Investigating the excitation mechanism of the Na D-line nightglow emission

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The Na D-line emission is one of the most prominent nightglow emission features in the terrestrial atmosphere. It was discovered in 1929 by Vesto Slipher and a first photochemical excitation scheme was proposed in 1939 by Sidney Chapman. Although the Na D-line nightglow emission has been studied for more than 80 years, its excitation mechanism is still not fully understood. Recent studies identified variations in the D2/D1 line ratio inconsistent with the original Chapman mechanism. In 2005 a modified mechanism was proposed by Slanger et al. (2005) that treats different electronically excited states of NaO separately. This mechanism has been tested using ground-based observations of the D2/D1 line ratio, but the vertical variation of the line ratio - a critical test of the modified Chapman scheme - has not yet been studied. The main science objectives of the research proposed here is to test our understanding of the Na D-line excitation mechanism using space-borne limb-emission observations with two instruments that are uniquely suited for this purpose, i.e. OSIRIS, the Optical Spectrograph and InfraRed Imager System on the Odin satellite and SCIAMACHY, the Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY on Envisat.Specifically, the satellite observations will be employed for the following purposes: 1) OSIRIS measurements - which have a very high signal-to-noise ratio - will be employed to empirically constrain the branching ratio f for the production of Na(2P) through the reaction of NaO and O, according to the original Chapman excitation mechanism. This will be done in combination with independent Na profile observations with ground-based Na LIDARs and available satellite data sets.2) SCIAMACHY nighttime limb-emission observations - that allow separating the D1 and D2 lines - will be employed to retrieve the vertical variation of the D2/D1 line ratio in the real atmosphere. SCIAMACHY is uniquely suited for this purpose (the spectral resolution of OSIRIS is not sufficient to separate the two Na D-lines). In addition, atomic oxygen density profiles in the mesopause region are available from SCIAMACHY nightglow measurements to interpret the inferred Na D-line ratios.With this approach the proposed research can make important and new contributions to improve our understanding of the Na D-line excitation mechanism. In addition, the results will consolidate the retrieval of Na density profiles in the mesopause region based on observations of the Na D-line emission. This will be achieved by providing an optimum value of the branching ratio f of the effective Chapman excitation scheme together with an estimate of its uncertainty.