日球层内外能量中性原子的探测

能量中性原子(Energetic Neutral Atoms, ENA, 简称能原子)是指在日球层内外空间, 拥有>0.1keV动能的原子.在此空间领域并没有温度>106K的中性气体, 但却充满动能>0.1keV的正离子.因此能原子A应该是A+离子与原地稀薄气体B原子或分子交换电荷所产生的, 即A++B→A+B+. 电荷交换涉及极小的动能变化, 新生的能原子A和离子B+基本上各自保持原有动能. 离子B+随即被当地磁场俘获, 能原子A则脱离磁场约束并携带其原属离子群的成分和能量信息而直线运动, 成为遥测空间等离子体的有效媒介. 美国人造卫星 IBEX (Interstellar Boundary Explorer) 直接探测得到来自日球层以外星际空间的能原子, 大幅延伸了利用能原子遥测空间等离子体的领域. 本文据此论述了空间能原子的发现, 综述了探测空间能原子的基本概念与实例、取得的主要成果、仪器设计和研制进展以及未来空间利用能原子遥测的发展趋势. 

Detecting Energetic Neutral Atoms in and out of the Heliosphere

By Energetic Neutral Atoms (ENA), we mean atoms having kinetic energies >0.1keV found in and out of the heliosphere. In the regions lacking any neutral gas of temperatures >106K, while abundant in ions of kinetic energies >0.1keV, surely an ENA has to be the product of charge exchange between an ion and an atom or molecule B of the local gas, i.e. A++B → A+B+. Because energy exchange is minute during charge exchange, the newly formed ENA A and ion B+ can be considered keeping their respective original kinetic energies. Instantly, ion B+ is trapped by the local magnetic field, but ENA A is liberated and flies yonder, to be a sample of its originating plasma in mass and energy, thus providing a precious medium for remote sensing the parent plasma. The recent success of NASA's IBEX (Interstellar Boundary EXplorer) Mission in directly detecting ENA coming from the local interstellar medium greatly expanded the scope of ENA remote sensing. Here, we review the discovery of ENA in high altitudes, the basic concept and existing examples of remote sensing of ENA, recent results, and the designs and development of the necessary instruments, and also offer a prospective on ENA-sensing in the near future.