Thermospheric no profiles observed at the diminishing phase of solar cycle 21

Two density profiles of the thermospheric nitric oxide were obtained by means of the γ(1,0) band airglow measured with rocket-home radiometers flown from Uchinoura, Japan (31°N) at around autumnal equinoxes in 1982 and 1983. The peak densities were found at altitudes of 105–110 km and are 9 × 10⁷ and 7 × 10⁷ cm⁻³, respectively. They are well reproduced by the variation of solar activity in terms of a one-dimensional photochemical-diffusive model, but the densities above 140 km under moderate solar activity differ considerably from the model prediction. A similar discrepancy has already been found in the NO density profile obtained by our previous experiment at solar maximum. These discrepancies infer a possibility either that our understanding of thermospheric nitrogen chemisty includes a serious error, or that the meridional circulation affects considerably the NO density profile even at altitudes above 140 km and at low latitudes.     

O(1S) quenching profile between 75 and 115 km

The rate at which O(¹S) is quenched in the atmosphere has been calculated as a function of altitude in the 75–115 km region. Recent measurements of the temperature-dependent O ₂ quenching rate coefficient have been used, while for quenching by O(³P), an expression combining new theoretical and experimental results is employed. For the O(³P) altitude profile, the Jacchia (1971) model is chosen. The quenching profile shows a pronounced minimum quenching rate at about 87 km. It is concluded that different studies carried out on pulsating Type-B red aurorae, which extract an O(¹S) quenching rate from the time lag between N ₂⁺(B?X) emission and 5577-Åemission, can now be interpreted as indicating an altitude range for these aurorae of 84–89 km. This conclusion is in accord with observations made on artificial aurorae.     

An Antarctic no density profile deduced from the gamma band airglow

The nitric oxide density profile between the altitudes 72 and 120 km was obtained by means of the airglow γ(1, 0) band measured with a rocket-borne radiometer flown at Syowa Station (69°S, 40°E). The NO density was found to have two peaks with a value of 1.5× 10⁸cm^?3 at 90 and 110 km, and is much larger than those in the middle and low latitudes. Because of a long lifetime of NO in the mesosphere, the observed NO enhancement may be due to the after-effect of the particle precipitation event which occurred within the half day before, despite no polar disturbance during the rocket flight.     

Transport of aurorally produced N(2D) by winds in the high latitude thermosphere

A time-dependent two-dimensional numerical model of the minor neutral constituents of the thermosphere NO, N(²D), and N(⁴S) is used to examine the effects of winds in transporting these constituents from their production region in auroral arcs. The calculations show that thermospheric winds flowing through regions of enhanced local auroral production produce downwind plumes of enhanced minor neutral constituent densities and that the densities depend upon the wind velocity. Below about 200 km N(²D) is in photochemical equilibrium and is not transported. Above 200 km N(²D) is transported by the wind and since quenching of N(²D) by O is small and the radiational lifetime is long, a downwind plume of emission at 5200 Å develops from the particle source region. We present data from a rocket flight in the vicinity of the magnetospheric cusp and data from the Atmosphere Explorer-D (AE-D) satellite that both show enhanced 5200 Å emission rates in a general downwind direction from a region of direct particle precipitation. The general wind speed and direction are obtained from predictions made by the NCAR thermospheric general circulation model. The results suggest that transport of N(²D) by the wind system is more important than the convection of O⁺ ions by electric fields in causing the enhanced 5200 Å emission rate in regions outside but in the vicinity of direct particle precipitation.     

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.     

Venus night airglow: Ground-based detection of OH, observations of O2 emissions, and photochemical model

Venus nightglow was observed at NASA IRTF using a high-resolution long-slit spectrograph CSHELL at LT = 21:30 and 4:00 on Venus. Variations of the O₂ airglow at 1.27 μm and its rotational temperature are extracted from the observed spectra. The mean O₂ nightglow is 0.57 MR at 21:30 at 35°S-35°N, and the temperature increases from 171 K near the equator to ∼200 K at ±35°. We have found a narrow window that covers the OH (1-0) P1(4.5) and (2-1) Q1(1.5) airglow lines. The detected line intensities are converted into the (1-0) and (2-1) band intensities of 7.2 ± 1.8 kR and <1.4 kR at 21:30 and 15.5 ± 2 kR and 4.7 ± 1 kR at 4:00. The f-component of the (1-0) P1(4.5) line has not been detected in either observation, possibly because of resonance quenching in CO₂. The observed Earth’s OH (1-0) and (2-1) bands were 400 and 90 kR at 19:30 and 250 and 65 kR at 9:40, respectively. A photochemical model for the nighttime atmosphere at 80-130 km has been made. The model involves 61 reactions of 24 species, including odd hydrogen and chlorine chemistries, with fluxes of O, N, and H at 130 km as input parameters. To fit the OH vibrational distribution observed by VEX, quenching of OH (v > 3) in CO₂ only to v ? 2 is assumed. According to the model, the nightside-mean O₂ emission of 0.52 MR from the VEX and our observations requires an O flux of 2.9 × 10¹² cm⁻² s⁻¹ which is 45% of the dayside production above 80 km. This makes questionable the nightside-mean O₂ intensities of ∼1 MR from some observations. Bright nightglow patches are not ruled out; however, the mean nightglow is ∼0.5 MR as observed by VEX and supported by the model. The NO nightglow of 425 R needs an N flux of 1.2 × 10⁹ cm⁻² s⁻¹, which is close to that from VTGCM at solar minimum. However, the dayside supply of N at solar maximum is half that required to explain the NO nightglow in the PV observations. The limited data on the OH nightglow variations from the VEX and our observations are in reasonable agreement with the model. The calculated intensities and peak altitudes of the O₂, NO, and OH nightglow agree with the observations. Relationships for the nightglow intensities as functions of the O, N, and H fluxes are derived.     

Isotope identification in NO as a chemical tracer in the middle atmosphere

The nitrogen isotope ratio of middle atmosphere nitrogen oxide is predicted as a function of altitude. Nitrogen oxides originate photochemically either from stratospheric nitrous oxide reacting with O(¹D) or in the mesosphere and thermosphere from direct dissociation of N₂ and ionization-initiated reactions involving O₂ and N₂. During its formation process, N₂O acquires a nitrogen isotopic composition of N isotopes different than N₂. Photodissociation within the stratosphere also modifies the proportion of isotopes. Reaction of stratospheric NO with O₃ produces NO₂, which when photodissociated yields NO depleted in ¹⁵N relative to NO₂ in laboratory air. The value of δ¹⁵NO in the stratosphere is −100‰. In the altitude region between 50 and 65 km, NO is transformed into NO₂ and then returned to NO by reaction of NO₂ with O and by NO₂ photodissociation. These reactions determine the isotopic makeup of NO. Above 65 km, nitric oxide is produced by local ionization processes and gas phase photochemical reactions involving N₂ and excited O₂. These processes determine the isotopic composition of NO in the upper mesosphere and thermosphere. Here δ¹⁵NO is 0‰. Air transported into the mesosphere above 65 km will reflect the NO isotopic values of the region below, while mesospheric NO transported below 65 km will not be distinguishable from NO originating in the stratosphere.     

The morphology of n and no in auroral substorms

The effects of a typical auroral electron precipitation substorm sequence on odd nitrogen species in the thermosphere have been investigated. The analysis makes use of the time dependent model of the aurora developed by Roble and Rees (1977), which couples the thermal properties to the ionospheric chemistry and transport self-consistently and includes diffusive transport of NO, N(²D) and N(⁴S). A substantial increase in the E-region density of NO or of N(⁴S) is predicted, with the result depending on the production ratio of N(²D) to N(⁴S) in the aurorally dominant source mechanism, electron impact dissociation of N₂. A production ratio that favors N(²D) by a factor of one half or larger leads to enhancement of NO, while a ratio of $\text{1}\text{4}\text{N}\text{(}{}^2\text{D}\text{)+}\text{3}\text{4}\text{N}\text{(}{}^4\text{S}\text{)}$ results in a buildup of N(⁴S). The cyclical behaviour of the substorm, i.e. alternate intervals of electron precipitation and quiet periods, accentuates the scavenging effect of the initially dominant odd nitrogen species upon the less abundant one.     

On the structure of Mars' atmosphere at 120–220 km

Altitude dependences of [CO₂] and [CO₂⁺] are deduced from Mariner 6 and 7 CO₂⁺ airglow measurements. CO₂ densities are also obtained from n_e radio occultation measurements. Both [CO₂] profiles are similar and correspond to the model atmosphere of Barth et al. (1972) at 120 km, but at higher altitudes they diverge and at 200–220 km the obtained [CO₂] values are three times less the model. Both the airglow and radio occultation observations show that a correction factor of 2.5 should be included into the values for solar ionization flux given by Hinteregger (1970). The ratio of [CO₂⁺]/n_e is 0.15–0.2 and, hence, [O]/[CO₂] is ～3% at 135 km. An atmospheric and ionospheric model is developed for 120–220 km. The calculated temperature profile is characterized by a value of T ≈ 370°K at h ? 220 km, a steep gradient (～2°/km) at 200-160 km, a bend in the profile at 160 km, a small gradient (～0.7°/km) below and a value of T ≈ 250°K at 120 km. The upper point agrees well with the results of the Lyman-α measurements; the steep gradient may be explained by molecular viscosity dissipation of gravity and acoustical waves (the corresponding energy flux is 4 × 10^?2 erg cm^?2sec^?1 at 180 km). The bend at 160 km may be caused by a sharp decrease of the eddy diffusion coefficient and defines K ≈ 2 × 10⁸cm²sec^?1; and the low gradient gives an estimate of the efficiency of the atmosphere heating by the solar radiation as ? ≈ 0.1.     

Excitation of O2 Herzberg Bands in the nightglow

The excitation mechanism for O₂ Herzberg Bands as given by Young and Black (1966) is examined. It is found that O₂ Herzberg Bands are heavily quenched by N atoms, while (0,0) and (0,1) Atmospheric Bands are quenched mainly by CO, NO, O₂ and N₂, NO, O₂ respectively. The emission of Herzberg Bands is found to arise from two layers centred at 80 and 100 km. The rate coefficients of a number of quenching reactions involving atmospheric gases are obtained theoretically.     

Characteristics of Saturn’s FUV airglow from limb-viewing spectra obtained with Cassini-UVIS

This study reports the analysis of far ultraviolet (FUV) limb spectra of the airglow of Saturn in the 1150-1850 Å spectral window, obtained with the Ultraviolet Imaging Spectrograph (UVIS) onboard Cassini, spanning altitudes from −1200 to 4000 km. The FUV limb emission consists of three main contributions: (1) H Ly-α peaking at 1100 km with a brightness of 0.8 kilo-Rayleighs (kR), (2) reflected sunlight longward of 1550 Å which maximizes at −950 km with 16.5 kR and (3) H₂ bands in the 1150-1650 Å bandwidth, peaking at 1050 km reaching a maximum of 3.9 kR.A vertical profile of the local H₂ volume emission rate has been derived using the hydrocarbon density profiles from a model of the Saturn equatorial atmosphere. It is well matched by a Chapman function, characterized by a maximum value of 3.5 photons cm⁻³ s⁻¹ in the 800-1650 Å UV bandwidth, peaking at 1020 km.Comparisons between the observed spectra and a first-order synthetic airglow H₂ model in the 1150-1650 Å bandwidth show that the spectral shape of the H₂ bands is accounted for by solar fluorescence and photoelectron excitation. The best fits are obtained with a combination of H₂ fluorescence lines and 20 eV electron impact spectra, the latter contributing ∼68% of the total H₂ airglow emission.     

Altitude variation of ultraviolet oxygen night airglow

Night airglow of oxygen 130.4 and 135.6 nm emissions was measured by a spectrophotometer aborad an S520 sounding rocket, launched at 19:50 JST (10:50 UT) on 14 February, 1982 from Kagoshima, Japan. The altitude variation of the emissions was obtained from 110 to 266 km at zenith angles of 35.5°±4°. The emission intensity around 260 km was about 160R and is roughly compatible with model calculations taking account of O⁺+e ^– radiative recombination as well as O⁺–O^– mutual neutralization. Some excess of about 50R, compared to the model calculation, was observed around 200 km. Possible explanations of the excess are: (i) remnant oxygen ions during the transition period from day to night and (ii) diffuse radiation from the background sky. Model calculations taking account of remnant oxygen ions were also performed by adding an excess electron density to the original density profile. However, it was found that an unreasonably large electron density is required around 200 km (5×10⁵ cm^–3) to produce the observed intensity. It is also probable that some contribution from the background sky is present in the observed intensity.     

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.     

Quenching of O+(2D) by electrons in the thermosphere

A major loss process for the metastable species, O⁺(²D), in the thermosphere is quenching by electrons

$\text{O}{}^{\mathrm{+}}\text{(}{}^2\text{D) + e → O}{}^{\mathrm{+}}\text{(}{}^4\text{S) + e}$

.To date no laboratory measurement exists for the rate coefficient of this reaction. Thermospheric models involving this process have thus depended on a theoretically calculated value for the rate coefficient and its variation with electron temperature. Earlier studies of the O⁺(²D) ion based on the Atmosphere Explorer data gathered near solar minimum, could not quantify this process. However, Atmosphere Explorer measurements made during 1978 exhibit electron densities that are significantly enhanced over those occurring in 1974, due to the large increases that have occurred in the solar extreme ultraviolet flux. Under such conditions, for altitudes ? 280 km, the electron quenching process becomes the major loss mechanism for O⁺(²D), and the chemistry of the N⁺₂ ion, from which the O⁺(²D) density is deduced, simplifies to well determined processes. We are thus able to use the in situ satellite measurements made during 1978 to derive the electron quenching rate coefficient. The results confirm the absolute magnitude of the theoretical calculation of the rate coefficient, given by the analytical expression k(T_e) = 7.8 × 10^?8 (T_e/300)^?0.5cm³s^?1. There is an indication of a stronger temperature dependence, but the agreement is within the error of measurement.     

On the temperature dependence of the reaction O + NO → NO12

The published data on the temperature dependence of the radiative combination of atomic oxygen with nitric oxide at pressures near 1 torr is examined. Arguments are advanced to suggest that radiation near the cut-off wavelength (～ 3875Å) is coming from the unstabilized activated complex, No¹₂. At 4000Å a positive activation energy of 1 kcal mole^?1 is deduced. Application of this temperature dependence with the rate coefficient at 5200Å is made to airglow measurements in aurora. The deduced NO concentration is about 10⁹ cm^?3, in general agreement with that deduced from the measured NO⁺/O⁺₂ ratio as well as an auroral model prediction.     

Winter anomalous D-region over Sardinia (40°N)

A simplified D-region model consisting of O₂⁺, NO⁺ and their respective cluster ions grouped as Z_o2⁺ and Z_NO⁺ is used to reproduce the available rocket data on positive ion relative composition and effective clustering rates for the height range 70–90 km. The results of this analysis for a winter anomalous day (Sardinia, 40°N) are in good agreement with the presently known ideas on NO densities, O₂⁺ production rates, mesospheric temperature, negative ion to electron density ratio and effective loss coefficient for electrons. Mesospheric nitric oxide density and temperature profiles from this study are in excellent agreement with the findings of Zbinden et al. (1975) and Hidalgo (1977) for the anomalous day at Sardinia.     

Observations and interpretation of spectra and rapid time variations of type-B aurora

Observations of type-B red and normal aurora were made with a high-speed multichannel photometer and a digital grating spectrometer. The ratio $\text{I}\text{(}\text{O}{}_2{}^{\mathrm{+}}\text{1N; 2, 0 + 3, 1)}\text{I}\text{(}\text{N}{}_2{}^{\mathrm{+}}\text{1N; 0, 3)}$ measured in the 5200–5300 Å region with the spectrometer was found to increase by about 16% from normal to type-B aurora. This small change is difficult to reconcile with a height below 90 km for the red border. In the type-B aurora, λ 5577 was weakened by a factor between 1.9 and 3.8 while the ratio $\text{I}\text{(}\text{N}{}_2\text{1P; 5, 2)}\text{I}\text{(}\text{N}{}_2{}^{\mathrm{+}}\text{1N)}$ was enhanced less than 20%. Rapid intensity variations in the type-B lower border were observed in the λ 5577 and other channels of the photometer. A revised time dependent auroral excitation-ion chemistry model is used in an attempt to reproduce the observations. The observed weakening of λ 5577 could be produced at heights equal to or less than 100 km while the short observed time lag of λ 5577 on the N₂⁺ 1N emission is easier to explain at 100 km than at 80 km. It is concluded that some type-B lower borders may occur near 100 km although it is recognized that there is good evidence rare deep crimson lower borders lie at 80 km or below. The mechanism for the excitation of O(¹S) is considered in the light of these results. None of the mechanisms examined is satisfactory on the basis of currently accepted atmospheric models and quenching rate coefficients.     

Mariner 9 ultraviolet spectrometer experiment: Structure of Mars' upper atmosphere

Analysis of 18 observations of the limb intensity profile of the CO Cameron bands in the Martian airglow shows that the equivalent subsolar zenith intensity, I_CAM, is related to the Ottawa 10.7 cm radio flux index, F_10.7, by the expression I_CAM = 0.062(74 + F_10.7)kR, with a correlation coefficient of 0.80. Comparison of averaged limb intensities of the CO₂⁺ doublet and the Cameron bands on four favorable occasions is consistent with the intensities being directly proportional, in the ratio 0.24:1. The mean of 18 Cameron band topside scale heights is 17.8 km, corresponding to an exospheric temperature of 325°K, and the largest and smallest values observed differ by 9.5 km. These observations are in accord with theoretical predictions within the uncertainties in the latter. However, the solar EUV flux used in these predictions is a factor of at least two too weak to produce the electron densities measured by the S-band occultation experiment.     

A sensitive search for nitric oxide in the lower atmospheres of Venus and Mars: Detection on Venus and upper limit for Mars

High-resolution spectra of Venus and Mars at the NO fundamental band at 5.3 μm with resolving power ν/δν=76,000 were acquired using the TEXES spectrograph at NASA IRTF on Mauna Kea, Hawaii. The observed spectrum of Venus covered three NO lines of the P-branch. One of the lines is strongly contaminated, and the other two lines reveal NO in the lower atmosphere at a detection level of 9 sigma. A simple photochemical model for NO and N at 50-112 km was coupled with a radiative transfer code to simulate the observed equivalent widths of the NO and some CO₂ lines. The derived NO mixing ratio is 5.5±1.5 ppb below 60 km and its flux is . Predissociation of NO at the (0-0) 191 nm and (1-0) 183 nm bands of the δ-system and the reaction with N are the only important loss processes for NO in the lower atmosphere of Venus. The photochemical impact of the measured NO abundance is significant and should be taken into account in photochemical modeling of the Venus atmosphere. Lightning is the only known source of NO in the lower atmosphere of Venus, and the detection of NO is a convincing and independent proof of lightning on Venus. The required flux of NO is corrected for the production of NO and N by the cosmic ray ionization and corresponds to the lightning energy deposition of . For a flash energy on Venus similar to that on the Earth (∼¹⁰⁹ J), the global flashing rate is ∼90 s⁻¹ and ∼6 km⁻² y⁻¹ which is in reasonable agreement with the existing optical observations. The observed spectrum of Mars covered three NO lines of the R-branch. Two of these lines are contaminated by CO₂ lines, and the line at 1900.076 cm⁻¹ is clean and shows some excess over the continuum. Some photochemical reactions may result in a significant excitation of NO (v=1) in the lowest 20 km on Mars. However, quenching of NO (v=1) by CO₂ is very effective below 40 km. Excitation of NO (v=1) in the collisions with atomic oxygen is weak because of the low temperature in the martian atmosphere, and we do not see any explanation of a possible emission of NO at 5.3 μm. Therefore the data are treated as the lack of absorption with a 2 sigma upper limit of 1.7 ppb to the NO abundance in the lower atmosphere of Mars. This limit is above the predictions of photochemical models by a factor of 3.     

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.