L. J. Paxton

A newly discovered 1000‐km scale longitudinal variation in ionospheric densities is an unexpected and heretofore unexplained phenomenon. Here we show that ionospheric densities vary with the strength of non‐migrating, diurnal atmospheric tides that are, in turn, driven mainly by weather in the tropics. A strong connection between tropospheric and ionospheric conditions is unexpected, as these upward propagating tides are damped far below the peak in ionospheric density. The observations can be explained by consideration of the dynamo interaction of the tides with the lower ionosphere (E‐layer) in daytime. The influence of persistent tropical rainstorms is therefore an important new consideration for space weather.

The Global Ultraviolet Imager (GUVI) instrument carried aboard the NASA TIMED satellite measures the spectral radiance of the Earth's 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's 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's dayglow, aurora, and ionosphere. Moreover, through forward modeling, the abundance of the major constituents of the thermosphere, O, N 2 , and O 2 and thermospheric temperatures can be retrieved from observations of the limb radiance. Variations of the column O/N 2 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 E o , and a parameter f o 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's limb and disk, respectively. Regions of depleted F‐region electron density are evident in the tropical Appleton anomaly regions, associated with so‐called ionospheric “bubbles.” Access to the GUVI data is provided through the GUVI website www.timed.jhuapl.edu\guvi .

Simultaneous measurements of disk‐viewing OI 135.6 nm and N 2 Lyman‐Birge‐Hopfield (LBH) dayglow can be used to monitor the solar EUV flux Q EUV and the column abundance of thermospheric O relative to N 2 (O/N 2 ). 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/N 2 . Model results show that the relationship between O/N 2 and 135.6/LBH is essentially independent of model atmosphere. Given either 135.6/LBH or O/N 2 , Q EUV can then be obtained directly from the absolute intensity of either 135.6 nm or LBH.