Oxygen airglow emission on Venus and Mars as seen by VIRTIS/VEX and OMEGA/MEX imaging spectrometers

Imaging spectrometers are highly effective instruments for investigation of planetary atmospheres. They present the advantage of coupling the compositional information to the spatial distribution, allowing simultaneous study of chemistry and dynamics in the atmospheres of Venus and Mars. In this work, we summarize recent results about the O₂(a¹Δ_g) night and day glows, respectively obtained by VIRTIS/Venus Express and OMEGA/Mars Express, the imaging spectrometers currently in orbit around Venus and Mars. The case of the O₂(a¹Δ_g - X³Σ_g⁻) IR emission at 1.27 μm on the night side of Venus and the day side of Mars is analyzed, pointing out dynamical aspects of these planets, like the detection of gravity waves in their atmospheres. The monitoring of seasonal and daily airglow variations provides hints about the photochemistry on these planets.     

Brightness of the O2 atmospheric bands in the daytime thermosphere

The Fabry-Perot interferometer on Dynamics Explorer 2 was used as a low sensitivity photometer to study the O₂ Atmospheric A band during the daytime. A study of the brightness of the emission showed that the assumed source of O₂(b¹Σ_g⁺) in the thermosphere, O(¹D), can account for the observed intensity up to about 250 km but with a significantly different scale height. This combined with an enhanced brightness above this altitude suggests an additional source for this emission.     

Association of atomic oxygen and airglow excitation mechanisms

An outline is presented of a method of calculating relative rates of association of oxygen atoms by energy transfer into the different bound states of molecular oxygen. The method takes account of the detailed form of the potential energy curves for the different states. Results are presented for seven bound states of O₂ at temperatures between 100 and 600 K and are compared with laboratory and airglow data. The ⁵Π_g state, not known experimentally, accounts for over 70% of the total association below 200 K. At higher temperatures it is redissociated, and the variation in its effective association rate is primarily responsible for the variation of the total rate of oxygen association with temperature. The ⁵Π_g state is probably involved in the production at night of the a¹Δ_g and b¹Σ_g⁺ states of O₂, and in the production of $\text{O(}{}^1\text{S)}$ in the airglow. It may also be produced by electron impact on O₂, and contribute to $\text{O(}{}^1\text{S)}$ production in aurorae.     

Fluctuations in mid-latitude 6300 Å airglow and their relationship to F-region irregularities

Observations of fluctuations in the 6300 Å airglow emission at night have been studied using a tilting filter photometer. Spectral analysis of the fluctuations shows that at times the fluctuations are primarily due to wavelike disturbances with relatively well defined periods. At other times the spectrum of the fluctuations contains significant power only at long periods and there is a relatively sharp cut-off at the short period end. Most spectra are of these two types and are probably indicative of gravity waves. A third category is observed in which the spectra are of the form $\text{P∽?}{}^{\mathrm{?1.3}}$ Fewer spectra of this type are observed when the magnetic activity increases and the periods of spectral peaks observed changes with magnetic activity. The total power in the airglow fluctuations increases with K_p and the occurrence of spread F.     

A measurement of the O2(b1Σg+−X3Σg−) atmospheric band and the OI(1S) green line in the nightglow

Simultaneous measurements of the nightglow profiles of the O₂(b¹Σ_g⁺?X³Σ_g^?) A-band, the atomic oxygen green line and the OH (8?3) Meinel band are presented. The altitude profiles are used to determine both the excitation mechanisms for the oxygen emissions and the atomic oxygen altitude distribution. It is shown that the measurements are consistent with a green line excitation through the Barth mechanism and that the molecular oxygen emission is excited through oxygen recombination and the reaction between OH¹ and atomic oxygen. The derived atomic oxygen concentrations,6.2 × 10¹¹cm^?3at 98km, are consistent with the Jacchia (1971) model.     

The excitation of O2(b1Σg+) in the nightglow

It is proposed that the available measurements of the O₂(b¹Σ_g⁺ ?X³Σ_g^?) atmospheric bands both in the nightglow and in the laboratory indicate that the excitation mechanism is a two-step process rather than the direct three body recombination of atomic oxygen. It is shown that such a two-step mechanism can explain observations of the atmospheric bands both in altitude and intensity.     

Changes in thermospheric composition inferred from twilight O+(2P) emission

Measurements of the twilight enhancement of airglow emission from O⁺(²P) near 7325 Å reveal major changes which accompany geomagnetic activity, no significant distance between evening and morning and an increase in brightness paralleling the approach to solar maximum. The principal source for O⁺(²P) is direct photoionization from O(³P) but at low solar activity there appears to be a contribution from another source in early twilight which may be local photoelectron ionization into O⁺(²P). The geomagnetic and solar effects appear to reflect changes in the O and N₂ density in the thermosphere; ground based twilight measurements of O⁺ emissions thus provide a simple means for monitoring thermospheric structure from 300 km to ～ 500 km at solar minimum and to ～600 km at solar maximum.     

Rocket measurements of oxygen and nitrogen emissions in the aurora

Altitude distributions of electronically excited atoms and molecules of oxygen and nitrogen in the aurora have been obtained by means of rocket-borne wavelength scanning interference filter photometers launched from Fort Churchill, Manitoba (58.4°N, 94.1°W) on January 23, 1974. Atomic oxygen densities derived from mass spectrometer measurements obtained during the flight are used in conjunction with the volume emission rate ratio of the N₂(C³Π_u?B³Π_g) (0-0) second positive and N₂(A³Σ_u⁺, v = 1?X¹Σ_g⁺) Vegard-Kaplan bands to derive a rate constant for quenching of the N₂(A³Σ_u⁺, v = 1) level with O(³P) of 1.7(±0.8) × 10^?11 cm³ s^?1 These data, together with O den derived from the O₂(b¹Σ_g⁺) state nightglow emission observed during the rocket ascent, suggest that quenching of the N₂(A³Σ_u⁺, v = 1) level by O₂ has a significant positive temperature dependence. The processes involved in the production and loss of the N₂(A³Σ_u⁺) state are considered and energy transfer from the N₂(A³Σ_u⁺) state to O(³P) is found to be a significant source of the OI 5577 Å green line in this aurora at altitudes below 130 km. Emission from the NO(A²Σ⁺?X²Π) gamma bands was not detected, an observation which is consistent with the mass spectrometer data obtained during the flight indicating that the NO density was <10⁸ cm³ at 110 km. On the basis of previous rocket and satellite measurements of the NO gamma bands, energy transfer from the N₂(A³Σ_u⁺) state to NO(X²Π) is shown to be an insignificant source of the gamma bands in aurora. Altitude profiles of the N₂(a¹Π_g?X¹Σ_g⁺) Lyman-Birge-Hopfield band system are presented.     

Spatial correlation of OH Meinel and O2 infrared atmospheric nightglow emissions observed with VIRTIS-M on board Venus Express

We present the two-dimensional distribution of the O₂ a¹Δ–X³Σ (0–0) band at 1.27 μm and the OH Δv = 1 Meinel airglow measured simultaneously with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express. We show that the two emissions present very similar spatial structures. A cross-correlation analysis indicates that the highest level of correlation is reached with only very small relative shifts of the pairs of images. In spite of the strong spatial correlation between the morphology of the bright spots in the two emissions, we also show that their relative intensity is not constant, in agreement with earlier statistical studies of their limb profiles. We conclude that the two emissions have a common precursor that controls the production of both excited species. We argue that atomic oxygen, which produces O₂ (¹Δ) molecules by three-body recombination and is the precursor of ozone formation, also governs to a large extent the OH airglow morphology through the H + O₃ → OH^* + O₂ reaction.     

Excitation of the oxygen nightglow on the terrestrial planets

A scheme of excitation, quenching, and energy transfer processes in the oxygen nightglow on the Earth, Venus, and Mars has been developed based on the observed nightglow intensities and vertical profiles, measured reaction rate coefficients, and photochemical models of the nighttime atmospheres of the Venus and Mars. The scheme involves improved radiative lifetimes of some band systems, calculated yields of the seven electronic states of O₂ in termolecular association, and rate coefficients of seven processes of electronic quenching of the Herzberg states of O₂, which are evaluated by fitting to the nightglow observations. Electronic quenching of the vibrationally excited Herzberg states by O₂ and N₂ in the Earth's nightglow is a quarter of total collisional removal of the O₂(A, A′) states and a dominant branch for the O₂(c) state. The scheme supports the conclusion by Steadman and Thrush (1994) that the green line is excited by energy transfer from the O₂(A³Σ_u⁺, v≥6) molecules, and the inferred rate coefficient of this transfer is 1.5×10⁻¹¹ cm³ s⁻¹. The O₂ bands at 762 nm and 1.27 μm are excited directly, by quenching of the Herzberg states, and by energy transfer from the O₂(⁵Π_g) state. Quenching of the O₂ band at 762 nm excites the band at 1.27 μm as well. Effective yield of the O₂(a¹Δ_g) state in termolecular association on Venus and Mars is ∼0.7. Quantitative assessments of all these processes have been made. A possible reaction of O₂(c¹Σ_u⁻)+CO is a very minor branch of recombination of CO₂ on Venus and Mars. Night airglow on Mars is calculated for typical conditions of the nighttime atmosphere. The calculated vertical intensity of the O₂ band at 1.27 μm is 13 kR, far below the recently reported detections.     

Ground-based observations of O2 (b1Σ+g−X3Σ−g) atmospheric bands in high latitude auroras

Analysis of observed spectrograms is based on comparison with synthetic spectra. The O₂(b¹Σ⁺_g?X³Σ^?_g Atm. (1,1) band in high latitude auroras observed from the ground is found to be the strongest in the Δv = 0 sequence. It is enhanced with altitude relative to the N₂ 1P(2, 0)and N⁺₂ M(2,0) bands, but the O₂ Atm. (2, 2) band has an unexpected low intensity. The range of rotational temperatures of the O₂ Atm. bands varies from approx. 200 to above 500 K which indicates that the altitude of the centroid of the emission region varies from about 100 km to the F-region. The highest temperature is found in the midday aurora associated with the magnetospheric cusp. Conspicuous relative variations between the intensities of N₂ and O₂ spectra are documented, but a satisfactory explanation of the variety is not given. Deviations of the observed O₂ Atm. band intensities from the vibrational intensity distribution predicted by Franck-Condor factors indicate that the excitation of the O₂ Atm. bands in aurora is not mainly due to particle impact on O₂, and the contribution due to energy transfer from hot O(¹D) atoms has to be found in future research.     

The altitude dependence of the O2(A3Σu+) vibrational distribution in the terrestrial nightglow

A simple vibrational relaxation model which reproduces the observed altitude integrated vibrational distribution of the Herzberg I bands in the nightglow is used to derive the altitude profiles of the individual vibrational levels at 1 km intervals in the 85–115 km height range. The possible errors associated with using rocket-borne photometer measurements of a limited number of bands in the O₂(A³Σ_u⁺?X³Σ_g^?) system to infer the total Herzberg I emission profile are assessed.     

Visible continuum emission and gravity waves

Observed oscillations in the visible continuum emission (5000 Å) are studied considering the usual visible emission mechanism, NO + O (both two-body and three-body paths). Characteristic parameters of internal gravity waves are obtained using Hines' linear theory. Values of the kinetic energy density ?, and temperature variations Δθ, due to gravity waves are calculated. The results (? ~ 10⁶cm²s^?2, Δθ = 4–9K) are in agreement with those obtained by means of different techniques reported in the literature. A similar analysis of the simultaneous green-line emission data is made and a comparison is drawn between the results obtained for both emissions. An expression relating relative brightness of continuum emission and relative perturbations of atmospheric density is proposed.     

Electron excitation of a Jovian aurora

The planet Jupiter, like the Earth, possesses a magnetic field, and, therefore, auroral activity is very likely. In this work, the auroral emissions due to electron precipitation are estimated for a model atmosphere with and without helium. The incident primary electrons, which are characterized by representative spectra, are degraded in energy by applying the continuous slow down approximation. All secondaries, tertiaries, and higher generation electrons are assumed to be absorbed locally. A compilation of excitation, dissociation, and ionization cross section data for H, H₂, and He are used to model all aspects of the energy deposition process. Volume emission rates are calculated from the total direct excitation rates, and appropriate corrections for cascading are applied. Integrated column intensities of several kiloRayleighs are obtained for the various vibrational levels of the Lyman and Werner bands of H₂, as well as the triplet continuum a³Σ_g⁺ → b³Σ_u⁺. Helium emissions are relatively small because the majority of electrons are absorbed above the region of maximum He concentration. Atomic hydrogen emissions are due mainly to dissociative excitation of molecular hydrogen rather than direct excitation.     

Airglow on Mars: Some model expectations for the OH Meinel bands and the O2 IR atmospheric band

This work presents model calculations of the diurnal airglow emissions from the OH Meinel bands and the O₂ IR atmospheric band in the neutral atmosphere of Mars. A time-dependent photochemical model of the lower atmosphere below 80 km has been developed for this purpose. Special emphasis is placed on the nightglow emissions because of their potential to characterize the atomic oxygen profile in the 50-80 km region. Unlike on Earth, the OH Meinel emission rates are very sensitive to the details of the vibrational relaxation pathway. In the sudden death and collisional cascade limits, the maximum OH Meinel column intensities for emissions originating from a fixed upper vibrational level are calculated to be about 300 R, for transitions v^′=9→v^′?8, and 15,000 R, for transitions v^′=1→v^′=0, respectively. During the daytime the 1.27 μm emission from O₂(), primarily formed from ozone photodissociation, is of the order of MegaRayleighs (MR). Due to the long radiative lifetime of O₂(), a luminescent remnant of the dayglow extends to the dark side for about two hours. At night, excited molecular oxygen is expected to be produced through the three body reaction O + O + CO₂. The column emission of this nighttime component of the airglow is estimated to amount to 25 kR. Both nightglow emissions, from the OH Meinel bands and the O₂ IR atmospheric band, overlap in the 50-80 km region. Photodissociation of CO₂ in the upper atmosphere and the subsequent transport of the atomic oxygen produced to the emitting layer are revealed as key factors in the nightglow emissions from these systems. The Mars 5 upper constraint for the product [H][O₃] is revised on the basis of more recent values for the emission probabilities and collisional deactivation coefficients.     

Further study of changes in thermospheric molecular oxygen abundance inferred from twilight 6300 Å airglow

Measurements of the [OI] 6300 Å twilight airglow during 1973 near Boulder, Colorado, show a strong dependence upon geomagnetic activity for the morning enhancement at solar depression angles where production of O¹D) is due primarily to photodissociation of O₂ and local photoelectron excitation. Analysis indicates that photodissociation is the dominant source; hence we infer a well defined magnetic dependence for the O₂/N₂ concentration ratio in the thermosphere. A seasonal variation in the twilight enhancement intensity is barely evident, in contrast with earlier observations made near solar maximum; the smaller variation is associated with a corresponding reduction in the seasonal variation of geomagnetic activity.     

Energy transfer from excited N2 and O2 as a source of O(1S) in the Aurora

It is proposed that energy transfer from excited O₂ contributes to the production of O(¹S) in aurora. An analysis is presented of the OI5577 Å emission in an IBC II⁺ aurora between 90 and 130 km. The volume emission rate of the emission at these altitudes is consistent with the production rate of O(¹S) by energy transfer to O(³P) from N₂ in the A³Σ₂⁺ state and O₂ in the A³Σ_u⁺, C³Δc¹Σ_u^? states, the N₂A state being populated by direct electron impact excitation and B → A cascade and the excited O₂ states by direct excitation. Above the peak emission altitude (~105 km), energy transfer from N₂A is the predominant production mechanism for O(¹S). Below it, the contribution from quenching of the O₂ states becomes significant.     

An interpretation of the rotational temperature of the airglow hydroxyl emissions

The rotational temperature of the airglow hydroxyl emissions arising from various schemes of vibrational transitions was obtained by using spectroscopic data from six observational sources. The rotational temperature was found to depend systematically on the quantum number (ν') of the upper vibrational level from which the relevant band originates. It has a doubly degrading characteristic with respect to ν' taking maximal values at ν' = 6 and 9, which exceed considerably the atmospheric temperature. It drops off quickly as ν' decreases from 9 to 7, and then from 6 to 3 after making an abrupt rise at ν' = 6. This ν'-dependence of the rotational temperature is in favor of the hypothesis that there are two routes of excitation of the hydroxyl airglow: O₃ + H = OH(ν ? 9) + O₂, and HO₂ + O = OH(ν ? 6) + O₂. The present result implies also that the relaxation time of rotation of OH in the upper mesosphere is as long as 0.1 sec; a value an order of magnitude larger than that inferred in earlier laboratory experiments.     

Nightglow (557.7 nm of OI) in the central polar cap

The 557.7 nm OI night airglow emission was measured in the central polar cap by ground-based photometric systems at Thule Air Base, Greenland during the winter seasons from 1972–1973 to 1974–1975 and at Thule-Qanaq, Greenland during the winter season of 1973–1974. The behavior of the 557.7 nm night airglow emission in the polar cap was found to be quite different from that observed at mid and low latitudes. No diurnal variation greater than ±5% exist in the data. Large amplitude variations in the 557.7 nm daily average emission intensities can change by up to a factor of approximately 8 over periods ranging from 4 to 19 days. These long-term airglow variations cover at least a 100 km horizontal range as determined by a correlation coefficient of 0.94 between daily average 557.7 nm airglow intensities observed at Thule Air Base and Thule-Qanaq. An interplanetary magnetic field sector related behavior is evident in the daily average intensities which shows an increase of intensity in a positive (+) sector and a decrease of intensity in a negative (?) sector. No significant correlation was found between the 557.7 nm daily average intensities and Zurich sunspot number R_Z, although a season to season positive trend was evident. Correlations between the 557.7 nm daily average intensities and planetary magnetic indices ΣK_p and A_p were found to be inconclusive due to sector related effects. The Barth and Chapman mechanisms are discussed as possible source mechanisms for the 557.7 nm airglow in the central polar cap, and a hypothesis is presented to explain the airglow variations.     

Dayglow emissions of the O2 Herzberg bands and the Rayleigh backscattered spectrum of the earth

Numerous fluorescent emissions from the Herzberg bands of molecular oxygen lie in the spectral region 242–300 nm. This coincides with the wavelength range used by orbiting spectrometers which observe the Rayleigh backscattered spectrum of the earth for the purpose of monitoring the vertical distribution of stratospheric ozone. Model calculations indicate that Herzberg band emissions in the dayglow could provide significant contamination of the ozone measurements if the quenching rate of O₂(A³Σ) is sufficiently small. This is especially true near 255 nm, where the most intense fluorescent emissions relative to the Rayleigh scattered signal are located and where past satellite measurements show a persistent excess radiance above that expected for a pure ozone absorbing and molecular scattering atmosphere. However, very small quenching rates are adequate to reduce the dayglow emission to negligible levels. Available laboratory data have not definitely established the quenching on the rate of O₂(A³Σ) as a function of vibrational level, and such information is required before the Herzberg band contributions can be evaluated with confidence.