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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A variety of chemical reactions occurring in the Earth’s upper atmosphere generate several emissions spanning a wide range of wavelengths. These are referred to as airglow in the literature and are responsible for the spectacular aurora at higher latitudes. Sophisticated instruments on-board satellites and ground-based systems have been used to monitor these emissions for investigating the regions where these emissions originate. Far ultraviolet emissions (FUV) have been used in remote sensing techniques for probing Earth’s thermosphere-ionosphere system, especially by satellites. These include contributions from OI 135.6 nm and N2 Lyman-Birge-Hopfield (LBH) vibrational bands. Spectral imaging in the FUV bands ~ 132 – 160 nm by the Global-Scale Observations of the Limb and Disk (GOLD) mission provides daytime measurements of temperature (TDisk) and composition of O/N2. To enable a more accurate determination of composition and temperature changes of Earth’s thermosphere-ionosphere using satellite-based missions, an accurate determination of the emission cross-sections and their radiative lifetime are necessary. The primary goal for this program is to determine the UV emission cross sections needed to accurately model remote sensing observations of the Earth’s dayglow. As Geophysical remote sensing techniques have improved and observations of the far ultraviolet (FUV: 125.0–250.0 nm) have led to important discoveries in Space Weather, UV spectroscopy methods with imaging capability have assumed an increasingly important role in both the laboratory and Terrestrial observations. The ongoing GOLD mission built at the University of Colorado (CU) uses the dayglow UV observations of the OI (135.6 nm) and N2 Lyman-Birge-Hopfield (LBH) band system (125-250 nm), both optically forbidden emissions. The proxy for the incident solar flux (QEUV) producing photoelectrons is derived using these emissions. In the dayglow, a unique signature of the O/N2 column density ratio are derived from satellite-based UV observations of the intensity ratio between the OI (135.6 nm) and N2 LBH band system (125-250 nm) both optically forbidden emissions. The O/N2 column density ratio and thermosphere temperature measurements are keys to understanding ionosphere and thermosphere composition and dynamical changes on a global scale under all geomagnetic conditions using Earth-orbiting satellites like GOLD. The failure to accurately measure the emission cross section contributes to the systematic uncertainty for O/N2 and QEUV retrievals (~ 30% reported for O/N2). The uniqueness of this proposal 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 an apparatus designed to account for cascade contributions (i.e., to eliminate common errors like wall collisions). These measurements will improve derivation of thermosphere-ionospheric parameters using satellite based terrestrial FUV measurements.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.