Lyman-Birge-Hopfield Emission Research Articles

Constraining the Upper Level Vibrational Populations of the N2 Lyman‐Birge‐Hopfield Band System Using GOLD Mission's Dayglow Observations

AbstractCurrent and previous thermospheric remote sensing missions use N2 Lyman‐Birge‐Hopfield (LBH) band dayglow emission measurements to retrieve line‐of‐sight thermospheric composition and temperature. The precision of thermospheric composition and temperature retrieved from observations depends on the uncertainty in the relative LBH vibrational populations. In the laboratory, electron impact induced LBH emission measurements have shown that the relative vibrational populations change with gas pressure. However, it is not fully understood how these populations change for dayglow observations where the emissions that contribute to the observations vary with solar illumination and line‐of‐sight geometry. We quantify the relative vibrational populations as a function of solar zenith angle (SZA) and tangent altitude using Global‐scale Observations of Limb and Disk mission's LBH dayglow observations. We find that, while some lower vibrational levels show potential enhancement with increasing pressure (decreasing altitude), in general, they do not change significantly with SZA or tangent altitude for dayglow observations. The vibrational populations can thus be assumed as fixed parameters when retrieving neutral disk temperatures from remotely sensed LBH dayglow observations.

Assimilative Mapping of Auroral Electron Energy Flux Using SSUSI Lyman‐Birge‐Hopfield (LBH) Emissions

AbstractFar ultraviolet (FUV) imaging of the aurora from space provides great insight into the dynamic coupling of the atmosphere, ionosphere, and magnetosphere on global scales. To gain a quantitative understanding of these coupling processes, the global distribution of auroral energy flux is required, but the inversion of FUV emission to derive precipitating auroral particles' energy flux is not straightforward. Furthermore, the spatial coverage of FUV imaging from Low Earth Orbit (LEO) altitudes is often insufficient to achieve global mapping of this important parameter. This study seeks to fill these gaps left by the current geospace observing system using a combination of data assimilation and machine learning techniques. Specifically, this paper presents a new data‐driven modeling approach to create instantaneous, global assimilative mappings of auroral electron total energy flux from Lyman‐Birge‐Hopfield (LBH) emission data from the Defense Meteorological System Program (DMSP) Special Sensor Ultraviolet Spectrographic Imager (SSUSI). We take a two‐step approach; the creation of assimilative maps of LBH emission using optimal interpolation, followed by the conversion to energy flux using a neural network model trained with conjunction observations of in‐situ auroral particles and LBH emission from the DMSP Special Sensor J and SSUSI instruments. The paper demonstrates the feasibility of this approach with a model prototype built with DMSP data from 17 February 2014 to 23 February 2014. This study serves as a blueprint for a future comprehensive data‐driven model of auroral energy flux that is complementary to traditional inversion techniques to take advantage of FUV imaging from LEO platforms for global assimilative mapping of auroral energy flux.

Variations of Lower Thermospheric FUV Emissions Based on GOLD Observations and GLOW Modeling.

Here we compare the global‐scale morphology of Earth's the Far‐Ultraviolet (FUV) emissions observed by NASA's Global‐scale Observations of Limb and Disk (GOLD) mission to those modeled using the Global Airglow (GLOW) code with atmospheric parameters provided by Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM). The O 5S oxygen (135.6 nm) and N2 Lyman‐Birge‐Hopfield (LBH) emissions are observed over the Western hemisphere every 30 min by the GOLD instrument. The FUV brightness of the thermosphere‐ionosphere is expected to vary in systemic ways with respect to geophysical parameters, solar energy input from above, and terrestrial weather input from below. In this paper we examine the O 5S oxygen emission and the N2 LBH emission brightnesses with local time, latitude, season, tides, geomagnetic activity, and solar activity based on GOLD observations and GLOW modeling. Early GOLD observations indicate that the model effectively reproduces the brightness variations with local time and latitude but is biased low in magnitude. However, the TIEGCM is unable to accurately represent the extraordinary nighttime equatorial ionization anomaly observed by GOLD. It is also expected from these results that the signal from geomagnetic storms may obscure tidal signals.

Global‐Scale Observations of the Limb and Disk Mission Implementation: 1. Instrument Design and Early Flight Performance

AbstractThe Global‐scale Observations of the Limb and Disk (GOLD) is a National Aeronautics and Space Administration mission of opportunity designed to study how the Earth's ionosphere‐thermosphere system responds to geomagnetic storms, solar radiation, and upward propagating atmospheric tides and waves. GOLD employs an instrument with two identical ultraviolet spectrographs that make observations of the Earth's thermosphere and ionosphere from a commercial communications satellite owned and operated by Société Européenne des Satellites (SES) and located in geostationary orbit at 47.5° west longitude (near the mouth of the Amazon River). They make images of atomic oxygen 135.6 nm and N2 Lyman‐Birge‐Hopfield (LBH) 137–162 nm radiances of the entire disk that is observable from geostationary orbit and on the near‐equatorial limb. They also observe occultations of stars to measure molecular oxygen column densities on the limb. Here, we provide an overview of the instrument and compare its prelaunch and early flight measurement performance. Direct comparison of LBH spectra of an electron lamp taken before launch with spectra on orbit provides evidence that both cascade and direct excitation are important sources of thermospheric LBH emission.

The UV Spectrum of the Lyman‐Birge‐Hopfield Band System of N2 Induced by Cascading from Electron Impact

AbstractWe have measured in the laboratory the far ultraviolet (FUV: 125.0–170.0 nm) cascade‐induced spectrum of the Lyman‐Birge‐Hopfield (LBH) band system ( ) of N2 excited by 30–200 eV electrons. The cascading transition begins with two processes: radiative and collision‐induced electronic transitions (CIETs) involving two states ( and w1Δu → a1Πg), which are followed by a cascade induced transition at the single‐scattering pressures employed here. Direct excitation to the a‐state produces a confined LBH spectral glow pattern around an electron beam. We have spatially resolved the electron‐induced glow pattern from an electron beam colliding with N2 at radial distances of 0–400 mm at three gas pressures. This imaging measurement is the first to isolate spectral measurements in the laboratory of single‐scattering electron‐impact‐induced fluorescence from two LBH emission processes: direct excitation, which is strongest in emission near the electron beam axis; and cascading‐induced, which is dominant far from the electron beam axis. The vibrational populations for vibrational levels from v′ = 0–2 of the a 1Πg state are enhanced by radiative cascade and CIETs, and the emission cross sections of the LBH band system for direct and cascading‐induced excitation are provided at 40, 50, 100, and 200 eV.

Upper Atmosphere Radiance Data Assimilation: A Feasibility Study for GOLD Far Ultraviolet Observations

AbstractFar ultraviolet observations of Earth's dayglow from the National Aeronautics and Space Administration (NASA) Global‐scale Observations of the Limb and Disk (GOLD) mission presents an unparalleled opportunity for upper atmosphere radiance data assimilation. Assimilation of the Lyman‐Birge‐Hopfield (LBH) band emissions can be formulated in a similar fashion to lower atmosphere radiance data assimilation approaches. To provide a proof‐of‐concept for such an approach, this paper presents assimilation experiments of simulated LBH emission data using an ensemble filter measurement update step implemented with National Oceanic and Atmospheric Administration (NOAA)'s Whole Atmosphere Model (WAM) and National Center for Atmospheric Research (NCAR)'s Global Airglow (GLOW) model. Primary findings from observing system simulation experiments (OSSEs), wherein “truth” atmospheric conditions simulated by NCAR's Thermosphere Ionosphere Electrodynamic General Circulation Model (TIEGCM) are used to generate synthetic GOLD data, are as follows: (1) Assimilation of GOLD LBH disk emission data can reduce the bias in model temperature specification (ensemble mean) by 60% under both geomagnetically quiet conditions and disturbed conditions. (2) The reduction in model uncertainty (ensemble spread) as a result of assimilation is about 20% in the lower thermosphere and 30% in the upper thermosphere for both conditions. These OSSEs demonstrate the potential for far ultraviolet radiance data assimilation to dramatically reduce the model biases in thermospheric temperature specification and to extend the utility of GOLD observations by helping to resolve the altitude‐dependent global‐scale response of the thermosphere to geomagnetic storms.

Electron impact study of the 100 eV emission cross section and lifetime of the Lyman‐Birge‐Hopfield band system of N2: Direct excitation and cascade

AbstractWe have measured the 100 eV emission cross section of the optically forbidden Lyman‐Birge‐Hopfield (LBH) band system (a1Πg → X1Σ+g) of N2 by electron‐impact‐induced fluorescence. Using a large (1.5 m diameter) vacuum chamber housing an electron gun system and the Mars Atmosphere and Volatile EvolutioN mission Imaging Ultraviolet Spectrograph optical engineering model, we have obtained calibrated spectral measurements of the LBH band system from 115 to 175 nm over a range of lines of sight to capture all of the optical emissions. These measurements represent the first experiment to directly isolate in the laboratory single‐scattering electron‐impact‐induced fluorescence from both direct excitation of the a1Πg state and cascading contributions to the a1Πg state (a′1Σ−u and w1Δu → a1Πg → X1Σ+g). The determination of the total LBH emission cross section is accomplished by measuring the entire cylindrical glow pattern of the metastable emission from electron impact by imaging lines of sight that measure the glow intensity from zero to ~400 mm radial distance and calculating the ratio of the integrated intensity from the LBH glow pattern to that of a simultaneously observed optically allowed transition with a well‐established cross section: Ni 120.0 nm. The “direct” emission cross section of the a1Πg state at 100 eV was determined to be σemdir = (6.41 ± 1.3) × 10−18 cm2. An important observation from the glow pattern behavior is that the total (direct + cascading) emission cross section is pressure dependent due to collision‐induced cascade transitions between close‐lying electronic states.

Production of [formula omitted] Vegard–Kaplan and Lyman–Birge–Hopfield emissions on Pluto

We have developed a model to calculate the emission intensities of various vibrational transitions of N2 triplet band and Lyman–Birge–Hopfield (LBH) band emissions in the dayglow of Pluto for solar minimum, moderate, and maximum conditions. The calculated overhead intensities of Vegard–Kaplan (A3Σu+–X1Σg+), First Positive (B3Πg–A3Σu+), Second Positive (C3Πu–B3Πg), Wu–Benesch (W3Δu–B3Πg), Reverse First Positive, and LBH (a1Πg–X1Σg+) bands of N2 are 17 (74), 14.8 (64), 2.4 (10.8), 2.9 (12.7), 2.9 (12.5), and 2.3 (10) R, respectively, for solar minimum (maximum) condition. We have predicted the overhead and limb intensities of VK (150–190nm) and LBH (120–190nm) bands of N2 on Pluto for the New Horizons (NH) flyby condition that can be observed by ALICE: the ultraviolet imaging spectrograph also know as P-ALICE. The predicted limb intensities of VK and LBH bands peak at radial distance of ∼2000km with the value of about 5 (13) and 9.5 (22) R for solar zenith angle 60° (0°), respectively. We have also calculated overhead and limb intensities of few prominent transition of CO Fourth Positive bands for NH flyby condition.

Statistical relationship between large‐scale upward field‐aligned currents and electron precipitation

AbstractSimultaneous observations of Birkeland currents by the constellation of Iridium satellites and N2 Lyman‐Birge‐Hopfield (LBH) auroral emissions measured by the Global Ultraviolet Imager (GUVI) onboard the Thermosphere, Ionosphere, and Mesosphere Energetics and Dynamics (TIMED) satellite are used to establish relationships between large‐scale upward field‐aligned currents and electron precipitation during stable current configurations. The electron precipitation was inferred from GUVI data using a statistical relationship between LBH intensity and electron energy flux. LBH emissions with >5% contribution from protons, identified by Lyman‐alpha intensity, were excluded from the analysis. The Birkeland currents were derived with a spatial resolution of 3° in latitude and 2 h in local time. For southward interplanetary magnetic field (IMF), the electron precipitation occurred primarily within and near large‐scale upward currents. The correspondence was less evident for northward IMF, presumably because the spatial variability is large compared to the areas of interest so that the number of events identified is smaller and the derived statistical distributions are less reliable. At dusk, the correlation between upward current and precipitation was especially high, where a larger fraction of the electron precipitation is accelerated downward by a field‐aligned potential difference. Unaccelerated electron precipitation dominated in the morning sector, presumably induced by scattering of eastward‐drifting energetic electrons into the loss cone through interaction with whistler‐mode waves (diffuse precipitation) rather than by field‐aligned acceleration. In the upward Region 1 on the dayside, where the electron precipitation is almost exclusively due to field‐aligned acceleration, a quadratic relationship between current density and electron energy flux was observed, implying a linear current‐voltage relationship in this region. Current density and electron energy flux in the regions of the large‐scale upward currents from pre‐midnight through dawn to noon are essentially uncorrelated, consistent with diffuse electron precipitation dominating the incident energy flux.

Modeled and observed N2Lyman‐Birge‐Hopfield band emissions: A comparison

[1] A thorough understanding of how the N2 Lyman-Birge-Hopfield (LBH) band emissions vary with altitude is essential to the use of this emission by space-based remote sensors. In this paper, model-to-model comparisons are first performed to elucidate the influence of the solar irradiance spectrum, intrasystem cascade excitation, and O2 photoabsorption on the limb profile. Next, the observed LBH emissions measured by the High resolution Ionospheric and Thermospheric Spectrograph aboard the Advanced Research and Global Observation Satellite are compared with modeled LBH limb profiles to determine which combination of parameters provides the best agreement. The analysis concentrates on the altitude dependence of the LBH (1,1) band, the brightest LBH emission in the observations. In the analysis, satellite drag data are used to constrain the neutral densities used for the data-to-model comparisons. For the average limb profiles on two of the three days analyzed (28, 29, and 30 July 2001), calculations using direct excitation alone give slightly better agreement with the observations than did calculations with cascading between the singlet electronic N2 states (a1Πg, a′Σ−u, and w1Δu); however, the differences between the observed profiles and either model are possibly greater than the differences between the models. Nevertheless, both models give excellent agreement with the observations, indicating that current models provide an adequate description of the altitude variation of the N2 LBH (1,1) band emissions. Consequently, when using the LBH bands to remotely sense thermospheric temperatures, which can be used to provide an unprecedented view of the thermosphere, the temperatures derived have a negligible dependence on the model used.

A downward revision of a recently reported proton auroral LBH emission efficiency

[1] A significantly higher N2 Lyman-Birge-Hopfield (LBH) emission efficiency for auroral proton precipitation compared to model calculations was reported by Knight et al. (2008) based on a statistical study utilizing coincident far ultraviolet and particle data from the sensors Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and Special Sensor J/5 (SSJ/5) on board the DMSP satellite F16. Here, the quantity of interest from that study is the median ratio of LBH column emission rates (CERs) from SSUSI and derived from SSJ/5 spectra using monoenergetic emission yields. The median ratio was found to be 2.83 for proton aurora, suggesting the need for significant increases in currently used LBH proton/H-atom impact cross sections. A key step in their analysis was extrapolation of SSJ/5 spectra above 30 keV. Limited testing of this algorithm using NOAA Polar Orbiting Environmental Satellites Total Energy Detector and Medium Energy Proton and Electron Detector (TM) data found no significant bias. This work reports on a more detailed investigation of the algorithm's performance, also using TM data, and has uncovered a bias that reduces the median column emission rates (CER) ratio to 1.75. Within expected uncertainties, including calibration, this still calls for cross section increases but to a lesser extent. The discovered bias becomes apparent with CER thresholding that was overlooked during testing by Knight et al. (2008). Thresholding at 400 Rayleighs (R) is necessary since Knight et al. excluded CERs < 400 R in deriving their median ratio. We show that the algorithm's performance degrades with increasing energy flux of the precipitation. A method is reported for eliminating most of the bias which utilizes auroral Ly α, whose emission strength is closely coupled to spectral hardness.

Laboratory studies of UV emissions from proton impact on N2: The Lyman-Birge-Hopfield band system for aurora analysis

[1] We have measured the emission cross sections of the Lyman-Birge-Hopfield (LBH) a 1Πg − X1Σg+ band system and several atomic nitrogen (N I) multiplets (1200, 1243, 1493 A) by H+ (proton) impact on N2 over an impact energy range of 1–7 keV. The peak proton-impact-induced emission cross section of the LBH band system (1260–2500 A) was measured to be 5.05 ± 1.52 × 10−17 cm2 at 7 keV. To the best of our knowledge, the present LBH emission cross sections are reported for the first time in the far ultraviolet (FUV) wavelength range of 1100–1600 A. The proton energy range in this study, when coupled with previously published 10–100 keV proton excited emissions of N I multiplets, provides a wide energy range of emission cross sections for proton energy loss transport codes. This energy range includes the peak cross section and the energy range for Born scaling. The reported measurements lead to an important component of monoenergetic yields for proton FUV auroral emission. Such yields, based on emission cross sections and transport modeling, allowed for convenient comparison of emission efficiencies between proton and electron aurora. In addition, we have measured the H Ly α, LBH, and N I multiplet emission cross sections for H2+ and H3+ ion impact on N2 at 5 keV and found that the magnitude of H Ly α emission cross section, σem(Ly α), follows in the order of impact ion mass H3+ > H2+ > H+.

Lyman–Birge–Hopfield emissions from electron-impact excited N2

Relative electron-impact-induced emission cross sections for the a 1Πg (v′ = 3)–X 1Σ+g (v″ = 0) and a 1Πg (v′ = 2)–X 1Σ+g (v″ = 0) transitions are presented. Critical comparison is made with existing cross sections showing significant discrepancy with the widely accepted excitation function of Ajello and Shemansky (1985 J. Geophys. Res. 90 9845–61) at energies below ∼80 eV. A series of extensive measurements are presented that were performed to rule out any possible systematic or random errors in the present experimental apparatus and methodology. These efforts lead to the conclusion that the current measurements are robustly reproducible and, thus, should supplant the LBH cross-section shape of Ajello and Shemansky (1985 J. Geophys. Res. 90 9845–61).

Molecular nitrogen LBH band system far‐UV emissions of sprite streamers

The time dependent optical emission model developed by Liu and Pasko (2004) is applied to studies of far‐UV emissions of sprite streamers due to N2 Lyman‐Birge‐Hopfield (LBH) band system. Modeling results indicate that the LBH emissions of sprite streamers at 70 km are generally stronger by up to a factor of 10 than those from the first negative band system of N2+, and experimental measurements of the ratio of these two emissions at sprite altitudes can be used to determine poorly known quenching altitude of the N2 (a1Πg) state.

The role of proton precipitation in the excitation of auroral FUV emissions

Far ultraviolet remote sensing from a high‐altitude satellite is extensively used to image the global aurora, derive its energetics, and follow its dynamical morphology. It is generally assumed that the observed emissions are dominated by the interaction of the precipitated electrons with the thermospheric constituents. A model to calculate far ultraviolet emissions excited by auroral electrons and protons and the secondary electrons they generate has been used to calculate the volume excitation rate of the H I Ly‐α, O I 1304 and 1356 Å, N 11493 Å multiplets, and the N2 Lyman‐Birge‐Hopfield (LBH) bands. The characteristic energy and the energy flux are derived from the observed statistical distribution of precipitated protons and electrons. This model is applied to the midnight aurora, the noon cusp, and a proton‐dominated aurora for moderately disturbed conditions. We show that in the first two cases, direct electron impact dominates the vertically integrated emission rate over the proton component, although proton excitation plays an important role at some altitudes in the daytime cusp. In afternoon regions of the auroral zone near the auroral boundary, secondary electrons due to proton ionization are the main source of FUV emissions. The energy dependence of the efficiency of LBH band emission viewed from high altitude is calculated for electron and proton precipitations. Maps of the N2 LBH emission excited by both components are obtained, and regions of proton‐dominated auroral emission are identified. It is found that the distribution of the ratio of proton‐induced to electron‐induced brightness resembles maps of the ratio of the respective precipitated energy fluxes. Proton‐dominated FUV emissions are thus located in a C‐shaped sector extending from prenoon to midnight magnetic local times with a maximum proton contribution near the equatorward boundary of the statistical electron oval. The distribution of the Ly‐α/LBH intensity ratio is found to mimic the ratio of the proton flux/total energy flux, although it is insufficient by itself to accurately determine the relative fraction of auroral energy carried by the protons.

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.

Upper limits on spacecraft‐induced ultraviolet emissions from the space shuttle (STS‐61C)

Ultraviolet spacecraft‐induced emissions from low Earth‐orbiting satellites have been reported by several investigators. Several R/Å of ultraviolet emission were observed from the S3‐4 satellite at altitudes between 180 and 250 km and from the Spacelab 1 shuttle mission at an altitude of 250 km. Conway et al. (1987) showed that N2 Lyman‐Birge‐Hopfield (LBH) emissions observed by S3‐4 at night are probably the result of spacecraft interaction with the atmosphere. We have searched for band emission of N2, OH, O2, and NO in nightglow spectra obtained in January 1986 with the Johns Hopkins ultraviolet background experiment (UVX) flown on the space shuttle Columbia (STS‐61C) at an altitude of 330 km. The experiment consisted of two Ebert monochromators spanning the spectral range from 1200 to 1700 Å at 17 Å resolution and from 1600 to 3200 Å at 29 Å resolution. The spectra yield 3σ upper limits for the following total band emission rates: NO δ, 0.6 R; NO γ, 0.7 R; NO β, 3.5 R; O2 Herzberg I, 4.5 R; OH (A ²Σu+ ‐ X²Π) (0,0) and (1,0), 0.1 R, and N2 LBH, 5.3 R. The upper limits for the N2 LBH emissions are consistent with the recent models of spacecraft induced LBH glow of Kofsky (1988), Swenson and Meyerott (1988), and Cuthbertson and Langer (1989) and with a [N2]³ or [N2]²[O] altitude dependence.

The dependence of modeled OI 1356 and N2 Lyman Birge Hopfield auroral emissions on the neutral atmosphere

Images of the entire auroral oval at carefully selected wavelengths contain information on the global energy influx due to energetic particles and some information on the characteristic energy of the precipitating particles. In this paper we investigate the sensitivity of selected auroral emissions to changes in the neutral atmosphere. In particular, we examine the behavior of OI 1356 Å and two Lyman Birge Hopfield (LBH) bands and their ratios to each other with changing atmospheric composition. The two LBH bands are selected so that one lies in the region of strong O2 absorption (1464 Å) and one lies at a wavelength where O2 absorption is effectively negligible (1838 Å). We find that for anticipated average uncertainties in the neutral atmosphere (factor of 2 at auroral altitudes), the resultant change in the modeled intensities is comparable to or less than the uncertainty in the neutral atmosphere. The smallest variations, for example, are for I 1838 (approximately 10 to 20%) while the largest variation is seen in the OI 1356 Å emission which is linear with [O] to within 20%. We have also investigated the dependence of these intensities, and their ratios, to much larger changes in the composition (i.e., [O]/[N2]) such as might be encountered in large magnetic storms, or over seasonal or solar cycle extremes. We find that the variation in the I 1356/I 1838 ratio over the equivalent of a solar cycle is less than 50%. The summer‐to‐winter changes are approximately a factor of 2. The I 1356/I 1838 ratio is a very sensitive indicator of the charactenstic energy, showing a change of 13 over the energy range 200 eV to 10 keV. The corresponding change in the LBH long‐to‐short wavelength ratio is much less (about a factor of 3). However, the latter is insensitive to changes in the neutral atmosphere (&lt;20% changes in LBH emission ratio for large changes in N2). The three emissions therefore potentially provide a most valuable diagnostic of particle characteristic energy and energy flux.

A surface chemistry model for the production of N 2 LBH spacecraft glow

Spacecraft-atmosphere interactions leading to N 2 Lyman-Birge-Hopfield (LBH) bands in the 1400–1700 Å ultraviolet (reported by Conway et at., 1987, Geophys. Res. Lett. 14, 628) have been proposed to result from surface recombination of N (Kofsky, 1988, Geophys. Res. Lett. 15, 241; Swenson and Meyerott, 1988, Geophys. Res. Lett. 15, 245). The surface recombination of N( 4S) can populate the a 1Π g , state (which leads to N 2 LBH emission) either (1) directly, or (2) by cascade from higher excited states of N 2. Gas phase recombination produces a 1Π q populations in υ = 4−6, whereas the observed spacecraft surface interactions lower the vibrational population. This study describes and explores the possible cascade (2) mechanism where the upper states of N 2 (particularly c′ 4 1 Σ + u ) are populated with the translational energy of ramming N on surface bound N, in recombination. The cascade mechanism would populate the a 1 Π g , υ = 0–1 directly. The proposed cascade mechanism predicts additional emissions at ∼ 3000 A ̊ (Gaydon Herman band system) and ∼ 958 A ̊ from the surface recombination. The 958 Å photons can resonant scatter in the spacecraft cloud and atmospheric N 2 which would result in extended spacecraft glows in low Earth orbit. The altitude profile is predicted and compared with observations. The predicted emissions offer criteria for experimental confirmation of the (1) “direct” vs (2) “cascade” mechanisms.

The far ultraviolet vehicle glow of the S3‐4 satellite

Nadir‐viewing far ultraviolet observations obtained on the S3‐4 satellite show that during the daytime the N2 Lyman‐Birge ‐Hopfield (LBH) emission exhibits a change in vibrational distribution and becomes substantially brighter than predicted from photoelectron theory when the satellite is near perigee. At night, the intensity depends on altitude and no emission is detected when the vehicle is above 230 km. The altitude variation of the signal is proportional to the cube of the N2 density or the product of the square of the N2 density and the O density. We conclude that there is an LBH emission which is excited by the interaction of the vehicle with the ambient atmosphere, and that it is this emission which was previously interpreted as nightglow. The vibrational distribution is sharply peaked at v′ = 0. The altitude variation of the emission suggests a three stage excitation process in which N2 is adsorbed, collisionally excited, and finally electronically excited and desorbed by ambient N2 collisions, but physically reasonable reaction rates fail to account for the observed intensities.