Planetary Corona Research Articles

KINETIC MODEL OF THE STELLAR WIND FORCING ON THE EXTENDED HYDROGEN ATMOSPHERE OF THE EXOPLANEt π Men c

In this paper, an extension of the kinetic model of the aeronomy of the upper atmosphere of an exoplanet is performed by including the processes of the effect of stellar wind plasma on the extended hydrogen corona of a hot sub-neptune. For this purpose, previously developed kinetic Monte Carlo models were used to study the precipitation of protons and hydrogen atoms with high energies into planetary atmospheres. The kinetic model is adapted to the upper atmospheres of hot sub-neptunes, which made it possible to calculate the rate of absorption of stellar wind plasma energy in the planetary corona and to refine estimates of the non-thermal loss rate of the atmosphere due to the influence of the stellar wind. The calculations carried out for the hot sub-neptune π Men c showed that the energy of a flux of energetic neutral hydrogen atoms (ENA H) penetrating the atmosphere, formed during the charge exchange of stellar wind protons with thermal hydrogen corona atoms, mainly goes to heating the hydrogen corona of a hot exoplanet.

Study of the non-thermal atmospheric loss for exoplanet π Men c

AbstractWe have studied the input of the exothermic photochemistry into the formation of the non-thermal escape flux in the transition H2 − H region of the extended upper atmosphere of the hot exoplanet - the sub-neptune π Men c. The formation rate and the energy spectrum of hydrogen atoms formed with an excess of kinetic energy due to the exothermic photochemistry forced by the stellar XUV radiation were calculated using a numerical kinetic Monte Carlo model of a hot planetary corona. The escape flux was estimated to be equal to 2.5×1012cm−2s−1 for the mean level of stellar activity in the XUV radiation flux. This results in the mean estimate of the atmospheric loss rate due to the exothermic photochemistry equal to 6.7×108g s−1. The calculated estimate is close to the observational estimates of the possible atmospheric loss rate for the exoplanet π Men c in the range less than 1.0×109gs−1.

Evaluation of hydrogen absorption cells for observations of the planetary coronas

Newly designed Lyman-alpha absorption cells for imaging hydrogen planetary corona were characterized using an ultra high resolution Fourier transform spectrometer installed on the DESIRS (Dichroïsme Et Spectroscopie par Interaction avec le Rayonnement Synchrotron) beamline of Synchrotron SOLEIL in France. The early absorption cell installed in the Japanese Mars orbiter NOZOMI launched in 1998 had not been sufficiently optimized due to its short development time. The new absorption cells are equipped with the ability to change various parameters, such as filament shape, applied power, H2 gas pressure, and geometrical configuration. We found that the optical thickness of the new absorption cell was ∼4 times higher than the earlier one at the center wavelength of Lyman-alpha absorption, by optimizing the condition to promote thermal dissociation of H2 molecules into two H atoms on a hot tungsten filament. The Doppler temperature of planetary coronas could be determined with an accuracy better than 100 K with the performance of the newly developed absorption cell.

Kinetic Monte Carlo models for the study of chemical reactions in the Earth’s upper atmosphere

A stochastic approach to study the non-equilibrium chemistry in the Earth’s upper atmosphere is presented, which has been developed over a number of years. Kinetic Monte Carlo models based on this approach are an effective tool for investigating the role of suprathermal particles both in local variations of the atmospheric chemical composition and in the formation of the hot planetary corona.

Suprathermal hydrogen produced by the dissociation of molecular hydrogen in the extended atmosphere of exoplanet HD 209458b

The ionization and dissociation of molecular hydrogen by the ultraviolet (UV) radiation of the parent star lead to the formation of hydrogen atoms with an excess of kinetic energy and, thus, are an important source of suprathermal hydrogen atoms in the upper atmosphere of exoplanet HD 209458b. Contemporary aeronomical models did not investigate these processes because they assumed the fast local thermalization of the hot atoms of hydrogen by elastic collisions. However, the kinetics and transfer of these atoms were not calculated in detail, because they require the solving of the Boltzmann equation for a nonthermal atom population. This work estimates the effect of the UV radiation of the parent star and the accompanying photocleacton flux on the production of the suprathermal fraction of atomic hydrogen in the H2 → H transition region. We also consider the formation of the escaping flux of Hatoms created by this effect in the upper atmosphere of HD 209458b. We calculate the production rate and energy spectrum of the hydrogen atoms with excess kinetic energy during the dissociation of H2. Using the numerical stochastic model created by Shematovich (2004) for a hot planetary corona, we investigate the molecular-scale kinetics and transfer of suprathermal hydrogen atoms in the upper atmosphere and the emergent flux of atoms evaporating from the atmosphere. The latter is estimated as 3.4 × 1012 cm−2 s−1 for a moderate stellar activity level of UV radiation, which leads to a planetary atmosphere evaporation rate of 3.4 × 109 g s−1 due to the process of the dissociation of H2. This estimate is close to the observational value of ∼1010 g s−1 for the rate of atmospheric loss of HD 209458b.

Stochastic models of hot planetary and satellite coronas: Total water loss in the Martian atmosphere

Estimates of the total thermal and nonthermal losses of hydrogen and the total nonthermal loss of oxygen from the atmosphere of Mars are discussed, and their ratio is analyzed. It is shown that an H to O ratio of 2:1 has not been achieved in any of the current models of various authors. The closest ratio, H:O = 4:1, has been obtained by Krestyanikova and Shematovich (2006) in the model of formation of a hot oxygen corona.

Stochastic models of hot planetary and satellite coronas: A hot oxygen corona of Mars

The processes of the kinetics and transport of hot oxygen atoms in the transition (between thermosphere and exosphere) region of the upper atmosphere of Mars are studied. The reaction of dissociative recombination of the main ionospheric ion O 2 + with thermal electrons in the ionosphere of Mars is considered as a source of hot oxygen atoms. The distribution of suprathermal oxygen atoms by kinetic energy is calculated. It is shown that the exosphere is populated by a considerable number of suprathermal oxygen atoms with kinetic energies just below the escape energy of 2 eV; that is, a hot oxygen corona of Mars is formed.

Stochastic models of hot planetary and satellite coronas: Atomic oxygen in Europa’s corona

This paper analyzes the formation, kinetics, and transport of hot oxygen atoms in the atmosphere of the Jovian satellite Europa. Atmospheric sources of suprathermal oxygen atoms are assumed to be represented by the processes of dissociation of molecular oxygen, which is the main component of the atmosphere, by solar UV radiation and electron fluxes from the inner magnetosphere of Jupiter, as well as by the reaction of dissociative recombination of the main ionospheric ion O2+ which thermal electrons. It is shown that dissociation in Europa’s near-surface atmosphere is balanced by the processes of the loss of atomic oxygen due to the effective escape of suprathermal oxygen atoms into the inner magnetosphere of Jupiter along the orbit of Europa and due to ionization by magnetospheric electrons and catalytic recombination of oxygen atoms on the icy surface of the satellite. It thus follows that atomic oxygen is only a small admixture to the main atmospheric component—molecular oxygen—in the near-surface part of the atmosphere. However, the outer exospheric layers of Europa’s atmosphere are populated mostly by suprathermal oxygen atoms. The near-surface molecular envelope of Europa is therefore surrounded by a tenuous extended corona of hot atomic oxygen.

Stochastic models of hot planetary and satellite coronas: a photochemical source of hot Oxygen in the upper atmosphere of Mars

The processes of the kinetics and transport of hot oxygen atoms in the upper atmosphere of Mars are studied. A reaction of dissociative recombination of the main ionospheric ion O 2 + with thermal electrons is considered as a photochemical source of suprathermal oxygen atoms. Oxygen atoms are formed in the dissociative recombination reaction with an excess of kinetic energy of about 0.4–4 eV and lose that energy in elastic and inelastic collisions with the ambient thermal atmospheric gas. The altitude distributions of the concentrations of neutral and ionized components, as well as their temperatures, were taken from Krasnopolsky (2002). Unlike the models published earlier, detailed calculations of the formation, collisional kinetics, and transport of suprathermal oxygen atoms in the thermosphere-exosphere transition region of the upper atmosphere of Mars have been made for the first time. For this, we used a stochastic model of the formation of a hot planetary corona (Shematovich, 2004). It has been shown that the considered photochemical source of suprathermal oxygen leads to the formation of the hot corona and to higher nonthermal losses of oxygen from the upper atmosphere of Mars due to escape fluxes. The detailed energy spectra of the fluxes of suprathermal atomic oxygen were calculated for the thermosphere-exosphere transition region of the Martian atmosphere.

Stochastic models of hot planetary and satellite coronas: a photochemical source of hot Oxygen in the upper atmosphere of Mars

Stochastic models of hot planetary and satellite coronas: a photochemical source of hot Oxygen in the upper atmosphere of Mars

Stochastic models of hot planetary and satellite coronas: A photochemical source of hot oxygen in the upper atmosphere of Mars

Stochastic models of hot planetary and satellite coronas: A photochemical source of hot oxygen in the upper atmosphere of Mars

Stochastic Models of Hot Planetary and Satellite Coronas: Suprathermal Nitrogen in Titan's Upper Atmosphere

The processes of dissociation and dissociative ionization of molecular nitrogen by solar UV radiation and by the accompanying flux of photoelectrons, as well as sputtering of the atmosphere by fluxes of magnetospheric ions and pick-up ions, are the main sources of translationally excited (hot, or suprathermal) nitrogen atoms and molecules in Titan's upper atmosphere. Since Titan does not possess an intrinsic magnetic field, ions from Saturn's magnetosphere can penetrate into the outer layers of Titan's atmosphere and sputter atoms and molecules from the atmosphere in momentum-transfer and charge exchange collisions. Atmospheric sputtering by corotating nitrogen ions and carbon-containing pick-up ions, as well as photodissociation-related losses, was considered previously by Lammer and Bauer (1993) and Shematovich et al. (2001, 2003). In this paper we investigate the processes of the formation and evolution of the fraction of suprathermal nitrogen atoms and molecules in the transition region of Titan's upper atmosphere using the previously developed Monte Carlo model for hot satellite and planetary coronas (Shematovich, 1999, 2004). It is established that the suprathermal nitrogen fraction in the transition region of Titan's upper atmosphere includes nitrogen atoms and molecules but the suprathermal nitrogen concentration is relatively small owing to high rates of escape from the atmosphere and to the efficient thermalization of suprathermal nitrogen at the altitudes of the relatively dense lower thermosphere. However, the scale height for suprathermal nitrogen in the transition region is much higher than that for the ambient atmospheric gas. Therefore, suprathermal nitrogen becomes one of the dominant components in the outer exosphere.

Stochastic Models of Hot Planetary and Satellite Coronas

The hot planetary and satellite coronas are populated by the suprathermal particles produced in the transition region between the collision-dominated and free-molecule atmospheric layers under the external effects of electromagnetic and corpuscular solar radiation and magnetospheric plasma. We construct a numerical stochastic model to investigate both the local formation and kinetics of suprathermal particles and their transport to exospheric heights from underlying atmospheric layers. In contrast to other commonly used approaches, the suggested numerical model is suitable for studying the flows of atmospheric gas weakly and strongly perturbed by suprathermal particles, i.e., for studying the formation of hot planetary and satellite coronas proper. Highly efficient Monte-Carlo algorithms with weighted particles underlie the numerical implementation of the model. This numerical model is used to investigate the following: (i) the hot oxygen corona of Europa, a Jovian satellite, which is an example of a highly nonequilibrium near-surface atmosphere; and (ii) the nonthermal losses of nitrogen from Titan, a Saturnian satellite, when suprathermal atoms and molecules of nitrogen are only a small admixture to the surrounding thermal molecular nitrogen—the main atmospheric component of Titan.

Production and imaging of energetic neutral atoms from Titan’s exosphere: a 3-D model

Energetic Neutral Atoms (ENAs) are formed when singly charged magnetospheric ions undergo charge exchange collisions with exospheric neutral atoms. The energy of the incident ions is almost entirely transferred to the charge exchange produced ENAs, which then propagate along nearly rectilinear ballistic trajectories. Thus the ENAs can be used like photons in order to form an image of the energetic ion distribution. The Cassini spacecraft is equipped with the Ion and Neutral Camera (INCA), a magnetospheric imaging ENA camera which is part of MIMI (Magnetospheric Imaging Instrument) [Mitchell, D.G., Cheng, A.F., Krimigis, S.M., Keath, E.P., Jaskulek, S.E., Mauk, B.H., McEntire, R.W., Roelof, E.C., Williams, D.J., Hsieh, K.C., Drake, V.A., 1993. INCA: the ion neutral camera for energetic neutral imaging of the Saturnian magnetosphere. Opt. Eng. 32, 3096; Krimigis, S.M., Mitchell, D.G., Hamilton, D.C. et al., 1998. Magnetospheric Imaging Instrument (MIMI) on the Cassini Mission to Saturn/Titan, Space Sci. Rev., submitted]. In this paper we study the production of energetic neutral atoms resulting from the interaction of Titan’s inner exosphere with Saturn’s magnetosphere. We then simulate the ENA images of this interaction, that we anticipate to get from INCA, by using a 3-D model of the ENA production. This first necessitated the development of a model for the altitude density profile and composition of the Titan exosphere [Amsif, A., Dandouras, J., Roelof, E.C., 1997. Modeling the production and the imaging of energetic neutral atoms from Titan’s exosphere. J. Geophys. Res. 102, 22,169]. We thus used the Chamberlain model [Chamberlain, J.W., 1963. Planetary corona and atmospheric evaporation. Planet. Space Sci. 11, 901] and included the five major species: H, H 2, N, N 2 and CH 4. The density and composition profiles obtained were then used to calculate the ENA production, considering a proton spectrum measured by Voyager in the Saturnian magnetosphere as the parent ion population. In order to generate simulated ENA images of the interaction of Titan’s exosphere with Saturn’s magnetosphere, we developed a model based on 3-D trajectory tracing techniques for the parent ions. Since the parent ions ( E>10 keV) have gyroradii comparable with the Titan diameter, the screening effect of Titan on the parent ion population was also taken into account. This effect results in highly anisotropic ion distributions, which produce ‘shadows’ in the ENA fluxes, in certain directions. These shadows depend on the ENA energy and on the relative geometry of Titan, the magnetic field and the Cassini spacecraft position. The INCA images will thus enable us to remotely sense the ion fluxes and spectra. They are also expected to give information about the magnetic field in the vicinity of Titan and thus to Titan’s interaction with the magnetosphere of Saturn.

Loss of Water on the Young Venus: The Effect of a Strong Primitive Solar Wind

An enhanced primitive solar wind, such as may have prevailed during the first few 100 million years of the solar system history, is shown to have had the potential to stimulate strong thermal atmospheric escape from the young Venus. Due to heating by solar wind bombardment of an extended dense planetary corona, typically 10 times more extensive than the solid planet, an escape flux of pure atomic hydrogen as large as 3 × 10 14cm −2sec −1is found to be possible, provided the solar wind was ≈10 3–10 4more intense than now. Even if escape was diffusion-limited, an enhanced primitive solar UV flux (a factor of ≈5 above present level), absorbed by ≈0.3 mbar of thermospheric water vapor, was able to supply the flow at the required rate. For these high escape rates, oxygen was massively dragged off along with hydrogen, and water molecules could be lost at a rate of ≈6 × 10 13molecules cm −2sec −1. Because, at this rate, a terrestrial-type ocean was completely lost in ≈10 million years, short compared to typical accretion and outgassing times, water was lost “as soon” as it was outgassed. This mechanism could explain the present lack of oxygen in the Venus atmosphere. Because it is expected to affect all sunlike stars in the early phase of planet formation, abiotic oxygen atmospheres could be rare in the universe.

Hydrodynamic escape of hydrogen from a hot water‐rich atmosphere: The case of Venus

A one‐dimensional model of hydrodynamic escape is used to study the loss of hydrogen from a hot, water‐rich atmosphere of the Venus type. A range of EUV heating rates corresponding to the present solar cycle fluctuations of the EUV flux and different possible heating efficiencies are considered. The present model takes into account the transition to the collisionless state at the exobase through a modified Jeans' approach. For the fluid inner planetary corona, the conservation equations are solved from the base of the expanding flow (z ≈ 200 km) up to the exobase, generally located at an altitude of ≈1 planetary radius. Solutions are found, for which the flux in the collisional region is equal to the Jeans escape flux at the exobase. The ionization state of the planetary wind is calculated, and the position of the obstacle is determined by equilibrating the solar wind ram pressure and the pressure of the expanding plasma. It is shown that ≈2/3 of the escape energy is supplied by energetic neutrals formed by charge exchange between escaping H atoms and solar protons in the heliosphere, a fraction of which intercepts the exobase and heats by collision the upper layers of the fluid planetary corona. Any planetary magnetic field pushing away the obstacle up to an altitude larger than ≈3 planetary radii would inhibit this effect. The combination of (1) the geometric amplification of solar EUV absorption at z > ≈2000 km due to the increase with altitude of the cross‐sectional area of the planetary corona and (2) the substantial inward flux of energy at the exobase through heating by energetic neutrals results in the formation of a stable hot region below the exobase, allowing substantial escape. A detailed energetic budget of the escape process is established. An upper limit of ≈3 × 1011 cm−2 s−1 on the escape flux is found. The ratio of the EUV flux to the solar wind strength is shown to be of prime importance, since the solar wind regulates the escape flux from outside, through the action of energetic neutrals, and the EUV flux acts from inside, by supplying atoms with the energy required to lift them up to the exobase.

Plasma Observations at Venus with Galileo

Plasma measurements were obtained with the Galileo spacecraft during an approximately 3.5-hour interval in the vicinity of Venus on 10 February 1990. Several crossings of the bow shock in the local dawn sector were recorded before the spacecraft passed into the solar wind upstream from this planet. Although observations of ions of the solar wind and the postshock magnetosheath plasmas were not possible owing to the presence of a sunshade for thermal protection of the instrument, solar wind densities and bulk speeds were determined from the electron velocity distributions. A magnetic field-aligned distribution of hotter electrons or ;;strahl'' was also found in the solar wind. Ions streaming into the solar wind from the bow shock were detected. Electron heating at the bow shock, </=20%, was notably small, with substantial density increases by factors of 2 to 3 at the day side of the shock that decrease for shock crossings further downstream from the planet. A search for pickup ions from the hot hydrogen and oxygen planetary coronas yielded an upper limit for these densities in the range of 10(-3) ion per cubic centimeter, which is consistent with densities expected from current models of neutral gas densities.

Doppler line profiles in a planetary corona: An extended approach

Measurements of the line profile of the geocoronal Lyman alpha emission by the hydrogen cell aboard Ogo 5 and a marked day‐night asymmetry in the measured intensity distribution have led Bertaux (1974, 1978) to conclude that the population of satellite particles in the exosphere cannot be described by any single value of the satellite critical radius (Chamberlain, 1963). Rather, he concludes (on the basis of a computer simulation of the effect) that Lyman alpha radiation pressure from the sun destroys satellites in a complicated manner which depends upon the initial orbital parameters of each particle and that the satellite critical radius formalism may no longer be a viable way to distinguish a satellite population. He does not, however, consider the alteration of the profile due to proton‐neutral charge exchange when the ionic temperature differs from that of the neutrals (Chamberlain, 1977; Prisco and Chamberlain, 1978), nor does he take into account the fact that the satellite temperature may differ from that of the remaining neutral population. This investigation takes the point of view that Bertaux's interpretation of his data may not be unique. In particular, the influence of these last two effects on the inferred profile (characterized by a ‘reduction factor’) is examined, and several conclusions are reached. First, we find that the perturbation to the reduction factor by charge exchange among the ballistic and escaping populations is negligible. Second, we are able to reproduce the Ogo 5 data satisfactorily by introducing a satellite population characterized by λcs=1.5 and a temperature which equals the neutral temperature beyond about 6 RE but increases inward to about 1.2 times the neutral temperature at 2 RE. We also argue that radiation pressure effects are symmetrical and hence are unlikely to be the cause of the observed asymmetry. Thus the question of how best to describe a satellite population appears still to be open.

Spectral line profiles in a planetary corona: A collisional model

The theory of spectral line profiles for a planetary corona (Chamberlain, 1976) is extended to include the effects of H‐H+ resonant charge exchange in which satellite particles are omitted (Chamberlain, 1977). A computational model is detailed, and sample profiles are presented. Observations of Lyman alpha or Balmer alpha profiles would have an important application to an understanding of the distribution of satellite particles and the nonthermal component of escaping hydrogen. The problem of satellites is seen to merit further investigation.

Charge exchange in a planetary corona: Its effect on the distribution and escape of hydrogen

The theory for a spherical collisionless planetary corona is extended to include charge-exchange collisions between H(+) and H, which are assumed to constitute intermingled gases with different kinetic temperatures. The treatment is based on the conventional concept of a critical level (or exobase) above which the only collisions considered in the Boltzmann equation are those that resonantly exchange charge. Although the geometry treated is an oversimplification for a real planet, numerical examples are given for an idealized earth and Venus. For earth, an ion temperature of 4 times the neutral temperature, an ion density at the exobase of 14,000 per cu cm, and a plasmapause at 1.5 earth radii will raise the escape flux of H by a factor of 6. The total H above the exobase is changed by less than 1%. For Venus, conditions are examined that would account for the peculiar H distribution observed from Mariner 5. The plasma conditions required are not obviously outrageous by terrestrial standards, but the Mariner 5 ionosphere measurements did not show a high plasmapause at, say, 1.25 or 1.5 planetary radii, a fact that might argue against a charge-exchange model.