彗星大气中的太阳风电荷交换。I. H $_2$ O中He和H粒子的电荷变化截面和电离截面
Solar wind charge exchange in cometary atmospheres. I. Charge-changing and ionization cross sections for He and H particles in H$_2$O
摘要：
太阳风电荷变化反应对彗星大气层的物理化学至关重要，通过将快速轻的太阳风离子有效转换为缓慢重的彗星离子，使太阳风负载质量。为了理解这些过程，并把它们放在太阳风等离子体与中性大气相互作用的背景下，数值或分析模型是必要的。这些模型的输入，如碰撞截面和化学，是至关重要的。本文讨论了水煤气中氢和氦粒子的电荷变化和电离截面的实验测量值的记录和拟合，重点是作为太阳风整体速度特征的低能/低速范围（$< 20$ keV u $^{-1}$ / $2000$ km s $^{-1}$）。我们提供了多项式拟合电荷变化和电离反应的截面，并列出了未来研究的实验需求。我们计算了麦克斯韦平均截面，并将它们与从$10^5$到$10^6$ K（$12-130$ eV）的太阳风温度的二元多项式拟合。在典型的太阳风速度下，He $^{2+}$的单电子和双电子捕获占主导地位。相应地，H $^+$的单电子捕获和H $^-$的单电子损失在这些速度下占主导地位，导致高能中性原子（ENA）的产生。电离截面都在高于$20$ keV的能量处达到峰值，并且预期在总离子产生中发挥中等作用。太阳风麦克斯韦温度的影响被认为是最大的截面峰值在较高的能量，这表明在彗星和行星环境中的冲击结构的局部加热可能有利于以前被认为是可以忽略不计的过程。
英文摘要：
Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet, mass-loading the solar wind through an effective conversion of fast light solar wind ions into slow heavy cometary ions. To understand these processes and place them in the context of a solar wind plasma interacting with a neutral atmosphere, numerical or analytical models are necessary. Inputs of these models, such as collision cross sections and chemistry, are crucial. Book-keeping and fitting of experimentally measured charge-changing and ionization cross sections of hydrogen and helium particles in a water gas are discussed, with emphasis on the low-energy/low-velocity range that is characteristic of solar wind bulk speeds ($< 20$ keV u$^{-1}$/$2000$ km s$^{-1}$). We provide polynomial fits for cross sections of charge-changing and ionization reactions, and list the experimental needs for future studies. We calculated Maxwellian-averaged cross sections and fitted them with bivariate polynomials for solar wind temperatures ranging from $10^5$ to $10^6$ K ($12-130$ eV). Single- and double-electron captures by He$^{2+}$ dominate at typical solar wind speeds. Correspondingly, single-electron capture by H$^+$ and single-electron loss by H$^-$ dominate at these speeds, resulting in the production of energetic neutral atoms (ENAs). Ionization cross sections all peak at energies above $20$ keV and are expected to play a moderate role in the total ion production. The effect of solar wind Maxwellian temperatures is found to be maximum for cross sections peaking at higher energies, suggesting that local heating at shock structures in cometary and planetary environments may favor processes previously thought to be negligible.
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