Dissociative Recombination Of O2 Research Articles

The formation of O and H radicals in a pulsed discharge in atmospheric pressure helium with water vapour admixtures

The spatio-temporal distribution of O and H radicals in a 90 ns pulsed discharge, generated in a pin–pin geometry with a 2.2 mm gap, in He + H2O (0.1% and 0.25%), is studied both experimentally and by 1D fluid modelling. The density of O and H radicals as well as the effective lifetimes of their excited states are measured using picosecond resolution two-photon absorption laser induced fluorescence. Good agreement between experiments and modelling is obtained for the species densities. The density of O and H is found to be homogenous along the discharge axis. Even though the high voltage pulse is 90 ns long, the density of O peaks only about 1 μs after the end of the current pulse, reaching 2 × 1016 cm−3 at 0.1% H2O. It then remains nearly constant over 10 μs before decaying. Modelling indicates that the electron temperature (Te) in the centre of the vessel geometry ranges from 6 to 4 eV during the peak of discharge current, and after 90 ns, drops below 0.5 eV in about 50 ns. Consequently, during the discharge (<100 ns), O is predominantly produced by direct dissociation of O2 by electron impact, and in the early afterglow (from 100 ns to 1 μs) O is produced by dissociative recombination of O2 +. The main loss mechanism of O is initially electron impact ionisation and once T e has dropped, it becomes mainly Penning ionisation with He2* and He* as well as three-body recombination with O+ and He. On time scales of 100–200 μs, O is mainly lost by radial diffusion. The production of H shows a similar behaviour, reaching 0.45 × 1016 cm−3 at 1 μs, due to direct dissociation of H2O by electron impact (<100 ns) followed by electron–ion recombination processes (from 200 ns to 1.5 us). H is dominantly lost through Penning ionisation with He* and He2* and by electron impact ionisation, and by charge exchange with O+. Increasing concentrations of water vapour, from 0.1% to 0.25%, have little effect on the nature of the processes of H formation but trigger a stronger initial production of O, which is not currently reproduced satisfactorily by the modelling. What emerges from this study is that the built up of O and H densities in pulsed discharges continues after electron-impact dissociation processes with additional afterglow processes, not least through the dissociative recombination of O2 + and H2 +.

Solar wind driven influences on the Martian oxygen corona: Constraints on atmospheric sputtering from a synthesis of MAVEN measurements during solar minimum

The prolonged low solar extreme ultraviolet (EUV) flux conditions provided by the deep solar minimum between solar cycles 24 and 25 can be expected to have amplified the relative influence of any solar wind driven effects on the Martian corona. As such, this solar minimum has provided an opportunity to study solar wind effects on processes that produce the energetic neutral particles that populate the oxygen corona, particularly atmospheric sputtering by precipitating energetic ions, and dissociative recombination of O2+. To probe the corona, we use a novel, fully empirical divergence-based technique for estimating coronal oxygen densities. This method solves the continuity equation for ions to obtain local ionization rates that are, in turn, converted to neutral densities by accounting for ionization via electron impact, charge exchange, and photoionization. We apply this technique to combined measurements of ion flux, electron flux, solar EUV flux, and magnetic fields from the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter at Mars. We separately combine MAVEN measurements during relatively low and high upstream solar wind flux, Fsw, conditions and derive the relevant fluxes for each range of conditions. We find that fluxes of planetward energetic O+ upstream of Mars increase by roughly ×3 during high Fsw conditions. We derive radial density profiles in the range 2.0–2.7 Mars radii and find that coronal hot oxygen densities scale with radial distance, R, as R−2 from 300cm−3 to 100cm−3 in this range. We perform χ2 parameter-space searches for exobase densities and effective temperatures to evaluate any potential differences in the hot O populations that populate the corona. Neither the radial profiles, nor the χ2 search, reveal any significant effects of increased Fsw on the Martian oxygen corona. The best estimates for exobase properties center around (6–7)×103atoms/cm3 and 0.3 eV/kB for both cases, with large correlated uncertainties. Based on the uncertainties inherent to the method and the measurements, we conclude that solar wind driven effects must be limited to roughly a factor of 2 and that further investigations to empirically constrain the corona over a wider range of altitudes are needed to investigate solar wind driven effects.

The altitude of green OI 557.7 nm and blue N2+ 427.8 nm aurora

Abstract. We have performed a large statistical study of the peak emission altitude of green O(1D2–1S0) (557.7 nm) and blue N2+ 1 N (427.8 nm) aurora using observations from a network of all-sky cameras stationed across northern Finland and Sweden recorded during seven winter seasons from 2000 to 2007. Both emissions were found to typically peak at about 114 km. The distribution of blue peak altitudes is more skewed than that for the green, and the mean peak emission altitudes were 114.84 ± 0.06 and 116.55 ± 0.07 km for green and blue emissions, respectively. We compare simultaneous measurements of the two emissions in combination with auroral modelling to investigate the emission production mechanisms. During low-energy electron precipitation (&lt;∼ 4 keV), when the two emissions peak above about 110 km, it is more likely for the green emission to peak below the blue emission than vice versa, with the difference between the two heights increasing with their average. Modelling has shown that under these conditions the dominant source of O(1S), the upper state of the green line, is energy transfer from excited N2 (A3Σu+), with a rate that depends on the product of the N2 and O number densities. Since both number densities decrease with higher altitude, the production of O(1S) by energy transfer from N2 peaks at lower altitude than the N2 ionisation rate, which depends on the N2 number density only. Consequently, the green aurora peaks below the blue aurora. When the two emissions peak below about 110 km, they typically peak at very similar altitude. The dominant source of O(1S) at low altitudes must not be energy transfer from N2, since the rate of that process peaks above the N2 ionisation rate and blue emission due to quenching of the long-lived excited N2 at low altitudes. Dissociative recombination of O2+ seems most likely to be a major source at these low altitudes, but our model is unable to reproduce observations fully, suggesting there may be additional sources of O(1S) unaccounted for.

Exosphere Modeling of Proxima b: A Case Study of Photochemical Escape with a Venus-like Atmosphere

Abstract Exoplanets orbiting M dwarfs within habitable zones are exposed to stellar environments more extreme than that terrestrial planets experience in our solar system, which can significantly impact the atmospheres of the exoplanets and affect their habitability and sustainability. This study provides the first prediction of hot oxygen corona structure and the associated photochemical loss from a 1 bar CO2-dominated atmosphere of a Venus-like rocky exoplanet, where dissociative recombination of O2 + ions is assumed to be the major source reaction for the escape of neutral O atoms and formation of the hot O corona (or exospheres) as on Mars and Venus. We employ a 3D Monte Carlo code to simulate the exosphere of Proxima Centauri b (PCb) based on the ionosphere simulated by a 3D magnetohydrodynamic model. Our simulation results show that variability of the stellar wind dynamic pressure over one orbital period of PCb does not affect the overall spatial structure of the hot O corona but contributes to the change in the global hot O escape rate that varies by an order of magnitude. The escape increases dramatically when the planet possesses its intrinsic magnetic fields as the ionosphere becomes more extended with the presence of a global magnetic field. The extended hot O corona may lead to a more extended H exosphere through collisions between thermal H and hot O, which exemplifies the importance of considering nonthermal populations in exospheres to interpret future observations.

Monte Carlo Calculations of Helium Escape on Mars via Energy Transfer from Hot Oxygen Atoms

Abstract Understanding He escape is crucial for deciphering the evolution of the He budget on Mars. A number of viable mechanisms have been proposed to drive He escape, with energy transfer from hot O generally thought to be the dominant one. This study is devoted to a state-of-the-art evaluation of the above process assuming hot O is exclusively produced from the dissociative recombination of O2 + in the Martian ionosphere. A Monte Carlo model is constructed, with model inputs optimized by the recent Mars Atmosphere and Volatile Evolution measurements. The model calculations reveal a dayside He escape flux of (1–2) × 106 cm−2 s−1 referred to the surface, for a possible range of H2 mixing ratio at an altitude of 80 km from 0 to 40 ppm. The computed He escape flux increases with increasing nascent O energy and decreasing atmospheric H2 and H abundances. The portion of the atmosphere below the exobase is found to make an exceptionally large contribution of 95% to He escape.

Low Electron Temperatures Observed at Mars by MAVEN on Dayside Crustal Magnetic Field Lines

AbstractThe ionospheric electron temperature is important for determining the neutral/photochemical escape rate from the Martian atmosphere via the dissociative recombination of O2+. The Langmuir Probe and Waves instrument onboard MAVEN (Mars Atmosphere and Volatile EvolutioN) measures electron temperatures in the ionosphere. The current paper studies electron temperatures in the dayside for two regions where (1) crustal magnetic fields are dominant and (2) draped magnetic fields are dominant. Overall, the electron temperature is lower in the crustal‐field regions, namely, the strong magnetic field region, which is due to a transport of cold electrons along magnetic field lines from the lower to upper atmosphere. The electron temperature is also greater for high solar extreme ultraviolet conditions, which is associated with the local extreme ultraviolet energy deposition. The current models underestimate the electron temperature above 250‐km altitude in the crustal‐field region. Electron heat conduction associated with a photoelectron transport in the crustal‐field regions is altered due to kinetic effects, such the magnetic mirror and/or ambipolar electric field because the electron mean free path exceeds the relevant length scale for electron temperature. The mirror force can affect the electron and heat transport between low altitudes, where the neutral density and related electron cooling rates are the greatest, and high altitudes, while the ambipolar electric field decelerates the electron's upward motion. These effects have not been included in current models of the electron energetics, and consideration of such effects on the electron temperature in the crustal‐field region should be considered for future numerical simulations.

First In Situ Evidence of Mars Nonthermal Exosphere

AbstractDedicated in situ measurements of Mars' exospheric Ar density by Neutral Gas Ion Mass Spectrometer/Mars Atmosphere and Volatile Evolution (NGIMS/MAVEN) highlight profiles displaying two slopes altitude variation. Below 230 km, the Ar density is characterized by scale height of 13.5 ± 1.7 km and above 400 km by scale height of 156 ± 25 km. A comparison with the model HELIOSARES suggests that the most probable origin of the Ar nonthermal component is the collisional interaction, below the exobase, between the atmospheric Ar and the nonthermal O atoms produced by the dissociative recombination of O2+. These measurements therefore provide a new approach to constrain one of the main sources of Mars' atmospheric escape. They also lead to set up an upper limit on the efficiency of pickup ion sputtering at Mars. The comparison with HELIOSARES suggests that NGIMS detection sensitivity might allow a positive detection of this mechanism if more measurements at high altitude deep in the nightside are performed.

On Mars's Atmospheric Sputtering After MAVEN's First Martian Year of Measurements

AbstractMars likely lost a significant part of its atmosphere to space during its history. The sputtering of the atmosphere, by precipitating planetary heavy pickup ions accelerated by the solar wind, is one of the processes that could have significantly contributed to this atmospheric escape, in particular since the cessation of its global magnetic field, 4.0–4.1 Gyr ago. We present a 2 year baseline analysis of Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of the precipitating flux. We use this measurement to model the expected escape rate and exospheric structure induced by this precipitation. We conclude that sputtering signatures in the dayside exosphere will be difficult to identify by MAVEN, and the induced atmospheric escape of O atoms remains orders of magnitude smaller than the expected rate induced by dissociative recombination of O2+ in Mars's ionosphere. On the contrary, deep in the nightside, Mars's sputtering might be the main source of the nonthermal part of the exospheric density profiles of species with mass larger or equal to Ar.

The Role of Excited Oxygen Molecules in the Formation of the Secondary Ozone Layer at 87 to 97 km

The secondary ozone layer is located at elevations of 87 to 97 km in the upper mesosphere – lower thermosphere. It overlaps with the ionospheric D-layer. Daytime intensive UV radiation is dissociating O2 molecules to O atoms and photoexcitating O2 molecules up to 11.07eV level. Ozone photolysis between the wavelengths of 118.7–121.6 nm produces three oxygen atoms from one ozone molecule. Collision reactions of and with produce additional oxygen atoms. The number of oxygen atoms is maintained at such a high level that a small but significant ozone concentration survives. UV radiation weakens radically during the night. The number of O atoms shows no diurnal variation in the MLT. This leads to a ten-fold increase of ozone concentration over the course of the night. Dissociative recombination of O2+ (entered via diffusion from above) and reactions of O (3P) atoms with excited O2 molecules generate O(1S) atoms. The quenching of O(1S)→O(1D) emits the green nightglow. The reactions of O(1D) with ozone and O2 absorption of UV nightglow produce . When these molecules relax, they emit the O2 UV nightglows. The relaxations of and emit infrared nightglows.

Escape of O(3P), O(1D), and O(1S) from the Martian atmosphere

We have computed here the escape probabilities, fluxes and rates for hot O atoms that are initially produced in the ground state and the first two excited metastable states, O(1D)and O(1S), in the Martian thermosphere by dissociative recombination of O2+. In order to compare our results with those of our previous calculations and with those of others, we have employed here the pre-MAVEN models that we have used previously. To compute the escape probabilities, we have employed the Monte Carlo escape code that has been described previously, but we here use for the first time energy-dependent elastic cross sections for collisions of the energetic O atoms with each of the twelve background species in our model. We also incorporate three mechanisms that interchange identities of the O(3P) and O(1D) atoms, including collisional excitation of O(3P) to O(1D), quenching of O(1D) to O(3P), and excitation exchange of O(1D) with O(3P). We find that the escape probabilities of O atoms that are produced initially as O(1D) are reduced compared to those in which these processes are not included, but the escape probabilities of O atoms that are initially produced as O(3P) are not significantly reduced. As a guide for our future research and those of other investigators, we review here what is known about the interactions of O atoms with other species in which the energies of the O atoms are altered, and several other sources of hot and escaping O, many of which have been suggested by other investigators. We will incorporate these data in a future MAVEN-like model.

Photochemical escape of oxygen from Mars: First results from MAVEN in situ data

AbstractPhotochemical escape of atomic oxygen is thought to be one of the dominant channels for Martian atmospheric loss today and played a potentially major role in climate evolution. Mars Atmosphere and Volatile Evolution Mission (MAVEN) is the first mission capable of measuring, in situ, the relevant quantities necessary to calculate photochemical escape fluxes. We utilize 18 months of data from three MAVEN instruments: Langmuir Probe and Waves, Neutral Gas and Ion Mass Spectrometer, and SupraThermal And Thermal Ion Composition. From these data, we calculate altitude profiles of the production rate of hot oxygen atoms from the dissociative recombination of O2+ and the probability that such atoms will escape the Mars atmosphere. From this, we determine escape fluxes for 815 periapsis passes. Derived average dayside hot O escape rates range from 1.2 to 5.5 × 1025 s−1, depending on season and EUV flux, consistent with several pre‐MAVEN predictions and in broad agreement with estimates made with other MAVEN measurements. Hot O escape fluxes do not vary significantly with dayside solar zenith angle or crustal magnetic field strength but depend on CO2 photoionization frequency with a power law whose exponent is 2.6 ± 0.6, an unexpectedly high value which may be partially due to seasonal and geographic sampling. From this dependence and historical EUV measurements over 70 years, we estimate a modern‐era average escape rate of 4.3 × 1025 s−1. Extrapolating this dependence to early solar system, EUV conditions gives total losses of 13, 49, 189, and 483 mbar of oxygen over 1–3 and 3.5 Gyr, respectively, with uncertainties significantly increasing with time in the past.

Climatology of thermospheric neutral winds over Oukaïmeden Observatory in Morocco

Abstract. In order to explore coupling between the thermosphere and ionosphere and to address the lack of data relating to thermospheric neutral winds and temperatures over the African sector, a new system of instruments was installed at the Oukaïmeden Observatory located in the high Atlas Mountains, 75 km south of Marrakesh, Morocco (31.206° N, 7.866° W, 22.84° N magnetic). In this work we present the first multi-year results of the climatology of meridional and zonal winds obtained during the period from January 2014 to February 2016, including observations from 648 nights. The measurements are obtained using an imaging Fabry–Pérot interferometer, which measures the 630.0 nm emissions caused by dissociative recombination of O2+. The basic climatology of the winds is as expected, showing zonal winds that are strongly eastward in the early evening just after sunset with a speed of 50 to 100 m s−1 decreasing in magnitude, and reversing directions in the local summer months, towards sunrise. The meridional winds are slightly poleward in the early evening during the local winter, before reversing directions around 21:00 LT. In the local summer months, the meridional winds are equatorward for the entire night, reaching a maximum equatorward speed of 75 m s−1. We compare the observed climatologies of neutral winds to that provided by the recently updated Horizontal Wind Model (HWM14) in order to validate that model's predictions of the thermospheric wind patterns over the eastern portion of Africa. The model captures much of the features in the observational climatologies. The most notable exception is for the zonal winds during local summer, when the maximum eastward wind in the observations occurs approximately 4 h later than seen in the model results.

Hot oxygen escape from Mars: Simple scaling with solar EUV irradiance

AbstractThe evolution of the atmosphere of Mars and the loss of volatiles over the lifetime of the solar system is a key topic in planetary science. An important loss process for atomic species, such as oxygen, is ionospheric photochemical escape. Dissociative recombination of O2+ ions (the major ion species) produces fast oxygen atoms, some of which can escape from the planet. Many theoretical hot O models have been constructed over the years, although a number of uncertainties are present in these models, particularly concerning the elastic cross sections of O atoms with CO2. Recently, the Mars Atmosphere and Volatile Evolution mission has been rapidly improving our understanding of the upper atmosphere and ionosphere of Mars and its interaction with the external environment (e.g., solar wind), allowing a new assessment of this important loss process. The purpose of the current paper is to take a simple analytical approach to the oxygen escape problem in order to (1) study the role that variations in solar radiation or solar wind fluxes could have on escape in a transparent fashion and (2) isolate the effects of uncertainties in oxygen cross sections on the derived oxygen escape rates. In agreement with several more elaborate numerical models, we find that the escape flux is directly proportional to the incident solar extreme ultraviolet irradiance and is inversely proportional to the backscatter elastic cross section. The amount of O lost due to ion transport in the topside ionosphere is found to be about 5–10% of the total.

Non-thermal production and escape of OH from the upper atmosphere of Mars

We present a theoretical analysis of formation and kinetics of hot OH molecules in the upper atmosphere of Mars produced in reactions of thermal molecular hydrogen and energetic oxygen atoms. Two major sources of energetic O considered are the photochemical production, via dissociative recombination of O2+ ions, and energizing collisions with fast atoms produced by the precipitating Solar Wind (SW) ions, mostly H+ and He2+, and energetic neutral atoms (ENAs) originating in the charge-exchange collisions between the SW ions and atmospheric gases. Energizing collisions of O with atmospheric secondary hot atoms, induced by precipitating SW ions and ENAs, are also included in our consideration. The non-thermal reaction O + H2(v, j) → H + OH(v′, j′) is described using recent quantum-mechanical state-to-state cross sections, which allow us to predict non-equilibrium distributions of excited rotational and vibrational states (v′, j′) of OH and expected emission spectra. A fraction of produced translationally hot OH is sufficiently energetic to overcome Mars’ gravitational potential and escape into space, contributing to the hot corona. We estimate its total escape flux from the dayside of Mars for low solar activity conditions at about 1.1 × 1023 s−1, or about 0.1% of the total escape rate of atomic O and H. The described non-thermal OH production mechanism is general and expected to contribute to the evolution of atmospheres of the planets, satellites, and exoplanets with similar atmospheric compositions.

Enhanced O2+ loss at Mars due to an ambipolar electric field from electron heating

AbstractRecent results from the MAVEN Langmuir Probe and Waves instrument suggest higher than predicted electron temperatures (Te) in Mars' dayside ionosphere above ~180 km in altitude. Correspondingly, measurements from Neutral Gas and Ion Mass Spectrometer indicate significant abundances of O2+ up to ~500 km in altitude, suggesting that O2+ may be a principal ion loss mechanism of oxygen. In this article, we investigate the effects of the higher Te (which results from electron heating) and ion heating on ion outflow and loss. Numerical solutions show that plasma processes including ion heating and higher Te may greatly increase O2+ loss at Mars. In particular, enhanced Te in Mars' ionosphere just above the exobase creates a substantial ambipolar electric field with a potential (eΦ) of several kBTe, which draws ions out of the region allowing for enhanced escape. With active solar wind, electron, and ion heating, direct O2+ loss could match or exceed loss via dissociative recombination of O2+. These results suggest that direct loss of O2+ may have played a significant role in the loss of oxygen at Mars over time.

The ionospheric source of the red and green lines of atomic oxygen in the Venus nightglow

We have modeled the nightside ionosphere of Venus for 11 solar zenith angles between 90° and 165° with a view toward determining the intensities of the atomic oxygen red, green and 2972Å features in the nightglow. The ionospheric source of these emissions is mainly dissociative recombination of O2+, which may proceed via four channels, which produce two O atoms in various combinations of the ground O(3P) state and the electronically excited O(1D) and O(1S) states. These excited states are the precursors to the nightglow features. We have modeled the O2+(v) vibrational distribution for each of the models, and we have used it to determine branching ratios for the DR channels. We have assumed that the ionosphere is produced by transport of atomic ions from the dayside, and that the ratios of the atomic ions are the same as the model values of the upward ion fluxes on the dayside. For each model, we have normalized the downward atomic ion fluxes to the peak O+ densities that were measured by the Pioneer Venus Orbiter Ion Mass Spectrometer. For the non-sunlit models, the O2+ densities peak in the altitude range 152–159km. We have computed the density profiles of O(1D) and O(1S), and the integrated overhead intensities of the transitions for each of the models. We find that the average intensities, weighted by the area of the solar zenith angle bins, for the red, green and 2972Å emissions are 26R, 4.6R, and 0.46R, respectively. Thus the average high solar activity nightside ionosphere cannot produce green line intensities that are as large as the maximum intensity of about 150R as measured from the ground. Our predicted red line intensities are, however, larger than the upper limits determined by ground based and spacecraft measurements. Possibilities for increasing the green line intensities and decreasing the red line intensities are discussed.

Hot oxygen atoms in the Venus nightside exosphere

The nightside oxygen exosphere of Venus is investigated for high and moderate solar activity by means of a Monte‐Carlo model. Hot O atoms are assumed to be produced by dissociative recombination of O2+ and NO+ molecular ions and by charge transfer processes between ionospheric O+ ions and neutral O and H atoms. The model considers rotational and vibrational excitation of the initial energy distribution of hot O atoms, includes elastic, inelastic, and quenching collisions between the suprathermal atoms and the ambient neutral atmosphere species, and uses differential cross sections for the determination of the scattering angle in the collisions. The results indicate that dissociative recombination of O2+ is, like at Venus' dayside, the most efficient source of hot O atoms at the planet's nightside. For high solar activity, the nightside exospheric density of hot O atoms is about one order of magnitude lower compared to the dayside, although between 2–10 times higher than in previous studies.

The theoretical impact polarization of the O I 6300 Å red line of Earth aurorae

Abstract. We are presenting a semi-classical theory of the impact polarization due to a quadrupolar electric excitation, which is the case of this forbidden line. In addition, this line is also radiatively forbidden being a triplet-singlet transition. This last feature is overcome by scaling the semi-classical result to a full quantum calculation at a single energy value. The cross-section and impact polarization are thus obtained as a function of energy, in agreement with the quantum calculations that exist only for the cross-section. The behavior of the impact polarization is found to be quite different than that of the usual dipolar electric interaction. Let us denote as radial the polarization parallel to the incident beam or magnetic field, and as tangential the perpendicular polarization. In the case of the dipolar electric interaction (permitted lines), the polarization is radial at low energy, and tangential at high energy, and it vanishes at energy about twelve times the threshold energy. In the case of the quadrupolar electric interaction, we observe quite different behavior, with the polarization vanishing point much closer to the threshold energy. This leads us to reanalyze the auroral red line polarization observation by Lilensten et al. (2008). From polarization observations made at Svalbard, they conclude to a rather strong tangential polarization observed during a 4-h recording including two auroral events. The existence of tangential polarization is questioned by our new theory, which leads to reconsidering the contribution of scattered parasitic light from a neighboring city that was mentioned but discarded by the authors. Finally, we conclude that the line is only weakly radially polarized by electron impact, and only during the auroral events. The weak polarization level leads to taking the competing depolarization by collisions with the neighboring O atoms into account, and by the competing isotropical (thus depolarizing) processes for populating the line upper level: the dissociative recombination of O2+ colliding with thermal electrons, and above all the reaction N(2D)+O2. The final diagnostic could be a density determination by depolarization, but it may be rather complicated because it involves several species.

Three‐dimensional study of Mars upper thermosphere/ionosphere and hot oxygen corona: 1. General description and results at equinox for solar low conditions

Unlike Earth and Venus, Mars with a weak gravity allows an extended corona of hot species and the escape of its lighter constituents in its exosphere. Being the most important reaction, the dissociative recombination of O2+ is responsible for most of the production of hot atomic oxygen deep in the dayside thermosphere/ionosphere. The investigation of the Martian upper atmosphere is therefore complicated by the change in the flow regime from a collisional to a collisionless domain. Past studies, which used simple extrapolations of 1‐D thermospheric/ionospheric parameters, could not account for the full effects of realistic conditions, which are shown to be of significant influence on the exosphere both close to and far away from the exobase. In this work, the combination of the new 3‐D Direct Simulation Monte Carlo kinetic model and the modern 3‐D Mars Thermosphere General Circulation Model is employed to describe self‐consistently the Martian upper atmosphere at equinox for solar low conditions. For the first time, a 3‐D analysis and shape of the Martian hot corona is provided, along with density and temperature profiles of cold and hot constituents as functions of position on the planet. Atmospheric loss and ion production (found to be more than an order of magnitude lower than the neutral escape), calculated locally all around the planet, provide valuable information for plasma models, refining the understanding of the ion loss, atmospheric sputtering, and interaction with the solar wind, in general.

On the elusive hot oxygen corona of Venus

After more than two years in orbit still no Venus Express observations were published concerning the hot oxygen corona of Venus which could verify the corresponding controversial observations of Venera 11 and PVO, three decades ago. Based on recent energy and mass dependent collision cross sections, the energy distributions of hot atomic oxygen created via dissociative recombination of O2+ are calculated in the daytime thermosphere by means of a 3D Monte Carlo approach. The exosphere density is obtained from the corresponding energy density and angular distribution at 240 km altitude by using a test particle model which traces the ballistic trajectories of hot O atoms in the exosphere. Our study indicates that upon taking into account proper input parameters, the hot oxygen corona appears to be substantially less dense than suggested by previous simulations.