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.     

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.     

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.     

Excitation of O(1S) and emission of 5577 Å radiation in aurora

The results of recent laboratory experiments suggest that the reaction N⁺ + O₂ → NO⁺ + O(¹S) is the principal source of O(¹S) in aurora. A negligible time delay between auroral ionization and O(¹S) production is associated with this indirect process, which is a necessary condition for a viable mechanism. The $\text{5577}\text{A}\text{?}\text{3914}\text{A}\text{?}$ volume emission rate ratio associated with this production source remains constant with altitude. The problems encountered by the currently accepted source of O(¹S), the reaction of N₂(A³Σ) molecules with atomic oxygen are explored, and the contributions of this and other reactions to the auroral green line emission are reevaluated.     

The excitation of atomic oxygen to the O(1S) level by energy transfer from N2(A3∑u+) molecules in aurora

The part that the energy transfer reaction $\text{N}{}_2\text{(A}{}^3\text{∑}{}_{\mathrm{u}}{}^{\mathrm{+}}\text{) + O(}{}^3\text{P) → N}{}_2\text{(X}{}^1\text{∑}{}_{\mathrm{g}}{}^{\mathrm{+}}\text{) + O(}{}^1\text{S)}$ plays in the excitation of the auroral green line has been investigated. The contribution is estimated to be 40 per cent in this case, containing pulsating aurora in class IBC 1. Due to greater quenching of the A³∑_u⁺ state, the centroid of the VK emission is displaced 10 km upwards of the green line height, which is centred at 110 km.     

Electron-driven excitation of O2 under night-time auroral conditions: Excited state densities and band emissions

Electron impact excitation of vibrational levels in the ground electronic state and seven excited electronic states in O₂ have been simulated for an International Brightness Coefficient-Category 2+ (IBC II+) night-time aurora, in order to predict O₂ excited state number densities and volume emission rates (VERs). These number densities and VERs are determined as a function of altitude (in the range 80-350 km) in the present study. Recent electron impact excitation cross-sections for O₂ were combined with appropriate altitude dependent IBC II+ auroral secondary electron distributions and the vibrational populations of the eight O₂ electronic states were determined under conditions of statistical equilibrium. Pre-dissociation, atmospheric chemistry involving atomic and molecular oxygen, radiative decay and quenching of excited states were included in this study. This model predicts relatively high number densities for the metastable electronic states and could represent a significant source of stored energy in O₂* for subsequent thermospheric chemical reactions. Particular attention is directed towards the emission intensities of the infrared (IR) atmospheric (1.27 μm), Atmospheric (0.76 μm) and the atomic oxygen ¹S→¹D transition (5577 Å) lines and the role of electron-driven processes in their origin. Aircraft, rocket and satellite observations have shown both the IR atmospheric and Atmospheric lines are dramatically enhanced under auroral conditions and, where possible, we compare our results to these measurements. Our calculated 5577 Å intensity is found to be in good agreement with values independently measured for a medium strength IBC II+ aurora.     

A discussion of the energy transfer from N2(A3Σu+) to O(1S) in pulsating aurora

In a recent paper, Brekke and Pettersen (1972) have introduced a method for estimating any indirect process in the production of the $\text{O}\text{(}{}^1\text{S}$) atoms in pulsating aurora; for 38 per cent of their data they found that the decay time for the indirect mechanism was shorter than the effective lifetime of the ${}^1\text{S}$ state. These data are related to the energy transfer from the N₂(A³Σ) molecules to the $\text{O}\text{(}{}^1\text{S)}$ state, and evidence is found for this process to contribute in the altitude range below 125 km.     

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.     

The height,spectrum and mechanism of type-B red aurora and its bearing on the excitation of O(1S) in aurora

The height of the lower red border of type-B aurora has been determined by triangulation using TV cameras at two ground stations. A mean height of 91.4 ± 1.1 km was determined from a set of 12 measurements made under ideal conditions. A TV spectrograph was used simultaneously to seek possible spectral changes between 6400 and 6900 Å which would be indicative of changes in the vibrational distribution in the N₂ First Positive bands. No significant difference was found in this distribution between the spectra from 93 and 122 km. The height distribution of contributions to the OI 5577 Å emission relative to the N⁺₂ First Negative emission was modelled from 80 to 160 km. Contributions from electron impact on atomic O, O⁺₂ dissociative recombination and N₂(A)O energy transfer were included. Account was taken of recent laboratory data on O(¹S) quenching. It was concluded that these processes could explain the excitation of O(¹S) in normal aurora and the height distribution of OI 5577 Å in type-B red aurora. It was confirmed that the lifetime ofO(¹S) in type-B red auroral rapid time variations is about 0.5 s and it was found from the model that the observed time variation can be reproduced by the mechanisms considered, provided the concentration of NO in the auroral atmosphere is about 1 × 10⁹ at 95 km. Before reasonable certainty can be attained in the correctness of the interpretation it will however be necessary to have reliable simultaneous observations of neutral atmospheric composition particularly for O and NO as well as unchallengeable measurements of the yields of O(¹S) for the processes considered and for several other processes which have been suggested recently.     

A comment on proposed mechanisms for the excitation of O(1S) in the aurora

A recent assessment by Rees (1984) of the contribution made to the excitation of O(¹S) in the aurora by the reaction of N₂(A³Σ⁺) with O(³P) is re-examined. It is demonstrated that the contribution attributed to this reaction may have been seriously under-estimated and it is shown that the results of recent laboratory investigations do not preclude this reaction as a major source of O(¹s) in the aurora.     

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.     

EUV resonance fluorescence of the c′4→X (0,v″) and b′→X (1,v″) transitions of N2

Extreme ultraviolet (EUV) resonance fluorescence of the (0,v″) bands of the c′₄¹Σ_u⁺→X¹Σ_g⁺ and the (1,v″) bands of the b′ ¹Σ_u⁺→X¹Σ_g⁺ transitions of N₂ has been observed by photon excitation of N₂ in the vicinity of 95.8 nm. The integrated fluorescence intensities of the c′₄→X (0,v″) emission become saturated at N₂ pressures higher than ∼0.16 mTorr. The emission features in the spectral region between 105 and 130 nm become progressively significant as the N₂ pressure is increased. The (1,v″) progression for v″ up to 11 of the b′→X transition and two progressions of the Lyman-Birge-Hopfield (LBH) system have been identified. The multiple scattering processes apparently cause significant reduction in the c′₄→X (0,0) emission rates. The present results may be useful in the explanation of the weak c′₄→X (0,0) fluorescence as well as the significant c′₄→X (0,v″) features in the dayglow of the Earth observed by the Far Ultraviolet Spectroscopic Explorer.     

An indirect mechanism for the production of O(1S) in the aurora

Recent laboratory measurements of the deactivation rate constants for O(¹S) have suggested that the dominant production mechanism for the green line in the nightglow is a two-step process. A similar mechanism involving energy transfer from an excited state of molecular oxygen is considered as a potential source of the OI (5577 Å) emission in the aurora. It is shown that the mechanism, $\text{O}{}_2\text{+ e → O}{}_2{}^1\text{+ e O}{}_2{}^1\text{+ O → O}{}_2\text{+ O(}{}^1\text{S}\text{)}$, is consistent with auroral observations; the intermediate excited state has been tentatively identified as the O₂(c¹∑^?_u) state. For the proposed energy transfer mechanism to be the primary source of the auroral green line, the peak electron impact cross-section for O₂¹ production must be approximately 2 × 10^?17 cm².     

Effects of ion excitation on charge transfer reactions of the Mars, Venus, and Earth ionospheres

The absolute reaction cross sections and reaction rate coefficients as a function of photoionisation energy for 25 ion-molecule reactions (charge transfer reactions except for one) have been measured between the most abundant species present as ions or neutral in the Mars, Venus and Earth ionospheres: O₂, N₂, NO, CO, Ar and CO₂.This study shows the strong influence of electronic as well as vibrational internal energy on most ion-molecule reactions. In particular endothermic charge transfer reactions are driven by electronic excitation of O₂⁺ and NO⁺ ions in their a⁴Π_u and a³Σ⁺ metastable states, respectively. Moreover, it is shown that lifetimes of these metastable states are sufficient to survive the mean free path in the lowest part of ionospheres and therefore express their enhanced reactivity. The reactions of O₂⁺ with NO as well as the reactions of CO₂⁺ with NO, O₂, CO and to a less extent N₂ are driven by vibrational excitation. N₂⁺ and CO⁺ reactions vary much less with photon energy than the other ones, except for the case of reactions with Ar. The effects of the molecular ion internal energy content on their reactivity must be included in the ionospheric models for most of the reactions investigated in the present work. It is also the case for the effect of collision energy on the CO⁺+M reactions as we expect that a significant proportion of these CO⁺ could be produced with translational energy by dissociation of doubly charged CO₂²⁺, in particular in the Mars ionosphere. Recommended effective rate constant values are given as a function of VUV photon energy.     

Possible new atmospheric sources and sinks of odd nitrogen: 2. A revisit with O2(B) and discussions of other possibilities

Recently, modelers have expressed a concern that the currently known chemistry of atmospheric NO_y is deficient. It is therefore necessary to explore possible sources and sinks of atmospheric NO_x that may have been overlooked. In this context, it is noteworthy that the experimentally observed, four-center, Woodward-Hoffman forbidden, reaction 0₂(B ³Σ) + N₂ → NO(X) + NO (X) is atmospherically significant. In the 20 to 30 km region NO_x production from this reaction may potentially exceed the production from the “classical” N₂0 + O(¹D) reaction, and may provide a new mechanism to couple the solar UV variability and stratospheric ozone. The avoidance of the non-conservation of the orbital symmetry via the production of one NO in the excited electronic state being endothermic, it appears that the interaction of 0₂(B ³Σ) with the adjoining ¹Λ, ³Λ and ³Σ_u⁺ states might be responsible for the reaction. Experimental studies of the reaction as a function of the vibrational levels of the B-state, temperature and pressure are needed for reliable atmospheric applications of this reaction. At altitudes greater than about 50 km the production of NO from 0₂(B) begins to decrease rapidly. The NO production from 0₂ (A ³Σ₊⁺) + N₂ → NO + NO reaction may become important here, if the reaction is confirmed by experiments. These new sources of NOx call for new sinks of this species. In the upper stratosphere and mesosphere the chemical acceleration of NO dissociation via the reactions of electronically and vibrationally excited NO with 0₂ may help. In the lower atmosphere there is a possibility of the annihilation of NO, N0₂pair leading to the recreation of a stable NN bond. This might happen if N₂0₃ from NO and N0₂ recombination may photodissociate as N₂0 + 0₂. Unfortunately the requirements are stringent, and only experiments can tell whether or not this mechanism operates in the atmosphere.     

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.     

Laboratory investigation of the ionospheric O2+(X2πg, v ≠ 0) reaction with NO

Laboratory measurements of the reaction of O₂⁺ with NO from thermal energy to 0.6 eV in an Ar buffered flow drift tube agree with similar measurements made earlier in the same drift tube with He buffer. Since the O₂⁺ ions are substantially vibrationally excited in Ar and not in He it follows that the reaction is not enhanced by vibrational excitation of the O₂⁺.     

Neutral temperatures and emission height changes in an E-region aurora

In this study a method is outlined which is capable of giving neutral temperatures and height changes in the aurora when the molecular emissions originate from the E-region.Absolute spectrometric measurements of N₂⁺ 1NG and O₂⁺ 1NG bands and the auroral green line are performed in a nightside aurora. Rotational temperatures and band intensities are deduced by a least-squares fit of synthetic spectra to observations. There is a close correlation between the variations in rotational temperatures and the relative intensity ratio of N₂⁺ 1NG(0,3) and O₂⁺ 1NG(1,0) bands. The change in the relative intensity ratio is similar to the intensity variation predicted by the changing N₂ and O₂ densities from 120 to 150 km, obtained from the MSIS 83 model atmosphere, and the derived neutral temperature variations are consistent with a similar change in emission height of the aurora. Therefore the changing temperature is most likely due to a changing emission height of the aurora, and no local heating can be inferred.     

Excitation of oxygen emissions in the night airglow of the terrestrial planets

The excitation, energy transfer and quenching of O₂ (A³ Σ_u⁺, C³ Δ_u, c¹ Σ_u^?) and O(¹S) are discussed, taking into account laboratory measurements and observations on the airglow of the Earth, Venus and Mars. The excitation of O(¹S) occurs by the Barth mechanism with O₂(c) as a precursor: the rate coefficient is 2.5 × 10^?12 cm³ s^?1 for υ > 0 and 2.5 × 10^?12 exp($\text{?1100}\text{T}$) cm₃ s^?1for υ = 0. The O₂(c) can be formed directly by recombination or by O₂(A) and O₂(C) colliding with other molecules; the O₂(c) yield through quenching of these states is about 0.3 in air and about 1.0 in carbon dioxide. The rate coefficients of some processes that control the molecular oxygen bands and the atomic oxygen green line are estimated.     

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.