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Browsing Journal Articles by Author "Shepherd, Gordon G."

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    Solar variablity of the daytime atomic oxygen O(¹S) emission in the middle and lower thermosphere.
    (The American Geophysical Union, 2004-03-13) Maharaj-Sharma, Rawatee; Shepherd, Gordon G.
    [1] The Wind Imaging Interferometer (WINDII) on board the Upper Atmosphere Research Satellite (UARS) measures winds from the Doppler shifts of airglow emissions, and as a by-product the volume emission rates (photon cm־³ sˉ¹) of those emissions. This includes the atomic oxygen O(¹S) emission at 557.7 nm, which during the daytime is emitted over a large altitude range from about 85 km to over 250 km. Two distinct peaks in altitude are formed, one near 100 km (herein denoted the E region) and one near 150 km (herein the F region). The daytime E region emission was virtually unknown prior to WINDII - it resembles its nighttime counterpart, but the emission rate is much higher and it is broader in vertical extent. Both regions are produced by direct and indirect processes, but the WINDII data show that both behave as Chapman layers, corresponding to production of emission by absorption of solar radiation. The F region processes are fairly well understood, but the direct E region process has not previously been identified. The data are consistent with excitation by photodissociation of O2 dominantly by Lyman-β. The solar influence is investigated through correlations of daily values of emission rate with six solar indices; the E region yields higher coefficients than the F region, with the highest value for solar Ly-α at 0.82, which is consistent with the Ly-β process. For one of the six indices, the GOES xl x-rays (0.1-0.8 nm), no correlation is evident, in part because the radiances are too low to produce the observed emission, but also because the radiances are seemingly random, not following the solar cycle variation. Variations of atmospheric origin with latitude and longitude are briefly reported upon, and an F region emission rate scale height correlation with temperature is also found.