Atmospheric Opacity. in the Schumann-Runge Bands and the Aeronomic Dissociation of Water Vapor

J. E. Frederick Laboratory for Planetary Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, MD 20771

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R. D. Hudson Laboratory for Planetary Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, MD 20771

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Abstract

Knowledge of the agronomic production of odd hydrogen in the dissociation of water vapor is limited by uncertainties in the penetration of solar irradiance in the Schumann-Rung bands of O2 and by incomplete information concerning the products of photolysis at Lyman alpha. Consideration of an error sources involved in computing the H2o dissociation rate in the wavelength region 175–200 nm leads to an estimated uncertainty of ±35% at an altitude of 90 km for an overhead sun. The uncertainty increases with decreasing altitude such that the true dissociation rate at 60 km for an overhead sun lies between 0.45 and 1.55 times the result computed using the best input parameters currently available. Calculations of the H2o dissociation rate by Lyman alpha should include the variation in O2 opacity across the solar line width. Neglect of this can lead to errors as large as 50% at altitudes where the process is the major source of odd hydrogen.

Abstract

Knowledge of the agronomic production of odd hydrogen in the dissociation of water vapor is limited by uncertainties in the penetration of solar irradiance in the Schumann-Rung bands of O2 and by incomplete information concerning the products of photolysis at Lyman alpha. Consideration of an error sources involved in computing the H2o dissociation rate in the wavelength region 175–200 nm leads to an estimated uncertainty of ±35% at an altitude of 90 km for an overhead sun. The uncertainty increases with decreasing altitude such that the true dissociation rate at 60 km for an overhead sun lies between 0.45 and 1.55 times the result computed using the best input parameters currently available. Calculations of the H2o dissociation rate by Lyman alpha should include the variation in O2 opacity across the solar line width. Neglect of this can lead to errors as large as 50% at altitudes where the process is the major source of odd hydrogen.

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