Simple models of the thermal structure of the Venusian ionosphere

Simple models, analytical and numerical, of plasma temperatures in the Venusian ionosphere are developed. They are applied to thermal-structure data obtained by the Pioneer Venus Orbiter to estimate the magnitudes of important plasma thermal parameters.     

Silicon ion chemistry in the ionosphere

Loss processes which remove Si⁺ ions selectively relative to other meteor-derived atomic ions in the E- and D-regions of the ionosphere have been identified and measured in the laboratory. The major Si⁺ loss in the E-region is the reaction Si⁺ + H₂O → HSiO⁺ + H (1) with a rate constant 2.3 ± 0.9 × 10^?10 cm³s^?1 at 300 K. The corresponding reactions with Fe⁺, Mg⁺ and other metallic meteor ions are endothermic. Presumably (1) is followed by a fast dissociative-recombination with electrons to produce neutral SiO or Si. At lower altitudes Si⁺ ions associate in a three-body reaction with O₂ with a much larger rate constant than the corresponding associations of Fe⁺ and Mg⁺ with O₂.     

The thermal state of the Venusian mantle

Assuming that the surface topography of the Ovda and Thetis plateaus, and that of the saddle region between Thetis and Atla plateaus, is due to surface subsidence of oceanic-type thermal boundary layers, we calculated the temperature at 200 km beneath the plateaus to be about 1600 C, and that beneath the saddle region to be about 1400–1500 C. The total subsidence of Ovda plateau in the last 3/4 of its existence, i.e. between 200 km and 800 km off the postulated ridge axis, suggests that the plateau is probably a young feature, less than 40 m.y. old. The spreading plate models imply a half-spreading rate of 2.3–5.3 cm/yr for the plateaus and 2–2.8 cm/yr for the saddle region.     

Influence of neutral transport on ion chemistry uncertainties in Titan ionosphere

Models of Titan ionospheric chemistry have shown that ion densities depend strongly on the neutral composition. The turbulent diffusion transport conditions, as modeled by eddy coefficients, can spectacularly affect the uncertainty on predicted neutral densities. In order to evaluate the error budget on ion densities predicted by photochemical models, we perform uncertainty propagation of neutral densities by Monte Carlo sampling and assess their sensitivity to two turbulent diffusion profiles, corresponding to the extreme profiles at high altitudes described in the literature. A strong sensitivity of the ion density uncertainties to transport is observed, generally more important than to ion–molecule reaction parameters themselves. This highlights the necessity to constrain eddy diffusion profiles for Titan ionosphere, which should progressively be done thanks to the present and future measurements of the orbiter Cassini.     

The electrostatic field in the ionosphere

Theoretical calculations of the electrostatic field in the ionosphere are presented for different seasons and longitude zones. The corresponding current systems have been shown earlier to give good agreement with the geomagnetic Sq variations. The question of whether the electrostatic field is generated by winds in the ‘dynamo region’ or by other processes higher in the magnetosphere is evaluated in the light of recent observations. Several details of the electrostatic field variation, such as an increase near sunset, are noted.     

Estimates of Venusian atmospheric torque

An estimate is derived of the solar gravitational torque on the thermal atmospheric tide of Venus. The value obtained is compared with the computed torque on the body of the planet itself caused by viscous coupling between it and the superrotating atmosphere. The comparison suggests that the solar thermal torque and the viscous torque are effective in the maintenance of the four-day superrotation of the Venusian atmosphere.UMIST, Department of Physics     

Formation of the plasma mantle in the Venusian magnetosphere

A model of the interaction of the solar wind with Venus is proposed including magnetic barrier formation, ionopause structure, plasma dynamics in the magnetic barrier, and the formation of the Venusian tail (wake). It is shown that under stationary conditions the ionopause is practically an equipotential boundary and its current is determined by a diamagnetic drift. The source of the plasma mantle can be provided by photoions appearing in the magnetic barrier and convecting toward the wake as a result of both magnetic pressure gradient and magnetic tension. The formation of the magnetic tail is determined by convection of magnetic barrier flux tubes in which the solar-wind plasma is replaced by ions of planetary origin. Compared to observational data the proposed model gives somewhat overestimated values of ion convective velocity and magnetic barrier thickness near the terminator and underestimated values of number density and magnetic field strength in the tail. Accordingly this suggests the possible influence of the anomalous ionization effects in the solar wind—Venus interaction.     

The slab thickness of the mid-latitude ionosphere

The thickness of the peak of the ionosphere depends primarily on the temperature T _n of the neutral gas, and corresponds approximately to an α-Chapman layer at a temperature of 0.87T _n. The overall slab thickness, as given by Faraday rotation measurements, is then τ =0.22 _n + 7km. Expansion of the topside ionosphere, and changes in the E-andFl-regions increase τ by about 20 km during the day in summer. Near solar minimum τ is increased by a lowering of the O ⁺/H ⁺ transition height; if the neutral temperature T _n is estimated, this height can be obtained from observed values of τ.Hourly values of slab thickness were determined over a period of 6 yr at 34°S and 42°S. Near solar maximum the night-time values were about 260 km in all seasons. The corresponding neutral temperatures agree with satellite drag values; they show a semiannual variation of 14 per cent and a seasonal change of 5 per cent. Daytime values of τ were about 230 km in winter and 320 km in summer, implying a seasonal change of 30 per cent in T _n. Temperatures increase steadily throughout the day in all seasons, with a rapid post-sunset cooling in summer. Downwards movements produce a large peak in τ at 0600 hr in winter. A large upwards flux, equal to about 40 per cent of the maximum (limiting) value, reduces τ for several hours after sunrise in winter. The slab thickness increases near solar minimum showing a reduction of the O ⁺/H ⁺ transition height to about 700 km in summer and 500 km in winter.     

Net global thermal emission from the Venusian atmosphere

Improved calculations of net emission from the northern hemisphere of Venus are presented. These are based on temperature profiles, water vapor mixing ratio profiles, and cloud models retrieved in 120 solar-fixed latitude-longitude bins from infrared measurements in six spectral channels made over a period of 72 days by the orbiter infrared radiometer (OIR) instrument of the Pioneer Venus mission. Only carbon dioxide, sulfuric acid cloud, and water vapor are considered as significant sources of atmospheric opacity, and the role of the latter component is found to be minor. The sensitivity of the calculations to extreme alternative cloud models, measurement errors, and calibration errors is also discussed. Net emission is found to be only weakly dependent on latitude and longitude during the period of observation with the exception of the high-latitude polar collar region, where emission is low. Mean net emission from the northern hemisphere is 157.0 ± 6.9 W.m^?2, corresponding to an equivalent temperature of 229.4 ± 2.5°K. If this figure is characteristic of the whole planet and if thermal balance is assumed, the bolometric albedo of Venus is 0.762 ± 0.011. This value is consistent with the latest estimates within experimental error.     

The spectrum of electron content fluctuations in the ionosphere

Continuous records of the electron content of the ionosphere, from 1965 to 1970, are used to obtain power spectra covering periods from 30 sec to 2 yr at latitudes of 34°S and 42°S. At periods up to 5 min, amplitudes were less than 0.2 per cent of the total electron content. Variations produced by gravity waves were very common in the range 20–80 min, with no preferred periods. The r.m.s. amplitude per octave A₀ was about 10¹⁵ electrons/m², or 0.6 per cent of the mean electron content. The amplitude increased during the day, particularly in winter when periodic components predominated. The cut-off at about 17 min was sharply defined, giving a mean scale height for the neutral atmosphere (at 300 km) of about 43 km in summer, 47 km on winter days and 42 km on winter nights.

From 12 hr to 1 month A₀ was about 12 per cent of the mean electron content in both summer and winter at 34°S, and 10 per cent at 42°S. The 24 hr and 27 day peaks were largest just before sunspot maximum, and almost disappeared near sunspot minimum. Variations between 1 and 27 days reflect the random occurrence of ionospheric storms and show no consistent peaks. Day to day and night to night variations were both about 10 per cent of the background content for periods from 2 days to 2 yr, apart from a slight decrease between 1 and 6 months.     

On the visibility of bright Venusian fireballs from Earth

The possibility of observing Venusian fireballs from Earth is examined. We estimate the steady-state flux of large, fireball-producing meteoroids at the orbit of Venus, and find that the prospects for observing such events from Earth with small, amateur-sized telescopes are not unreasonable.     

The lower ionosphere of Venus

Galactic cosmic ray bombardment provides a permanent background ionosphere in planetary atmospheres. A transport technique is used to compute the cosmic ray ionization rate profile in a model of the Venusian atmosphere at altitudes between 55 and 100 km. These ionization rates are then applied to a model of ion chemistry to predict equilibrium electron and ion density profiles. Ionization rates for typical solar flare proton events are available from earlier calculations and have been included.     

The lower ionosphere of Titan

Ionization of the atmosphere of Titan by galactic cosmic rays is a very significant process throughout the altitude range of 100 to 400 km. An approximate form of the Boltzmann equation for cosmic ray transport has been used to obtain local ionization rates. Models of both ion and neutral chemistry have been employed to compute electron and ion density profiles for three different values of the H₂/CH₄ abundance ratio. The peak electron density is of the order 10³ cm^?3. The most abundant positive ions are C₂H₉⁺ and C₃H₉⁺, while the predicted densities of the negative ions H^? and CH₃^? are very small (<10^?4 that of the positive ions). It is suggested that inclusion of the ion chemistry is important in the computation of the H and CH₃ density profiles in the lower ionosphere.     

The upper ionosphere of Titan

Photoionization of the upper atmosphere of Titan by sunlight is expected to produce a substantial ionospheric layer. We have solved one-dimensional forms of the mass, momentum, and energy conservation equations for ions and electrons and have obtained electron number densities of about 10³ cm^?3, using various model atmospheres. The significant ions in a CH₄H₂ atmosphere are H⁺, H₃⁺, CH₅⁺, CH₅⁺, CH₃⁺, and C₂H₅⁺. Electron temperatures may be as high as 1000°K, depending on the abundance of hydrogen in the high atmosphere. Interaction of the solar wind with the ionosphere is also discussed.     

The post-terminator ionosphere of Venus

We have modeled the near and post-terminator thermosphere/ionosphere of Venus with a view toward understanding the relative importance of EUV solar fluxes and downward fluxes of atomic ions transported from the dayside in producing the mean ionosphere. We have constructed one-dimensional thermosphere/ionosphere models for high solar activity for seven solar zenith angles (SZAs) in the dusk sector: 90°, 95°, 100°, 105°, 110°, 115° and 125°. For the first 4 SZAs, we determine the optical depths for solar fluxes from 3 Å to 1900 Å by integrating the neutral densities numerically along the slant path through the atmosphere. For SZAs of 90°, 95°, and 100°, we first model the ionospheres produced by absorption of the solar fluxes alone; for 95°, 100°, and 105° SZAs, we then model the ion density profiles that result from both the solar source and from imposing downward fluxes of atomic ions, including O⁺, Ar⁺, C⁺, N⁺, H⁺, and He⁺, at the top of the ionospheric model in the ratios determined for the upward fluxes in a previous study of the morphology of the dayside (60° SZA) Venus ionosphere. For SZAs of 110°, 115° and 125°, which are characterized by shadow heights above about 300 km, the models include only downward fluxes of ions. The magnitudes of the downward ion fluxes are constrained by the requirement that the model O⁺ peak density be equal to the average O⁺ peak density for each SZA bin as measured by the Pioneer Venus Orbiter Ion Mass Spectrometer. We find that the 90° and 95° SZA model ionospheres are robust for the solar source alone, but the O⁺ peak density in the “solar-only” 95° SZA model is somewhat smaller than the average value indicated by the data. A small downward flux of ions is therefore required to reproduce the measured average peak density of O⁺. We find that, on the nightside, the major ion density peaks do not occur at the altitudes of peak production, and diffusion plays a substantial role in determining the ion density profiles. The average downward atomic ion flux for the SZA range of 90–125° is determined to be about 1.2 × 10⁸ cm⁻² s⁻¹.     

Motion of Venusian clouds by the deposition of meteoroids

The theory of superrotation of the Earth's atmosphere by global deposition of meteoroids recently developed by the author (Mitra, 1974) is extended after a slight refinement to explain the rotation period of Venusian clouds. A satisfactory agreement with observations is obtained.     

The horizontal propagation of Pc1 pulsations in the ionosphere

The propagation properties of the various elements of the plane-wave angular spectrum of a Pc1 pulsation signal in the ionosphere are determined by a full-wave numerical analysis. A spectral component is characterized by the wave-vector azimuthal direction, and the Snell constant S. The isotropic R-mode transmission coefficient to ground is fairly flat for S ? 400, but thereafter (S > 500) drops rapidly with increasing S. Coupling of energy from the field-guided L-mode to the R-mode occurs along the entire length of the L-mode trajectory within the ionospheric duct in which the R-mode can propagate. Within the duct, the R-mode attenuation is determined largely by R to L-mode coupling, which is larger for E-W than for N-S azimuths, especially for steep angles of incidence (S < 100). This should lead to enhanced injection of energy into E-W high altitude, high velocity paths, but to higher E-W attenuation at oblique angles. For oblique propagation (S ? 200) horizontal group velocities are slightly higher than the Alfvén phase velocity at the F-layer peak, but about twice as high for steep angles (S ≈ 100).     

The penetration of soft electrons into the ionosphere

Calculations are presented of energy spectra and angular and spatial distributions of electron fluxes in the ionosphere resulting from precipitation ofmonoenergetic (E = 25, 50 and 100 eV) electrons. The incident electrons are assumed to be isotropic over the downward direction. It is found that the resulting steady-state electron fluxes above ca. 300 km are highly anisotropic, and that the pitch angle distribution is energy dependent. About 15 per cent of the incident electrons are backscattered elastically to the protonosphere. A much larger number of electrons escape after they have deposited a part of their energy in the atmosphere. The mean energy of the escaping electrons is about half that of the incident electrons. About 50% of the incident energy is absorbed in the atmosphere, the remainder being returned to the protonosphere. The rate of absorption of energy is a maximum at heights between 300 and 400 km. Most of the energy is absorbed in ionization and excitation of atomic oxygen. An appreciable amount of energy is, however, absorbed as heat by the ambient electron gas. Altitude profiles are presented of the rates of ionization, excitation, and electron heating caused by soft electron precipitation.     

Temperature domains in the ionosphere

The non-linear stationary temperature waves (domains) is analysed. The exact analytical solutions of the non-linear equation of the heat conductivity determine the region of existence of such domains and the critical values of plasma parameters, correspond to the increase of the periodical temperature profiles in the plasma. A stationary source of heating (photo-electrons or electric fields) may stimulate the existence of domains, when the power of the source reaches a critical value. Conditions in the F-region of the ionosphere near the equator favour the increase of the domains.