X-Ray and extreme ultraviolet emissions from comets
X-Ray and extreme ultraviolet emissions from comets
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彗星的 X 射线和极紫外线发射
DOI：
10.1023/b:spac.0000046754.75560.80
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发表时间：
2004
影响因子：
10.3
通讯作者：
P. Stancil
中科院分区：
文献类型：
--
作者：
V. Krasnopolsky;J. Greenwood;P. Stancil
自 1996 年发现彗星以来，在 X 射线和 EUV 发射的观测、理论和理解方面取得了重大进展。这一发现非常令人费解，因为彗星的 X 射线发射效率似乎比月球高 80 万倍。检测到的发射是彗星的一般特性，目前已在五个轨道天文台的 13 颗彗星中进行了检测和分析。 2000年之前的观测研究基于X射线相机和低分辨率（E/δE≈1.5-3）仪器，重点关注X射线的形态、它们与彗星中气体和尘埃产生以及太阳X射线和太阳风的相关性。即使这些观测结果也使得选择太阳风重离子和彗星中性粒子之间独特的电荷交换作为主要激发过程成为可能。最近发布的光谱质量要好得多，可以识别由太阳风带到彗星的 O、C、Ne、Mg 和 Si 多电荷离子的发射。观测到的光谱已用于研究太阳风的成分及其变化。对彗星中电荷交换引起的 X 射线和 EUV 光子激发、阿克尘埃粒子对太阳光子的散射、高能电子撞击和轫致辐射、彗星与行星际尘埃之间的碰撞以及彗星中的太阳 X 射线散射和荧光进行了理论分析。这些分析证实电荷交换是主要的激发机制，它负责观察到的发射的 90% 以上，而其他每个过程仅限于百分之几或更少。考虑了电荷交换理论和电荷交换的不同计算方法。回顾了与彗星条件相关的电荷交换的实验室研究。实验室测量的电荷交换的总横截面和状态选择性横截面被制成表格。讨论了彗星中电荷交换合成光谱的模拟。彗星的 X 射线和 EUV 发射涉及彗星物理学、基础物理学、X 射线光谱学和空间物理学等不同学科和领域。
There is significant progress in the observations, theory, and understanding of the x-ray and EUV emissions from comets since their discovery in 1996. That discovery was so puzzling because comets appear to be more efficient emitters of x-rays than the Moon by a factor of 80 000. The detected emissions are general properties of comets and have been currently detected and analyzed in thirteen comets from five orbiting observatories. The observational studies before 2000 were based on x-ray cameras and low resolution (E/δE≈1.5–3) instruments and focused on the morphology of x-rays, their correlations with gas and dust productions in comets and with the solar x-rays and the solar wind. Even those observations made it possible to choose uniquely charge exchange between the solar wind heavy ions and cometary neutrals as the main excitation process. The recently published spectra are of much better quality and result in the identification of the emissions of the multiply charged ions of O, C, Ne, Mg, and Si which are brought to comets by the solar wind. The observed spectra have been used to study the solar wind composition and its variations. Theoretical analyses of x-ray and EUV photon excitation in comets by charge exchange, scattering of the solar photons by attogram dust particles, energetic electron impact and bremsstrahlung, collisions between cometary and interplanetary dust, and solar x-ray scattering and fluorescence in comets have been made. These analyses confirm charge exchange as the main excitation mechanism, which is responsible for more than 90% of the observed emission, while each of the other processes is limited to a few percent or less. The theory of charge exchange and different methods of calculation for charge exchange are considered. Laboratory studies of charge exchange relevant to the conditions in comets are reviewed. Total and state-selective cross sections of charge exchange measured in the laboratory are tabulated. Simulations of synthetic spectra of charge exchange in comets are discussed. X-ray and EUV emissions from comets are related to different disciplines and fields such as cometary physics, fundamental physics, x-rays spectroscopy, and space physics.