Quiet-time seasonal behavior of the thermosphere seen in the far ultraviolet dayglow

The TIMED/GUVI instrument is a far ultraviolet spectrograph that obtains images in five spectrally resolved wavelength channels. These images yield information on the dayside composition, temperature, solar EUV flux, large-scale wave structures, and auroral processes. In this paper we present an overview analysis of Earth-disk images for four seasons (March, July, and September 2002 and January 2003). Days were selected during geomagnetically quiet periods when the Sun was nearly in the orbital plane (noon orbits). Two of GUVI's five channels (designated as 135.6 and LBHS and dominated by OI 135.6 nm and short wavelength N-2 LBH band emission, respectively) are used when the instrument is in its imaging mode. These data are used to derive O/N-2 (column density ratio referenced to an N-2 column density of 10(17) cm(-2)). The AURIC model is used to generate a lookup table that relates O/N-2 to the ratio of 135.6 to LBHS for a given solar zenith angle. Global images of derived O/N-2 (designated as GUVI O/N-2) are presented for the 4 days. The initial validation of the retrieved composition ratio comes from comparison with the NRLMSIS model. Good overall qualitative agreement is obtained between GUVI and NRLMSIS. Both data and model exhibit similar latitudinal behaviors on the near-solstice days, namely a distinct gradient with O/N-2 decreasing from the winter to the summer hemisphere. Reductions in O/N-2 in the vicinity of magnetic poles are seen in both GUVI and NRLMSIS images. Globally, O/N-2 is smaller at the solstices and may be explained by the "thermospheric spoon" mechanism discussed by Fuller-Rowell [1998]. Alternatively, the greater overall values at the equinoxes may arise in part from global response to greater Joule heating at these times of the year. The sensitivity of O/N-2 to scalings of the N-2 LBH cross section and solar EUV below 20 nm is also addressed in response to recent papers on these topics. This initial look at the GUVI data demonstrates great promise of FUV remote sensing as a way to observe thermospheric composition changes over broad geographic scales.