Initial observations with the Global Ultraviolet Imager (GUVI) in the NASA TIMED satellite mission

The Global Ultraviolet Imager (GUVI) instrument carried aboard the NASA TIMED satellite measures the spectral radiance of the Earth\textquoterights far ultraviolet airglow in the spectral region from 120 to 180 nm using a cross-track scanning spectrometer design. Continuous operation of the instrument provides images of the Earth\textquoterights disk and limb in five selectable spectral bands. Also, spectra at fixed scanning mirror position can be obtained. Initial results demonstrate the quantitative functionality of the instrument for studies of the Earth\textquoterights dayglow, aurora, and ionosphere. Moreover, through forward modeling, the abundance of the major constituents of the thermosphere, O, N2, and O2\ and thermospheric temperatures can be retrieved from observations of the limb radiance. Variations of the column O/N2\ ratio can be deduced from sunlit disk observations. In regions of auroral precipitation not only can the aurora regions be geographically located and the auroral boundaries identified, but also the energy flux Q, the characteristic energy Eo, and a parameter fo\ that scales the abundance of neutral atomic oxygen can be derived. Radiance due to radiative recombination in the ionospheric F region is evident from both dayside and nightside observations of the Earth\textquoterights limb and disk, respectively. Regions of depleted F-region electron density are evident in the tropical Appleton anomaly regions, associated with so-called ionospheric \textquotedblleftbubbles.\textquotedblright Access to the GUVI data is provided through the GUVI website\ www.timed.jhuapl.edu\guvi.