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Abstract. We present a technique for using the measured variations of ultraviolet emissions produced by radiative recombination at 911 and 1356 • to determine the nighttime altitude distribution of F region O + ions and electrons. The algorithm uses an iterative scheme based on discrete inverse theory to determine the best fit to the data. We present the results of simulations that demonstrate the convergence properties of the algorithm and the fidelity with which it reproduces the input ionosphere. The algorithm was tested against more realistic simulated "data" generated using the international reference ionosphere (IRI-90) [Bilitza, 1990]. The algorithm accurately retrieved the nighttime F region electron density at midlatitudes (_+25ø-65øN) over a wide range of solar and geomagnetic activity and local time.

Tomographic imaging of the ionosphere is a recently developed technique that uses integrated measurements and computer reconstructions to determine electron densities. The integral of electron density along vertical or oblique paths is obtained with radio transmissions from low-earth-orbiting (LEO) satellite transmitters to a chain of receivers on the earth’s surface. Similar measurements along horizontal paths can be made using transmissions from Global Position System (GPS) navigation satellites to GPS receivers on LEO spacecraft. Also, the intensities of extreme ultraviolet (EUV) emissions can be measured with orbiting spectrometers. These intensities are directly related to the integral of the oxygen ion and electron densities along the instrument line of sight. Two-dimensional maps of the ionospheric plasma are produced by analyzing the combined radio and EUV data using computerized ionospheric tomography (CIT). Difficulties associated with CIT arise from the nonuniqueness of the reconstructions, owing to limited angle measurements or nonoptimal receiver location. Improvements in both reconstruction algorithms and CIT measurement systems are being implemented to overcome these difficulties. New imaging systems being developed employ CIT for large area mapping of the plasma densities in the ionosphere.

and PIM models o, we work a difficult test case following from recent comments on the concept of 834-A remote sensing of ionospheric O +. We find that the DIT method can correctly distinguish between distinctly different F layers that produce nearly identical intensity profiles, consistent with instrument specifications for future missions.

[1] Ultraviolet dayglow observed using the Ionospheric Spectroscopy and Atmospheric Chemistry (ISAAC) satellite instrument is used to determine the concentration of nitric oxide (NO) in the upper mesosphere and thermosphere. ISAAC was launched in February 1999 on board the Advanced Research and Global Observing Satellite (ARGOS). We focus on ISAAC dayside limb observations at tangent altitudes between 80 and 200 km and over the latitude range 40°N-70°S. The data analyzed here were obtained in November and December 1999 during a period of moderate to high solar activity. The NO density retrievals include constraints on instrument sensitivity, in-flight assessments of stray light, and determinations of Rayleigh-scattered background contributions. Residual spectra contain strong dayglow signatures of the NO g bands which are used for inversion of measured radiances to absolute NO concentrations. Comparison between ISAAC measurements and photochemical calculations indicates improved agreement above 130 km using O 2 and temperatures from the new NRLMSISE-00, as compared to the MS model. In the lower thermosphere, observed daily variations in low-latitude nitric oxide are strongly correlated with variations in the solar soft X-ray flux.