Collaborative Research: Laboratory Measurements of Oxygen (O) and Nitrogen (N2) Ultraviolet (UV) Cross Sections by Particle Impact for Remote Sensing of Thermosphere O/N2 Variation

天基遥感被广泛用于测量地球大气层。这些测量大多是基于探测大气分子和原子自然产生的光发射。为了正确地解释这些测量结果，理解这些粒子中的光发射是如何发生的是至关重要的。一个称为发射截面的量是描述发射过程的关键参数。虽然该参数有时可以从观察中推断出来，但在受控环境中的实验室测量对于提供该参数的确定估计至关重要。该项目的目标是确定NASA航天器遥感观测地球日光所需的紫外线发射截面。在日光中，O/N2柱密度比的一个独特特征来自于基于卫星的紫外线观测，来自OI （135.6 nm）和N2 Lyman-Birge-Hopfield （LBH）波段系统（125-250 nm）的强度比，两者都是光学禁止发射。O/N2柱密度比是利用地球轨道卫星（如GOLD (global -scale Observation of The Limb and Disk)）了解所有地磁条件下全球尺度电离层和热层组成变化的关键。该团队在最后一个资助期的研究表明，过去50年的实验室光谱学未能测量级联诱导的紫外光谱，也未能确定LBH的振动强度或级联发射截面，这占地球最强FUV发射总发射截面的约30%。由于地面气辉观测和正演模式计算之间的二分法，这一失败在文献中引发了一场持续了十多年的争议。项目组在实验室测量了30 - 200ev电子激发的N2 LBH带系统的FUV级联诱导光谱。级联跃迁开始于两个过程：涉及两个状态（a和w）的辐射和碰撞诱导电子跃迁（CIETs），随后是级联诱导跃迁x。在这个项目中，团队将研究10-30 eV的阈值发射截面。该项目的独特之处在于，在相同的实验条件下，利用设计了比以往实验室测量大10倍的碰撞室的特殊装置，更准确地测量了原子O和分子N2的绝对Qem（总发射截面）和Qcasc（级联诱导截面），以适当地考虑级联的贡献。LASP的实验室光谱学迈出了里程碑式的一步，首次测量了N2的光禁级联诱导紫外光谱。然而，要完成对LBH和其他重要的光禁跃迁的研究还需要做很多工作，这些跃迁始于第一个腔室（0.75 m半径）。第二个腔室半径为2米（是第一个腔室的两倍多），为原子和分子物理学开辟了一个全新的领域。在过去的50年里，单散射电子冲击诱导荧光光谱的寿命范围为1-100ns（主要允许紫外电子跃迁）。科罗拉多大学的两个大型真空室甚至可以研究寿命为10ms的更严格禁止的过渡，以捕获100ms的全光发射和部分光发射，并能够建模到1000ms。这个涉及光学禁跃迁的新物理学领域允许研究以前从未在单散射条件下测量过的光谱，例如LBH， CO的Cameron波段和N2的Vegard-Kaplan波段系统。该提案的pi有50年的广泛跟踪记录，测量地球和行星大气感兴趣的原子和分子的绝对Qem和Qcasc。实验和建模的多方面方法为了解地球热层并将其范围扩展到其他行星大气提供了一个关键的热层参数（O/N2）。项目组的数据为当前和未来的卫星任务（例如，在国际空间站携带一套仪器的大气-空间相互作用监测仪）进行高分辨率研究提供了基准参考。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Space-based remote sensing is widely used to measure the Earth’s atmosphere. Most of these measurements are based on detection of naturally occurring light emissions from the atmospheric molecules and atoms. To interpret these measurements correctly, understanding how the light emission occurs in these particles is of fundamental importance. A quantity called emission cross section is a key parameter that describes the emission process. While this parameter can sometimes be inferred from observation, laboratory measurement in a controlled environment is essential to provide a definitive estimate for such parameter. The goal of this project is to determine the UV emission cross sections needed for remote sensing observations of the Earth’s dayglow by NASA spacecraft. In the dayglow, a unique signature of the O/N2 column density ratio, derived from satellite-based UV observations, comes from the intensity ratio of the OI (135.6 nm) and N2 Lyman-Birge-Hopfield (LBH) band system (125-250 nm), both optically forbidden emissions. The O/N2 column density ratio is key to understanding ionosphere and thermosphere composition changes on a global scale under all geomagnetic conditions from Earth-orbiting satellites, e.g. GOLD (Global-scale Observation of the Limb and Disk). The team’s research in the last funding period shows laboratory spectroscopy for the past 50-years has failed to measure the cascade-induced UV spectrum and determine LBH vibrational intensities or cascade emission cross sections, which accounts for ~30% of the total emission cross section, of the Earth’s strongest FUV emission. This failure has precipitated a controversy in the literature that has persisted for over a decade due to the dichotomy between terrestrial airglow observations and forward model calculations. The project team have measured in the laboratory the FUV cascade-induced spectrum of the LBH band system of N2 excited by 30–200 eV electrons. The cascading transition begins with two processes: radiative and collision-induced electronic transitions (CIETs) involving two states (a and w), which are followed by a cascade induced transition a X. In this project, the team will investigate the threshold emission cross sections from 10-30 eV. The uniqueness of this project is the measurement of both the atomic O and molecular N2 absolute Qem (total emission cross section) and Qcasc (cascade-induced cross section) more accurately with a special apparatus designed with a ten times bigger collision chamber than previous laboratory measurements to properly account for the cascade contributions under the same experimental conditions.Laboratory spectroscopy at LASP has made a monumental step by measuring the optically-forbidden cascade-induced UV spectrum of N2 for the first time. However, much work needs to be done to complete the study of LBH and other important optically forbidden transitions that began in the first chamber (0.75 m radius). The second chamber with a radius of 2 m (more than double that of the first chamber) allows a whole new realm of atomic and molecular physics. The lifetime ranges available for laboratory studies of single scattering electron impact induced fluorescence spectra were 1-100ns (mainly allowed UV electronic transitions) for the past 50 years prior. With two large vacuum chambers at the University of Colorado even more highly forbidden transitions with lifetimes to 10ms can be studied to capture the full light emission and partial light emission to 100ms with an ability to model to 1000ms. This new field of physics involving optically forbidden transitions allows the study of spectra never before measured in single-scattering conditions such as that from LBH, the Cameron Bands of CO and Vegard-Kaplan band system of N2. The PIs of this proposal have an extensive 50 year track record of measuring the absolute Qem and Qcasc for atoms and molecules of interest to Earth and planetary atmospheres. The multi-facet approaches of both experiment and modeling promise a critical thermosphere parameter (O/N2) for an understanding of the Earth’s thermosphere and extending in scope to other planetary atmospheres. The project team’s data provides a bench-mark reference for high resolution studies by current and future satellite missions (e.g., Atmosphere-Space Interactions Monitor (ASIM) carrying a suite of instruments on the International Space Station).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.