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

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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.