Advances in soft X-ray emission of the Earth's magnetosphere
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摘要: 太阳风-磁层耦合和地球空间的动力学过程是空间天气的基本驱动要素，在系统尺度上认知这些过程对于空间物理和空间天气的研究至关重要. 太阳风电荷交换（solar wind charge exchange, SWCX）机制的提出，为磁层大尺度特性研究提供了一种全新的探测方式，即地球磁层的软X射线成像. SWCX发生在太阳风中的高价态重离子（例如C6+、N7+、O7+、O8+等）和中性原子或分子（例如地球空间中的中性氢原子，日球层中的中性氢原子和氦原子，彗星和其它行星上的水分子、CO2等）发生碰撞时. 太阳风离子得到一个或多个电子后进入激发态，随后在回到基态的过程中释放出一个或多个软X射线波段的光子. 地球磁层的SWCX软X射线辐射主要发生在日侧的磁鞘和极尖区，因此利用软X射线大范围成像技术可以对磁层进行远距离全景成像，从而在大尺度上认知太阳风-磁层相互作用的基本模式. 在此背景下，中欧联合空间科学卫星计划太阳风-磁层相互作用全景成像卫星（Solar wind Magnetosphere Ionosphere Link Explorer, SMILE）得到立项和实施. SMILE卫星将针对日下点附近区域的磁层顶、弓激波、部分极尖区和地球极光进行成像探测，同时对太阳风等离子体和磁场进行原位测量. SMILE卫星计划于2024—2025年发射. 本文将阐述地球磁层软X射线辐射的机制、回顾磁层软X射线辐射观测证据及辐射特性方面的研究、总结磁层信号的模拟仿真进展、介绍磁层成像探测计划，并提出未来行星磁层软X射线成像探测的概念.Abstract:
The coupling between solar wind and the magnetosphere and the dynamic processes in geo-space are the basic driving factors of space weather. Understanding these processes on the system level is essential to the studies of space physics and space weather. Recent solar wind charge exchange (SWCX) X-ray emission discoveries provide a novel approach to detect the large-scale magnetosphere through soft X-ray imaging. SWCX occurs when high-charge heavy ions such as C6+, N7+, O7+, and O8+ in the solar wind interact with neutral atoms or molecules, such as neutral hydrogen atoms in near-Earth space, neutral hydrogen and helium atoms in the heliosphere, and H2O and CO2 molecules on comets and other planets. Solar wind ions become excited by receiving one or more electrons, and then return to the ground state by releasing one or more photons in the soft X-ray band. The characteristics of the SWCX emissions include the X-ray spectrum showing line emissions corresponding to different species of solar wind particles and neutrons, and fast time variations closely related to solar wind variations. The SWCX soft X-ray emission of the Earth's magnetosphere mainly occurs in the magnetosheath on the dayside and the cusp regions. Therefore, the magnetosphere can be remotely imaged using large-scale soft X-ray imaging technology, allowing the fundamental modes of the interaction between the solar wind and magnetosphere to be recognized on a systematic scale. In this context, the European Space Agency (ESA) and Chinese Academy of Sciences (CAS) jointly proposed the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE). SMILE was approved in 2016 and implemented thereafter. SMILE aims to provide remote sensing measurements of the magnetopause and bow shock around the subsolar region, part of the cusp regions, and the aurora, as well as simultaneous in situ observations of the solar wind plasma and magnetic field. SMILE is planned to be launched in 2024–2025. Other missions have also been proposed to image the Earth's magnetosphere, such as the GEOspace X-ray imager (GEO-X) in Japan, Lunar Environment heliospherics X-ray Imager (LEXI) in the United States, and Lunar-based soft X-ray imager (LSXI) in China. With the advent of soft X-ray imaging missions to detect the magnetopause, evaluating the expected soft X-ray images and developing appropriate techniques to extract 3-dimentional boundary information from the 2-dimentional images is essential. Global magnetohydrodynamic (MHD) and instrument simulations are used to generate expected X-ray images under different solar wind conditions with different viewing geometries. Based on these images, several approaches have been developed to analyze the signals, composing the 'arsenal' or 'toolkit' for magnetopause reconstruction. Each approach has its own advantages and disadvantages, applicable to different situations. This paper introduces the mechanism of magnetospheric X-ray emission, reviews the research on the observational evidence and characteristics of SWCX X-ray emissions, summarizes the progress of simulation studies, presents the missions (or concept) of the magnetospheric X-ray detection, and discusses the possibility of future implementation of X-ray imaging for detecting other planets. -
Key words:
- solar wind-magnetosphere coupling /
- X-ray imaging /
- numerical simulation
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图 1 地球磁层结构示意图. P是热压
Figure 1. Schematic plot of the Earth's magnetosphere. P is the thermal pressure
图 2 慢速太阳风中计算出的SWCX发射光谱. 横轴是能量，纵轴是X射线强度，不同颜色对应不同离子的发射线（修改自Koutroumpa et al., 2009）
Figure 2. Spectrum of SWCX emission calculated for the slow solar wind. The horizontal axis is energy and the vertical axis is X-ray intensity. Different colors correspond to emission lines of different ions (modified from Koutroumpa et al., 2009)
图 3 XMM-Newton卫星在0.3～2.0 keV波段的X射线图像. 从左到右依次为MOS1、MOS2和PN，其中带红色斜线的小圆圈表示要从视野中去除的天文点源，大的黄色圆圈表示用于分析的视野区域
Figure 3. X-ray images of XMM-Newton in the 0.3–2.0 keV band. From left to right: MOS1, MOS2, and PN. The small circle with red diagonal line represents the astronomical point source to be removed from the field of view, and the large yellow circle represents the field of view for analysis
图 4 1/4 keV ROSAT全天空调查图. （a）、（b）分别是移除LET前和移除LET后的全天空X射线背景图（修改自Snowden et al., 2009）
Figure 4. 1/4 keV ROSAT all-sky survey map. (a) and (b) are the all-sky X-ray background images before and after the removal of LET, respectively (modified from Snowden et al., 2009)
图 5 SWCX光谱模型拟合到XMM-Newton在ICME中观测到的光谱. 横轴是能量，上面板的纵轴是强度，下面板的纵轴是模型和观测的拟合方差. 不同颜色的高斯线对应不同离子的发射线（修改自Carter et al., 2010）
Figure 5. SWCX spectral model fitted to the spectra observed by XMM-Newton in an ICME. The horizontal axis is energy, the vertical axis of the upper panel is X-ray intensity, and the vertical axis of the lower panel is the fitting variance of the model and observation. Gaussian lines of different colors correspond to emission lines of different ions (modified from Carter et al., 2010)
图 6 ICME驱动的鞘区（a）和ICME本体（b）作用下磁层X射线辐射示意图. 白球和彩球分别表示太阳风中的质子和离子. ACE卫星位于太阳风中，XMM-Newton卫星位于磁层中，其中白色虚线是它的视线（修改自Zhang et al., 2022b）
Figure 6. Schematic diagram of magnetospheric X-ray emission during the impact of the ICME-driven sheath (a) and the ICME (b). The white and colored balls represent the protons and ions in the solar wind, respectively. The ACE satellite is located in the solar wind and the XMM-Newton satellite is located in the magnetosphere, where the white dotted line is its line of sight (modified from Zhang et al., 2022b)
图 7 SWCX信号强度与太阳风质子通量的统计关系. 中间面板的横轴是太阳风质子通量，纵轴是相关性系数. 顶部面板中的直方图表示线性相关系数的绝对值，星号曲线表示该相关性的百分比. 右面板中的直方图表示事件数，星号曲线表示每个相关系数区间的平均太阳风质子通量. 红色、黑色和橙色分别表示正相关、不相关和负相关. 绿色和蓝色分别表示太阳风质子通量的突然增加和减少. 粉色代表所有情况的总和（修改自Zhang et al., 2022a）
Figure 7. Statistical relationship between SWCX signal intensity and solar wind proton flux. The horizontal axis of the middle panel is the solar wind proton flux and the vertical axis is the correlation coefficient. The histogram in the top panel represents the absolute value of the linear correlation coefficient and the asterisk curve represents the percentage of this correlation. The histogram in the right panel represents the number of events and the asterisk curve represents the average solar wind proton flux in each correlation coefficient interval. Red, black, and orange indicate positive correlation, uncorrelated, and negative correlation, respectively. Green and blue indicate the sudden increase and decrease of solar wind proton flux, respectively. Pink represents the sum of all cases (modified from Zhang et al., 2022a)
图 8 SMILE任务X射线成像仿真图. 从左到右太阳风等离子体数密度分别为：5、20、35 cm-3. 白色曲线所示为极尖区边界，红色曲线为赤道面和子午面内的磁层顶位置（修改自Sun et al., 2019）
Figure 8. Prediction of the X-ray image to be observed by the SMILE mission. From left to right: the solar wind number density is 5, 20, and 35 cm−3. The white curves depict the boundaries of cusps and the red curves are the magnetopause position in the equatorial and meridian plane (modified from Sun et al., 2019)
图 9 SXI视场内X射线强度图（a1-a3）、光子计数图像（b1-b3）及切向拟合法获得的磁层顶位置（c1-c3）. 由左到右太阳风等离子体密度逐渐增强（修改自Guo et al., 2022）
Figure 9. Predicted X-ray intensity image (a1-a3), photon counts image (b1-b3), and magnetopause position derived from the photon counts image via Tangent Fitting Approach (c1-c3). The solar wind number density increases from left to right (modified from Guo et al., 2022)
图 10 三维磁层反演的方法总结. 从左到右分别是反演用到的信息，反演的方法和需要的假设. 反演用到的信息为一幅、两幅或多幅图. 反演的方法有边界拟合法（BFA）、切向拟合法（TFA）、切线方向法（TDA）和计算机断层分析法（CTA）. 需要的假设有磁层顶位型函数、最大X射线强度在切线方向、磁层顶位型恒定等（修改自Sun et al., 2020）
Figure 10. Summary plot of the 3D magnetopause reconstruction approach from X-ray images. From left to right: information used for reconstruction, reconstruction approaches, and required assumptions. The information of reconstruction is from one, two, to a series of pictures. The reconstruction approaches include the Boundary Fitting (BFA), Tangent Fitting (TFA), Tangential Direction (TDA), and Computed Tomography Approaches (CTA). The required assumptions are the forms of magnetopause, the maximum X-ray intensity is in the tangent direction, and different images are taken for the same magnetopause profile (modified from Sun et al., 2020)
图 11 使用TFA方法由X射线图像提取磁层顶边界. （a）SMILE视场内的X射线强度仿真图，黑色曲线为基于X射线图像得到的磁层顶切线方向，红线为通过TFA找到的最佳匹配的切线方向. （b）TFA反演得到的三维磁层顶. （c）不同平面内的等离子体热压图，白线和红线分别为MHD和TFA反演得到的磁层顶位置（修改自Sun et al., 2020）
Figure 11. Magnetopause position derived by utilizing the Tangent Fitting Approach. (a) Simulated X-ray image inside the FOV. The black curve represents the tangent directions derived from the X-ray image, and the red curve shows the best match calculated from the parameterized function. (b) Reconstructed 3-D magnetopause. (c1-c4) Contours of plasma thermal pressure on different planes rotating around the x axis by 0°, 30°, 60°, and 90°, respectively, starting at the equatorial plane. The white and red curves show the MHD and reconstructed magnetopause, respectively (modified from Sun et al., 2020)
图 12 STORM仪器原型及SMILE软X射线成像仪3D模型. 右图中从左到右依次是前端电子设备、望远镜结构、龙虾眼望远镜和杂散光挡板（修改自Collier et al., 2015; Raab et al., 2016）
Figure 12. Prototype of STORM and 3D model of SMILE SXI. From left to right: front-end electronics, telescope structure, lobster-eye telescope, and straylight baffle (modified from Collier et al., 2015; Raab et al., 2016)
图 13 SMILE任务概念图（修改自Branduardi-Raymont et al., 2016）
Figure 13. Concept of the SMILE mission (modified from Branduardi-Raymont et al., 2016)
图 14 月基软X射线成像仪观测地球磁层概念图（修改自Guo et al., 2021）
Figure 14. Sketch of the lunar based X-ray imager observing the Earth's magnetopause (modified from Guo et al., 2021)
表 1 天文X射线卫星
Table 1. Astronomical X-ray satellite
卫星 科研单位 任务时间 仪器 能量范围/keV 视场 伦琴（ROSAT） 德、美、英 1990-06-01至1999-02-12 位置敏感比例计数器（PSPC） 0.1～2.5 ~2° 钱德拉X射线天文台（Chandra） 美 1999-07-23 至今 高级CCD成像光谱仪（ACIS-I） 0.2～10 $ 16\text{'}\times 16\text{'} $ 高级CCD成像光谱仪（ACIS-S） $ 8\text{'}\times 48\text{'} $ 多镜片X射线观测卫星（XMM-Newton） 欧 1999-12 至今 欧洲光子成像相机（EPIC-MOS） 0.1～15 $ 33\text{'}\times 33\text{'} $ 欧洲光子成像相机（EPIC-PN） $ 27.5\text{'}\times 27.5\text{'} $ 朱雀（Suzaku） 日、美 2005-07-10至2015-09-02 X射线成像光谱仪（XIS） 0.2～12 $ 18\text{'}\times 18\text{'} $ 
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表 2 国际磁层成像观测计划
Table 2. Imaging missions of the Earth's magnetopause
观测计划 国家 平台类型 发射时间 观测视场 运行轨道 微笑卫星（SMILE） 中+欧 小型卫星 2024—2025 16°×27° 高地球轨道 空间X射线成像仪（GEO-X） 日本 科学卫星 2023—2025 4°×4° 高地球轨道/近月轨道 月基软X射线成像仪（SXI） 中国 月球科研站 待定 15°×30° 月表 月球环境日球层X射线成像仪（LEXI） 美国 月球着陆点 待定 ~9°×9° 月表
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