ratio of integrated kinetic to magnetic energy Ek/Em provides a reasonable proxy of this force ratio. Given that Ek/Em ≈ 10−4 − 10−3 in the Earth’s core, the geodynamo is expected to operate far from the dipole–multipole transition. It hence appears that the occurrence of geomagnetic reversals is unlikely related to dramatic and punctual changes of the amplitude of inertial forces in the Earth’s core, and that another mechanism must be sought.

中文翻译：

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头重脚双扩散地球发电机模型中的地磁相似性和偶极-多极转变

总结 地球液态外核中的对流是由热和化学扰动驱动的。本研究的主要目的是通过 3-D 全球地球发电机模型检查双扩散对流对磁场产生的影响，在所谓的双扩散对流“头重脚轻”状态下，当两个热和成分背景梯度不稳定。使用线性特征求解器，我们首先确认，与标准的单扩散配置相比，添加第二个浮力源有助于对流的开始。接下来，我们通过执行 79 次数值发电机模拟来进行系统参数调查。我们表明，对于热成分和化学成分之间的对流输入功率的任何分配，都可以获得模拟磁场和地磁场之间的良好一致性。相反，发现以偶极子为主的发电机和多极发电机之间的过渡强烈依赖于浮力强迫的性质。预计控制这种转变的经典参数，例如局部罗斯比数（惯性力与科里奥利力之比的代表）或流动的赤道对称程度，未能捕捉到偶极子击穿。相反，力平衡的尺度相关分析表明，当主要长度尺度上的惯性力与洛伦兹力之比达到 0.5 时，无论浮力的分配如何，都会发生转变。综合动能与磁能的比率 Ek/Em 提供了该力比率的合理代表。鉴于地核中的 Ek/Em ≈ 10-4 - 10-3，预计地球发电机将在远离偶极-多极转变的地方运行。因此，似乎地磁反转的发生不太可能与地核中惯性力幅度的剧烈和准时变化有关，必须寻找另一种机制。