Dissociative Recombination Of O2 Research Articles

Mars solar wind interaction: Formation of the Martian corona and atmospheric loss to space

A three‐dimensional (3‐D) atomic oxygen corona of Mars is computed for periods of low and high solar activities. The thermal atomic oxygen corona is derived from a collisionless Chamberlain approach, whereas the nonthermal atomic oxygen corona is derived from Monte Carlo simulations. The two main sources of hot exospheric oxygen atoms at Mars are the dissociative recombination of O2+between 120 and 300 km and the sputtering of the Martian atmosphere by incident O+pickup ions. The reimpacting and escaping fluxes of pickup ions are derived from a 3‐D hybrid model describing the interaction of the solar wind with our computed Martian oxygen exosphere. In this work it is shown that the role of the sputtering crucially depends on an accurate description of the Martian corona as well as of its interaction with the solar wind. The sputtering contribution to the total oxygen escape is smaller by one order of magnitude than the contribution due to the dissociative recombination. The neutral escape is dominant at both solar activities (1 × 1025s−1for low solar activity and 4 × 1025s−1for high solar activity), and the ion escape flux is estimated to be equal to 2 × 1023s−1at low solar activity and to 3.4 × 1024s−1at high solar activity. This work illustrates one more time the strong dependency of these loss rates on solar conditions. It underlines the difficulty of extrapolating the present measured loss rates to the past solar conditions without a better theoretical and observational knowledge of this dependency.

Martian corona: Nonthermal sources of hot heavy species

We have studied the production of hot O and C atoms, and hot CO2 and CO molecules in the Martian upper atmosphere and exosphere by dissociative recombination (DR) of O2+ and CO+ ions, and sputtering of the atmosphere by incident O+ pick‐up ions. Production and collisional thermalization of the hot particles in the upper atmosphere are described by using a unique Monte Carlo test particle approach to simulate both nonthermal processes. Velocity distributions, atmospheric loss rates, and density profiles are derived for suprathermal O, C, CO, and CO2 at low and high solar activity. At high solar activity the hot oxygen escape rate estimated from DR of O2+ is found to be less than two times the sputtering rate. Sputtering is found to efficiently populate the corona with molecular species such as CO and CO2 at high solar activity and also to produce a carbon escape rate that is comparable to that derived from the major photochemical sources. Dissociation of CO2 molecules by the impacting pick‐up ions flux are found to produce about 50% of the sputtered exospheric oxygen density at high solar activity. Collisions of the background atmospheric gas with hot O atoms produced by DR of O2+ produce densities of hot CO2 and CO molecules larger than 102 cm−3 for altitudes lower than 1000 km, at both high and low solar activity. Interestingly, the hot CO2 density scale height is observed to be process dependent. The hot oxygen energy distributions associated with sputtering and DR near the exobase are also found to follow distinct decreasing energy laws. We suggest that the effects of the solar zenithal angle (SZA), crustal magnetic fields, and atmospheric tides on the ionospheric structure may produce exospheric signatures.

First observations of detached equatorial ionospheric plasma depletions using OI 630.0 nm and OI 777.4 nm emissions nightglow imaging

Near‐simultaneous all‐sky (180° field of view) observations of the OI 630.0 nm and OI 777.4 nm nightglow emissions are being carried out on a routine basis at “Laboratorio Nacional de Astrofisica (LNA)”, Brazópolis (22.5° S, 45.6° W; dip latitude 17.5° S; altitude 1860 m), Brazil, since September 2002. The all‐sky imaging observations of the OI 630.0 nm and OI 777.4 nm emissions, which arise from the dissociative recombination of O2+ ions and radiative recombination of O+ ions (mainly), respectively, are used to monitor the morphology and dynamics of the equatorial F‐region at the respective emission heights. In this paper we present and discuss the first observations showing detached equatorial F‐region plasma depletions or bubbles. Three cases are presented from observations during the years 2002 and 2003. We suggest a possible mechanism for their generation.

Discrepancy between the nighttime molecular ion composition given by the International Reference Ionosphere model and airglow measurements at low latitudes

The 630‐nm nighttime airglow is radiated by O(1D) atoms, which are produced by the dissociative recombination of O2+ ions. The typical approach used to calculate the red line emission rate at night is based on the assumption that O2+ is mainly produced by the reaction of O+ with molecular oxygen. In the case that the O2+ density is much smaller than the O+ density, [O+] = ne in the F2 region. Good agreement between measured nighttime integrated emission rates and the emission rates calculated by this typical approach, using both electron densities measured by incoherent scatter radars and given by the International Reference Ionosphere (IRI) model, has been shown. However, the O2+ densities given by the IRI model are much higher than the densities produced by the reaction of O+ with O2, and these densities do not correspond to the condition [O+] = ne. In this case, the typical approach cannot be applied and molecular ions must be included in the emission rate calculations. The integrated emission rates calculated including the molecular ion density given by the IRI model have been found to be much higher than the measured 630.0‐nm emission rates. This discrepancy takes place at latitudes below about ±30° in the western longitude sector, mainly for the period from March to November, and the disagreement is higher than 1 order of magnitude at the equator. In addition, we model the F2 region green line O(1S) emission at 557.7 nm resulting from the dissociative recombination of O2+. Using measurements of this volume emission rate made by the Wind Imaging Interferometer (WINDII) satellite, we are able to show that IRI overestimates the O2+ density (and ion fraction) on the bottomside of the F2 region. A revision of the ion composition in the IRI model on the bottomside seems to be needed on the basis of these results. Airglow measurements may be useful in constraining such a revision. A revision could utilize the formulae for the relationship between the molecular ion densities and neutral densities derived here, using the Mass Spectrometer Incoherent Scatter (MSIS) neutral densities and the IRI electron density. These calculations are based on the assumption that O2+ and NO+ are only produced through ion‐molecular reactions. Such a revision would correct the magnitude and altitudinal dependence of the molecular ion fraction in the IRI model.

Electron energy-dependent product state distributions in the dissociative recombination of O2+

We present product state distributions and quantum yields from the dissociative recombination reaction of O2+ in its electronic and vibrational ground states as a function of electron collision energy between 0 and 300 meV. The experiments have been performed in the heavy-ion storage ring, CRYRING, and use a cold hollow-cathode discharge source for the production of cold molecular oxygen ions. The branching fractions over the different dissociation limits show distinct oscillations while the resulting product quantum yields are largely independent of electron collision energy above 40 meV. The branching results are well reproduced assuming an isotropic dissociation process, in contrast with recent theoretical predictions.

Position- and time-sensitive coincident detection of fragments from the dissociative recombination of O2+ using a single hexanode delay-line detector

This contribution describes the use of the RoentDek hexanode delay line detector to detect fragments from the dissociative recombination of O2+ in an experiment at the ion storage ring CRYRING, Manne Siegbahn Laboratory, Stockholm. In this experiment, the fragments have a maximum time- and position-separation of 20 ns and 40 mm, respectively. The position resolution obtained was 0.16 mm (FWHM) on the 67 mm diameter detector. The time resolution obtained from the time-of-arrival difference between the product fragments was about 1 ns. The detector system handles event rates as large as 30 kHz. Techniques for the calibration of the absolute position of particles on the detector are discussed.

Vibrational dependence in the dissociative recombination of O2+

We report data on a study into the vibrational dependence in the dissociative recombination of O+2 (X2Πg). The experiment has been performed with a merged-beam apparatus in the heavy-ion storage ring, CRYRING. We present the total rate coefficients as function of collision energy for five different vibrational populations of the ion beam and the partial rate coefficients and branching fractions near 0 eV collision energy for the vibrational levels v = 0, 1 and 2. Furthermore, we report on the control over and characterization of vibrational populations. The total rate coefficient is found to be weakly dependent on the vibrational excitation. The rate decreases upon increasing vibrational excitation, though only within an order of magnitude. The partial rate coefficients are strongly dependent on the vibrational level; with the v = 0 oxygen ions recombining the fastest. The partial branching fractions are also found to be strongly dependent on the vibrational level, specifically the O(1S) quantum yield which increases strongly upon increasing vibrational level.

Vibrationally resolved rate coefficients and branching fractions in the dissociative recombination of O2+

We have studied the dissociative recombination of the first three vibrational levels of O(2) (+) in its electronic ground X (2)Pi(g) state. Absolute rate coefficients, cross sections, quantum yields and branching fractions have been determined in a merged-beam experiment in the heavy-ion storage ring, CRYRING, employing fragment imaging for the reaction dynamics. We present the absolute total rate coefficients as function of collision energies up to 0.4 eV for five different vibrational populations of the ion beam, as well as the partial (vibrationally resolved) rate coefficients and the branching fractions near 0 eV collision energy for the vibrational levels v=0, 1, and 2. The vibrational populations used were produced in a modified electron impact ion source, which has been calibrated using Cs-O(2)(+) dissociative charge transfer reactions. The measurements indicate that at low collision energies, the total rate coefficient is weakly dependent on the vibrational excitation. The calculated thermal rate coefficient at 300 K decreases upon vibrational excitation. The partial rate coefficients as well as the partial branching fractions are found to be strongly dependent on the vibrational level. The partial rate coefficient is the fastest for v=0 and goes down by a factor of two or more for v=1 and 2. The O((1)S) quantum yield, linked to the green airglow, increases strongly upon increasing vibrational level. The effects of the dissociative recombination reactions and super elastic collisions on the vibrational populations are discussed.

Height‐resolved ionospheric drifts at low latitudes from simultaneous OI 777.4 nm and OI 630.0 nm imaging observations

Transequatorial F region plasma bubbles are large‐scale ionospheric depleted regions that develop in the bottomside of equatorial F region due to plasma instability processes. Simultaneous all‐sky imaging observations of the OI 630.0 nm and OI 777.4 nm nightglow emissions were carried out at São José dos Campos (23.21°S, 45.86°W; dip latitude 17.6°S), Brazil, during the years 2000 and 2001, a period of high solar activity. In this work we present and discuss the height‐resolved nocturnal F region zonal drift velocities obtained from plasma bubbles observed in imaging observations of these two emissions for several days of 2000 and 2001. It should be pointed out that the two emissions result from different excitation mechanisms (OI 630.0 nm by dissociative recombination of O2+ and OI 777.4 nm by radiative recombination of O+) and come from different F region heights, separated by ∼50–80 km. We have investigated the nighttime zonal plasma drift variations using fixed emission peak altitudes, used by earlier investigators, as well as emission peak altitudes based on simultaneous ionospheric sounding observations. The average maximum and minimum zonal plasma drift velocities inferred for both the emissions, using emission peak altitudes based on simultaneous ionospheric observations, (OI 630.0 nm: 172 ± 2 and 89 ± 15 m/s; OI 777.4 nm: 184 ± 12 and 103 ± 16 m/s) are lower and with less scatter than those using fixed emission peak altitudes (OI 630.0 nm: 185 ± 10 and 104 ± 18 m/s; OI 777.4 nm: 202 ± 19 and 121 ± 20 m/s). Also, the nocturnal variations of the zonal plasma drift velocities obtained for the two emissions with peak altitudes based on simultaneous ionospheric observations show better agreement than for the case with fixed emission peak altitudes (h = 380 km for OI 777.4 nm and h = 300 km for OI 630.0 nm).

On the onset of HF‐induced airglow at HAARP

Observations of airglow at 630 nm (red line) and 557.7 nm (green line) during the February 2002 campaign at the High Frequency Active Auroral Research Program (HAARP) heating facility are analyzed. We find that during injections toward magnetic zenith (MZ) the green and red lines gain ∼5 R within ∼1 s and ∼20 R within ∼10 s, respectively. We term this period the onset of the HF‐induced airglow. A model of the onset at magnetic zenith is developed. It accounts for background photoelectrons and dissociative recombination of O2+. It is shown that heating and acceleration of background electrons dominate the airglow onset. We propose a scenario for the generation of strong Langmuir turbulence for injections outside the Spitze region, including magnetic zenith.

Distributions of hot oxygen for Venus and Mars

Hot atomic oxygen velocity distributions and corresponding profiles of density and temperature have been calculated for the Venus and Mars exospheres by Monte Carlo simulation. The Venus results are realistically based on well‐established models of the Venus atmosphere and ionosphere that were derived from Pioneer Venus orbiter measurements near solar maximum. The confidence level is lower for Mars because the only viable data come from Viking entry measurements in daytime at solar minimum, and the global morphologies of the atmosphere and ionosphere are poorly understood. What is clear is that planetary differences arise because the exothermal velocities of hot oxygen created by dissociative recombination of O2+ are below satellite speeds on Venus and greater than escape on Mars. As a result, the distribution of velocities on Venus tends to be nearly isotropic, whereas it is grossly anisotropic on Mars, favoring the vertical with increasing distance from the planet. The calculated lower bound for the Martian oxygen escape rate indicates that the lifetime of CO2 in the Mars atmosphere is probably less than 10 Myr.

Modeling the OI 630.0 and 557.7 nm thermospheric dayglow during EISCAT‐WINDII coordinated measurements

The 630.0 and 557.7 nm thermospheric dayglow was modeled at high‐latitude using an eight‐moment fluid model, from measurements coordinated between the European Incoherent Scatter (EISCAT) radar and the Wind Imaging Interferometer (WINDII). The emission computation in particular included the electron (suprathermal and thermal) impact whose cross sections have been updated, the dissociative recombination of O2+ ion described for the first time by a theoretical rate coefficient, the photodissociation of molecular oxygen, and some relevant chemical reactions. The neutral atmosphere was adjusted by calibrating the ionospheric model outputs to EISCAT data. Slight adjustments were needed in order to reach a good agreement. The results were successfully compared to WINDII observations. Our present study shows that simultaneous EISCAT‐WINDII measurements can be used to reduce uncertainties due to the neutral composition and that new observations of the EUV solar spectrum are still needed.

Solar cycle variability of hot oxygen atoms at Mars

The population of hot oxygen atoms in the Martian exosphere is reexamined using newly calculated hot O production rates for both low and high solar cycle conditions. The hot oxygen production rates are assumed to result from the dissociative recombination of O2+ ions. These calculations take into account the calculated vibrational distribution of O2+ and the new measured branching ratios. Furthermore, these calculations also consider the variation of the dissociative recombination cross section with the relative speed of the participating ions and electrons, the rotational energy of the O2+ ions, and the spread of the ion and electron velocities. These production rates were next used in a two‐stream model to obtain the energy dependent flux of the hot oxygen atoms as a function of altitude. Finally, the calculated flux at the exobase was input into an exosphere model, based on Liouville's theorem, to calculate the hot oxygen densities as a function of altitude in the exosphere and the resulting escape flux. It was found that hot oxygen densities vary significantly over the solar cycle; the calculated densities vary from about 2×103 to 6×103 cm−3 at an altitude of 1000 km. The escape flux also varies from about 3×106 to 9×106 cm−2s−1.

Hot oxygen corona at Europa

A model of the hot oxygen exosphere at Europa was constructed. The source term for the hot atoms was assumed to be dissociative recombination of O2+ and Liouville's theorem was used to calculate their altitude distribution. It was found that near the surface the hot oxygen density is in excess of 200 cm−3, dropping to a value on the order of 50 cm−3 at 1500 km. These calculations indicate that the hot atomic oxygen densities are considerably less than the thermal molecular oxygen ones, but slightly larger than the measured sodium values. The escape flux of the hot oxygen atoms was calculated to be on the order of 1.4×108 atoms cm−2 sec−1. This corresponds to a global escape rate of 4.4×1025 atoms sec−1, which is more than an order of magnitude less than the estimated atmospheric sputtering rate.

Mechanism for the Green Glow of the Upper Ionosphere

The generation of the green line of atomic oxygen by dissociative recombination of O 2 + occurs by the capture of an electron into a repulsive state of O 2 followed by dissociation along another state of a different electronic symmetry. The two states are coupled together by mixed symmetry Rydberg states. Quantum chemical calculations give a rate coefficient at room temperature of (0.39 −0.19 +0.31 ) × 10 −8 cubic centimeters per second. The quantum yield of excited oxygen is within the range deduced from ground, rocket, and satellite observations. The rate coefficients and yields are needed in models of the optical emission, chemistry, and energy balance of planetary ionospheres.

Spectrum of hot O at the exobases of the terrestrial planets

The distribution of energetic O produced in dissociative recombination of O2+ at the exobases of the terrestrial planets is important in determining the structure of the outer hot O coronas and for the escape flux of O from Mars. Using recently measured values for the branching ratios of the energetically allowed channels in O2+ dissociative recombination, along with models of the vibrational distribution of O2+, we compute the velocity distribution of hot O atoms produced at the exobases of Mars, Venus, and Earth. We take into account the effects of the ion and electron temperatures, the rotational energy of the ions, and the decrease of the dissociative recombination cross section with relative velocity of the ion and electron. We find that distinct peaks appear at the energies associated with the various channels in the Martian spectrum but that for Venus, the features are less distinct, and for Earth, the distribution appears as a single, but broad and asymmetrical peak. For Mars, we compute the fractions of atoms produced with speeds greater than the escape velocity for 16O, 17O, and 18O of 0.68, 0.64, and 0.58, respectively. Thus there is a small isotope effect inherent in the dissociative recombination mechanism that has not been taken into account in models of Martian atmospheric evolution or in estimates of the total inventory of oxygen that is implied by the observed lack of enhancement of the heavier isotopes relative to the terrestrial values.

The Source of Green Light Emission Determined from a Heavy-Ion Storage Ring Experiment

The final electronic states of oxygen atoms formed by dissociative recombination of O 2 + with cold electrons have been measured by an imaging technique at a heavy-ion storage ring. The 3 P , 1 D , and 1 S quantum yield for O 2 + in the vibrational ground level was found to be 0.86, 1.09, and 0.05, respectively. The yield of the 1 S state, which is responsible for the 5577 angstrom green light from upper planetary atmospheres, was more than an order of magnitude larger than the theoretical value used in atmospheric models. The results may help refine the models of the chemistry and dynamics of the ionosphere and provide guidance for further quantum-chemistry calculations.

New sources for the hot oxygen geocorona: Solar cycle, seasonal, latitudinal, and diurnal variations

This paper demonstrates the variability of thermospheric sources of hot oxygen atoms. Numerical calculations were performed for day and night, high and low solar activity, summer and winter, and low‐ and middle‐latitude conditions. Under most conditions, reactions involving metastable species are more important hot O sources than previously considered dissociative recombination of O2+ and NO+. All the hot O sources are an order of magnitude lower at midnight than at noon. At night, dissociative recombination of O2+ and NO+ are the most important sources. Quenching of vibrationally excited N2 (N2*) by O is the most important metastable source at night. Above 300 km, hot O sources increase by an order of magnitude between solar minimum and solar maximum. For a given level of solar activity, the high‐altitude total production rate of hot O kinetic energy is greater during winter than during summer, indicating a dominance of cooler hot O sources during summer. The N2* source dominates at low altitudes. At high altitudes it is almost negligible at solar minimum, but increases to become the dominant source at solar maximum. Atomic oxygen quenching of N(²D) is a large source at solar minimum and is still important at solar maximum. Overall, seasonal variations are small compared to solar cycle, diurnal and latitudinal variations. While quenching of metastable species is more important at midlatitudes than at low latitudes, there is little latitudinal variation in hot O production due to dissociative recombination of NO+ and O2+.

A kinetic model of the formation of the hot oxygen geocorona: 1. Quiet geomagnetic conditions

A model of the hot oxygen geocorona in the transition region near the exobase is described. It is based on a Monte Carlo solution of the nonlinear Boltzmann equation for hot oxygen atoms produced by chemical processes usually considered as a source of hot oxygen (photodissociation of O2 and dissociative recombination of O2+ and NO+ ions). The evolution of the system is described stochastically as a series of random Markovian processes. The energy distribution function of the thermal and non‐thermal O(³P) atoms and of the nonthermal O(¹D) atoms is calculated from the thermospheric collision‐dominated region to the exosphere where the gas flow is virtually collisionless. The model is applied to equatorial latitudes for conditions of low solar and geomagnetic activity. Numerical simulations show that the distribution function of thermal oxygen is increasingly perturbed by collisions with the hot oxygen population at high altitudes and departs significantly from a Maxwellian distribution at all altitudes. The number density and temperature of the nonthermal oxygen atoms are derived from their microscopic distribution function and found to be in qualitative agreement with previous theoretical and experimental estimates.

On the escape of oxygen and hydrogen from Mars

Escape rates of oxygen atoms from dissociative recombination of O2+ above the Martian exobase are computed in light of new information from ab initio calculations of the dissociative recombination process, and our recently revised understanding of the Martian dayside ionosphere. Only about 60% of the dissociative recombinations occur in channels in which the O atoms are released with energies in excess of the escape velocity. Futhermore, we find that the computed escape fluxes for O depend greatly on the nature of the ion loss process that has been found necessary to reproduce the topside ion density profiles measured by Viking. If it is assumed that the ions are not lost from the gravitational field of the planet, as required by an analysis of nitrogen escape, the computed average O escape rate is 3 × 106 cm−2 s−1, much less than half the H escape rates inferred from measurements of the Lyman alpha dayglow, which are in the range (1 – 2) × 108 cm−2 s−1. Suggestions for restoring the relative escape rates of H and O to the stoichiometric ratio of water are explored. The final resolution of this problem may have to await a future aeronomy mission to Mars.