A Novel Faraday Filter-based Spectrometer for Studying the Sodium Airglow and Measuring the Atomic Oxygen Mixing Ratio in the Upper Mesosphere

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This project will develop and deploy a new optical instrument, a compact spectrometer, that will enable new measurements of the mesospheric sodium nightglow. One of the primary aims is to determine the ratio of two sodium emission lines, the D2 and D1. Early studies predicted that the D2/D1 ratio might differ from its quantum statistical value of 2 due to chemical interactions. However, measurements taken three decades ago using a Fabry-Perot instrument reported a value of 1.98±0.1; since then, 2 has taken to be the correct value without further investigation. Recently, T. G. Slanger, J. M. C. Plane and others have searched through sky spectra from Keck telescopes taken over 8 years, and deduced that depending on atmospheric conditions, the value NaRD = I(D2)/I(D1) lies between 1.2 and 1.8. A clear seasonal dependence of this ratio, with higher values in equinox and lower values in solstice, was noted as well. They proposed a modified Chapman mechanism which suggests competing pathways of NaO+O reactions produce Na nightglow emission. Substantiated by laboratory experiments, they concluded that the ratio NaRD is a reliable proxy for the measurement of the [O]/[O2] ratio. Since atomic oxygen is probably the most important species in determining the chemistry and dynamics of the mesosphere and lower thermosphere (MLT), even an indirect real-time determination of [O]/[O2] could greatly advance our understanding of the MLT energy balance, especially since there is no direct ground-based method for observing the atomic oxygen concentration, [O]. Through this project, a novel, compact spectrometer, based on a pair of modified Na vapor Faraday filters, will be designed to measure Na nightglow and to determine NaRD. In addition to NaRD, the method can also determine the pathway fractions of the two possible reactions in the modified Chapman process. The NaRD spectrometer will be deployed at the Colorado State University lidar site for intitial testing and calibration. The project involves a research scientist as Co-PI and a graduate student for his Ph.D thesis. The research would lead to the development of an inexpensive compact spectrometer that could be used at other observatories with a simple sodium lidar. The project will contribute to studies of atomic oxygen in the mesopause region as more data are gathered from various sites in the future. Such measurements will improve the understanding of the aeronomy of airglow processes, most of which involve interactions with O.