I.R. Radiation of the upper atmosphere

A theory of the i.r. radiation (2–20 μ) of the upper atmosphere (90–250 km height) has been developed. It includes the calculation of concentrations and temperatures as well as the analysis of atomic and molecular level population kinetics. Various excitation and quenching processes are analysed. Results are given for the following bands: NO (5.3μ), NO⁺(4.3μ.), CO (4.7 μ), N¹⁴N¹⁵ (4.4 μ), CO₂(4.3 and 15 μ), H₂O(2.7 and 6.3 μ), N₂O(4.5; 7.8 and 17μ), O₃(9.6 and 14.4 μ). The energy aspect of the problem is discussed. It is found that at a height of 120 km intensity in the region of 2 to 20 μ 3 to 10 is that of the 63 μ line of atomic oxygen. The comparison of theory with the experiment was carried out and satisfactory agreement obtained. The correlations of intensities in i.r. bands and emissions in visible and u.v. spectra were considered.     

Thermal structure of Uranus' atmosphere.

Application of a radiative-convective equilibrium model to the thermal structure of Uranus' atmosphere evaluates the role of hazes in the planet's stratospheric energy budget and places a lower limit on the internal energy flux. The model is constrained by Voyager and post-Voyager observations of the vertical aerosol and radiative active gas profiles. Our baseline model generally reproduces the observed tropospheric and stratospheric temperature profile. However, as in past studies, the model stratosphere from about 10(-3) to 10(-1) bar is too cold. We find that the observed stratospheric hazes do not warm this region appreciably and that any postulated hazes capable of warming the stratosphere sufficiently are inconsistent with Voyager and ground-based constraints. We explore the roles played by the stratospheric methane abundance, the H2 pressure-induced opacity, photochemical hazes, and C2H2, and C2H6 in controlling the temperature structure in this region. Assuming a vertical methane abundance profile consistent with that found by the Voyager UVS occultation observations, the model upper stratosphere, from 10 to 100 microbar, is also too cold. Radiation in the 7.8-micrometers band from a small abundance of hot methane in the lower thermosphere absorbed in this region can warm the atmosphere and bring models into closer agreement with observations. Finally, we find that internal heat fluxes < or approximately 60 erg cm-2 sec-1 are inconsistent with the observed tropospheric temperature profile.     

High-resolution calculations of asteroid impacts into the Venusian atmosphere.

We present results from a number of 2D high-resolution hydrodynamical simulations of asteroids striking the atmosphere of Venus. These cover a wide range of impact parameters (velocity, size, and incidence angle), but the focus is on 2-3 km diameter asteroids, as these are responsible for most of the impact craters on Venus. Asteroids in this size range are disintegrated, ablated, and significantly decelerated by the atmosphere, yet they retain enough impetus to make large craters when they meet the surface. We find that smaller impactors (diameter <1-2 km) are better described by a "pancaking" model in which the impactor is compressed and distorted, while for larger impactors (>2-3 km) fragmentation by mechanical ablation is preferred. The pancaking model has been modified to take into account effects of hydrodynamical instabilities. The general observation that most larger impactors disintegrate by shedding fragments generated from hydrodynamic instabilities spurs us to develop a simple heuristic model of the mechanical ablation of fragments based on the growth rates of Rayleigh-Taylor instabilities. Although in principle the model has many free parameters, most of these have little effect provided that they are chosen reasonably. In practice the range of model behavior can be described with one free parameter. The resulting model reproduces the mass and momentum fluxes rather well, doing so with reasonable values of all physical parameters.     

Peculiarities of convective motions in the upper solar atmosphere. I

A convective field of intensities and velocities between the levels of continuum origination and the temperature minimum is investigated based on spectral observations of iron lines performed near the center of the solar disc using the 70-cm German vacuum tower telescope (VTT) located at del Teide observatory of the Institute of Astrophysics on the Canaries (Tenerife island). Convective elements in the process of their upward and downward motion change with height not only the sign of the relative contrast but also the direction of motion. The height at which this reversal occurs strongly depends on the velocity and contrast of the convective elements which they had at the level of continuous spectrum formation. On average, the reversal of the velocity takes place at the height of 240 ± 130 km, and that of the contrast occur at the height of 200 ± 65 km.     

Photochemistry of the Martian atmosphere

A variety of models are explored to study the photochemistry of CO₂ in the Martian atmosphere with emphasis on reactions involving compounds of carbon, hydrogen, and oxygen. Acceptable models are constrained to account for measured concentrations of CO and O above 90 km, with an additional requirement that they should be in accord with observations of CO, O₂, and O₃ in the lower atmosphere. Dynamical mixing must be exceedingly rapid at altitudes above 90 km, with effective eddy diffusion coefficients in excess of 10⁷ cm² sec^?1. If recombination of CO₂ is to occur mainly by gas phase chemistry, catalyzed by trace quantities of H, OH, and HO₂, mixing must be rapid over the altitude interval 30 to 40 km. The value implied for the diffusion coefficient in this region is a function of assumptions made regarding the rates for reaction of OH with HO₂ to form H₂O and of the rate for reaction of HO₂ with itself to form H₂O₂. If rates for these reactions are taken to have values similar to rates used in current models for the Earth's stratosphere, the eddy diffusion coefficient at 40 km on Mars should be about 5 × 10⁷ cm² sec^?1, consistent with Zurek's (1976) estimate for this parameter inferred from tidal theory. Surface chemistry could have an influence on the abundances of atmospheric CO and O₂, but a major effect would imply sluggish mixing at all altitudes below 50 km and in addition would carry implications for the magnitude of the rates for reaction of OH with HO₂ and HO₂ with itself.     

Seismology of the solar atmosphere

We describe a new instrument for seismically probing the properties of the Sun's lower atmosphere, and present some first results from an observational campaign carried out at the geographic South Pole during the austral summer of 2002/2003. A preliminary analysis of the data (simultaneous, high-cadence observations of the velocity signals from the photosphere and low chromosphere) shows that the well-known suppression of acoustic power in regions of strong magnetic field, and enhancement of high-frequency power around active regions (acoustic halos), are both consistent with a spreading out of the magnetic field lines with increasing height in the atmosphere. The data have also revealed some unexpected wave behavior. First, evanescent-like waves are found at frequencies substantially above the acoustic cut-off frequency in regions of intermediate magnetic field. Second, upward- and downward-propagating waves are detected in areas of strong magnetic field such as sunspots and plage: even at frequencies below the acoustic cut-off frequency. Third, the wave behavior in regions of strong magnetic field can change over periods of a few hours from propagating to evanescent. While we have no concrete explanation for the first two results, the latter result opens up the question of whether sound waves are involved in short-term events such as flares or CME's.     

Structure of the Venus atmosphere

The structure of the Venus atmosphere is discussed. The data obtained in the 1980s by the last Soviet missions to Venus: orbiters Venera 15, 16 and the entry probes and balloons of Vega 1 and 2 are compared with the Venus International Reference Atmosphere (VIRA) model. VIRA is based on the data of the extensive space investigations of Venus in the 1960s and 1970s. The results of the IR Fourier Spectrometry experiment on Venera 15 are reviewed in detail. This instrument is considered as a precursor of the long wavelength channel of the Planetary Fourier Spectrometer on Venus Express.     

Formation of the lunar atmosphere

Measurements of⁴⁰Ar and helium made by the Apollo 17 lunar surface mass-spectrometer are used in the synthesis of atmospheric supply and loss mechanisms. The argon data indicate that about 8% of the⁴⁰Ar produced in the Moon due to decay of⁴⁰K is released to the atmosphere and subsequently lost. Variability of the atmospheric abundance of argon requires that the source be localized, probably in an unfractionated, partially molten core. If so, the radiogenic helium released with the argon amounts to 10% of the atmospheric helium supply. The total rate of helium escape from the Moon accounts for only 60% of the solar windα particle influx. This seems to require a nonthermal escape mechanism for trapped solar-wind gases, probably involving weathering of exposed soil grain surfaces by solar wind protons.     

Some properties of convective motions in the upper solar atmosphere. II

Using a three-dimensional hydrodynamical model of the solar atmosphere, we calculated profiles of the Fe I λ 639.361 nm and Fe II λ 523.462 nm lines with consideration for deviations from the local thermodynamic equilibrium. We determined heights for intensity contrast reversal and for velocity sign reversal of convective elements. Both of these parameters depend strongly on the convective velocity and intensity measured in the continuum. The larger the parameters, the greater is the atmosphere altitude where the reversal takes place. We compared all the calculated relations with the observations obtained at the German Vacuum Tower Telescope in Izana (Tenerife, Spain). In our opinion, the 3D hydrodynamical model of the solar atmosphere satisfactorily describes all the main features of observed convective velocities and intensities.     

The rotation of the solar atmosphere

A model of the solar atmosphere is presented in which we discuss the conservation of angular momentum for the two basic states in which the solar gas can be: namely, either confined by closed field lines or outflowing along open magnetic field lines. It can be shown that the boundary conditions are in general different for these two cases. From this we obtain the results that in the closed configuration the gas can corotate at the solar surface with the magnetic field lines and its angular velocity will then increase with height, whereas for a gas flowing along an open field line the angular velocity will decrease. An exception to the latter case can be found where the open magnetic field lines are strongly nonradial and where the density is a slowly varying function of radius. In such regions the angular velocity may initially increase with height, reach a maximum and then decrease.Kitt Peak National Observatory Contribution No. 439.Operated by The Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Germane in the atmosphere of Jupiter

High-altitude spectra of Jupiter obtained from the Kuiper Airborne Observatory are analyzed for the presence of germane (GeH₄) in Jupiter's atmosphere. Comparison with laboratory spectra shows that the strong Q branch of the ν₃ band of germane at 2111 cm^?1 is prominent in the Jovian spectra. The abundance of germane in Jupiter's atmosphere is 0.006 (±0.003) cm-am corresponding to a mixing ratio of 0.6 ppb. This trace amount of germane is consistent with chemical equilibrium calculations if the germane present at ～1000°K is carried up by convection to the spectroscopically observable region at ～300°K.     

Models of the open solar atmosphere

McWhirter et al. (1975) have presented a standard model for the transition region and inner corona that matches with the Harvard Smithsonian Reference Atmosphere. They assume an open field line configuration and solve numerically the equations of energy and hydrostatic equilibrum. The purpose of the present paper is to generalise their model for the temperature and density as functions of height in several ways and, in particular, to determine the temperature maxium and its location. The effect of varying the following characteristics of the model is determined:

1. Boundary conditions on temperature and density;
2. magnitude of the heating;
3. form of the heating term;
4. divergence of the field lines;
5. presence of subsonic flows, either upward or downward.

If the heating is localised at great altitudes, it tends to produce a narrower and larger temperature maximum at a greater altitude than a uniform heating and even more so than a heating proportional to density. For fixed base conditions, an increase in heating or field line divergence or downflow decreases the coronal temperature and reduces the height of the temperature maximum, while a steady upflow has the opposite effects. A maximum possible upflow was found, beyond which a catastrophe occurs so that no steady hot solution exists.     

Potassium in the atmosphere of Mercury

The discovery of potassium in the atmosphere of Mercury is reported. The average column density is approximately 10⁹ atoms cm⁻² column, which is about one percent of the average sodium column density.     

The structure of the Uranus atmosphere

A series of radiative/connvective models is presented for the Uranus atmosphere for various methane-to-hydrogen mixing ratios and internal heat fluxes. The variation of flux through the atmosphere, which is largely defined by absorption of sunlight in methane bands, the partial pressure of methane, which is taken to be limited by saturated vapor pressure, and the temperature structure are all constrained to be self-consistent. From model spectra calculated for the visible, thermal infrared, and microwave regions, it is concluded that the methane-to-hydrogen mixing ratio is greater than 0.01 and probably less than 0.10. The lower limit to the internal heat flux is nonzero but less than ～1/2000th of the total flux. In addition, the specific heat of the molecular hydrogen is found to be very close that that for normal hydrogen, as suggested previously by Trafton. Peculiarities in thermal structure are found to be of no help in understanding the microwave spectrum, but H₂S-to-NH₃ mixing ratios somewhat greater than unity are almostt as good in explaining the spectrum as the precisely unity case ey S. Gulkis, M. A. Janssen and E. T. Olsen (1978, Icarus34, 10–19).     

Physical parameters of the martian atmosphere

The following physical parameters have been computed from 0 to 200 km altitude; (1) pressure, (2) density (3), speed of sound, (4) density, (5) number density, (6) mean free path, (7) viscosity, (8) pressure scale, (9) mean particle velocity, (10) collisional frequency and (11) columnar mass.The Viking measurements have been used as input data. A critical comparison of the computed pressures and densities, is given, useful for future explorations.     

Physical parameters of the Jovian atmosphere

The following physical parameters have been computed for the Jovian atmosphere between 270 and ?300 km: (1) Pressure, (2) Density, (3) Speed and sound, (4) Number density, (5) Density scale. It has considered that the top of the clouds is at 0 km. For the calculations of these parameters we have used:

1. for the altitudes 270-0 km data from Voyager I and II.
2. for the altitudes ?300–0 km data from Voyager II and spectroscopic observations.

     

Currents in the cometary atmosphere

If the structure of the magnetic field and electric current in the cometary type I tail can be represented by an electric current circuit, disruption of the cross-tail current system may lead to a current discharging through the cometary ionosphere, and the dissipation of the magnetic energy stored in the tail. From the point of view of energy budget, a tail-aligned magnetic field on the order of 10γ will be sufficient to produce a strong ionization effect of the cometary atmosphere.