Simultaneous measurements of disk‐viewing OI 135.6 nm and N2 Lyman‐Birge‐Hopfield (LBH) dayglow can be used to monitor the solar EUV flux QEUV and the column abundance of thermospheric O relative to N2 (O/N2). We report on a study that quantifies the relationships between these emissions and the above parameters. Emission is considered from 134.5 to 139.0 nm (designated 135.6 nm) and from 155.0 to 170.0 nm (designated as LBH) at a resolution of 3.6 nm. The intervals and resolution were chosen for analysis of satellite dayglow data to be reported in the companion paper by Evans et al. (this issue). The first interval is dominated by OI 135.6 nm with minor contributions from LBH 135.4 and 138.3 nm. The second interval contains only LBH. An important finding is that 135.6/LBH is essentially independent of the solar EUV spectrum from low to high activity based on using the Hinteregger formulation for characterizing spectral changes with solar activity. Given this behavior in 135.6/LBH, one can then unambiguously interpret changes in this ratio in terms of changes in O/N2. Model results show that the relationship between O/N2 and 135.6/LBH is essentially independent of model atmosphere. Given either 135.6/LBH or O/N2, QEUV can then be obtained directly from the absolute intensity of either 135.6 nm or LBH.

The Earth's thermosphere and ionosphere constitute a dynamic system that varies daily in response to energy inputs from above and from below. This system can exhibit a significant response within an hour to changes in those inputs, as plasma and fluid processes compete to control its temperature, composition, and structure. Within this system, short wavelength solar radiation and charged particles from the magnetosphere deposit energy, and waves propagating from the lower atmosphere dissipate. Understanding the global-scale response of the thermosphere-ionosphere (T-I) system to these drivers is essential to advanc- ing our physical understanding of coupling between the space environment and the Earth's atmosphere. Previous missions have successfully determined how the "climate" of the T-I system responds. The Global-scale Observations of the Limb and Disk (GOLD) mission will determine how the "weather" of the T-I responds, taking the next step in understanding the coupling between the space environment and the Earth's atmosphere. Operating in geostationary orbit, the GOLD imaging spectrograph will measure the Earth's emissions from 132 to 162 nm. These measurements will be used image two critical variables-thermospheric temperature and composition, near 160 km-on the dayside disk at half-hour time scales. At night they will be used to image the evolution of the low latitude ionosphere in the same regions that were observed earlier during the day. Due to the geostationary orbit being used the mission observes the same hemisphere repeatedly, allowing the unambiguous separation of spatial and temporal variability over the Americas.

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The Global Ultraviolet Imager (GUVI) on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission will determine the variability in thermospheric composition, and its response to auroral inputs as well as measuring those inputs. GUVI is the result of twenty years of work in designing large field of regard far ultraviolet (1 10 -1 80 nm) imagers for spaceflight. These systems are based on the concept of a horizon-to-horizon "monochromatic" imager. The field of view of a spectrograph is swept from horizon to horizon using a scan mirror. The spectrograph uses a grating to spectrally disperse the light. A two-dimensional detector is used to record spatial and spectral information simultaneously. Images are obtained at discrete wavelengths without the use of filters; this reduces if not eliminates much of the concern about instrumental bandpasses, out-of-band rejection, and characterization of filter responses. Onboard processing is used to bin the spectral information into "colors" thereby reducing the overall data rate required. The spectral bandpass is chosen to lie in the far ultraviolet so that the sunlit and dark aurora can be imaged. We review the instrument's as delivered performance and the TIMED science requirments. TIMED will be launched May 18, 2000 and will inaugurate the Solar-Terrestrial Connections program at NASA.

In this paper we examine the relationship among certain prominent auroral FUV emission features, the incident electron spectrum, and the model neutral atmosphere. Given the neutral atmosphere, we show that for simple models of the incident electron spectrum (Maxwellian and Gaussian in energy), satellite measurements of FUV emission features, in principle, determine the incident electron spectrum. We also discuss the relationship between the incident electron spectrum and the E region plasma density profile for the continuous (diffuse) aurora and for a stable arc.