Pioneer Venus Radio Occultation Research Articles

Radio Scintillations in Venus's Atmosphere: Application of a Theory of Gravity Wave Generation

Radio scintillations in Pioneer Venus radio Occultation data are simulated assuming that the index of refraction fluctuations in Venus's atmosphere responsible for the scintillations are directly caused by gravity wave fluctuations. The gravity waves are created by a global convection layer between 50- and 55-km attitude in Venus's atmosphere and propagate vertically. The authors compare the simulated scintillations with data from Pioneer Venus. These gravity waves can explain the spectral shape and amplitude of the radio scintilations. The shape at high frequencies is controlled by wave breaking, which yields a saturated spectrum. The amplitude is subject to parameters such as the intensity of the convection, the angle between the zonal winds and the beam path, and the zonal wind profile at polar latitudes. To match the observed amplitude of the scintillations, the velocity variations of the energy-bearing eddies in the convection must be at least 2 m s−1. This value is consistent with the Venus balloon results of Sagdeev et al. and is in the middle of the range considered by Leroy and Ingersoll in their study of convectively generated gravity waves. The later study, combined with the lower bound on velocity from the present study, then yields lower bounds on the vertical fluxes of momentum and energy in the Venus atmosphere.

The Downward Flux of O + over the Nightside of Venus

We have constructed a map of the downward flux of O + over the nightside of Venus at high and low solar activities through a combination of modeling and analysis of Pioneer Venus ion mass spectrometer data. O + density profiles were obtained for almost 400 inbound or outbound segments of orbits from the first 2 years of the mission and about 40 more in the recent reentry phase of the mission. We have determined the nearly linear relationship between the O + maximum density and the downward O + flux for several solar zenith angles and local times by constructing models of the nightside ionosphere of Venus for a range of downward ion fluxes at the upper boundaries. We find that the largest downward fluxes occur near the terminators, and the fluxes fall off sharply toward the antisolar point. Although the standard deviations in the data are large, there is a suggestion of a local maximum near 155°, and the location of this maximum correlates fairly well with structure in the peak electron density as a function of solar zenith angle reported for the Pioneer Venus radio occultation experiment. The average downward ion flux is inferred to be about 1.7 × 10 8 cm -2 sec -1 over the nightside hemisphere at solar maximum and a factor of 7 less at moderately low solar activity.

Model calculations of the dayside ionosphere of Venus at solar minimum

The dayside ionosphere of Venus at solar minimum was studied using one‐dimensional MHD model. The observed electron density profiles were reproduced reasonably well, assuming that the ionosphere of Venus at solar minimum is magnetized by the solar wind. A bump or a ledge structure of the electron density profile at the altitude range of 170–200 km observed by the Pioneer Venus radio occultation measurements were also reproduced under magnetized conditions. It was found that the bump structure is more apparent and more realistic for lower electron temperatures, suggesting that electron temperatures at low altitudes (<200 km) significantly decrease from a solar maximum period to a solar minimum period.

Magnetization of the ionospheres of Venus and Mars: Results from radio occultation measurements

In situ measurements by the Pioneer Venus orbiting spacecraft, conducted during solar maximum only, have shown that magnetization (permeation of large‐scale magnetic fields) of the ionosphere of Venus occurs under high solar wind dynamic pressure and that this takes place most frequently near the subsolar region. Magnetization is also revealed in electron density profiles by the presence of disturbed topside plasma and/or a ledge near the altitude of 200 km. In this paper we use remote sensing radio occultation measurements to study magnetization of the ionospheres of Venus and Mars based on these characteristics. For Venus we take advantage of the unique data set consisting of 148 electron density profiles deduced from Pioneer Venus radio occultation measurements. We demonstrate that radio occultation measurements yield results on frequency of occurrence of magnetization during solar maximum that are similar to those obtained from the Pioneer Venus in situ magnetic field measurements. During solar minimum, for which direct ionospheric measurements have never been made, we find that magnetization of the Venus ionosphere is more pervasive than at solar maximum. Magnetization extends to higher solar zenith angles (SZA) and appears stronger than at solar maximum. These results confirm that during solar minimum, the high solar wind dynamic pressure state is more prevalent at Venus because the ionospheric plasma pressure is weaker than at solar maximum. Comparison of a large number of electron density profiles of Mars (deduced from radio occultation measurements by the Viking 1 and 2 and Mariner 9 spacecraft for SZA > 46°) with those of Venus shows an absence of the ledge and disturbed topside plasma observed in the Venus profiles. These results, however, do not constitute evidence against magnetization of the ionosphere of Mars, as Shinagawa and Cravens (1989) have shown in their one‐dimensional MHD models that, even when the ionosphere of Mars is highly magnetized, the magnetic structure differs from that at Venus, and a ledge does not form in its electron density profiles.

The effect of the solar cycle on the maintenance of the nightside ionosphere of Venus

Pioneer Venus radio occultation measurements of the nightside ionosphere of Venus collected from 1979 to 1986 have made it possible to study its behavior at times of both solar maximum and solar minimum. Although some solar maximum measurements are similar in nature to those observed at solar minimum, which have an average peak density of about 7×10³ cm−3, others show much higher peak densities, reaching values of about 4×104 cm−3. These elevated peak densities also occur at higher altitudes. The integrated electron column densities for these measurements are also much higher, indicating the presence of substantial ionization above the main peak. The magnitudes of both the peak density and the integrated content above the peak are anticorrelated with the solar wind dynamic pressure, leading to the interpretation that these enhancements during solar maximum are due to transterminator transport of O+ ions from the dayside when the solar wind dynamic pressure is low enough to permit a sufficiently high dayside ionopause. The resulting ionization peak can be many times the concentration produced by energetic electron fluxes impacting the neutral atmosphere on the nightside, which apparently form the remaining source of the nightside peak at such times during solar maximum, when transterminator flow is cut off by high solar wind pressure depressing the dayside ionopause, and during solar minimum, when the ionopause is always depressed.

Results for 13-cm absorptivity and H 2SO 4 abundance profiles from the season 10 (1986) Pioneer Venus Orbiter radio occultation experiment

Recent radio astronomical observations of Venus have suggested that the abundance and distribution of sulfur-bearing gases below the main cloud layer may vary significantly both in time and with position. To determine whether this is the case for sulfuric acid vapor (H 2SO 4), data from 23 Pioneer Venus Orbiter radio occultation experiments have been processed to obtain 13-cm absorptivity profiles. The data were obtained from radio occultation season 10 (late 1986 and early 1987). During the course of this season the occultation geometry allowed the spacecraft signal to probe the atmosphere over a wide range of latitudes in the northern hemisphere, and to altitudes as low as 39.4 km (above a mean radius of 6051.92 km). Since sulfuric acid vapor is the most opaque constituent in Venus' atmosphere at this wavelength, these profiles have been used to infer upper limits for abundance profiles of sulfuric acid vapor. Error bars have been placed on the absorptivity and abundance profiles by considering the propagation of statistical uncertainties through the steps of the signal inversion process (as per B. Lipa and G. L. Tyler, 1979, Icarus 39, 192–208). The results confirm the presence of statistically significant variations in the abundance and distribution of gaseous H 2SO 4 between the equatorial latitudes (11° to 25°N) and latitudes above 36°N. The results also suggest that the abundance of sulfuric acid vapor may have declined since the first Pioneer Venus radio occultation experiments in 1979, but additional work is needed to determine if the variations are statistically significant.

Solar-cycle changes in the thermal structure of the Venus dayside ionosphere

Vertical profiles of electron density in the dayside ionosphere of Venus, measured by the Pioneer Venus radio occultation experiment, averaged in bins of solar zenith angle, have been used to examine the behavior of the ionospheric thermal structure with the solar cycle. It was found that the plasma scale height decreased by a factor ranging from 1.5 at 190 km to about 3.0 at 300 km altitude as the solar input went from maximum to minimum ( F 10.7 at 1 a.u. changed from 226 to 76). If the assumption of diffusive equilibrium holds above h = 180 km for both the times of solar maximum and solar minimum, similar decreases in the plasma temperature and, consequently, the electron and ion temperatures, are inferred. If, however, diffusive equilibrium conditions do not apply at the time of solar activity minimum, perhaps due to the effects of enhanced solar wind interaction, no information on the plasma temperatures is contained in the data on the topside electron density structure.

Zonal Winds and the Angular Momentum Balance of Venus’ Atmosphere within and above the Clouds

Temperatures and pressures inferred from radio occultation data acquired by the Pioneer Venus orbiter between September 1982 and November 1983 are used to derive cyclostrophic zonal winds in the middle atmosphere of Venus (1350 to 2.1 mb, 10 deg to 70 deg latitude). The main feature of the wind field is a jet positioned just above the cloud tops at 70 km and approximately 48 deg latitude. The maximum speed of the jet is about 130 m/s. A comparison with results of similar analyses on Pioneer Venus radio occultation data obtained between December 1978 and October 1981 suggests an equatorward shift of the jet and a decrease in jet speed during this five-year time interval. It is proposed that the poleward transport of westward zonal momentum by the upper branch of the cloud level Hadley cell supplies the excess momentum of the jet and maintains it against dissipation. The location of the jet thereby provides a minimum estimate of the latitudinal extent of the Hadley cell. Cyclostrophic zonal wind velocities decrease with height above about 70-75 km. It is suggested that this deceleration of the superrotation in equatorial latitudes is due to the dissipation of vertically propagating thermal tides forced primarily at altitudes around 65 km.

Venus atmospheric circulation: Observations and implications of the thermal structure

Continued analysis of Pioneer Venus imaging and polarimetry data indicates that the average cloud-top level circulation is mainly zonal (east to west) with a small meridional component. Presence of planetary scale waves and a possible sun-related component are evident in the data. If the tracked features refer to the same vertical level, then some variability of the circulation would have to be present to account for the Pioneer and Mariner 10 cloud-tracking results. However, the implied balanced flow from the observed thermal structure analysis strongly suggests that at least some of the variations in these observations is due to apparent cloud-top variations and that the circulation itself is relatively stable. Direct cyclostrophic calculations based on the observed thermal structure of the atmosphere yield a balanced zonal circulation with distinct mid-latitude jets (peak velocities about 110–120 ms −1) located between 50 and 40 mb in each hemisphere of the planet near 45° latitude. The calculations which extend to about 40 km altitude from 80 km above the surface agree well with the observed entry probe zonal components and indicate breakdown of the balance condition near the upper and lower boundaries at low latitudes. The balanced flow results are consistent with the Mariner 10 and Pioneer cloud tracked estimates of the zonal circulation provided the effective altitude of the tracked features is slightly different at different observation periods. The features in the Pioneer Venus data would then lie on a sloping surface that extends from about 68 km (40 mb) at low latitudes to about 75 km (10 mb) in mid-latitudes. The polarization features would occur on a roughly parallel surface that is 1–2 km above the effective cloud-height surface, and Mariner 10 features would have effective altitudes somewhat lower than the Pioneer ultraviolet features. A slight asymmetry is evident in the balanced zonal circulation arising out of an asymmetry in the thermal field. Finally, the solenoids formed by intersecting isobaric and isosteric (constant specific volume) surfaces deduced from the Pioneer Venus radio occultation data show distinct evidence of a direct meridional circulation that may be important in sustaining the Venus atmospheric circulation.

Zonal Winds in the Middle Atmosphere of Venus from Pioneer Venus Radio Occultation Data

The present investigation is concerned with the derivation of zonal winds for the middle atmosphere (40-80 km) of Venus, using Pioneer Venus (PV) radio occultation data. The considered altitude range is of particular interest because the atmospheric circulation of Venus is uncertain above the cloud tops (65-70 km). The Pioneer Venus orbiter began circling the planet every 24 hours on December 4, 1978. Four seasons of occultation data have now been collected. It is found that the assumption of cyclostrophic balance, in conjunction with radio occultation derived temperature data, yields a zonal wind field for most of the middle atmosphere of Venus. An intense midlatitude jet at and above the cloud tops is the most notable feature of the winds.

Eddy diffusion coefficient for the atmosphere of Venus from radio scintillation measurements

Estimates of the vertical mass eddy diffusion coefficient K are necessary to understand the dynamics and photochemistry of a planetary atmosphere. Most previous estimates for the atmosphere of Venus1–10, pertain to the upper atmosphere. In the lower atmosphere (≲80 km), information on small-scale (≲1 km) temperature fluctuations has been provided by radio scintillations observed during radio occultation by the atmosphere11–18. Here we relate the vertical mass eddy diffusion coefficient to these radio scintillation observations so that reliable estimates can be obtained near the tropopause level of 60 km for the first time. Using the recent Pioneer Venus radio occultation measurements16, and assuming that the turbulence is generated by shear instability, we find that K = 4 × 104 cm2 s−1, the energy dissipation rate ɛ = 20 cm2 s−1, and the dissipation rate of temperature fluctuations Nθ = 10−3 K2 s−1. These results indicate that small-scale turbulence probably plays a major part in vertical transport near the tropopause of Venus.

Radio occultation measurements of turbulence in the Venus atmosphere by Pioneer Venus

As in the cases of Mariner 5 and 10 and Venera 9, the Pioneer Venus radio occultation measurements of Venus show an upper region of turbulence located in the vicinity of 60 km. Estimates of the deduced intensity of turbulence are consistent with the upper bound obtained earlier from the complementary Pioneer Venus probe measurements. Comparison with the Pioneer Venus in situ temperature measurements of 1 km scale size also shows very good agreement. It is clear that for scale sizes smaller than the Fresnel size (∼1 km), the refractive index irregularities are represented by a continuum of scale sizes having a spatial wave number spectrum that is power law and close to Kolmogorov. These irregularities are elongated in the horizontal direction (axial ratios are large, ≳10). The fact that the upper region of turbulence coincides with a region of high stability marked by one or more peaks in the stability profile suggests that the turbulence is associated with trapped gravity waves. The amplitude scintillations appear to increase poleward of about 70° which is interpreted as an increase in turbulence level with latitude.