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

A stationary, nonlinear collisional-radiative model for high-temperature atomic oxygen is presented. Populations of electrons, ions, and excited atoms and intensities of spectral, continuum, and dielectronic recombination lines are calculated in a wide range of conditions. Transport of radiation is included by coupling the rate equations for production of the electrons, ions, and excited atoms with the concept of the escape factors that are not constant but dependent upon plasma conditions. The calculated total continuum emission is in good agreement with existing measurements.

Thermal nonequilibrium in partially ionized atomic oxygen

Electron scattering with atomic oxygen has been studied using the $B$-spline $R$-matrix-with-pseudostates method. Cross sections for elastic scattering, excitation, emission, and ionization processes are presented. The excitation cross sections have been calculated for transitions between the $2{s}^{2}2{p}^{4}$ and $2{s}^{2}2{p}^{3}3l$ states of oxygen in the energy range from threshold to 200 eV. The present work differs from numerous previous studies due to the inclusion of a large number of pseudostates in the calculation. We included a total of 1116 spectroscopic bound, core-excited autoionizing, and target continuum states in the close-coupling expansion. The atomic oxygen structure model has been described by combining the multiconfiguration Hartree-Fock and the $B$-spline box-based multichannel methods. The inclusion of a large number of pseudostates representing the target continuum has a major impact, especially on the theoretical prediction of the excitation cross sections for many transitions at intermediate energies. A large reduction in excitation and emission cross sections has been noted due to the inclusion of coupling to the ionization continuum. The calculated cross sections are now in better agreement with available experimental results. The ionization cross sections for the ground $2{s}^{2}\phantom{\rule{0.16em}{0ex}}2{p}^{4}{\phantom{\rule{0.16em}{0ex}}}^{3}P$ and metastable $2{s}^{2}2{p}^{4}{\phantom{\rule{0.16em}{0ex}}}^{1}D$ and $^{1}S$ states are also presented. The electron-impact-induced emission cross sections for the $(2{s}^{2}2{p}^{3}3s){\phantom{\rule{0.16em}{0ex}}}^{3}{S}^{o}--(2{s}^{2}2{p}^{4}){\phantom{\rule{0.16em}{0ex}}}^{3}P$ (130.4 nm), $(2{s}^{2}2{p}^{3}3d){\phantom{\rule{0.16em}{0ex}}}^{3}{D}^{o}--(2{s}^{2}2{p}^{4}){\phantom{\rule{0.16em}{0ex}}}^{3}P$ (102.7 nm), $(2{s}^{2}2{p}^{3}3{s}^{\ensuremath{'}}){\phantom{\rule{0.16em}{0ex}}}^{3}{D}^{o}--(2{s}^{2}2{p}^{4}){\phantom{\rule{0.16em}{0ex}}}^{3}P$ (98.9 nm), and $(2{s}^{2}2{p}^{3}3{s}^{\ensuremath{'}\ensuremath{'}}){\phantom{\rule{0.16em}{0ex}}}^{3}{D}^{o}--(2{s}^{2}2{p}^{4}){\phantom{\rule{0.16em}{0ex}}}^{3}P$ (87.8 nm) transitions have been calculated and compared with the available experimental results.

/pdf/b-spline-r-matrix-with-pseudostates-approach-for-excitation-1q2kxljfqp.pdf

B -spline R -matrix-with-pseudostates approach for excitation and ionization of atomic oxygen by electron collisions

The Pioneer Venus Orbiter Ultraviolet Spectrometer has recorded continuous but variable emissions of atomic oxygen at 1304 and 1356 A in images of the nightside of Venus. We show that the observed intensities are consistent with the presence of precipitation of soft electrons into the nightside thermosphere. Model calculations are presented in which upper and lower limits to the magnitude of the electron flux necessary to produce the observed intensities are derived. Constraints are imposed on the energy spectrum of the electrons by the measured ion densities and by the predicted intensities of other emissions that have not been detected.

/pdf/the-venus-ultraviolet-aurora-a-soft-electron-source-3kcqpleczd.pdf

The Venus Ultraviolet Aurora: A Soft Electron Source

A nonlinear time-dependent collisional-radiative model for atomic oxygen is presented. Effective ionization and three-body recombination rate coefficients are calculated for either optically thin or thick plasmas where ${10}^{18}{\mathrm{m}}^{\mathrm{\ensuremath{-}}3}l~{n}_{e}l~{10}^{21}{\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$ and $8000\mathrm{K}l~{T}_{e}l~20000\mathrm{K}.$ In order to implement these coefficients easily in plasma flow codes, simple analytical expressions are proposed. Significant discrepancies with existing literature values are pointed out and discussed. The time-dependent approach confirms the importance of a quasi-steady-state condition to derive meaningful rate coefficients. Therefore the relaxation time necessary for the system to reach a quasi-steady-state limits the validity of the rate coefficients determined in this study. In optically thick cases, this relaxation time in either recombining or ionizing plasmas depends weakly on the electron temperature but strongly increases as the electron number density decreases. In optically thin cases, it is more difficult to derive general results since the relaxation time depends on the electron temperature, on densities, but also on the initial distribution on atomic levels.

Ionization and recombination rates of atomic oxygen in high-temperature air plasma flows

A new apparatus designed specifically for the study of electron scattering from unstable species at very low incident energies has been constructed and put into operation. Remeasurement of the O I ³P → ³S0 (1304 A) differential and integral cross sections (DCS and ICS) at energies below 30 eV has provided new values for the DCS and ICS for this cross section as well as for the ³P → ³D0 (989 A) transition. These new values replace the cross sections reported by Vaughan and Doering (1987) which were measured in the original apparatus designed for high incident energy work. The two sets of measurements agree at incident energies 30 eV and above. However, the new values for the ³P → ³S0 transition at 20 and 16.5eV are larger than the previous measurements by a factor of 2 to 3. Extension of the work on this transition down to 13.87 eV shows that the cross section remains large to within a few eV of threshold, in agreement with the theoretical predictions of Rountree and Henry (1972) and Smith (1976). Similar behavior is observed for the ³P → ³D0 (989 A) transition down to 20 eV.

Absolute differential and integral electron excitation cross sections for atomic oxygen, 5, revised values for the ³P → ³S0 (1304 Å) and ³P → ³D0 (989 Å) transitions below 30 eV

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Absolute differential and integral electron excitation cross sections for atomic oxygen, 5, revised values for the ³P → ³S0 (1304 Å) and ³P → ³D0 (989 Å) transitions below 30 eV