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NASA NTRS1979

Electron temperatures and densities in the Venus ionosphere - Pioneer Venus orbiter electron temperature probe results

The Pioneer Venus orbiter electron temperature probe was used to obtain altitude profiles of electron temperature and density in the ionosphere of Venus. Elevated temperatures at times of low solar wind flux might indicate support for a certain model. According to this model, less than 5% of the solar wind energy is deposited at the ionopause and is conducted downward through an unmagnetized ionosphere to the region below 200 km where electron cooling to the neutral atmosphere proceeds rapidly. The patterns of electron temperatures and densities at higher solar wind fluxes are considered, the variability of the ionopause height in the late afternoon is noted, and the role of an induced magnetic barrier in the neighborhood of the ionopause is discussed.

Brace, L. H.↗

NASA NTRS1979

The polar ionosphere of Venus near the terminator from early Pioneer Venus orbiter radio occultations

Fourteen profiles of electron density in the ionosphere of Venus were obtained by the dual-frequency radio occultation method with the Pioneer Venus orbiter between 5 and 30 December 1978. A region of almost constant electron density above approximately 250 km was detected. The ionopause height is found to vary from about 300 to 700 km. The structures of the profiles are consistent with models in which O2(+) dominates near the ionization peak and is replaced by O(+) at higher altitudes.

Kliore, A. J.↗

NASA NTRS1979

Venus upper atmosphere neutral composition - Preliminary results from the Pioneer Venus orbiter

Measurements in situ of the neutral composition and temperature of the thermosphere of Venus are being made with a quadrupole mass spectrometer on the Pioneer Venus orbiter. The presence of many gases, including the major constituents CO2, CO, N2, O, and He has been confirmed. Carbon dioxide is the most abundant constituent at altitudes below about 155 kilometers in the terminator region. Above this altitude atomic oxygen is the major constituent, with O/CO2 ratios in the upper atmosphere being greater than was commonly expected. Isotope ratios of O and C are close to terrestrial values. The temperature inferred from scale heights above 180 kilometers is about 400 K on the dayside near the evening terminator at a solar zenith angle of about 69 deg. It decreases to about 230 K when the solar zenith angle is about 90 deg.

Niemann, H. B.↗

NASA NTRS1979

Infrared image of Venus at the time of Pioneer Venus probe encounter

An image of the infrared emission from the Earth-facing hemisphere of Venus was obtained at the time the Pioneer Venus probes penetrated the atmosphere. The thermal structure of the atmosphere at the 85-millibar level included regions of rapidly varying polar features, a solar-related postdawn warm area, and a nonsolar-fixed nighttime warm area. The probes succeeded in entering each of these three thermal regions.

Apt, J.↗

NASA NTRS1979

Composition and structure of the Venus atmosphere - Results from Pioneer Venus

The composition of the Venusian atmosphere was studied using a mass spectrometer on the Pioneer Venus sounder probe. The single-focusing magnetic-sector spectrometer scanned the mass range from hydrogen through mercury with a dynamic range of six decades. Data taken by the mass spectrometer were compared with those of a gas chromatograph, resulting in slight discrepancies due to the use of a sputter ion pump acting as a sink for entry of rare gases through the inlet leak. A surprisingly large concentration of primordial Ar-36 and Ar-38 was discovered in a ratio of 5 to 1. It was concluded that the large excess of primordial argon was a valid result and that the mixing of HCl in the lower Venusian atmosphere was less than a few parts per million. Arguments against the sun as the source for excess primordial gases on Venus were presented. Concentrations of other elements such as Ne, Kr, He, S, and O were discussed. Although the mass peaks in the spectrum were real, it was not clear whether all of the chemical reactions (i.e., COS production) actually occurred in the atmosphere. Until further analysis can be made, it will be uncertain how the inlet system, which is at atmospheric temperature, affected the results.

Hoffman, J. H.↗

NASA NTRS1980

Data acquisition for measuring the wind on Venus from Pioneer Venus

The data acquisition and processing technqiues used in the Pioneer Venus differential long baseline interferometry experiment are discribed. The experiment was designed to measure the motion in three dimensions of the Pioneer probes during their fall to the surface of Venus, using a combination of Doppler and long baseline ratio interferometric methods. The design of the experiment and the equipment and software techniques that were developed specially for this experiment are also described.

Smith, J. R.↗

NASA NTRS1979

Position and shape of the Venus bow shock - Pioneer Venus Orbiter observations

Magnetometer data from the Pioneer Venus Orbiter is used to examine the position and shape of this planet's bow shock. Utilizing crossings identified on 86 occasions during the first 65 orbits a mean shock surface is defined for sun-Venus-satellite angles of 60-110 deg. Both the shock shape and variance in location are found to be very similar to the terrestrial case for the range in SVS angle considered. However, while the spread in shock positions at the earth is due predominantly to the magnetopause location varying in response to solar wind dynamic pressure, ionopause altitude variations can have little effect on total obstacle radius. Thus, the Cytherean shock is sometimes observed much closer to or farther from the planet than previously predicted by gasdynamic theory applied to the deflection of flow about a blunt body which acts neither as source nor sink for any portion of the flow.

Slavin, J. A.↗

NASA NTRS1980

The location of the dayside ionopause of Venus - Pioneer Venus Orbiter magnetometer observations

The location of the dayside Venus ionopause, as observed by the Pioneer Venus Orbiter, is shown to depend on the magnetic pressure in the shocked, highly compressed solar wind plasma just outside the ionopause. Assuming a balance exclusively between this external magnetic pressure and internal ionospheric thermal pressure, invariance of ionospheric conditions, and an isothermal ionosphere, it is possible to determine pressure scale heights for various solar zenith angle intervals. These scale heights yield ionospheric temperatures which agree with direct measurements obtained independently. Not surprisingly, the average ionopause altitude is higher near the terminator, where the average external magnetic pressure is lower. The near-terminator ionopause has much greater positional variability than that at lower solar zenith angles; this appears to be due principally to concomitant variations in the external magnetic pressure, presumably related to solar wind pressure changes.

Elphic, R. C.↗

NASA NTRS1980

Pioneer Venus plasma wave observations - The solar-wind-Venus interaction

The Pioneer Venus plasma wave instrument is described with a discussion of wave observations throughout the typical near-noon and near-midnight orbits. This is followed by a comparison of the bow shock turbulence characteristics at earth and at Venus. The wave-particle interactions detected near the dayside ionopause are analyzed showing that the whistler mode Landau damping develops when the B field direction changes so that the whistler becomes oblique.

Scarf, F. L.↗

NASA NTRS1982

Structure of the Venus mesosphere and lower thermosphere from measurements during entry of the Pioneer Venus probes

Data on the thermal structure of the nightside middle atmosphere of Venus, from 84 to 137 km altitude, have been obtained from analysis of deceleration measurements from the third Pioneer Venus small probe, the night probe, which entered the atmosphere near the midnight meridian at 27 deg S latitude. Comparison of the midnight sounding with the morning sounding at 31 deg S latitude indicates that the temperature structure is essentially diurnally invariant up to 100 km, above which the nightside structure diverges sharply from the dayside toward lower temperatures. Very large diurnal pressure differences develop above 100 km with dayside pressure ten times that on the nightside at 126 km altitude. This has major implications for upper atmospheric dynamics. The data are compared with the measurements of Keating et al. (1980) above 140 km, with theoretical thermal structure models of Dickinson, and with data obtained by Russian Venera spacecraft below 100 km. Midnight temperatures are approximately 130 K, somewhat warmer than those reported by Keating et al.

Seiff, A.↗

NASA NTRS1982

The distant interplanetary wake of Venus - Plasma observations from Pioneer Venus

Observations by the Pioneer Vernus Orbiter plasma analyzer during passages through the Venus solar wind wake in June 1979 are reported. The spacecraft traveled 8-12 Venus radii behind the planet from the sun, detecting frequent, episodic disappearances of the plasma. The disappearance occurred inside the magnetotail, indicating a tattered, filamentary extension of the solar wind cavity. Positive oxygen ions were observed inside and outside of the magnetotail on some days, and were absent on others. The data indicate that the cavity contracts during high solar wind dynamic pressure, although the presence of 0(+) ions is not correlated well with the solar wind dynamic pressure. The most intense bursts originated from a flux of 10 million ions/sq cm per sec, with an 0(+) kinetic energy estimated to be 100,000-1,000,000 K.

Mihalov, J. D.↗

NASA NTRS1982

Thermal structure of the atmosphere of Venus from Pioneer Venus radio occultations

The processing and analysis results from 87 radio occultation experiments performed with the Pioneer Venus Orbiter are detailed. Atmospheric structure data were obtained with S-band (2293 MHz) and X-band (8407 MHz) signals transmitted through the 40 and 85 km levels of the Venus atmosphere. Latitudes of -68 to 88 deg at solar zenith angles of 8-166 deg were explored. The conversion of the data into acceptable form for the barometric equation to use the perfect gas law to obtain the temperature profiles is described. The temperature structures were more dependent on latitude than solar illumination conditions, with the greatest changes occurring in the circumpolar region. The polar tropopause altitude was about 4.8 km above its value at lower altitudes, with a temperature drop of 60 K and a pressure drop of 240 mb.

Kliore, A. J.↗

NASA NTRS1984

The dayside Venus ionosphere. I - Pioneer-Venus retarding potential analyzer experimental observations

The median values of the principal ionospheric quantities of the Venus dayside ionosphere are presented. The values are derived from the quantities measured by the Pioneer-Venus orbiter retarding potential analyzer over a period of two earth years at solar cycle maximum. Quantities reported are total ion density, O(+) density, O2(+) density, sum density (NO(+) + N2(+) + CO(+)), CO2(+) density, ion temperature, electron temperature, and plasma particle pressure. The data are organized to reveal altitude, solar zenith angle, solar longitude, and latitude dependences. The O(+) density exhibits both a solar longitude and a latitude dependence which may be caused by superrotation of the thermosphere and/or ionosphere. Asymmetry between the dawn and dusk terminator regions in the behavior of other quantities is also described.

Miller, K. L.↗

NASA NTRS1985

Pioneer Venus suprathermal electron flux measurements in the Venus umbra

Statistical properties of the two Maxwellian electron densities and temperatures characterizing the differential suprathermal electron fluxes measured by the Pioneer-Venus Lockheed-IPW retarding potential analyzer in the Venus umbra between approximately 1000 and 2000 km altitude and nightward of the 130 deg solar zenith angle are reported. It is found that the median density n2 and temperature T2 of the principal Maxwellian component are 3/cu cm and 14 eV, respectively, and do not exhibit an altitude or solar zenith angle dependence within the umbra region studied. Two thirds of the measured values of n2 and T2 typically fall within plus or minus a factor of two of the median values. The second suprathermal component density n3 is less well defined but has a median density of 0.06/cu cm and similarly exhibits no clear altitude or solar zenith angle dependence.

Knudsen, W. C.↗

NASA NTRS1985

Deuterium on Venus - Model comparisons with Pioneer Venus observations of the predawn bulge ionosphere

A self-consistent model of the Venus ionosphere in the predawn bulge region where the mass-two ion density is observed to be maximum was prepared in order to examine the question of mass-two ion identification in detail. The model calculations are compared to the Pioneer Venus observations of ion composition and structure in the 153-250 km altitude range. The observed densities of major ions O2(+) and O(+) are used to constrain the source of ionization. Once the source is determined, the density distribution of D(+) and H2(+) is calculated for various values of D and H2 in the atmosphere. It is found that mass-two ion is clearly due to deuterium and that the H2 contribution to the mass-two ion is small, if present at all.

Kumar, S.↗

NASA NTRS1985

Recent results on the Venus atmosphere from Pioneer Venus radio occultations

Temperature structure of the Venus atmosphere was derived from radio occultation data collected by the Pioneer Venus spacecraft during seven occultation seasons in the 1978-1983 period. The measurements, which cover latitudes from both poles to the equator from 40 km to 85 km altitudes, show a latitudinal dependence. At latitudes below 45 deg, there is a smooth temperature transition from the troposphere to the mesosphere. Between 60 and 80 deg, in the 'collar cloud' region, a strong temperature inversion (up to 30 K) appears, which disappears again in the polar areas (80-90 deg), where the mesosphere becomes isothermal without inversion. This temperature behavior is related to the persistent circulation pattern, in which a zonal retrograde motion below 45 deg changes gradually to a circumpolar vortex at the 'collar cloud'. A considerable temporal variability was observed in the thermal structure above the tropopause, suggesting a weather-like variability in the persistent circulation pattern.

Kliore, A. J.↗

NASA NTRS1987

Aeolian abrasion on Venus: Preliminary results from the Venus simulator

The role of atmospheric pressure on aeolian abrasion was examined in the Venus Simulator with a constant temperature of 737 K. Both the rock target and the impactor were fine-grained basalt. The impactor was a 3 mm diameter angular particle chosen to represent a size of material that is entrainable by the dense Venusian atmosphere and potentially abrasive by virtue of its mass. It was projected at the target 10 to the 5 power times at a velocity of 0.7 m/s. The impactor showed a weight loss of approximately 1.2 x 10 to the -9 power gm per impact with the attrition occurring only at the edges. Results from scanning electron microscope analysis, profilometry, and weight measurement are summarized. It is concluded that particles can incur abrasion at Venusian temperatures even with low impact velocities expected for Venus.

Marshall, J. R.↗

NASA NTRS1987

Further information on structure of the atmosphere of Venus derived from the Vega Venus balloon and lander mission

Continued analysis of the pressure and temperature data returned by the two Vega mission balloons has revealed an apparently significant difference in mean atmospheric static stability between the two data sets. Furthermore, the stability is time dependent within each data set. The 6.5 K temperature contrast between the two balloons remains, and appears to have a counterpart in the contrast between two of the Pioneer Venus probes at these levels, which has been attributed to planetary scale waves. Comparisons of the Vega 2 lander data with those of the Pioneer Venus Large Probe shows relatively close agreement in the state properties and in the atmospheric static stability profiles as well.

Seiff, A.↗