Pioneer Venus Orbiter Data Research Articles

Meteoric ion layers in the ionospheres of venus and mars: Early observations and consideration of the role of meteor showers

Layers of metal ions produced by meteoroid ablation have been known in Earth’s ionosphere for decades, but have only recently been discovered at Venus and Mars. Here we report the results of a search for meteoric layers in earlier datasets from Venus and Mars. We find 13 candidates at Venus in Mariner 10, Venera 9/10, and Pioneer Venus Orbiter data that augment the 18 previously identified in Venus Express data. We find 8 candidates at Mars in Mariner 7 and Mariner 9 data that augment the 71 and 10 previously identified in Mars Global Surveyor and Mars Express data, respectively. These new findings extend the ranges of conditions under which meteoric layers have been observed, support studies of the temporal variability of meteoric layers, and (for Venus) independently confirm the existence of meteoric layers. One of the proposed causes of temporal variations in the occurrence rate of meteoric layers is meteor showers. This possibility is controversial, since meteor showers have minimal observed effect on meteoric layers in Earth’s ionosphere. In order to aid progress towards a resolution of this issue, we present a series of tests for this hypothesis.

Location of the bow shock and ion composition boundaries at Venus—initial determinations from Venus Express ASPERA-4

For the first time since 1992 when the Pioneer Venus Orbiter (PVO) ceased to operate, there is again a plasma instrument in orbit around Venus, namely the ASPERA-4 flown on Venus Express (inserted into an elliptical polar orbit about the planet on April 11, 2006). In this paper we report on measurements made by the ion and electron sensors of ASPERA-4 during their first five months of operation and, thereby, determine the locations of both the Venus bow shock (BS) and the ion composition boundary (ICB) under solar minimum conditions. In contrast to previous studies based on PVO data, we employ a 3-parameter fit to achieve a realistic shape for the BS. We use a different technique to fit the ICB because this latter boundary cannot be represented by a conic section. Additionally we investigate the dependence of the location of the BS on solar wind ram pressure (based on ASPERA-4 solar wind data) and solar EUV flux (using a proxy from Earth).

Venus/Mars pickup ions and ionosheath wave structures

A search for ULF waves associated with pickup ions was undertaken for Venus and for Mars. The Pioneer Venus Orbital Plasma Analyzer (OPA) measurements of ion energy per unit charge spectra provided signatures of the probable presence of pick-up ions. At the locations of these energetic ion signatures we derived power spectra and propagation characteristics of structures in the local magnetic field from measurements by the Pioneer Venus Orbiter Magnetometer. Some of these features are characteristic of effects previously identified in the ionosheath of Venus, analogous to wave activity properties in the Earth’s magnetosheath. However, for the first time examples have been found at Venus of ULF waves near the ion gyrofrequencies in the ionosheath and in the far magnetic tail. Both linear and elliptically polarized waves are found in the vicinity of ion pickup detections. The signatures of these features are presented using one orbit of Pioneer Venus Orbiter data. The same wave analysis techniques are applied to the magnetic field data measured on one characteristic orbit from the Mars Global Surveyor Magnetometer. This is used to demonstrate how a comparative analysis of the two planets can be useful.

Magnetic field near Venus: A comparison between Pioneer Venus Orbiter magnetic field observations and an MHD simulation

Pioneer Venus Orbiter (PVO) measurements revealed the shape and the changing location of the Venus bow shock with solar cycle and provided a detailed picture of the magnetic field pileup in the dayside magnetosheath. Nevertheless, the reason for the increase of the terminator shock position to the observed distances has evaded our understanding, and the “magnetic barrier” region has been studied primarily by comparisons with gasdynamic models due to the difficulty of using more sophisticated treatments. In this study we investigate the extent to which a three‐dimensional magnetohydrodynamic (MHD) model of the Venus‐solar wind interaction, with and without “mass loading” by photoionization of the atomic oxygen upper atmosphere, can reproduce some of the basic features of the dayside magnetic field observed on PVO. The ideal MHD model uses a conducting sphere to represent the basic Venus ionospheric obstacle to the solar wind flow. We adopt the viewpoint that during solar maximum, a conducting obstacle with oxygen mass loading is appropriate, while a no‐mass loading case is a good first approximation to the solar minimum situation. The MHD simulations are found to give a realistic picture of both the shape of the bow shock and its observed elliptical cross section at the terminator. The introduction of the oxygen mass loading moves the shock position to that observed at solar maximum. The magnetic field strength on the dayside has a dependence on solar zenith angle similar to that found in statistical analyses of the PVO data, although the field is stronger than that measured. The mass loading creates a layer near the planet where the magnetic pressure is replaced with the thermal pressure much like observed. Our studies also raise the question of the role of nightside flow vortices in the formation of the effective obstacle boundary. Overall, our results illustrate that many features seen in the MHD model are consistent with the previously reported observations of the Pioneer Venus Orbiter.

An extension of the VIRA electron temperature and density models to include solar cycle variations

The original VIRA ionosphere model was based primarily on the Pioneer Venus Orbiter (PVO) data obtained at solar maximum (F10.7∼200) in 1979 and 1980 when periapsis was being maintained deep in the Venusian ionosphere. In situ measurements provided data on temperature, composition, density, and drift velocity, while the radio occultation method provided height profiles of electron density, N e. The solar cycle variation was deduced by comparison with the Venera 9 and 10 occultation data from the previous solar minimum. No data were available on the solar cycle variations of other ionospheric parameters, because periapsis had already risen out of the ionosphere by the time solar activity began to decline early in 1983. During the Entry period in the Fall of 1992, however, PVO got a brief glimpse of the nightside ionosphere at lower solar activity (F10.7∼120). During the intervening decade important in situ data were obtained on the upper nightside ionosphere that extends far down stream from the planet. This region was found to be highly sensitive to solar wind interactions and solar activity. In this paper, we discuss ways in which the later PVO data can be used to extend the VIRA model to higher altitudes and to include the solar cycle variations. As an example, we present some pre-entry Orbiter Electron Temperature Probe measurements that provide new clues as to the dayside T e behavior at low solar activity.

Three‐dimensional MHD simulations of the interaction between venus and the solar wind

We have developed a three‐dimensional, single‐fluid, MHD code which has successfully simulated the major qualitative features of the Venus‐solar wind interaction. A bow shock forms at a height above the ionosphere close to that deduced from Pioneer Venus Orbiter (PVO) observations. The bow shock shows an asymmetry in the terminator plane qualitatively similar to results obtained from PVO data analyses. The magnetic field drapes over the planet and forms a two‐lobed magnetotail. The magnetic field configuration near the planet is very similar to that recorded by PVO as well. We model the Venus obstacle as a hard, conducting sphere. We simulate the case in which the IMF is tilted with respect to the incoming solar wind velocity at approximately the Parker spiral angle; this is the first simulation of this particular case which includes all spatial regions of the interaction.

Venus gravity and topography: 60th degree and order model

We have combined the most recent Pioneer Venus Orbiter (PVO) and Magellan (MGN) data with the earlier 1978–1982 PVO data set to obtain a new 60th degree and order spherical harmonic gravity model and a 120th degree and order spherical harmonic topography model. Free‐air gravity maps are shown over regions where the most marked improvement has been obtained (Ishtar‐Terra, Alpha, Bell and Artemis). Gravity versus topography relationships are presented as correlations per degree and axes orientation.

A high resolution gravity model for Venus: GVM‐1

A spherical harmonic model of the gravitational field of Venus complete to degree and order 50 has been developed using the S‐band Doppler tracking data of the Pioneer Venus Orbiter (PVO) collected between 1979 and 1982. The short wavelengths of this model could only be resolved near the PVO periapse location (∼14°N latitude), therefore a priori constraints were applied to the model to bias poorly observed coefficients towards zero. The resulting model has a half‐wavelength resolution of 400 km near the PVO periapse location, but the resolution degrades to greater than 1000 km near the poles. This gravity model correlates well with a degree 50 spherical harmonic expansion of the Venus topography derived from a combination of Magellan and PVO data. New tracking data from Magellan's gravity mission should provide some improvement to this model, although a complete model of the Venusian gravity field will depend on tracking of Magellan after the circularization of it's orbit using aerobraking.

Estimation of local planetary gravity fields using line of sight gravity data and an integral operator

The gravity data coming from planetary probes are Doppler tracking observations which are sometimes reduced to Line-Of-Sight (LOS, the line joining the probe and the observer) gravity data. This direction-dependent representation is quantitatively difficult to use for geodetic and geophysical purposes. We present a new method of mapping LOS gravity data as gravity disturbances (gravity anomalies along a radial direction at a constant altitude). For this, we use an inversion procedure derived from Hotine's integral operator, with a Tikhonov-Arsenine regularization method. Different choices of the regularization parameter are presented in relation to data errors and method errors. After a study of a synthetic case, we apply the method to a real case, the region of Gula Mons (Venus), using Pioneer-Venus orbiter data.

Necessity of evolution in cosmological γ-ray bursts

RESULTS from the Burst and Transient Source Experiment (BATSE)1 on the Compton Gamma-Ray Observatory show a relative deficiency of faint γ-ray bursts without any detectable anisotropy. This gives strong support to suggestions that the bursts originate at cosmological distances2, such that cosmic expansion reduces the detectability of faint (and therefore distant) sources relative to a uniformly filled euclidean space. But the Pioneer Venus Orbiter (PVO) observations of brighter bursts show no such deficiency, indicating that the bursts that it sees are at redshifts less than about one. The best-fit cosmological model based on PVO data (peak luminosity L0 = 2 x 1050erg s-1) with no evolution of the source population predicts, when extrapolated to the fainter sources, a factor of 40 more events than BATSE sees. Even the 3a limit from PVO (L0 = 9.1 x 1051 ergs-1) is barely consistent with the BATSE result. Evolution of the cosmological γ-ray burst sources with epoch therefore seems necessary if the PVO and BATSE data are to be reconciled. The required evolutionary trend is for fewer or fainter bursts at larger redshift.

Induced magnetic fields at the surface of Venus inferred from pioneer Venus orbiter near‐periapsis measurements

Although magnetic field measurements have not been made on the surface of Venus or, for that matter, below the Pioneer Venus Orbiter (PVO) periapsis altitude of ∼150 km, the theory of the large‐scale ionospheric magnetization that results from the solar wind interaction can be used in concert with the available data to infer the existence of nonzero fields down to the surface. Here it is argued that magnetometer measurements in the upper part of the “diffusion region” of the ionosphere can be extrapolated to lower altitudes because of the particularly simple physics that determines the altitude profile of the large‐scale magnetic field in this region. The magnetic field at the bottom of the ionosphere inferred by this method using the PVO data is up to tens of nanoteslas when large‐scale ionospheric fields are present, suggesting the likely presence of fields of similar strength below. Given the negligible intrinsic field of Venus inferred from the PVO measurements, these fields must result from the solar wind interaction. The implications for Mars, which may have an analogous solar wind interaction, is that magnetometers on the surface could regularly observe varying fields of similar magnitude and origin.

Ulf waves upstream of the Venus bow shock: Properties of one‐hertz waves

Pioneer Venus orbiter data are used to examine the properties of a class of ULF upstream waves with relatively high observed frequencies (1.1–1.3 Hz). In the spacecraft frame these waves are most often left‐hand elliptically polarized. They have amplitudes up to 3 nT (near the bow shock) and propagate obliquely to the magnetic field at angles from 10° to 50°. These waves show significant similarity in their properties to “one‐Hertz” waves identified at the Earth in the ISEE 1 and 2 observations and the whistler waves identified earlier with IMP 6 observations. The waves appear almost immediately after the spacecraft crosses the magnetic field tangent line to the bow shock surface into the region of connected field lines. The amplitude of these waves decreases with distance from the shock measured along the magnetic field line. We have used the cold plasma dispersion relation in order to study the propagation of these waves and to explain their observed polarization. Calculated group velocities indicate that those waves have sufficient upstream velocities to propagate from the shock into the solar wind. The totality of observations, including the observed wave damping and the observation of right‐handed waves, seems best explained by a source of right‐handed whistler mode waves at the bow shock.

Steady‐state plasma transition in the Venus ionosheath

The results of an extended analysis of the plasma and electric field data of the Pioneer Venus Orbiter (PVO) are presented. We report the persistent presence of a plasma transition embedded in the flanks of the Venus ionosheath between the bow shock and the ionopause. This transition is identified by the repeated presence of characteristic bursts in the 30 kHz channel of the electric field detector of the PVO. The observed electric field signals coincide with the onset of different plasma conditions in the inner ionosheath where more rarified plasma fluxes are measured. The repeated identification of this intermediate ionosheath transition in the PVO data indicates that it is present as a steady state feature of the Venus plasma environment. The distribution of PVO orbits in which the transition is observed suggests that it is more favourably detected in the vicinity of and downstream from the terminator.

Comparison of observed and predicted gravity profiles over Aphrodite Terra, Venus

We compare observed Pioneer Venus orbiter (PVO) gravity profiles over Aphrodite Terra to profiles predicted from models of thermal isostasy, mantle convection, and Airy compensation. Similar approaches are used in order to investigate how well the models can be distinguished with the PVO data. Topography profiles across Aphrodite are compared to model spreading ridge profiles in order to further assess this model. Airy compensation depths and convection layer thicknesses are greater under eastern Aphrodite than western Aphrodite. Compensation depths in the east are greater than most estimates of lithospheric thickness, suggesting that this part of the ridge is dynamically supported. In parts of western Aphrodite, the spreading ridge model gravity provides a better fit to the data than either Airy compensation or mantle convection. Best‐fit spreading rates are between 0.3 and 1.6 cm/yr. Airy compensation and mantle convection cannot be distinguished in most places using only PVO data.

Electrons in the ionopause boundary layer of Venus

Measurements made by the retarding potential analyzer on board the Pioneer Venus orbiter (PVO) have indicated the presence of both magnetosheath electrons and atmospheric photoelectrons in the plasma mantle region of Venus, which lies just outside the ionopause. A theoretical model of the spatial and energy distribution of electrons in the plasma mantle has been constructed using the two‐stream electron transport method and the time‐dependent electron energy equation. Magnetic field lines parallel to the ionopause are assumed. Transport and collisional processes, including electron scattering by neutrals and ions, are taken into consideration. The calculated electron spectra in the mantle show signatures of both solar wind electrons and photoelectrons. Cold ionospheric electrons are present only for altitudes below the ionopause. The calculated electron temperatures agree very well with the PVO data both in the mantle region and in the magnetized ionosphere.

Crustal spreading on Venus: evidence from topography, morphology, symmetry, and map patterns

Evidence for numerous features similar to terrestrial divergent plate boundaries and spreading centers are present in the topography and morphology of Western Aphrodite Terra, Venus (Head and Crumpler, 1987). Linear discontinuities cutting across the strike of Western Aphrodite Terra (Cross-Strike Discontinuities or CSDs) were examined in further detail. It was found that: 1. (1) They represent the most prominent regional structural trend in Western Aphrodite and are readily distinguishable from linear patterns caused by ground tracks in the Pioneer-Venus orbiter data; 2. (2) individual CSDs are either deep troughs or steep regional scarps and steps; 3. (3) the spacing between the more prominent CSDs ranges from about 500 to 1500 km (average spacing is 850 km); and 4. (4) there is a distinctive structural trend orthogonal to the strike of the CSDs. Additional smaller scarps and slopes with characteristics of known CSDs occur in the Arecibo altimetric profiles at spacings of as little as 50 km, in agreement with previous suggestions of Sotin et al. (1989) that other smaller CSDs with less offset may occur in domains between the more prominent ones previously mapped. The detailed characteristics of the CSDs (detailed trough and scarp-like form, association with ridge offsets, linear terminations, linear slope traces, parallelism, great length) are all very similar to the characteristics of transform faults and fracture zones on the Earth's seafloor, and this additional analysis supports the earlier interpretation that CSDs represent analogs to terrestrial oceanic fracture zones. Assessment of bilateral symmetry of topography and other elements across Western Aphrodite Terra indicate that: 1. (1) an accurate prediction of the large and small-scale symmetrical elements in both the altimetry data and radar images within each domain on one flank of the highlands may be made by substitution of a mirror image of the opposite flank, and 2. (2) smaller wavelength symmetrical elements are revealed in the altimetric profiles by subtracting the background regional symmetry and examining the residuals. Residual altimetry profiles obtained in this manner show that individual altimetry features several hundred kilometers wide and several hundred meters high are frequently located at the same distance from the axis of regional symmetry on the north and the south flanks of the highlands. Topography and geometric features such as ridge crests and transforms have specific predictable relationships in a crustal spreading environment. Tests were made in Western Aphrodite for such organized relationships predicted to occur in association with ridge crest offsets at transform faults and fracture zones. It was found that: 1. (a) topographic stepdowns occur across CSDs in accordance with the general consequences of juxtaposing lithospheres of different relative ages, 2. (b) the correct direction of stepdown (down toward older crust and lithosphere) occurs relative to that predicted from ridge crest offsets, and 3. (c) the observed amount of stepdown is similar in many cases to that predicted from the observed ridge crest offsets. These analyses of cross-strike discontinuities, topographic and image symmetry across Aphrodite, and predictions of topography based on ridge crest and fracture zone geometric relationships further support the earlier interpretation that the 7500 km extent of Western Aphrodite Terra represents the site of crustal spreading and displays many of the characteristics of terrestrial divergent plate boundaries.

The compressional ULF foreshock boundary of Venus: Observations by the PVO magnetometer

Entries and exits of the compressional magnetic wave segments of the Pioneer Venus Orbiter data are identified as crossings of a foreshock boundary. When these crossings are plotted in an appropriate coordinate system, they form scatter diagrams representing the geometry of a composite boundary covering all solar wind conditions of the sample. When the orientation of the boundary for the average interplanetary magnetic field direction at Venus is extracted, the boundary is similar, although not identical within present uncertainties, to that found for the analogous foreshock boundary of the earth.

Periodicity of the γ-ray transient event of 5 March 1979

An unusual gamma-ray burst event was observed on 5 March 1979 by nine different spacecraft. The position of the event has been accurately determined as r.a. = 5 h 25.95 min, dec = -66 deg 07.1 arcmin (epoch 1950.0), coincident with the location of the supernova remnant N49 in the Large Magellanic Cloud. The burst was of very high intensity, and if isotropic and located at the distance (approximately 55 kpc) of N49 had a peak luminosity of greater than 10 to the 44th erg/sec. Even more interesting is the obvious 8-s periodicity of the event, following the initial very intense outburst. The time history and power spectrum of this event as determined from Pioneer Venus Orbiter data is here reported.