A sounding rocket measurement of the ultraviolet, atomic oxygen dayglow reveals an excess of emission compared to standard thermospheric model calculations at exospheric altitudes. We explore two explanations for this discrepancy: a breakdown of the radiative transfer model due to nonlocal thermal equilibrium (non-LTE) conditions above the exobase and a hot atomic oxygen geocorona. In particular, the effects of non-LTE on the 3 P2,1,0 sublevel populations are modeled, and a hot O component in the upper thermosphere and lower exosphere is added to investigate the effects on the modeled emissions. For both cases, the data are reanalyzed and compared with the results using a standard LTE model. A hot O geocorona having a peak density of 106 cm -3 at 550 km and a temperature of 4000 K is consistent with the data and appears to be the most reasonable explanation of the high-altitude enhanced emissions observed in the data.

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.

Abstract. We present a statistical reconstruction framework for space-based extreme ultraviolet (EUV) ionospheric tomography. The EUV technique offers a means to invert the nighttime F region electron density on global scales from a single spaceborne spectrograph, using prominent optically thin emissions produced by radiative recombination of O +. Since the EUV technique does not rely on ground receivers to make the measurements, the observations do not suffer from limitations on the viewing angles. The EUV tomography is an ill-conditioned inverse problem in the sense that its solution is sensitive to perturbations of the measured data. With large condition numbers of a typical projection matrix, simple least squares inversion techniques yield unacceptable results in the presence of noise. This reflects the fact that more degrees of freedom are being sought than are supported by the noisy data. To overcome this limitation, we cast the tomographic inverse problem in a stochastic framework and incorporate a statistical prior model. In doing so we also obtain measures of estimation uncertainty for the solutions. Through simulations, we demonstrate the applicability of these techniques in the context of a space mission designed for EUV ionospheric tomography, namely, the Tomographic Experiment Using Radiative Recombinative Ionospheric EUV and Radio Sources (TERRIERS). The simulations show promising results for EUV tomography as a viable ionospheric remote sensing technique.

The Berkeley extreme-ultraviolet airglow rocket spectrometer (BEARS) made spectroscopic measurements of far and extreme UV, atomic oxygen emissions from a Black Brant XII (12.041 WT) sounding rocket launched from Wallops Island, Virginia, on September 30, 1988. BEARS' primary instrument, a near-normal Rowland mount spectrometer, measured several atomic oxygen and molecular nitrogen dayglow features at high spectral resolution (1.5 •): O I (989, 1027, 1304, and 1356 •); and N2 Lyman-Birge-Hopfield (4,0) and (3,0) bands at 1325 and 1354 •. The instrument collected over 800 s of data spanning altitudes of 150-963 km with look directions between 95 ø and 125 ø from zenith. We have analyzed the data using electron and radiative transport models in a forward modeling approach. The model and data are generally in good agreement. However, there are some discrepancies, which are discussed in terms of remote sensing capabilities and improvements to the models. In particular, the data indicate an optically thick cascade contribution of 40% to the O I (1304 •) emission. There is a significant contribution to the O I (1027 •) feature due to Lyman/3 resonant scattering and an inconsistency in the modeled and measured intensities in the lower thermosphere. cally thin lines such as the 6300-.• emission [Solomon, 1987]. Instead, one must take a forward modeling approach. This strategy is based on a physical model that calculates the initial excitation (e.g., resulting from solar resonance scat-•Now at tering or impact excitation) and the subsequent radiative transport of the photons which produce the emissions. By adjusting the input parameters, model atmosphere, energy input, temperature, etc., and then iterating, one can arrive at a set of inputs that is consistent with the data. Much progress has been made in the last decade in this approach to modeling, specifically in the area of radiative transport. Anderson et al. [1980] analyzed O I (989, 1027, 1152, 1304, and 1356 •) data from a rocket spectrometer launched at White Sands Missile Range in 1978 [Gentieu et al., 1979] using a complete frequency redistribution, isothermal radiative transfer model, and measured photoelectron fluxes. They showed that the large optical depths of the emissions dictated the need for improving the radiative transfer model. Meier and Lee [1982] subsequently reanalyzed the 989-and 1304-• data with a Monte Carlo partial frequency redistribution radiative transfer code including temperature gradients and provided a means for the appropriate modeling of these optically thick lines. Since then, other radiative transport codes have been developed and employed successfully [Gladstone et al., 1987]. With the development of electron transport models, direct measurements of the photoelectron fluxes can be proxied by an appropriate solar EUV flux, since this allows one to derive the photoelectron flux directly from the solar EUV radiation. Models by Strickland and Meier [1982] and Link [1982] that use this technique have been used to model all of the emissio...

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