Visible Airglow Experiment Research Articles

Oxygen green and red line emission and O 2+ dissociative recombination

The O( 1S) quantum yield in O 2 + dissociative recombination f( 1S) in the nocturnal F-region and processes O + + O 2 → O + O 2 +( ν ⩽ 7) and O 2 + ( ν) + O → O 2 +( ν′ < ν) + Space Sci. 36, 621) and Yee et al. (1989, in Dissociative Recombination: Theory, Experiment and Applications, World Scientific, New Jersey) have earlier concluded the values of f( 1S) obtained from the Visible Airglow Experiment do not seem explicable in terms of the ab initio results of Guberman (1987, Nature 327, 408) on the α( 1S, ν) direct dissociative recombination coefficients. It is argued that the discrepancy cannot be attributed to an overlooked O( 1S) source in the F-region. Doubt is thrown on the correctness of the results of the Visible Airglow Experiment by a recent determination of the f( 1S)/f( 1D) ratio by Takahashi et al. (1990, Planet. Space Sci. 38, 547) from equatorial F-region profile measurements. These are consistent in general with theoretical expectation. However, discrepancies appear in detailed facets of the data. If they are real they may conceivably be due to the computed α( 1S, ν) being less accurate than has been supposed. The possibility is raised that α( 1S, 0), in particular, is affected by the neglected indirect dissociative recombination. Laboratory measurements on f( 1S) by Queffelec et al. (1989, J. chem. Phys. 91, 5335) are considered. They conflict with so much independent evidence that they must be regarded with reserve.

Auroral excitation of the N2 2P(0,0) and VK(0,9) bands

The low‐energy secondary electron flux caused by auroral electron precipitation is examined using data from the Atmosphere Explorer C satellite. An energetic electron transport algorithm is used to compute the differential electron flux produced by measured primaries. Emissions of N2 in the 2P(0,0) band at 337 nm and the VK(0,9) band at 335 nm predicted by the model are compared with photometric observation of their combined volume emission rate altitude profile made by the visible airglow experiment. Reasonable correspondence between model and measurement is obtained. Ratios of emissions at 337 nm and 630 nm to the N2+ 1N(0,0) band at 428 nm are also studied. It is concluded that the 337/428 nm ratio responds to changes in the characteristic energy of primary auroral electrons only insofar as part of the 337 nm brightness is due to N2 VK(0,9) emission. The 630/428 nm ratio, which is strongly dependent on characteristic energy, also varies significantly with changes in atomic oxygen density.

Diurnal and seasonal variation of the nighttime OH (8‐3) emission at low latitudes

Nighttime morphological maps of the hydroxyl emission (8–3 band) measured by the Visible Airglow Experiment on board the Atmosphere Explorer E satellite are presented for solstice and equinox conditions. The volume emission rate profile of the emission was determined by inverting satellite perigee observations. This profile was later used to convert limb measurements into zenith intensities of the emission. A theoretical model has been used to correct twilight observations for the presence of the thermospheric O+(²P) emission at 7320–7330 Å. The derived morphology agrees well with ground based measurements and theoretical simulations.

The 630 nm dayglow

The daytime thermospheric emission at 630 nm from the (³P‐¹D) transition of atomic oxygen is examined using data from the Atmosphere Explorer C and E spacecraft. Observed altitude distributions of the emission rate measured using the Visible Airglow Experiment are compared with those calculated from in situ measurements of ion and neutral densities and temperatures, and from a model of the photon and photoelectron flux. Good agreement is obtained for most orbits with photoelectron impact on O, photodissociation of O2, and dissociative recombination of O2+ providing most of the production. Implications for some of the controversial points of O(¹D) chemistry, such as the solar EUV and Schumann‐Runge continuum flux, the yield of O(¹D) from the reaction of N(²D) with O2, the value of spontaneous transition coefficients, and the rate of quenching by O(³P) are discussed.

Atmosphere Explorer observations of the geocoronal H Balmer α nightglow

The Balmer α nightglow emission of geocoronal atomic hydrogen has been investigated using photometric data obtained by the Visible Airglow Experiment on board the Atmosphere Explorer E satellite. The measured H Balmer α intensities are presented as a function of solar depression angle, observation zenith angle, and azimuth relative to the Sun for solar minimum conditions. The results are at variance with the earlier D2A satellite observations but in better agreement with previous ground‐based studies. As a result of a comparison with theoretical predictions, the present observations reaffirm the findings of the ground‐based investigations in implying that values for the Lyman β line center flux are a factor of 2 greater than those derived from direct measurements.

The auroral 6300 Å emission: Observations and modeling

Measurements of the auroral atomic oxygen (³P‐¹D) emission line at 6300 Å made by the Atmosphere Explorer visible airglow experiment are analyzed using a tomographic inversion. Emission altitude profiles are compared to the results from an electron transport and chemical reaction model. The model incorporates measurements of the energetic electron flux, neutral composition, ion composition, and electron density. Reasonable correspondence is obtained using primarily the “classical” sources of O(¹D) excitation: electron impact on atomic oxygen and dissociative recombination of O2+. The reaction of N(²D) with O2 is considered to be a minor source. Small contributions are also calculated for cascade from O(¹S), electron impact dissociation of O2, reaction of N+ with O2, and energy transfer from O+(²D) and thermal electrons to O(³P). A possible minor source from the quenching of N(²D) or N(²P) by O(³P) is also discussed.

The visible airglow experiment—a review

Contributions of the Visible Airglow Experiment (VAE) to the understanding of thermospheric aeronomy are summarized and discussed. The importance of instrumental design and operation is considered and particular attention is given to the relationship between observation and inversion for optical measurements of light emissions from a diffuse medium. The specific influence of the VAE investigation on the present understanding of the physical mechanisms which produce and destroy the excited species, O + ( 2P), O( 1D), O( 1S), N( 2D), N 2 + (B 2 Σ u +), N 2(C 3 π u ) and Mg + ( 2 P 1 2 ) , is discussed in detail.

Mesospheric 5577Å green line and atmospheric motions—Atmosphere explorer satellite observations

Photometric measurements of the 5577Å O( 1S) green line mesospheric emission obtained by the Visible Airglow Experiment (VAE) on board the Atmosphere Explorer ( AE) satellite have been analyzed. The inverted volume emission rate profiles showed a peak at approximately 96–97 km with a half-width of ∼8 km. The diurnal variation of the intensity indicates the presence of a wave component with 10 ∼ 12 h period, probably of solar semi-diurnal tide. Shorter time scale variations due to the presence of travelling waves were also observed.

Modeling the 6300‐Å low‐latitude nightglow

Observations of the 6300‐Å nightglow from the Visible Airglow Experiment (VAE) instrument on AE‐E are presented for spring equinox, solar cycle maximum conditions. The data comprise altitude profiles and integrated column brightness maps from ∼1800 to 0400 LT and within ±30° of the dip equator. The data clearly show near‐midnight enhancements of the 6300‐Å emission. Attempts to model the column brightness maps indicated that these enhancements are due to tidal effects: the enhancements were only reproduced in the theoretical calculations which included upward propagating tidal components in the neutral winds. Further, low equatorial intensities were observed by the VAE which could only be simulated by assuming that the phase of the E × B drift be shifted 1 hour LT; i.e., upward drift persists until 2000 LT instead of 1900 LT. The VAE observations could be reasonably simulated with the phase shift in the E × B drift and with the dip and geographic equators offset. The major discrepancy is in the magnitude of the nightglow maxima: the calculated intensities are a maximum of 2 times too large. Possible sources are uncertainties in the neutral densities, chemistry, and rate coefficients and in the neutral winds.

The quenching rate of O( 1D) by O( 3P)

The rate coefficient for the quenching ofO( 1D) by O( 3P) has recently been calculated by Yee et al. (1985). Their results indicate that quenching by atomic oxygen should not be ignored in the analysis of the 6300 Å emission airglow. Data obtained by the Visible Airglow Experiment (VAE) on board the AE satellites have been reanalyzed to determine the quenching rate of O( 1D) by atomic oxygen. The results of this analysis are presented.

Characteristics of the spacecraft optical glow

The optical glow induced by spacecraft‐environment interaction under daytime conditions is analyzed using photometric data obtained by the Visible Airglow Experiment on board the Atmosphere Explorer Satellite. Because of the increased ambient oxygen density, the glow is more intense in the daytime. The daytime production efficiency, measured in Rayleighs per oxygen atom, is similar to the nighttime value. No dependence of the glow intensity on the ambient plasma density was found. The production efficiency increased with time throughout the duration of the spacecraft in orbit.

Tomographic inversion of satellite photometry

An inversion algorithm capable of reconstructing the volume emission rate of thermospheric airglow features from satellite photometry has been developed. The accuracy and resolution of this technique are investigated using simulated data, and the inversions of several sets of observations taken by the Visible Airglow Experiment are presented.

Comments on the interpretation of 3371 Å filter-photometer observations and its implications for the AE-E photoelectron fluxes

Modeling of the dayglow spectrum in the wavelength region between 3300 and 3500 Å indicates that the N 2 second positive (0,0) band at 3371 Å is blended with the Vegard-Kaplan (0,9) band. A recent analysis of rocket observations of the dayglow shows that 20–30% of a 3371 Å narrow band filter-photometer signal is due to the VK emission (Conway and Christensen, 1983). Kopp et al. (1977) and Hernandez et al. (1983) reported analyses of 3371 Å photometer observations from the Visible Airglow Experiment on the Atmospheric Explorer-C (AE-C) satellite which did not consider the Vegard-Kaplan (VK) emission. The observations were compared to theoretical estimates of the second positive volume emission rate based on a photoelectron model and on absolute fluxes measured by the Photoelectron Spectrometer experiments on AE-C and AE-E. Inclusion of the VK band in the AE analysis would bring the reported photoelectron theory into agreement with the airglow observations. However, the overestimate of the N 2 second positive airglow predicted by the AE-E photoelectron flux measurements increases to a factor of nearly two rather than the 20–30% reported by Hernandez et al. (1983).

The surface glow of the Atmosphere Explorer C and E satellites

Detailed line intensities of the rotational‐vibrational spectrum of OH are calculated for various rates of population of sixteen bound vibrational levels in which the rotational level populations are in thermal equilibrium. The line intensities are convolved with the instrument function of the Visible Airglow Experiment on the AE C and E satellites to obtain spectra for comparison with the atmospheric data. Within the uncertainties, a model can be constructed that reproduces the observed spectrum. Synthetic spectra are computed for the OH Meinel system between 2000 Å and 5.5µ. The maximum intensity occurs between 1µ and 2µ.

The dissociative recombination of O2+: The quantum yield of O(¹S) and O(¹D)

Data from the visible airglow experiment on the Atmosphere Explorer‐E satellite have been used to determine the quantum yield of O(¹S) and O(¹D) from the dissociative recombination of O2+. A range of values between 0.09 and 0.23 has been obtained for the quantum yield of O(¹S). It is shown that the quantum yield of O(¹S) depends on the ratio of electron density to atomic oxygen density. This suggests that the quantum yield of O(¹S) may depend on the degree of vibrational excitation of the recombining O2+. The quantum yield of O(¹D) has been measured to be 1.23±0.42, with no dependence on the electron‐oxygen ratio.

Theoretical modeling of low‐latitude Mg+

F region metal ion morphology is important because these long‐lived ions act as tracers of the dynamical processes. The behavior of Mg+ was inferred from the 2800 Å resonance emission observed by the Visible Airglow Experiment on the Atmosphere Explorer E satellite in equatorial orbit. Data were obtained over 5 months near winter solstice during solar maximum conditions. Theoretical modelling of the Mg+ densities was undertaken by solving the coupled continuity and momentum equations for one major and one minor ion. A uniform layer of 100 metal ions cm−3 at the lower boundary of 125 km was assumed. The numerical value of the density at the boundary is not critical, as all results are linearly scaled to the boundary density. The calculations reproduced the observed appearance of largest metal ion densities in the early afternoon in winter solstice conditions, and number density profiles that are nearly altitude independent near the equator near noon. Longitudinal effects are apparent mainly at high altitudes (≳350 km) near dusk. Correlations of high altitude densities with season and solar cycle are supported: larger densities at fixed altitudes are seen during solar maximum conditions, and at equinox near sunset. Neutral winds, particularly in the E region, were found to be very important in determining the metal ion distribution at high altitudes. The E region tidal oscillations especially seem to be the largest single factor controlling the F region metal ion behavior.

Visible glow induced by spacecraft‐environment interaction

Illumination was found recently in the ram direction of the Shuttle Orbiter arising from interaction with the ambient surrounding atmosphere. We have examined a similar optical glow by analyzing photometric data taken by the Visible Airglow Experiment on board the Atmosphere Explorer Satellites. Analysis at six different wavelengths from the ultraviolet to the near infrared suggests that the glow has a diffuse band or continuum spectrum which peaks in the red and probably in the infrared.

Volume emission rate profiles of the 6300‐Å tropical nightglow obtained from the AE‐E satellite: Latitudinal and seasonal variations

Surface brightnesses at 6300‐Å measured by the visible airglow experiment on board the AE‐E satellite have been inverted to obtain altitude profiles of volume emission rate. A morphological picture of the emission at low latitudes is developed and qualitatively interpreted in terms of the behavior of electric fields and neutral winds in the nighttime ionosphere. Global pictures of the emission are presented for solstice and equinox conditions between 1800 and 0400 LT. Observed regions of depleted and enhanced emission are in agreement with the emission pattern expected from the effect of E × B drifts on the ionization. Other features observed are interpreted in terms of a transequatorial wind and the presence of a pressure maximum near local midnight. These data suggest that the location of the pressure bulge changes seasonally with respect to the geographic equator. Average bottomside electron density altitude profiles are derived from the 6300‐Å volume emission profiles. The local time variation of the height of the peak electron density deduced from the airglow measurements is in agreement with incoherent scatter measurements made at Arecibo, Puerto Rico, during the same time period.

Galactic and zodiacal light surface brightness measurements with the Atmosphere Explorer Satellites

Galactic and zodiacal light surface maps are presented at 7320, 6300, 5577, 5200, and 4278 A. These were prepared from measurements made with the Visible Airglow Experiment on board the Atmosphere Explorer-C, -D, and -E satellites.

The production efficiency of O+(²P) ions by auroral electron impact ionization

A set of complementary observations obtained by experiments on the Atmosphere Explorer D satellite was used to deduce the production efficiency of O+(²P) by electron impact ionization. An efficiency of 18% was derived.