Storm time neutral density assimilation in the thermosphere ionosphere with TIDA Journal of Space Weather and Space Climate

To improve Thermosphere–Ionosphere modeling during disturbed conditions, data assimilation schemes that can account for the large and fast-moving gradients moving through the modeled domain are necessary. We argue that this requires a physics based background model with a non-stationary covariance. An added benefit of using physics-based models would be improved forecasting capability over largely persistence-based forecasts of empirical models. As a reference implementation, we have developed an ensemble Kalman Filter (enKF) software called Thermosphere Ionosphere Data Assimilation (TIDA) using the physics-based Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model as the background. In this paper, we present detailed results from experiments during the 2003 Halloween Storm, 27–31 October 2003, under very disturbed (Kp = 9) conditions while assimilating GRACE-A and B, and CHAMP neutral density measurements. TIDA simulates this disturbed period without using the L1 solar wind measurements, which were contaminated by solar energetic protons, by estimating the model drivers from the density measurements. We also briefly present statistical results for two additional storms: September 27 – October 2, 2002, and July 26 – 30, 2004, to show that the improvement in assimilated neutral density specification is not an artifact of the corrupted forcing observations during the 2003 Halloween Storm. By showing statistical results from assimilating one satellite at a time, we show that TIDA produces a coherent global specification for neutral density throughout the storm – a critical capability in calculating satellite drag and debris collision avoidance for space traffic management.

为改善扰动条件下的热层-电离层（Thermosphere–Ionosphere）建模工作，亟需能够处理模拟区域内大尺度且快速演变的梯度的资料同化方案。我们认为，这一需求需要采用具备非平稳协方差的物理驱动背景场模型。相较于经验模型大多基于持续性的预报方式，采用物理驱动模型的额外优势在于其预报性能可得到显著提升。

作为参考实现方案，我们以物理驱动的耦合热层-电离层-等离子体层电动力学（Coupled Thermosphere Ionosphere Plasmasphere electrodynamics，CTIPe）模型作为背景场，开发了一款名为热层电离层资料同化（Thermosphere Ionosphere Data Assimilation，TIDA）的集合卡尔曼滤波（ensemble Kalman Filter，enKF）软件。

本文展示了2003年10月27日至31日2003年万圣节风暴事件期间的实验详细结果，该事件处于极强扰动（Kp = 9）条件下，实验中同化了GRACE-A、GRACE-B以及CHAMP的中性密度观测数据。TIDA无需使用受太阳高能质子污染的L1波段太阳风观测数据，而是通过中性密度观测数据估算模型驱动参数，以此模拟该扰动时段。

我们还简要展示了另外两次风暴事件的统计结果：2002年9月27日至10月2日，以及2004年7月26日至30日，以证明同化中性密度分析结果的性能提升并非2003年万圣节风暴事件中强迫观测数据失真所带来的假象。通过展示单次同化单颗卫星观测数据的统计结果，我们证明TIDA能够在整个风暴事件期间生成一致的中性密度全球分析场——这一能力对于计算卫星阻力、开展空间交通管理中的碎片碰撞规避工作至关重要。