COV-OBS.x2: 180 years of geomagnetic field evolution from ground-based and satellite observations,Earth, Planets and Space

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COV-OBS.x2: 180 years of geomagnetic field evolution from ground-based and satellite observations
Earth, Planets and Space ( IF 3.0 ) Pub Date : 2020-10-23 , DOI: 10.1186/s40623-020-01194-2
Loïc Huder ₁ , Nicolas Gillet ₁ , Christopher C Finlay ₂ , Magnus D Hammer ₂ , Hervé Tchoungui ₁

Affiliation

We present the geomagnetic field model COV-OBS.x2 that covers the period 1840–2020. It is primarily constrained by observatory series, satellite data, plus older surveys. Over the past two decades, we consider annual differences of 4-monthly means at ground-based stations (since 1996), and virtual observatory series derived from magnetic data of the satellite missions CHAMP (over 2001–2010) and Swarm (since 2013). A priori information is needed to complement the constraints carried by geomagnetic records and solve the ill-posed geomagnetic inverse problem. We use for this purpose temporal cross-covariances associated with auto-regressive stochastic processes of order 2, whose parameters are chosen so as to mimic the temporal power spectral density observed in paleomagnetic and observatory series. We aim this way to obtain as far as possible realistic posterior model uncertainties. These can be used to infer for instance the core dynamics through data assimilation algorithms, or an envelope for short-term magnetic field forecasts. We show that because of the projection onto splines, one needs to inflate the formal model error variances at the most recent epochs, in order to account for unmodeled high frequency core field changes. As a by-product of the core field model, we co-estimate the external magnetospheric dipole evolution on periods longer than 2 years. It is efficiently summarized as the sum of a damped oscillator (of period 10.5 years and decay rate 55 years), plus a short-memory (6 years) damped random walk.

中文翻译：

COV-OBS.x2：来自地面和卫星观测的 180 年地磁场演化

我们提出了涵盖 1840 年至 2020 年期间的地磁场模型 COV-OBS.x2。它主要受到天文台系列、卫星数据以及较早调查的限制。在过去的二十年里，我们考虑了地面站（自 1996 年以来）4 个月平均值的年差异，以及从卫星任务 CHAMP（2001-2010 年）和 Swarm（自 2013 年以来）的磁数据得出的虚拟观测站系列。需要先验信息来补充地磁记录所携带的约束并解决不适定的地磁反问题。为此，我们使用与 2 阶自回归随机过程相关的时间互协方差，其参数的选择是为了模拟在古地磁和天文台系列中观察到的时间功率谱密度。我们的目标是通过这种方式获得尽可能真实的后验模型不确定性。这些可用于通过数据同化算法推断核心动态，或短期磁场预测的包络线。我们表明，由于投影到样条上，需要夸大最近时期的形式模型误差方差，以便考虑未建模的高频核心场变化。作为核心场模型的副产品，我们共同估计了超过 2 年周期的外部磁层偶极子演化。它可以有效地概括为阻尼振荡器（周期为 10.5 年，衰减率为 55 年）加上短记忆（6 年）阻尼随机游走的总和。

更新日期：2020-10-23

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