The Earth's ultraviolet airglow contains fundamental diagnostic information about the state of its upper atmosphere and ionosphere. Our understanding of the excitation and emission processes which are responsible for the airglow has undergone dramatic evolution from the earliest days of space research through the past several years during which a wealth of new information has been published from high-resolution spectroscopy and imaging experiments. This review of the field begins with an overview of the phenomenology: how the Earth looks in the ultraviolet. Next the basic processes leading to excitation of atomic and molecular energy states are discussed. These concepts are developed from first principles and applied to selected examples of day and night airglow; a detailed review of radiation transport theory is included. This is followed by a comprehensive examination of the current status of knowledge of individual emission features seen in the airglow, in which atomic physics issues as well as relevant atmospheric observations of major and minor neutral and ionic constituents are addressed. The use of airglow features as remote sensing observables is then examined for the purpose of selecting those species most useful as diagnostics of the state of the thermosphere and ionosphere. Imaging of the plasmasphere and magnetosphere is also briefly considered. A summary of upcoming UV remote sensing missions is provided.

Ultraviolet spectroscopy and remote sensing of the upper atmosphere

The analytic characterizations of electron impact cross sections for important atmospheric gases (namely, O2, N2, O, CO, CO2, and He) are updated. With these cross sections it is simple to communicate massive quantities of experimental and theoretical results. In addition, these forms are convenient for applications in energy degradation calculations, including a new approach described in a companion paper.

Electron impact on atmospheric gases, I. Updated cross sections

To develop heat shields for space vehicles, materials must be characterized in simulation conditions close to those in space environments. The most important conditions for simulating the Earth re-entry phase of space vehicles (high temperatures, low pressure air plasma,…) are achieved through the MESOX set-up associating a solar radiation concentrator and a microwave plasma generator. After determining the thermal recombination flux transferred to the material (thermal approach, accommodation), we developed a chemical approach for evaluating the recombination coefficient of atomic oxygen γO by Optical Emission Spectroscopy. We also measured the gas temperature above the material, close to the surface where recombination occurs, using the rotational temperature of nitrogen. The recombination coefficients of atomic oxygen were determined in function of the temperatures of two types of silica materials. Finally, the activation energy of the atomic oxygen recombination reaction was obtained for each material. Results showed that the microstructure of the topmost layer has an important effect and that the recombination coefficient of β-cristobalite is four times higher than that of quartz.

Recombination coefficient of atomic oxygen on ceramic materials under earth re-entry conditions by optical emission spectroscopy

Resonance absorption at 130 nm was used to measure absolute oxygen atom concentrations in O2‐containing distributed electron cyclotron resonance plasmas. The dissociation fraction [O]/[O2] in pure O2 plasmas (1–6 mTorr) was in the range 0.01–0.06, but was significantly increased by the addition of SF6, N2 or Kr. At 2 mTorr total pressure a maximum [O]/[O2] of 0.3 was observed for 10% SF6 added. The results were compared to those obtained by optical emission actinometry measurements. The quantity I0 (844 nm)/IAr (750 nm) (with 0.1 mTorr Ar added) was poorly correlated with [O] but well correlated with [O2]. This suggests that, for dissociation fractions lower than 0.1, dissociative excitation, O2+e→O*(3p 3P)+O, is the most important mechanism for the production of 844 nm emission.

Oxygen atom actinometry reinvestigated: Comparison with absolute measurements by resonance absorption at 130 nm

Revised cross-section values for the excitation of three O I resonance transitions at 1304, 1027, and 989 A, by electron impact on atomic oxygen are presented from threshold to 300 eV. These results are smaller than the excitation cross sections used in some airglow models by a factor of about 2.8. The revised values are in good agreement with recent quantum-scattering calculations. The downward revision is required by new laboratory studies in which the direct and dissociative cross sections for 1304 A excitation were normalized with small probable error to the O and O2 ionization cross sections. The results also reflect new advances in VUV optical calibration techniques. A number of outstanding airglow problems are simplified by these revisions.

Electron impact excitation of atomic oxygen: Revised cross sections

The role of cascading in auroral or photoelectron excited atomic oxygen emission is investigated by calculating distorted wave electron impact cross sections for some 24 excited levels in O I. Cascading contributes approximately 75 percent of the effective emission cross section for 1304 A and 1356 A. About 98 percent of the cascade component to these ultraviolet lines is fed through the respective 8446- and 7774-A transitions, with the consequence that the 4368- and 3947-A emissions are very weak. The cascade component to 1304 A is strongly enhanced in an optically thick medium due to radiation trapping of the lines from the 3s' 3D, nd/sup 3/D, and ns/sup 3/S (n greater than or equal to 4) levels to the ground state. The 1304-A emission cross section appropriate in the optically thick atmosphere is predicted to be a factor of 2 larger than the emission cross section measured in an optically thin laboratory experiment. Consideration of this effect is necessary to understand the measured atmospheric 1304-/1356-A ratio in terms of laboratory cross sections. Several recent atmospheric measurements on electron excited atomic oxygen 1304-, 1356-, 4368-, 7774-, and 1152-A lines appear to be consistent with calculated results, which imply that the Stonemore » and Zipf (1974) 1304- and 1356-A laboratory cross sections may be systematically too large by as much as a factor of 2.« less

Cascade and radiation trapping effects on atmospheric atomic oxygen emission excited by electron impact

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Cascade and radiation trapping effects on atmospheric atomic oxygen emission excited by electron impact