| 基于X射线能谱拟合的地球中性大气数密度反演模拟及误差分析 |
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| 引用本文： | 余道淳, 李保权, 刘亚宁, 李海涛. 2022. 基于X射线能谱拟合的地球中性大气数密度反演模拟及误差分析. 地球物理学报, 65(11): 4163-4173, doi: 10.6038/cjg2022Q0158 |
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| 作者姓名： | 余道淳 李保权 刘亚宁 李海涛 |
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| 作者单位： | 中国科学院国家空间科学中心,北京 100190;中国科学院大学,北京 100049;中国科学院国家空间科学中心,北京 100190 |
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| 基金项目： | 国家重点研发计划(2017YFB0503300)，国家自然科学基金(Nos.41604152，U1938111)，中国科学院空间科学战略性先导科技专项(XDA04060900，XDA15020800，XDA15072103)和中国科学院青年创新促进会(2018178)资助 |
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| 摘 要： | 利用X射线掩星技术可以反演得到地球高中层和低热层总的中性大气密度，这是其他手段很难探测到的区域.通过模拟NICER望远镜观测蟹状星云(Crab Nebula)的地球大气掩星过程，在1~10 keV能量范围内，对X射线地球大气掩星期间的光变曲线和能谱建模，通过对不同海拔高度范围内的能谱模型添加噪声，生成能谱仿真数据，通过非线性最小二乘拟合方法对模型和数据进行拟合，得到地球大气密度反演结果.作为NICER望远镜主要的科学仪器，X射线定时仪(XTI)拥有很高的吞吐量和背景噪声的低敏感性，可以模拟生成信噪比很高的光变曲线和能谱数据.通过光变曲线的模拟结果发现，对于能量范围在1~10 keV的X射线，掩星发生在100~200 km的海拔高度范围内，对该掩星过程中1~10 keV的衰减能谱进行提取，并用能谱模型对其进行拟合从而可以反演得到地球中性大气密度.本文中，NRLMSISE-00模型的大气密度被选为真值，用来生成能谱仿真数据，NRLMSIS 2.0模型的大气密度被选为的初值，用来构建模型能谱并与能谱仿真数据进行拟合.通过对不同海拔高度范围内的模型能谱和仿真数据的拟合，得到修正因子γ的最佳拟合值，计算修正因子γ的最佳拟合值与初值的乘积，得到地球大气密度的反演结果.对不同海拔高度范围内的模型能谱和仿真数据的拟合结果进行统计学分析，计算最佳拟合模型和仿真数据之间的2/dof和p值(p-value)，从而评价最佳拟合模型和仿真数据之间的拟合优度，并计算地球大气密度的反演结果与真值之间的测量误差，发现100~115 km高度范围内的大气密度反演结果与模型真值之间的测量误差介于-0.067%~10.22%之间，115~180 km高度范围内的大气密度反演结果与真值之间的测量误差介于-3.67%~1.92%之间，180~200 km高度范围内的大气密度反演结果与真值之间的测量误差介于-8.03%~0.305%之间，最后分析了影响反演结果测量误差的因素.
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| 关 键 词： | X射线掩星 能谱拟合 地球大气密度反演 误差分析 |
| 收稿时间： | 2022-03-09 |
| 修稿时间： | 2022-05-06 |
Simulationand error analysis on the retrieval of Earth neutral atmosphere number density based on X-ray energy spectrum fitting |
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| YU DaoChun, LI BaoQuan, LIU YaNing, LI HaiTao. 2022. Simulationand error analysis on the retrieval of Earth neutral atmosphere number density based on X-ray energy spectrum fitting. Chinese Journal of Geophysics (in Chinese), 65(11): 4163-4173, doi: 10.6038/cjg2022Q0158 |
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| Authors: | YU DaoChun LI BaoQuan LIU YaNing LI HaiTao |
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| Affiliation: | 1. National Space Science Center, Chinese Academy of Sciences, Beijing 100190, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China |
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| Abstract: | X-ray occultation technique can be used to retrieve the Earth's total neutral atmospheric density in the upper mesosphere and lower thermosphere, which is difficult to detect with other methods. By simulating the Earth's atmospheric occultation process of the Crab Nebula observed by the NICER telescope, we model the light curve and energy spectrum during the X-ray Earth's atmospheric occultation in the energy range of 1~10 keV. The energy spectrum simulation data are generated by adding noise to the energy spectrum model at different altitudes. The model and data are fitted by nonlinear least square fitting method, and the retrieved results of Earth's atmospheric density are obtained. As the principal scientific instrument of the NICER telescope, the X-ray Timing Instrument (XTI) has high throughput and low sensitivity to background noise, and the light curves and energy spectrum data of high signal-to-noise ratio can be simulated. Through the simulation results of light curve, it is found that the occultation range of X-rays in the energy range of 1 keV to 10 keV is 100~200 km. The attenuation energy spectrum from 1 to 10 keV in the occultation process is extracted, and the energy spectrum model is used to fit it so that the Earth's neutral atmospheric density could be inverted. In this paper, the atmospheric density of the NRLMSISE-00 model is selected as the truth value to generate energy spectrum simulation data, and the atmospheric density of the NRLMSIS 2.0 model is selected as the initial value of the model energy spectrum, and then the energy spectrum model is used to fit the energy spectrum simulation data to obtain the retrieved results. By fitting model energy spectrum and simulation data in different altitude range, the best fitting value of the correction factor γ is obtained, and the product of the best fitting value of the correction factor γ and the initial value is calculated, and the retrieved result of Earth's atmospheric density is obtained. Statistical analysis is made on the fitting results of model energy spectrum and simulation data in different altitude ranges, and the 2/dof and p-value between the best fitting model and simulation data are calculated, so as to evaluate the goodness of fit between the best fitting model and simulation data. Based on the best fitting of the above simulation data and the energy spectrum of the model, the measurement error between the retrieved result and the truth value of the Earth's atmospheric density is calculated. It is found that the measurement error between the retrieved result and the truth value of the Earth's atmospheric density is -0.067%~10.22% in the altitude range of 100~115 km. The measurement error between the atmospheric density retrieved results and the true values in the altitude range of 115~180 km is -3.67%~1.92%. The measurement error between the atmospheric density retrieved results and the true values in the altitude range of 180~200 km is -8.03%~0.305%. Finally, the factors affecting the measurement error of retrieved results are analyzed. |
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| Keywords: | X-ray occultation Spectrum fitting Retrieval of Earth's atmospheric density Error analysis |
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