Photoelectron Impact Excitation Research Articles

Pressure-Driven Switching of Photoelectron Impact Ionization-Chemical Ionization/Penning Ionization in Vacuum Ultraviolet Photoionization Mass Spectrometry.

Vacuum ultraviolet photoionization (VUV-PI) is a soft ionization technique that operates under pressures ranging from vacuum to ambient pressure. VUV-PI has played an essential role in direct sampling mass spectrometry. In this study, new ionization processes initiated by photoelectrons have been studied through the inclusion of a radio frequency (RF) electric field at different pressures. After deducting the contribution of single photoionization (SPI), the signal intensity of 1 ppmv toluene (C7H8+) in Ar was approximately 5-fold higher than that in N2. Mixed gases with different ionization energies (IEs) and excitation energies (EEs) were further investigated to reveal that metastable species were involved in the enhancement process. Reactant ions were produced by photoelectron impact ionization (PEI), which further triggered ion-molecule reactions, i.e., chemical ionization (CI). Metastable species were produced by photoelectron impact excitation (PEE), which further triggered Penning ionization (PenI). Analytes with IEs above 10.6 eV, such as CO2 (IE = 13.78 eV) and CHCl3 (IE = 11.37 eV), could be sensitively ionized by PenI with a sensitivity comparable to SPI. Except for the contribution of SPI, the dominant ionization process was switched from PEI-CI to PenI when the pressure was elevated from 50 to 500 Pa, as the electron energy gradually decreased and was only able to produce metastable states based on the kinetic energy balance equation of electrons. The conversion processes and conditions from PEI-CI to PenI will provide novel insights to develop new selective and sensitive VUV-PI sources and understand the ionization mechanism in other discharge ionization sources.

Efficient Indirect Interatomic Coulombic Decay Induced by Photoelectron Impact Excitation in Large Pure Helium Nanodroplets.

Ionization of matter by energetic radiation generally causes complex secondary reactions that are hard to decipher. Using large helium nanodroplets irradiated by extreme ultraviolet (XUV) photons, we show that the full chain of processes ensuing primary photoionization can be tracked in detail by means of high-resolution electron spectroscopy. We find that elastic and inelastic scattering of photoelectrons efficiently induces interatomic Coulombic decay (ICD) in the droplets. This type of indirect ICD even becomes the dominant process of electron emission in nearly the entire XUV range in large droplets with radius ≳40 nm. Indirect ICD processes induced by electron scattering likely play an important role in other condensed-phase systems exposed to ionizing radiation as well, including biological matter.

Reducing the Ionospheric Contamination Effects on the Column O/N2 Ratio and Its Application to the Identification of Non‐Migrating Tides

AbstractPrior investigations have attempted to characterize the longitudinal variability of the column number density ratio of atomic oxygen to molecular nitrogen (O/N2) in the context of non‐migrating tides. The retrieval of thermospheric O/N2 from far ultra‐violet (FUV) emissions assumes production is due to photoelectron impact excitation on O and N2. Consequently, efforts to characterize the tidal variability in O/N2 have been limited by ionospheric contamination from O+ + e radiative recombination at afternoon local times (LT) around the equatorial ionization anomaly. The retrieval of O/N2 from FUV observations by the Ionospheric Connection Explorer (ICON) provides an opportunity to address this limitation. In this work, we derive modified O/N2 datasets to delineate the response of thermospheric composition to non‐migrating tides as a function of LT in the absence of ionospheric contamination. We assess estimates of the ionospheric contribution to 135.6 nm emission intensities based on either Global Ionospheric Specification (GIS) electron density, International Reference Ionosphere (IRI) model output, or observations from the Extreme Ultra‐Violet imager (EUV) onboard ICON during March and September equinox conditions in 2020. Our approach accounts for any biases between the ionospheric and airglow datasets. We found that the ICON‐FUV data set, corrected for ionospheric contamination based on GIS, uncovered a previously obscured diurnal eastward wavenumber 2 tide in a longitudinal wavenumber 3 pattern at March equinox in 2020. This finding demonstrates not only the necessity of correcting for ionospheric contamination of the FUV signals but also the utility of using GIS for the correction.

Thermospheric density enhancement and limb O 130.4 nm radiance increase during geomagnetic storms

We explore a connection between thermospheric density enhancement and increase in thermospheric O 130.4 nm radiance. We observe TIMED/GUVI enhancements in the limb 130.4 nm radiances at ∼400 and ∼520 km on the dayside during four intense geomagnetic storms in 2003 and 2004. The enhancements were well correlated with Dst and CHAMP total neutral density at 400 km which represents O density as O is the dominant species at those altitudes. At the 400 and 520 km altitudes, O 130.4 nm emissions are mostly created by two comparable sources: solar resonance scatter and photoelectron impact excitation. The coincident disk 130.4 nm radiances, mostly due to emissions below 200 km (peaked around 130–140 km), were not clearly correlated with the limb radiances. Because the limb 130.4 nm radiances depend on O density, solar EUV and 130.4 nm fluxes, variations in the limb 130.4 nm radiance respond mostly to changes in O density when the solar EUV and 130.4 nm fluxes are stable. This explains the good correlation (correlation coefficients up to 0.98) between the limb 130.4 nm radiance and CHAMP neutral density. Once a quantitative relationship is established between GUVI limb 130.4 nm radiance and neutral density under both quiet and disturbed conditions and at different altitude levels through empirical or radiative transfer modeling, the limb 130.4 nm radiances can be used to retrieve O density profiles in the upper thermosphere.

The day-glow data application of FY-3D IPM in monitoring O/N2

The Ionosphere Photometer (IPM) is a far ultraviolet nadir-viewing photometer that flew aboard the second-generation, polar-orbiting Chinese meteorological satellite FY-3D, which was launched on November 25th, 2017. The F2 layer peak electron density (NmF2) and O/N2 the column density ratio can be deduced by far ultraviolet measurements of night-time OI 135.6 nm emissions, which are produced by the recombination of O+ ions and electrons, and the day-time OI 135.6 nm and N2LBH wavelength emissions produced by the photoelectron impact excitation of atomic oxygen O and molecular nitrogen N2. Analysis of the IPM data indicated the stray light at 135.6 nm wavelength is under deducted. Based on comparison with the emissive intensity calculation of AURIC model, the day-time data was conducted to make a revision of stray light. The reliable factor (1.13) of stray light subtraction was obtained by fitting the median values of the observed data points per each latitude bin to the model data. According to the O/N2 error estimation from the estimated factor of the stray light, when the factor does not change by more than 0.03, the error of O/N2 ratio from the factor will not exceed 20%. The O/N2 changes retrieved from revised IPM data during storms in April and August 2018 are well correlated with Dst indices. The research results show that the IPM data could provide a good monitoring of O/N2 changes during magnetic storms.

A Comparative Analysis of the OI 130.4‐nm Emission Observed by NASA's TIMED Mission Using a Monte Carlo Radiative Transfer Model

AbstractRemote sensing of the OI 130.4‐nm emission is potentially a useful means for routine monitoring of the atomic oxygen abundance in the upper atmosphere, especially for altitudes above 300 km where the OI 135.6‐nm emission becomes too dim to be useful. However, to date, the interpretation of the OI 130.4‐nm emission as a proxy for the O density remains ambiguous in that the relative contribution of the external and internal sources to the production of this emission has not been fully understood. In this study, we perform a comparative analysis of the OI 130.4‐nm dayglow observed by NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission using a Monte Carlo radiative transfer model to investigate the consistency between models and the TIMED/Global UltraViolet Imager (GUVI) data. The solar 130.4‐nm flux is measured by TIMED/Solar EUV Experiment (SEE). The Global Airglow (GLOW) and the Atmospheric Ultraviolet Radiance Integrated Code (AURIC) models are used to calculate the initial volume emission rates due to photoelectron impact excitation, and the NRLMSISE‐00 model is used to provide the atmospheric density and temperature profiles. Our data‐model comparisons suggest that the model predicted contributions from photoelectron impact excitation need to be scaled by a factor of 0.1–0.5 to achieve good fits with the data. Moreover, the modeled solar contributions need to be scaled up/down to fit the observations at small ( 20 )/large ( 60 ) solar zenith angles, with a factor of 0.5–1.7. These scale factors are larger than the uncertainties in the GUVI and SEE instrument calibrations, which might suggest inaccurate model predictions of the dayside O density and temperature variations with solar zenith angles.

The O(1S) 297.2‐nm Dayglow Emission: A Tracer of CO2 Density Variations in the Martian Lower Thermosphere

AbstractThe O(1S) metastable atoms can radiatively relax by emitting airglow at 557.7 and 297.2 nm. The latter one has been observed with the Imaging Ultraviolet Spectrograph onboard the Mars Atmosphere and Volatile Evolution Mars orbiter since 2014. Limb profiles of the 297.2‐nm dayglow have been collected near periapsis with a spatial resolution of 5 km or less. They show a double‐peak structure that was previously predicted but never observed during earlier Mars missions. The production of both 297.2‐nm layers is dominated by photodissociation of CO2. Their altitude and brightness is variable with season and latitude, reflecting changes in the total column of CO2 present in the lower thermosphere. Since the lower emission peak near 85 km is solely produced by photodissociation, its peak is an indicator of the unit optical depth pressure level and the overlying CO2 column density. Its intensity is directly controlled by the Lyman‐α solar flux reaching the Martian upper atmosphere. We take advantage of the Lyman‐α flux measurements of the solar Extreme Ultraviolet Monitor instrument onboard Mars Atmosphere and Volatile Evolution to model the observed OI 297.2‐nm limb profiles. For this, we combine photodissociation sources with chemical processes and photoelectron impact excitation. To determine the relative importance of the excitation processes, we apply the model to the atmospheric structure measured by the Viking 1 lander before applying it to a model atmosphere. We find very good agreement with the lower peak structure and intensity if the CO2 density provided by the Mars Climate Database is scaled down by a factor between 0.50 and 0.66. We also determine that the previously uncertain quantum yield for production of O(1S) atoms by photodissociation of CO2 at Lyman‐α wavelength is about 8%.

Ionospheric‐thermospheric UV tomography: 3. A multisensor technique for creating full‐orbit reconstructions of atmospheric UV emission

AbstractWe present the Volume Emission Rate Tomography (VERT) technique for inverting satellite‐based, multisensor limb and nadir measurements of atmospheric ultraviolet emission to create whole‐orbit reconstructions of atmospheric volume emission rate. The VERT approach is more general than previous ionospheric tomography methods because it can reconstruct the volume emission rate field irrespective of the particular excitation mechanisms (e.g., radiative recombination, photoelectron impact excitation, and energetic particle precipitation in auroras); physical models are then applied to interpret the airglow. The technique was developed and tested using data from the Special Sensor Ultraviolet Limb Imager and Special Sensor Ultraviolet Spectrographic Imager instruments aboard the Defense Meteorological Satellite Program F‐18 spacecraft and planned for use with upcoming remote sensing missions. The technique incorporates several features to optimize the tomographic solutions, such as the use of a nonnegative algorithm (Richardson‐Lucy, RL) that explicitly accounts for the Poisson statistics inherent in optical measurements, capability to include extinction effects due to resonant scattering and absorption of the photons from the lines of sight, a pseudodiffusion‐based regularization scheme implemented between iterations of the RL code to produce smoother solutions, and the capability to estimate error bars on the solutions. Tests using simulated atmospheric emissions verify that the technique performs well in a variety of situations, including daytime, nighttime, and even in the challenging terminator regions. Lastly, we consider ionospheric nightglow and validate reconstructions of the nighttime electron density against Advanced Research Project Agency (ARPA) Long‐range Tracking and Identification Radar (ALTAIR) incoherent scatter radar data.

The effect of geomagnetic‐storm‐induced enhancements to ionospheric emissions on the interpretation of the TIMED/GUVI O/N2 ratio

AbstractWe examine the consequence of enhanced atomic oxygen (OI) 135.6 nm emissions due to the recombination of O+ with electrons on the column number density ratio of atomic oxygen to molecular nitrogen (O/N2 ratio) provided by Global Ultraviolet Imager (GUVI) on board the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics satellite. GUVI O/N2 ratio is derived from the measurements of OI 135.6 nm and N2 Lyman‐Birge‐Hopfield airglow emissions. The OI 135.6 nm emission arises from two sources: photoelectron impact excitation of neutral atomic oxygen and the radiative recombination of O+ with electrons. We estimate the O/N2 ratio disturbance associated with the O+ density enhancement during geomagnetic storms through the case study of the storms on 20 November 2003 and 8 November 2004. The OI 135.6 nm emission enhancement originating from the ionosphere is derived using the Utah State University Global Assimilation of Ionospheric Measurements model ionosphere. Our results show that the O/N2 ratio increase from the equator to middle latitudes during the storm periods is primarily associated with thermospheric neutral composition disturbances. However, the contribution of the OI 135.6 nm emission originating from the ionosphere to the storm time O/N2 ratio increase is substantial in the northern low‐middle latitude regions where severe plasma density enhancements occur during the main phase of the storms. Therefore, the ionospheric contribution should be considered for an accurate assessment of the storm time O/N2 ratio increase at low‐middle latitudes during these large storm events.

Model for Cameron-band emission in comets: a case for the EPOXI mission target comet 103P/Hartley 2

Abstract The CO2 production rate has been derived in comets using Cameron-band (a3Π→X1Σ) emission of CO molecules, assuming that photodissociative excitation of CO2 is the main production mechanism of CO in the a3Π metastable state. We have developed a model for the production and loss of CO(a3Π), which has been applied to comet 103P/Hartley 2: the target of the EPOXI mission. Our model calculations show that photoelectron impact excitation of CO and dissociative excitation of CO2 can together contribute about 60–90 per cent to Cameron-band emission. The modelled brightness of (0–0) Cameron-band emission on comet Hartley 2 is consistent with Hubble Space Telescope observations for 3–5 per cent CO2 (depending on the model input solar flux) and 0.5 per cent CO relative to water, where the photoelectron impact contribution is about 50–75 per cent. We suggest that estimation of CO2 abundances on comets using Cameron-band emission may be reconsidered. We predict a height-integrated column brightness of the Cameron band of ∼1300 Rayleigh during the EPOXI mission encounter period.

Photoelectron impact excitation of OI 8446 Å emission observed from Arecibo Observatory

Photoelectron (PE) impact on thermospheric oxygen atoms is a major source of OI 8446 Å emission excitation at midlatitudes. However, historical discrepancies between observed twilight emission brightnesses and PE model predictions have not only prompted speculation regarding secondary sources of excitation but also precluded the use of observed brightness as a much‐needed diagnostic of thermospheric O density. In an effort to improve understanding of the physical mechanisms responsible for its excitation, this paper presents new photometric measurements of twilight OI 8446 Å emission brightness, together with calculations using the Field Line Interhemispheric Plasma (FLIP) PE model, acquired from Arecibo Observatory under geomagnetically quiet conditions during three winter campaigns from 1999–2002. Winter is a particularly favorable season for twilight 8446 Å observation in the northern hemisphere since significant PE production, and thus 8446 Å excitation, persists for several hours in the fully illuminated, geomagnetic conjugate hemisphere in the absence of any local thermospheric illumination. The winter 8446 Å brightness data are fully consistent with a dominant PE impact excitation source. However, the new data also confirm the Lancaster et al. (2000) report of excess early morning brightness at Arecibo with respect to PE models which use a tilted‐dipole approximation to the geomagnetic field. By using the International Geomagnetic Reference Field (IGRF) geomagnetic field model to refine Arecibo's conjugate point location specified in the FLIP PE model, we demonstrate significantly improved agreement between the modeled and observed brightness decay profiles during both morning and evening twilight intervals.

Precipitation of energetic electrons in Io's atmosphere: Production of UV emissions

Abstract Recent observations of Io by Galileo spacecraft showed the presence of a dense ionosphere, which is identified by a cold dense plasma that is at rest with respect to Io, and a tentative possibility of an intrinsic magnetic field. Also, there are measurements of intense magnetic field-aligned energetic bidirectional electron beams of tens of keV in the wake region of Io. These energetic electrons may enter into the atmosphere of Io, where they can degrade their energy via collisions with the neutral atmospheric gases. A Monte Carlo model has been generated to study the spatial degradation of electrons of energy ≤ 10 keV in the SO 2 -dominated atmosphere of Io. This model is employed to study the excitation of Io's atmosphere. The intensities of neutral atomic oxygen (OI) and sulfur (SI) ultraviolet emissions due to the energetic electron impact on SO 2 are calculated and are compared with the Hubble Space Telescope-observed intensities and the intensities produced due to photoelectron impact excitation.

CO Rydberg series converging to the D and C states observed by VUV-fluorescence spectroscopy

The undispersed and dispersed vacuum ultraviolet fluorescence ( - 200 nm) after neutral photodissociation excitation of CO has been measured as a function of the exciting-photon energy in the region between 18.5 and 36 eV. The target-gas pressure dependence of the fluorescence intensities for energies below 23 eV was observed to be linear. For exciting-photon energies above 23 eV photoelectron impact excitation of neutral CO from the ground to the CO(A ) state causes a nonlinear pressure dependence of the undispersed fluorescence intensity. CO Rydberg states converging to the D and C states have been assigned on the basis of the recorded spectra. Previous assignments of the (3s,4s,3p,4p) and (3s,4s) Rydberg series could be supported and the identification of their vibrational levels have been considerably extended. Two new progressions ((5s,6s)) have been assigned.

Simulations of DE 1 UV airglow images

In this paper we present simulations of the UV airglow observed in selected images obtained by the spinscan auroral imager experiment on the Dynamics Explorer 1 satellite. In particular, we present one example each of observed and modeled images for the dayglow and for the nightglow. The model dayglow emissions include the O I 130.4 nm triplet (excited by both photoelectron impact excitation of O atoms and resonant scattering of the solar O I 130.4 nm triplet), the O I 135.6 nm doublet (excited by photoelectron impact excitation of O atoms), and the N2 Lyman‐Birge‐Hopfield (LBH) molecular bands (excited by photoelectron impact excitation of N2 molecules). The model neglects minor contributions from H I 121.6 nm and N I 149.3 nm dayglow emissions. The model nightglow emissions are comprised of the same O I 130.4 nm and 135.6 nm features, but on the nightside they are produced through recombination of O+ ions. The model neglects a contribution to the nightglow O I 130.4 nm and 135.6 nm emissions due to recombination of O+ and O− ions. The results presented here demonstrate that the dayglow emissions are well understood and can be simulated with high accuracy. This allows us to determine relative contributions of the various dayglow emission sources over the disk of the Earth. For example, while the O I 130.4 nm emission is very bright and fairly uniform at about 15‐20 kR over much of the disk, the optically thin LBH emissions become much larger than the O I 130.4 nm emission at the limb, with a band‐integrated maximum brightness of about 35 kR. Although the image simulation code used is fairly CPU intensive, it should prove useful in studying variations in the abundances of O and N2 following geomagnetic storms or in extracting the dayglow contribution from images of the sunlit auroral oval, for instance. The nightglow emissions are not as well fit by the model, indicating that the O+‐O− mutual neutralization source is significant and should be included in future simulations. The observed tropical arcs are somewhat narrower than produced by the model simulation, probably due to the limited spatial resolution of the model. The model results indicate that the O I 130.4 nm and 135.6 nm recombination nightglow can reach several hundred rayleighs in the middle of the tropical arcs. Although swamped by dayglow emissions, the O I recombination glow on the dayside must typically exceed a kilorayleigh.

Thermospheric O I 844.6‐nm emission in twilight

The thermospheric O I 844.6‐nm column emission rate was measured over Toronto, a midlatitude station, in the autumn of 1991 using an imaging Fabry‐Perot spectrometer. Twilight decay curves were measured on four clear evenings when the solar zenith angle was between 95° and 104°, giving corresponding column emission rates between 874 R and 130 R at 20° elevation angle in the azimuth of the Sun. The expected decay curves were calculated from the field line interhemisperic plasma model assuming only photoelectron impact excitation as the production mechanism with a cross section appropriate to an optically thin atmosphere. The agreement was good when the solar and geomagnetic activity levels were low to moderate, but the emission rate was overestimated during high activity periods. The comparison indicates that the photoelectron impact mechanism with a thin‐atmosphere cross section is sufficient to explain the twilight decay of the thermospheric O I 844.6‐nm emission.

Interpretation of satellite airglow observations during the March 22, 1979, magnetic storm, using the coupled ionosphere‐thermosphere model developed at University College London

The University of California, Berkeley, extreme ultraviolet spectrometer aboard the U.S. Air Force STP 78‐1 satellite measured emission features in the Earth's dayglow due to neutral and ionized species in the atmosphere, in the 35‐140‐nm range. The spectrometer was operating between March 1979 and March 1980, including the period of the magnetic storm on March 22, 1979. Some of these measurements are interpreted using the predictions of the three‐dimensional time‐dependent coupled ionosphere‐thermosphere model developed at University College London. The observations show a reduction in the atomic oxygen 130.4‐nm airglow emission at high northern latitudes following the storm. Model simulations show that this reduction in 130.4‐nm emission is associated with an increase in the O2/O ratio. Analysis of model results using electron transport and radiative transport codes shows that the brightness of 130.4‐nm emission at high latitudes due to resonantly scattered sunlight is approximately twice that due to photoelectron impact excitation. However, the observed decrease in the brightness at high northern latitudes is mainly due to a change in the photoelectron impact source, which contributes approximately 75% of the total, as well as its multiple scattering component; for the photoelectron impact source at 70° latitude and 200 km altitude, the reduction in multiple scattering is 1.5 times greater than the reduction in the initial excitation. The reduction in the airglow emission is visible only in the northern hemisphere because the south pole was not sunlit over the storm period. The comparison of model results with observations suggests that 130.4‐nm emission may be a useful as a tracer for global changes in the concentration of atomic oxygen.

Special Sensor Ultraviolet Limb Imager: an ionospheric and neutral density profiler for the Defense Meteorological Satellite Program satellites

The Naval Research Laboratory is developing a series of far- and extreme-ultraviolet spectrographs (800 to 1700 Å) to measure altitude profiles of the ionospheric and thermospheric airglow from the U.S. Air Force Defense Meteorological Satellite Program's Block 5D3 satellites. These spectrographs, which comprise the Special Sensor Ultraviolet Limb Imager (SSULI), use a near-Wadsworth optical configuration with a mechanical grid collimator, concave grating, and linear array detector. To image the limb, SSULI employs a rotating planar SiC mirror that sweeps the field of view perpendicular to the limb of the Earth. In the primary operating mode, the mirror sweeps the instrument field of view through 17 deg to view tangent heights from about 50 to 750 km. The SSULI detectors use microchannel plate intensification and wedge-and-strip decoding anodes to resolve 256 pixels in wavelength dispersion. The detector is windowless and uses an o-ring sealed door to protect the Csl photocathode from exposure prior to insertion in orbit. The altitude distributions of the airglow measured by the SSULI sensors will be used to infer the altitude distributions of electrons and neutral species. At night, electron densities will be determined by measurement of ion recombination nightglow. Daytime electron densities will be obtained from measurements of multiple resonant scattering of O⁺ 834-Å radiation produced primarily by photoionization excitation of atomic oxygen. Dayside neutral densities and temperatures will be inferred from the measurement of dayglow emissions from N₂ and O produced by photoelectron impact excitation.

Analysis and interpretation of observations of airglow at 297 nm in the Venus thermosphere

Limb profiles obtained by the Pioneer Venus orbiter ultraviolet spectrometer (PVOUVS) are analyzed. The analysis is confined to those data obtained when the spacecraft was near periapsis (∼150‐200 km altitude) and the PVOUVS grating is set to a center‐of‐band‐pass wavelength of 297 nm. Sources are identified which give rise to the observed signal as they fall within the 1.3‐nm instrumental band pass: Rayleigh scattering, [O I] 2972 Å, and C I 2967 Å. Additionally, contamination from off‐axis scattering and the cosmic ray background are evaluated. Below 175 km, O(¹S) is produced through the photo‐dissociative excitation of CO2. Above 175 km, O(¹S) produced through the dissociative recombination of O2+ and photoelectron impact excitation of C(5S°) become important sources of airglow emissions. Structure in the [O I] 2972‐Å emission profile in the lower thermosphere is explained in terms of large temperature excursions which alter the rate of quenching of O(¹S) by CO2. This phenomenon suggests a new technique for remotely sensing Venus lower thermosphere temperatures. The deduced temperature profile is consistent with in situ measurements which have been interpreted as evidence of the propagation of atmospheric gravity waves.

Some molecular nitrogen emission from Titan‐solar EUV interaction

Voyager 1 explored the satellite Titan at a solar zenith angle of 73°. Photoelectron fluxes in the atmosphere of Titan for this same angle (73°) have been calculated for an atmospheric model with molecular nitrogen. The Photoelectron impact excitation rates of a′πg and b′πu states of N2 are also calculated.

Pioneer Venus Orbiter ultraviolet spectrometer limb observations: Analysis and interpretation of the 166‐ and 156‐nm data

Pioneer Venus orbiter ultraviolet spectrometer (PVOUVS) limb observations at 166 and 156 nm have been analyzed, and emission mechanisms at these wavelengths have been modeled. The observations prove to be diagnostic of both the thermal carbon density and the O2 to CO2 mixing ratio. The photons seen by the PVOUVS at 156 and 166 nm arise from a variety of sources: photoelectron impact dissociative excitation of CO and CO2, photodissociative excitation of CO and CO2, photoelectron impact excitation of C, solar resonance scattering by C, and the underlying CO fourth‐positive bands (CO4PG). Solar resonant scattering by atomic carbon is an important contribution to the observed intensity and, furthermore, a diagnostic of the O2 to CO2 mixing ratio. An O2 to CO2 mixing ratio of 0.3% yielded the best agreement between the modeled and observed limb intensities for both the 156‐ and 166‐nm data sets. For that O2 to CO2 mixing ratio, the atomic carbon density reaches a maximum at 155 km of 5×106 cm−3.