NSF Award
0850348
As remote sensing techniques have been refined and the desire for global imaging has increased, spectroscopic observations of atomic oxygen (O) and molecular nitrogen (N2) in the far ultraviolet (FUV) have assumed an important role. The central objective of this project is to determine accurate (~ 15%) electron-induced emission cross-sections for the most important atomic and molecular emissions used for remote sensing of the O/N2 density ratio in the upper atmosphere. This ratio is a primary tool for observing the large scale changes in thermospheric composition that result from geomagnetic and solar variability. The density ratio of O/N2 during the daytime is determined directly from analysis of the intensity ratio of electron-induced ultraviolet emissions of their long-lived excited states, such as the OI emission at 135.6 nm and those from the N2 Lyman-Birge-Hopfield (LBH) bands. For OI 135.6 nm only one measurement and one theoretical calculation of cross-section, both reported more than 30 years ago, are found in the literature and they differ by a factor of two. For N2 LBH there is only one direct measurement of the emission cross-sections, published more than two decades ago. Subsequent empirical calculations based on aeronomical observations gave LBH cross-sections approximately 1.6 times higher. It was suggested that this difference could be due to either limitations in the experimental techniques used when measuring the emissions from one of the long-lived states or a failure to properly account for the cascade contributions of some of the upper states or both. To best fit remote sensing observations, aeronomy modelers have sometimes adjusted the existing cross-sections by as much as 200%. It is uncertain whether to scale to the cross-section of OI 135.6 nm (decreasing it) or to the cross-section of N2 LBH (increasing it) in order to obtain agreement for O/N2 between observations from the TIMED satellite's Global Ultraviolet Imager instrument and the empirical MSIS (Mass-Spectrometer-Incoherent-Scatter) model. As the ratio O/N2 plays a pivotal role in Space Weather studies, accurate absolute emission cross-sections of OI and LBH are important. The project will therefore undertake two tasks: (1) To simultaneously measure the absolute cross-sections of both the OI 135.6 and N2 LBH emissions using the same state-of-the-art experimental facilities at the Laboratory for Atmospheric and Space Physics (LASP) of University of Colorado at Boulder (CU) and the Jet Propulsion Laboratory (JPL); and (2) To use the new cross-sections in an analysis to be performed at the Florida Space Institute (FSI in collaboration with Computational Physics Inc. (CPI), in order to improve use and understanding of OI 135.6 nm and N2 LBH observations. The broader impacts of the project include the provision of high-accuracy cross-sections to the community; the cross sections will enable improved understanding of Earth's atmosphere as well as other oxygen- and N2-bearing planetary atmospheres. Students will participate in the research activities which focus on UV spectroscopy, an important remote sensing technique used by the Earth-orbiting Great Observatories.
