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.     

Changes in thermospheric molecular oxygen abundance inferred from twilight 6300 A airglow

When the local solar zenith angle, χ_L, is < 105° the 6300 A line is much stronger than expected on the basis of F region ionic recombination alone. Between 95 and 105° the additional intensity is quantitatively explained by production of O(¹D) from photolysis of O₂ in the Schumann-Runge continuum, (λλ 1300–1750 A) using current values for solar flux, atmospheric composition and quenching of O(¹D) by N₂. The Schumann-Runge (SR) component exhibits a large seasonal variation with a maximum in summer. We interpret this variation as implying a seasonal change in thermospheric O₂ abundance; the change seems largely to reflect a variation in O₂ density at the base of the diffusive regime although some contribution may come from changes in thermospheric temperature structure. Large changes in the SR component exist from day to day and with a 27 day period following a major magnetic storm. The photodissociation source becomes inadequate when x_l < 95°; at 90° more than half of the intensity comes from still another source which we identify as local photoelectron excitation of O atoms.     

N2 emission in the twilight

About a year's observations of the N₂⁺ band (3914 Å) at Kitt Peak (latitude 32°) are reported. Morning intensities are the same throughout the year, but there is a strong winter maximum in the evening. It is suggested that the additional ionization is produced by photoelectrons from the magnetic conjugate point. Heights are estimated by the zenith-horizon method, which gives 235 km for the constant component and 350 km during the evening enhancement. The intensity variation through twilight is therefore entirely due to changes of the N₂⁺ concentration; each ion scatters light at a constant rate. The rotational distribution resembles that for a temperature of 1600°K, much higher than the temperature of the atmosphere. It is suggested that part of the ions may be produced by charge transfer from metastable O⁺(²D). N₂⁺ concentrations resulting from photoionization are calculated; they give a fair account of the observed horizon intensities, but not the zenith. Non-local electrons from higher in the atmosphere are suggested as a possible extra source; alternatively, the zenith measurements may be perturbed by scattered horizon light. The band intensity in the nightglow cannot be measured; the upper limit is 1 R.     

Relative flow of H+ and O+ ions in the topside ionosphere at mid-latitudes

Theoretical results on the daily variation of O⁺ and H⁺ field-aligned velocities in the topside ionosphere are presented. The results are for an L = 3 magnetic field tube under sunspot minimum conditions at equinox. They come from calculations of time-dependent O⁺ and H⁺ continuity and momentum balance in a magnetic field tube which extends from the lower F2 region to the equatorial plane (Murphy et al., 1976).There are occasions when ion counterstreaming occurs, with the O⁺ velocity upward and H⁺ velocity downward. The conditions causing this counterstreaming are described: the H⁺ layer is descending whilst O⁺ is supplied from below either to increase the O⁺ concentration at fixed heights or to replace O⁺ ions lost by charge exchange with neutral H. It is suggested that the results of observations at Arecibo by Vickrey et al. (1976) of O⁺ and H⁺ concentrations and counterstreaming velocities are significantly affected by $\text{E}\text{×}\text{B}$ drift.     

On the diurnal and seasonal variations of H+, He+, N+, O+, and Ne at 1400-km altitude

The diurnal and seasonal variations of H⁺, He⁺, N⁺, O⁺ and Ne are analyzed at 1400-km altitude. Using longitudinally averaged observations of ISIS-2 (April 1971 to December 1972), the ion and electron densities are decomposed via spherical harmonics and Fourier series into time-independent, seasonal and diurnal terms. The time-independent terms of H⁺ and He⁺ show a plateauor trough-like structure at medium to low latitudes and a strong decrease towards the poles; N⁺ and O⁺, on the other hand, yield an almost inverse picture with a density increase at high latitudes. All constituents, except He⁺, show at polar latitudes an enhancement during local summer conditions and a depletion during local winter conditions; He⁺, however, exhibits a winter bulge and a density minimum during local summer. The diurnal variations are strongly latitude dependent; while the amplitudes (relative) of H⁺, He⁺, and Ne are rather small, the heavier ions N⁺ and O⁺ show a deep minimum early in the morning and a high but flat maximum during daytime.     

Energy transfer of O(1S) atoms in collision with O(3P) atoms

Elastic scattering and excitation transfer collision cross-sections in O(¹S)-O(³P) collisions are calculated. These cross-sections are needed in determining the degree of thermalization of the O(¹S) atoms in the nighttime thermosphere. A formula is given for the rate coefficient for the production of an O(¹S) atom with a specific energy in collisions involving an O(¹S) atom of a given initial energy and the ground state O(³P) atoms of a thermal gas. Effective elastic scattering and excitation transfer cross-sections are defined and calculated to be 1.71 × 10^?15 cm² and 6.67 × 10^?16 cm² respectively at a relative collision energy of 0.41 eV.     

Linewidth studies on the NI(4S−4P) resonance multiplet

We have measured the linewidths of the NI multiplets $\text{[2p}{}^2\text{3p}{}^4\text{D}{}^0\text{?2p}{}^2\text{3s}{}^4\text{P, λ8691}\text{A}\text{?}\text{; 2p}{}^2\text{3p}{}^4\text{P}{}^0\text{?2p}{}^2\text{3s}{}^4\text{P, λ8212}\text{A}\text{?}\text{; 2p}{}^2\text{3s}{}^4\text{P?2p}{}^3{}^4\text{S}{}^0\text{, λ1200}\text{A}\text{?}\text{]}$ produced in the dissociative excitation of N₂ by energy electrons. The infrared transitions excite the N(⁴P) resonance state by cascade and they account for > 50% of the total N(⁴P) cross section at 100 eV. Both the i.r. and v.u.v. lines are found to be highly Doppler broadened ( ～ 25 times the thermal Doppler line width). These results indicate that dissociative excitation of N₂ produces N (⁴P) atoms with sufficient kinetic energy so that the $\text{λ1200}\text{A}\text{?}$ resonance radiation $\text{[2p}{}^2\text{3s}{}^4\text{P ?2p}{}^3{}^4\text{S}{}^0\text{]}$ emitted by these excited atoms would be optically thin in the Earth's upper atmosphere. We also found that the line strength ratios for the resolved components of the $\text{λ1200}\text{A}\text{?}$ triplet excited by dissociative excitation differ from those predicted by the multiplicities of the states involved and used in current entrapment models; the intensity ratios also vary with the energy of the incident electron. These developments introduce new complications into the analysis of the terrestrial ultraviolet dayglow.     

Further quantification of the sources and sinks of thermospheric O1D) atoms

In this paper we confirm an earlier finding that the reaction

constitutes a major source of OI 6300 Å dayglow. The rate coefficient for this reaction is found to be consistent with an auroral result, namely k₁ ≈ 6 × 10^?12cm³s^?1. We correct an error in an earlier publication and demonstrate that reaction (1) is consistent with the laboratory determined quenching rate for the reaction

where $\text{k}{}_2\text{= 2.3 × 10}{}^{\mathrm{?11}}\text{cm}{}^3\text{s}{}^{\mathrm{?1}}$. Dissociative recombination of O⁺₂ with electrons is found to be a major daytime source in summer above ～220 km.     

Aeronomical determination of the efficiency of O1D) production by dissociative recombination of O2+

Simultaneous measurements of the 6300 Å airglow intensity, the electron density profile, and F-region ion temperatures and vertical ion velocities taken at the Arecibo Observatory in March 1971 are utilized in the height integrated continuity equation to extract the number of photons'of 6300 Å emitted per recombination. After accounting for quenching of $\text{O}\text{(}{}^1\text{D}\text{)}$ and the electrons lost via NO⁺ recombination, the efficiency of $\text{O}\text{(}{}^1\text{D}\text{)}$ production by the dissociative recombination of O₂⁺ is determined to be 0.6 ± 0.2 including cascading from the $\text{O}\text{(}{}^1\text{S}\text{)}$ state. The uncertainty includes both random measurement errors and estimates of possible systematic errors.     

Reaction paths leading from O+2 to water clusters under cold mesospheric conditions

Arnold and Krankowsky (Int. Symp. Solar-Terrestrial Physics, Sao Paulo, 1974) have reported D-region positive ion measurements in which a number of new cluster ions of minor abundance were apparent. These ions, which they attributed to clusters with N₂, O₂, and CO₂ ligands, were observable due to enhanced O⁺₂ production and to the low temperatures during the flight. Here we consider these in situ ion data in view of recent laboratory ion-molecule reaction experiments which cast light on the mechanism leading from O⁺₂ to water clusters in air mixtures. Possible intermediates are discussed in terms of ion stability and existence of effective reaction paths under the given atmospheric conditions. These proposed intermediates are then fitted into a coherent reaction mechanism resulting in significant new pathways for the formation of protonated water clusters. A semiquantitative measure of the importance of each of the pathways is then calculated by the use of signal flow graph theory.     

Energy distribution function of translationally hot O(3P) atoms in the atmosphere of Earth

Translationally hot O(³P) atoms are produced in the atmosphere of Earth by photolysis of O₂ and O₃ and quenching of O(¹D). A rigorous kinetic theory analysis of this problem is developed and compared with the approach previously employed by Logan and McElroy [Planet. Space Sci.25, 117 (1977)]. It is shown that the kinetic theory employed by the previous workers is somewhat deficient. With the line-of-centers cross-section, the rates of reactions of the translationally hot O(³P) atoms with other atmospheric gases are calculated and found to be in some instances many orders of magnitude larger than the equilibrium rates. Though the non-equilibrium reaction rates with O(³P) are substantially increased, they are still not competitive with the corresponding reaction rates with O(¹D).     

On the backscatter of solar HeII, 304åradiation from interplanetary He +

Backscatter of solar HeII, 304Åradiation by interplanetary He ⁺ is shown to be insufficient to account for recent observations.     

A source of extended HCO+ emission in young stellar objects

Anomalous molecular line profile shapes are the strongest indicators of the presence of the infall of gas that is associated with star formation. Such profiles are seen for well-known tracers, such as HCO⁺, CS and H₂CO. In certain cases, optically thick emission lines with appropriate excitation criteria may possess the asymmetric double-peaked profiles that are characteristic of infall. However, recent interpretations of the HCO⁺ infall profile observed towards the protostellar infall candidate B335 have revealed a significant discrepancy between the inferred overall column density of the molecule and that which is predicted by standard dark cloud chemical modelling.
This paper presents a model for the source of the HCO⁺ emission excess. Observations have shown that, in low-mass star-forming regions, the collapse process is invariably accompanied by the presence of collimated outflows; we therefore propose the presence of an interface region around the outflow in which the chemistry is enriched by the action of jets. This hypothesis suggests that the line profiles of HCO⁺, as well as other molecular species, may require a more complex interpretation than can be provided by simple, chemically quiescent, spherically symmetric infall models.
The enhancement of HCO⁺ depends primarily on the presence of a shock-generated radiation field in the interface. Plausible estimates of the radiation intensity imply molecular abundances that are consistent with those observed. Further, high-resolution observations of an infall-outflow source show HCO⁺ emission morphology that is consistent with that predicted by this model.     

Vibrational temperature and excess vibrational energy of molecular nitrogen in the ground state derived from N2+ emission bands in aurora

A method to estimate the vibrational distribution for N₂ in the ground state by using the intensities of allowed transitions, excited by direct particle impact, is developed. The method is tested on a rather limited amount of data, but the results seem to be reasonable and indicate that 10% of the particle energy is stored as vibrational energy by N₂ molecules, and that the vibrational temperature is increased more than 1000K relative to the kinetic temperature on the dayside.     

SAR arcs and 6300 Å emission by dissociative recombination of O2+ observed from Africa during severe magnetic storms

SAR arcs were observed from Southern Africa on 17/18 December 1971, 4/5 August 1972 and 1/2 April 1973 with the equatorwards edge at L = 1.8. Simultaneous with the latter event the intertropical arc was observed at an equatorial station. There was no apparent relationship. Calculations show that while the entire observed inter-tropical emission results from dissociative recombination of O₂⁺ this process may, in some cases, account for only a fraction of a percent of the observed SAR arc emission. More than five years of geomagnetic storm data shows that Southern African SAR arcs are unlikely unless disturbances exceed 150 γ. For very severe 300 γ disturbances main phase SAR arcs may be observed. Estimates of the fraction of storm energy used in production of the present arcs indicate they are inefficient sinks for magnetic storms.     

Frank-Condon factors for H2O+ molecular bands

Frank-Condon factors for H₂O⁺ bands have been calculated. They are used to estimate the photon scattering coefficient g_8.0 for the (8,0) band at 6158 Å.     

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.     

Line shape of the non-thermal 6300 Å O(1D) emission

The line shape of the non-thermal O(¹D) 6300 Å emission is calculated using the two population model of Schmitt, Abreu and Hays (Planet. Space Sci.29, 1095, 1981). The calculated line shapes simulate observations made from a space platform at different zenith angles and altitudes. The non-thermal line shapes observed at zenith angles other than the local vertical have been obtained by using the Addition theorem for spherical harmonics of a Legendre polynomial expansion of the non-thermal population distribution function.     

The dissociative recombination of O2+ in the ionosphere

Aeronomical determinations of the dissociative recombination reaction rate coefficient for O₂⁺, α, depend directly on a knowledge of the rate coefficient for the charge exchange of O⁺ with O₂, k. A re-evaluation of the aeronomical determination of α using Atmosphere Explorer satellite data is necessary in the light of a subsequent laboratory measurement of k. The results reported here are in reasonable agreement with laboratory determinations to within the uncertainty of the analysis for night-time conditions. However, for data obtained under sunlit conditions the aeronomical determination differs significantly from the laboratory measurements. The results imply that the state of the O₂⁺ molecule resulting from the major thermospheric processes requires further examination.