Pioneer Venus Orbiter Measurements Research Articles

Influence of Solar Wind Variations on the Shapes of Venus’ Plasma Boundaries Based on Venus Express Observations

The interaction between the solar wind and Venus creates an induced magnetosphere. The regions of the induced magnetosphere are separated by plasma boundaries, where their shapes and sizes are influenced by variations in the surrounding environment. Investigations of the boundaries and their variability can help us understand the solar wind’s effect on Venus and unmagnetized planets in general. In this study, the bow shock and ion composition boundary locations are investigated using, for the first time, the full data set of plasma and magnetic field measurements by Venus Express taken during 2006–2014. The locations of the boundaries are examined with respect to upstream conditions and the solar cycle. We find, in agreement with previous studies using Pioneer Venus Orbiter measurements, that the bow shock location is mostly sensitive to the solar cycle and the dynamic pressure and that it exhibits asymmetries in the terminator plane, depending on the direction of the interplanetary magnetic field. The asymmetries are mainly attributed to the asymmetry in the pickup ion distribution and thus mass loading in the magnetosheath. The ion composition boundary on the dayside is found to decrease in altitude with increasing dynamic pressure during solar maximum (2006–2011), but shows no clear trends during solar minimum (2011–2014) conditions.

Asymmetry of the Venus nightside ionosphere: Magnus force effects

A study of the dawn–dusk asymmetry of the Venus nightside ionosphere is conducted by examining the configuration of the ionospheric trans-terminator flow around Venus and also the dawn-ward displacement of the region where most of the ionospheric holes and the electron density plateau profiles are observed (dawn meaning the west in the retrograde rotation of Venus and that corresponds to the trailing side in its orbital motion). The study describes the position of the holes and the density plateau profiles which occur at neighboring locations in a region that is scanned as the trajectory of the Pioneer Venus Orbiter (PVO) sweeps through the nightside hemisphere with increasing orbit number. The holes are interpreted as crossings through plasma channels that extend downstream from the magnetic polar regions of the Venus ionosphere and the plateau profiles represent cases in which the electron density maintains nearly constant values in the upper ionosphere along the PVO trajectory. From a collection of PVO passes in which these profiles were observed it is found that they appear at neighboring positions of the ionospheric holes in a local solar time (LST) map including cases where only a density plateau profile or an ionospheric hole was detected. It is argued that the ionospheric holes and the density plateau profiles have a common origin at the magnetic polar regions where plasma channels are formed and that the density plateau profiles represent crossings through a friction layer that is adjacent to the plasma channels. It is further suggested that the dawn–dusk asymmetry in the position of both features in the nightside ionosphere results from a fluid dynamic force (Magnus force) that is produced by the combined effects of the trans-terminator flow and the rotational motion of the ionosphere that have been inferred from the PVO measurements.

Viscous Magnus Force for the Rotating Venus Ionosphere

The Magnus force applicable to the Venus ionosphere is calculated for the high-altitude transterminator flow that streams around the low-altitude ionospheric plasma that is subject to rotating motion. The Magnus force can account for the observed dawn-dusk (east-west) asymmetry of the Venus nightside ionosphere and is suitable to describe conditions in astrophysical problems in which the plasma of a rotating body interacts with a directional plasma flow. The Magnus force equation is derived including the viscous force term in the momentum equation of the transterminator flow in the Venus upper ionosphere and is examined by comparing the expected dawnward-directed displacement of that flow with the Pioneer Venus Orbiter measurements. Values of the coefficient of the Magnus force equation suitable for the dawnward deflection of the transterminator flow are comparable to those of a rotating sphere subject to a streaming flow in fluid dynamics problems and imply Re ~ 200-300 values of the Reynolds number for that flow using the ~600 km width of the region where it is measured. Such values of the Reynolds number are 1 order of magnitude larger than those inferred from the viscous transport of solar wind momentum to the Venus upper ionosphere that is implied by the velocity boundary layer observed along the flanks of the Venus ionosheath. With values of the coefficient of the Magnus force equation for the Venus ionosphere that are similar to those applicable to ordinary fluid dynamic problems it is possible to suggest a common quality in the response of rotating obstacles subject to a streaming flow in fluid dynamics and in astrophysical problems.

Modelling dications in the diurnal ionosphere of Venus

Context.Previous studies have shown that doubly-charged positive ions, including molecular ions, can have detectable densities in the ionospheres of Mars, Titan, and the Earth. Aims.In the present paper, we continue our approach by modelling the Venusian ionosphere. The neutral atmosphere of Venus has a similar composition to the ones encountered in the other terrestrial planets, with carbon dioxide (as on Mars), nitrogen (as on Earth and Titan), and atomic oxygen (as on Earth). Methods. We computed the doubly-charged positive ion production through photoionisation (primary production) and electron impact ionisation (secondary production). We computed the densities under the photochemical equilibrium assumption. Results. We predict a ${\rm CO_2}^$ layer centred around $140~{\rm km}$ altitude with a density reaching 30 dications per ${\rm cm}^{3}$ in active solar conditions and a ${\rm N_2}^$ layer centred around a $140{-}150~{\rm km}$ altitude with density reaching $0.3~{\rm cm}^$. We find good agreement between the modelled ${\rm O}^$ densities and Pioneer Venus Orbiter measurements. Finally, we investigate the problem of possible detection of these ions by the European spacecraft Venus Express. Conclusions. Although molecular doubly-charged positive ion densities are low, these ions cannot be considered as negligible compared to other minor ion species. Their detection should be possible in the future through remote sensing or in-situ methods including ion mass spectrometry and UV spectroscopy.

Distributions of hot oxygen for Venus and Mars

Hot atomic oxygen velocity distributions and corresponding profiles of density and temperature have been calculated for the Venus and Mars exospheres by Monte Carlo simulation. The Venus results are realistically based on well‐established models of the Venus atmosphere and ionosphere that were derived from Pioneer Venus orbiter measurements near solar maximum. The confidence level is lower for Mars because the only viable data come from Viking entry measurements in daytime at solar minimum, and the global morphologies of the atmosphere and ionosphere are poorly understood. What is clear is that planetary differences arise because the exothermal velocities of hot oxygen created by dissociative recombination of O2+ are below satellite speeds on Venus and greater than escape on Mars. As a result, the distribution of velocities on Venus tends to be nearly isotropic, whereas it is grossly anisotropic on Mars, favoring the vertical with increasing distance from the planet. The calculated lower bound for the Martian oxygen escape rate indicates that the lifetime of CO2 in the Mars atmosphere is probably less than 10 Myr.

A new look at Venus' thermosphere H distribution

Pioneer Venus Orbiter measurements in the ionosphere, at low altitudes in 1978–1980 and from the entry phase of the mission in late 1992, are used to determine the neutral H concentrations, on an orbit by orbit basis, in the altitude region where H + is in chemical equilibrium. The chemical equilibrium region is selected based on the characteristic chemical and diffusion times for each measurement. This results in a dataset of H concentrations at Venus from in situ measurements covering all local times and diverse solar activity conditions. A comparison of H with He measurements shows parallel local time behavior. The H concentrations in the bulge region may have a slight inverse correlation with F10.7, whereas the He bulge data (from ONMS) near 175 km do not.

The nightward ion flow scenario at Venus revisited

In this paper, we indicate how existing Pioneer Venus Orbiter (PVO) data might be used to gain a better understanding of nightward ion flow in the Venusian ionosphere. Calculations based on PVO measurements made at solar maximum suggest that the global nightward flow of O + may be significantly greater than is required to maintain the observed nightside ionosphere densities. The validity of this conclusion depends upon (1) the accuracy with which the flow can be determined from the PVO ion density and velocity measurements and (2) the validity of the ionosphere theory used to estimate the required downward O + flux on the night side. If the measurements and theory are assumed to be accurate, the excess nightward flow implies a significant rate of ion escape from the planet, particularly at times of low solar wind dynamic pressure, Psw, when the ionopause rises to allow increased nightward flow. To illustrate a potentially important mechanism for ion escape from Venus, we present Orbiter Electron Temperature Probe (OETP) observations of plasma clouds and scavenged ionospheric plasma observed above the ionopause. We then employ OETP and Orbiter Retarding Potential Analyzer (ORPA) data to reexamine the global ion flow for average Psw conditions, and we find the net flow to be in agreement with previous estimates. We also find that the net transterminator flow at average Psw approaches the limiting upward ion flow available from the dayside ionosphere, a limit which is imposed by O + collisions with O 2 +. OETP measurements in the terminator ionosphere show that N e declines at times of low Psw. This suggests that the dayside ionosphere is unable to supply the increased demand when the ionopause rises above its average height, thus requiring an upward diversion of some of the nightward flow to populate the overlying, newly-created ionosphere. We predict that the ion velocities will also be found to decrease at times of low Psw, in part due to the source-limited flow, and in part because the expansion of the ionosphere diminishes the “nozzle effect” that has been identified as a source of nightward ion acceleration. Collectively, these effects act to limit the increase in the net nightward ion flow at times of low Psw. Since O +-O 2 + collisions will impart an upward velocity to the O 2 + ions, the O + diffusion limit may be a soft one, so the nightward flow can continue to increase somewhat as Psw declines. The collision process should also enhance the O 2 + concentration in the dayside upper ionosphere and, to the extent that the O 2 + participates in the nightward flow, should produce O 2 + enhancements on the night side as well. These expectations could be verified by an examination of the orbit-to-orbit changes in ion composition and velocity in response to Psw variations.

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.

Small‐scale plasma, magnetic, and neutral density fluctuations in the nightside Venus ionosphere

Pioneer Venus orbiter measurements have shown that coherent small‐scale waves exist in the electron density, the electron temperature, and the magnetic field in the lower ionosphere of Venus just downstream of the solar terminator (Brace et al., 1983). The waves become less regular and less coherent at larger solar zenith angles, and Brace et al. suggested that these structures may have evolved from the terminator waves as they are convected into the nightside ionosphere, driven by the day‐to‐night plasma pressure gradient. In this paper we describe the changes in wave characteristics with solar zenith angle and show that the neutral gas also has related wave characteristics, probably because of atmospheric gravity waves. The plasma pressure exceeds the magnetic pressure in the nightside ionosphere at these altitudes, and thus the magnetic field is carried along and controlled by the turbulent motion of the plasma, but the wavelike nature of the thermosphere may also be coupled to the plasma and magnetic structure. We show that there is a significant coherence between the ionosphere, thermosphere, and magnetic parameters at altitudes below about 185 km, a coherence which weakens in the antisolar region. The electron temperature and density are approximately 180° out of phase and consistently exhibit the highest correlation of any pair of variables. Waves in the electron and neutral densities are moderately correlated on most orbits, but with a phase difference that varies within each orbit. The average electron temperature is higher when the average magnetic field is more horizontal; however, the correlation between temperature and dip angle does not extend to individual wave structures observed within a satellite pass, particularly in the antisolar region. The importance of fluctuations in the magnetic dip angle is that they inhibit vertical conduction and produce a small effective dip angle which reduces the heat flux necessary to maintain the observed temperatures.

The ionotail of Venus: Its configuration and evidence for ion escape

The Pioneer Venus Orbiter measurements of the plasma and magnetic environment of the near tail of Venus show that the ionosphere becomes increasingly filamentary with increasing altitude, apparently forming cometlike tail rays that extend several thousand kilometers behind the planet. We call this region the ionotail of Venus. The tail rays are envisioned as plumes of high‐beta plasma of ionospheric origin that are surrounded by regions of low‐density, low‐beta plasma. The ionotail appears to be in quasi‐equilibrium, with the plasma pressure in the rays approximately balanced by the magnetic pressure of the region surrounding the rays. The magnetic field in this region is approximately sunaligned as we assume are the tail rays. Magnetic field reversals observed in the tail ray boundaries suggest the presence of strong current sheets there. Unlike the lower ionosphere whose major ion is thermal O+, a detailed study of tail ray plasma between 2000 and 2500 km altitude shows that the major ions are superthermal O+, with energies in the range of 9‐16 eV. The electrons are much cooler, with energies of about 1 eV. A minor, more energetic ion component, having energies exceeding 40 eV is also observed within the tail rays and occasionally between the rays as well. These Pioneer Venus Orbiter measurements reveal an ionotail that is highly dynamic, a region in which solar wind induced magnetic fields configure the ionospheric structures and accelerate the ions beyond the planetary escape velocity. We estimate a total planetary O+ escape rate of 5 × 1025 ions/s, and we infer an H+ escape rate of about half that value, about a factor of 2 below the hydrogen escape rate due to H+ charge exchange with the hydrogen exosphere of Venus.

Possible effects of solar flares on the ionosphere of Venus from Pioneer Venus Orbiter measurements

The sharp enhancements in ionization seen in the outbound profiles around 200 km, for three orbits, during the second and the third sets of dayside periapsis passes, have been identified as possible ionospheric effects of three class 1 solar flares, simultaneously monitored on the earth. Two of these flares were accompanied by sudden ionospheric disturbances on the earth also. The effects of these flares could be observed by the orbiter, which subsequently sampled the ionosphere after a time comparable to the relaxation time of the ionosphere at these heights.

Venus nightside ionospheric troughs: Implications for evidence of lightning and volcanism

Ionization troughs are frequently observed in the Venus nightside ionosphere by the orbiter ion mass spectrometer (OIMS) on the Pioneer Venus Orbiter (PVO). These events typically exhibit density depletions which may range from a factor of 2 to an order of magnitude or more, often feature sharp density gradients, and frequently are associated with superthermal ion fluxes. Although their full spatial extent cannot be determined, the ion trough dimensions as perceived along the PVO orbit track extend from 1° to more than 10° in latitude and longitude. Troughs have been encountered at altitudes ranging from more than 1000 to less than 250 kilometers. The nightside trough boundaries are similar to both the ionization gradients which mark the Venus dayside ionopause and the ionization gradients which define the high‐latitude ionization trough in the earth ionosphere. In each of these Venus and earth phenomena there is a close association between sharp gradients in thermal ion distributions and energetic and dynamic processes stimulated by the interaction between the solar wind/interplanetary magnetic field (IMF) and the thermal ionosphere. A close correlation is identified between ion trough events and prominent published examples of 100‐Hz plasma waves detected by the orbiter electric field detector (OEFD), which have been interpreted by Scarf and Russell (1983) and by Ksanfomaliti et al. (1983) as whistler mode propagation stimulated by lightning in the lower atmosphere and frequently occurring over mountains. While the occasional coincidence between the detection of whistler waves and the overflight of Venus mountain ranges is also observed for the ion troughs, neither phenomenon is seen to exhibit convincing evidence of an association with the mountains. Overall, these findings lead us to believe that many of the 100‐Hz noise bursts previously attributed to lightning are, like the ion troughs, actually the result of the interaction between the ionosphere and the combined effects of the solar wind and the draped IMF. If this interpretation is correct, there may be little evidence for lightning from the PVO measurements. Similarly, a supporting argument for active volcanism as a possible source for the deduced lightning events would also be discounted.

New empirical models of the electron temperature and density in the Venus ionosphere with application to transterminator flow

The continuing rise of periapsis has allowed the Pioneer Venus orbiter (PVO) to encounter vast regions of the Venus ionosphere that were not sampled earlier when propulsion was being used to maintain periapsis deep in the ionosphere. We have employed the orbiter electron temperature probe measurements from the first seven Venus years (December 1978–December 1982) to devise new empirical models of electron temperature Te and density Ne. Departing from our earlier use of spherical harmonics to describe the solar zenith angle variations of Ne (Theis et al., 1980), we find that this more complete data set can be described better by error functions and exponentials in solar zenith angle and power law variations in altitude. This model represents a significant improvement over the previous models, as it allows the dawn and dusk sectors to be modeled separately, providing more realistic representations of the gradients near the terminators as well as the variation of those gradients with altitude. We employ these models to perform a two‐dimensional solution of the momentum equation for the nightward ion flow velocities believed to be largely responsible for the maintenance of the nightside ionosphere. The velocities at the terminator rise from the neutral atmospheric wind velocity (∼300 m/s) at 150 km to a peak velocity exceeding 2000 m/s above 500 km, in general agreement with PVO measurements of ion drift in that region.

The influence of thermospheric winds on exospheric hydrogen on Venus

Monte Carlo models of the distribution of atomic hydrogen in the exosphere of Venus were computed which simulate the effects of thermospheric winds and the production of a “hot” hydrogen component by charge exchange of H + and H and O in the exosphere, as well as classic exospheric processes. A thermosphere wind system that is approximated by a retrograde rotating component with equatorial speed of 100 m/sec superimposed on a diurnal solar tide with cross-terminator day-to-night winds of 200 m/sec is shown to be compatible with the thermospheric hydrogen distribution deduced from Pioneer Venus orbiter measurements.

Plasma clouds above the ionopause of Venus and their implications

Early Pioneer Venus orbiter measurements by the Electron Temperature Probe (OETP) have revealed wavelike structures at the ionopause and clouds of plasma above the ionopause, features which may represent ionospheric plasma at different stages in its removal by solar wind-ionosphere interaction processes. Continuing operation of the orbiter through three Venus years has now provided enough additional examples of these features to permit their morphologies to be examined in some detail. The global distribution of the clouds suggests that they originate at the dayside ionopause as wavelike structures which may become detached and swept downstream in the ionosheath flow. Alternatively the clouds may actually be attached streamers analogous to cometary structure. Estimates of the total ion escape rate from Venus by this process yields values up to 7 × 10 26 ions s −1, based on their measured transit times, their probability of occurrence, their statistical distribution and their average electron density. Preliminary analysis shows that such an excape flux could be supplied by the upward diffusion limited flow of 0 + from the entire dayside ionosphere. Observed distortions of dayside ionosphere height profiles suggest that such flows may be present much of the time. If such an escape flux were to continue over the entire lifetime of Venus, the effects upon the evolution of its primitive atmosphere may have been significant.

H2 abundance in the atmosphere of Venus

The in situ measurements from the Pioneer Venus Orbiter (PVO) Ion Mass Spectrometer have led to the detection of H2+ ions in the Venus ionosphere. Although H2+ is a minor ion, its measurement provides the first direct clue for determining the H2 abundance in the upper atmosphere of Venus. A photochemical model using PVO measurements is used to derive an H2 mixing ratio of 10 ppm below 140 km altitude. The presence of this much H2 confirms our previous prediction that the reactions of O+ with H2 and subsequent recombination of OH+ provide an important source of nonthermal H observed in the Venus exosphere. The estimated escape flux for H is ∼108 cm−2 s−1