Earth’s internal magnetic field is generated through motion of the electrically conductive iron-alloy fluid comprising its outer core. Temporal variability of this magnetic field, termed secular variation (SV), results from two processes: one is the interaction between core fluid motion and the magnetic field, the other is magnetic diffusion. As diffusion is widely thought to take place over relatively long, millennial time scales, it is common to disregard it when considering yearly to decadal field changes; in this frozen-flux approximation, core fluid motion may be inferred on the core–mantle boundary (CMB) using observations of SV at Earth’s surface. Such flow models have been used to forecast variation in the magnetic field. However, recent work suggests that diffusion may also contribute significantly to SV on short time scales provided that the radial length scale of the magnetic field structure within the core is sufficiently short. In this work, we introduce a hybrid method to forecast field evolution that considers a model based on both a steady flow and diffusion, in which we adopt a two-step process: first fitting the SV to a steady flow, and then fitting the residual by magnetic diffusion. We assess this approach by hindcasting the evolution for 2010–2015, based on fitting the models to CHAOS-6 using time windows prior to 2010. We find that including diffusion yields a reduction of up to 25% in the global hindcast error at Earth’s surface; at the CMB this error reduction can be in excess of 77%. We show that fitting the model over the shortest window that we consider, 2009–2010, yields the lowest hindcast error. Based on our hindcast tests, we present a candidate model for the SV over 2020–2025 for IGRF-13, fit over the time window 2018.3–2019.3. Our forecasts indicate that over the next decade the axial dipole will continue to decay, reversed-flux patches will increase in both area and intensity, and the north magnetic (dip) pole will continue to migrate towards Siberia.

中文翻译：

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通过连续估计核心流和磁扩散预测年度地磁变化

地球的内部磁场是通过构成其外核的导电铁合金流体的运动产生的。这种磁场的时间变化，称为长期变化 (SV)，由两个过程产生：一个是核心流体运动与磁场之间的相互作用，另一个是磁扩散。由于扩散被广泛认为发生在相对较长的千年时间尺度上，因此在考虑每年到十年的场变化时通常会忽略它；在这种冻结通量近似中，可以使用对地球表面 SV 的观测来推断地核-地幔边界 (CMB) 上的地核流体运动。这种流动模型已被用于预测磁场的变化。然而，最近的工作表明，只要核心内磁场结构的径向长度尺度足够短，扩散也可能在短时间尺度上对 SV 有显着贡献。在这项工作中，我们引入了一种混合方法来预测场演化，该方法同时考虑了基于稳态流和扩散的模型，其中我们采用了两步过程：首先将 SV 拟合到稳态流，然后拟合残差通过磁扩散。基于使用 2010 年之前的时间窗口将模型拟合到 CHAOS-6，我们通过后报 2010-2015 年的演变来评估这种方法。我们发现，包括扩散在内的地球表面全球后报误差减少了 25% ; 在 CMB，这种误差减少可以超过 77%。我们表明在我们考虑的最短窗口上拟合模型，2009-2010，产生最低的后报错误。根据我们的后测测试，我们为 IGRF-13 提供了 2020-2025 年 SV 的候选模型，适用于 2018.3-2019.3 时间窗口。我们的预测表明，在接下来的十年中，轴向偶极子将继续衰减，反通量斑块的面积和强度都将增加，而北磁（倾角）极将继续向西伯利亚迁移。