Pioneer Venus Orbiter Research Articles

Stream Interactions and Interplanetary Coronal Mass Ejections at 0.72 AU

We present comprehensive surveys of 203 stream interaction regions (SIRs) and 124 interplanetary CMEs (ICMEs) during 1979 – 1988 using Pioneer Venus Orbiter (PVO) in situ solar-wind observations at 0.72 AU and examine the solar-cycle variations of the occurrence rate, shock association rate, duration, width, maximum total perpendicular pressure (P t), maximum dynamic pressure, maximum magnetic field intensity, and maximum velocity change of these two large-scale solar-wind structures. The medians, averages, and histogram distributions of these parameters are also reported. Furthermore, we sort ICMEs into three groups based on the temporal profiles of P t, and we investigate the variations of the fractional occurrence rate of three groups of ICMEs with solar activity. We find that the fractional occurrence rate of magnetic-cloud-like ICMEs declined with solar activity, consistent with our former 1-AU results. This study at 0.72 AU provides a point of comparison in the inner heliosphere for examining the radial evolution of SIRs and ICMEs. The width of SIRs and ICMEs increases by 0.04 and 0.1 AU, respectively, and the maximum P t decreases to about 1/3 from Venus to Earth orbit. In addition, our work establishes the statistical properties of the solar-wind conditions at 0.72 AU that control the solar-wind interaction with Venus and its atmosphere loss by related processes.

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.

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).

The Venus ultraviolet oxygen dayglow and aurora: Model comparison with observations

We compare the intensity of the OI 130.4 and 135.6 nm emissions calculated using the soft electron precipitation measured on board the Pioneer Venus (PV) Orbiter with the auroral brightness observed with the ultraviolet spectrometer (OUVS) on board the PV. For this purpose, we use a new electron transport model based on a Monte Carlo implementation of the Boltzmann equation and a multi-stream radiative transfer model to calculate the effects of multiple scattering on the intensity field of the 130.4-nm triplet. We show that the consideration of the enhancement of the emergent 130.4-nm to the 135.6-nm intensity by multiple scattering in the optically thick Venus atmosphere increases the auroral 130.4/135.6 ratio by a factor of about 3. We find agreement with the mean 130.4/135.6 ratio observed with PV-OUVS using the typical suprathermal electron energy spectrum reported from PV in situ measurements showing a characteristic energy of about 14 eV. To account for the average OI auroral emissions, the required precipitated energy flux is 2×10 −3 mW m −2, that is about 30% of the measured suprathermal night-side soft electron spectrum used as a reference. The calculated brightness of the CO Cameron bands is about twice as large as the weak observed emission, but within the error bars of the observations and the uncertainties of the dissociative excitation cross-section of CO 2. The electron transport model, coupled with calculations of excitation processes is also applied to an analysis of the FUV oxygen day airglow observations made with PV-OUVS and the Hopkins Ultraviolet Telescope (HUT) spectrograph. Comparisons indicate that the model accounts for both the disc-averaged intensities observed with the HUT spectrograph, the limb scans and the 130.4-nm images obtained with PV-OUVS. The relative contribution of resonance scattering of the solar line and photoelectron impact to the excitation of the 130.4-nm triplet depends on the altitude, but is globally dominated by resonance scattering. The intensity of the 130.4-nm dayglow emission does not vary proportionally with the O density in the lower thermosphere, but provides nevertheless a useful tool to remotely probe the atomic oxygen density and its variations.

Hybrid modelling the Pioneer Venus Orbiter magnetic field observations

This study presents comparisons between the Pioneer Venus Orbiter (PVO) magnetometer (OMAG) observations and the HYB-Venus hybrid simulation code. The comparisons are made near periapsides of four PVO orbits using the full resolution PVO/OMAG data. Also, the statistics of the solar wind and interplanetary magnetic field (IMF) conditions at Venus are studied using the PVO interplanetary dataset. The statistics include the histograms and the probability density maps of the selected upstream parameters. The confidence intervals derived from the upstream statistics demonstrate the nominal simulation input parameter space. Moreover, the probability density maps give the dependencies between the upstream parameters. The comparisons between the simulation code and the data along the spacecraft trajectory show that the basic, large scale, trends seen in the magnetic field can be understood by the current simulation runs. The discrepancies between the simulation and the data were found to arise at low altitudes close to the planetary ionosphere in the region which cannot be resolved in detail by the grid size of the runs.

Near‐terminator Venus ionosphere: Evidence for a dawn/dusk asymmetry in the thermosphere

Recent models of the near‐terminator ionosphere of Venus constructed using neutral density profiles from the VTS3 model of Hedin et al. (1983) have shown that altitudes of the electron density peaks are in agreement with those measured by Pioneer Venus (PV) Orbiter Radio Occultation (ORO) and other radio occultation profiles in the solar zenith angle (SZA) range 60 to 70°, where they are near 140 km (Fox, 2007). The model peaks in the 75–85° range, however, do not decrease in altitude to near 135 km, as do the PV ORO electron density peaks shown in the study of Cravens et al. (1981). We investigate here possible reasons for this decrease. The PV Orbiter Neutral Mass Spectrometer (ONMS) measured densities of CO2, O, CO, N2, N, and He for many of the first 600 orbits. We have chosen 10 orbits in the dawn sector and 12 orbits in the dusk sector for which the solar zenith angles at periapsis were in the 75–85° range, and we have examined the ONMS density profiles reported in the PV Unified Abstract Data System. We find that for most of the orbits, the appropriately normalized ONMS measured densities for CO2 and O are, however, either similar to or larger than those generated from the VTS3 model for the same solar zenith angle and F10.7 flux, and the use of these densities in our models would therefore produce a higher, rather than a lower, peak. The VTS3 models are, however, not expected to be accurate in the terminator region because of the small number of spherical harmonics used in the models and the large density changes that are expected near the terminators. We have also investigated a possible dawn/dusk asymmetry in the ionosphere. All the low‐altitude PV radio occultation electron density peaks reported in the study of Cravens et al. (1981) in the 70 to 85° range were in the dawn sector at high latitudes. In the VTS3 models, the exospheric temperatures are predicted to be smaller at dawn that at dusk, but the asymmetries are confined to the region above ∼165 km. Thus use of the VTS3 model densities and temperatures in the near‐terminator dawn sector models cannot produce electron density peaks that are lower in altitude than those in the dusk sector. We suggest that there is a high‐latitude asymmetry between the dawn and dusk neutral densities that extends down to within ∼20 km above the expected altitude of the electron density peaks, and that produces a significantly asymmetrical ionosphere.

Space weather at Venus and its potential consequences for atmosphere evolution

Space weather storms at the Earth are dominated by the magnetosphere's response to coronal mass ejections, or CMEs, whose disturbances propagate through the solar wind to 1 AU, and to a lesser extent by the pressure ridges associated with the interactions of solar wind streams with different properties. The interplanetary signatures of these events include high solar wind dynamic pressures and high interplanetary magnetic fields, the same parameters that cause compression and/or magnetization of the ionosphere at weakly magnetized Venus. The nature of Venus' response to these events is also expected to include increased atmosphere erosion by the solar wind interaction, a matter of potential interest for historical extrapolations of atmosphere escape. We consider the possible evidence for enhanced escape during these disturbances in the data from the Pioneer Venus Orbiter (PVO). Available magnetometer and plasma analyzer results are used to identify periods of interest and the interplanetary characteristics of the disturbed periods. It is found that the suprathermal (>36 eV) ion measurements from the Pioneer Venus neutral mass spectrometer (ONMS) provide convincing evidence of the related enhanced escape of mostly O+ suprathermal ions. The results provide the first direct demonstration that space weather has played an important role in Venus (and other weakly magnetized planet) atmosphere loss through time. It is important to pursue study of these effects with the new measurements from Venus Express and, in light of the approaching solar activity cycle rising phase, extremely timely.

Near‐terminator Venus ionosphere: How Chapman‐esque?

We have modeled the near‐terminator ionosphere of Venus for solar zenith angles χ between 60 and 85° in 5° increments, and from 86 to 90° in 1° increments. The most important neutral densities of the background thermospheres have been adopted from the VTS3 model of Hedin et al. (1983), which is based on densities from the Pioneer Venus (PV) Orbiter Neutral Mass Spectrometer (e.g., Niemann et al., 1980) that are normalized to the PV Orbiter Atmospheric Drag data (e.g., Keating et al., 1980). We compare the ion density profiles to those of a Chapman layer and to those obtained from radio occultation data. We determine the best fit values of nmax,0i and k in the Chapman expression nmax,χi = nmax,0i cos(χ)k for each solar zenith angle interval; using a linear least squares regression for log nmax,χi as a function of log cos(χ) we also derive the best fit values for the group of ten models for solar zenith angles from 60 to 89°. For a theoretical Chapman layer with plane parallel geometry, k = 0.5, and nmax,0i is the peak electron density at the subsolar point. In the near‐terminator region, the production rates and densities must be computed using spherical geometry, and this alone will cause the derived value of k to be reduced below the theoretical factor of 0.5. For the solar zenith angle models in the range 60–85°, we find that the magnitudes of the both the main (F1) and lower (E) peaks decrease as the solar zenith angle increases in a somewhat Chapman‐like manner, but the altitudes of the F1 and E peaks increase only slowly from 140 to 144 km and from 125 to 129.5 km, respectively. The nearly constant altitude of the F1 peaks has been attributed to the collapse of the thermosphere as it merges into the nighttime cryosphere (Cravens et al., 1981). This behavior is quite different from that of Mars, where the electron density peak altitudes have been observed to rise monotonically as the solar zenith angle increases. The predicted behavior for the Venus ionosphere is in general agreement with radio occultation data, but some differences also are observed. We discuss the differences between the modeled and observed peak altitudes and we attribute the differences partly to deficiencies in the VTS3 models and partly to uncertainties in the electron temperatures. There also may be some errors introduced into the radio occultation profiles in the very near‐terminator region due to deviations from spherical symmetry. In addition, differences exist between the measured and model peak magnitudes at solar zenith angles near 60° that we ascribe partly to the use of the S2K v2.22 solar flux model of Tobiska (2004), for which the EUV and soft X‐ray photon fluxes are significantly smaller than those of the S2K v1.24 or the Hinteregger solar flux models.

Evolution of solar wind structures from 0.72 to 1 AU

Understanding the evolution of solar wind structures in the inner heliosphere as they approach the Earth is important to space weather prediction. From the in situ solar wind plasma and magnetic field measurements of Pioneer Venus Orbiter (PVO) at 0.72 AU (1979–1988), and of Wind/Advanced Composition Explorer (ACE) missions at 1 AU (1995–2004), we identify and characterize two major solar wind structures, stream interaction regions (SIRs) and interplanetary coronal mass ejections (ICMEs). The average percentage of SIRs occurring with shocks increases significantly from 3% to 24% as they evolve from 0.72 to 1 AU. The average occurrence rate, radial extent, and bulk velocity variation of SIRs do not change from 0.72 to 1 AU, while peak pressure and magnetic field strength both decrease with the radial evolution of SIRs. Within the 0.28 AU distance from the orbit of Venus to that of Earth, the average fraction of ICMEs with shocks increases from 49% to 66%, and the typical radial extent of ICMEs expands by about a fraction of 1.4, with peak pressure and magnetic field strength decreasing significantly. The mean occurrence rate and expansion velocity of ICMEs do not change from 0.72 to 1 AU.

Effects of Low Energetic Neutral Atoms on Martian and Venusian Dayside Exospheric Temperature Estimations

The heating of the Upper atmospheres and the formation of the ionospheres on Venus and Mars are mainly controlled by the solar X-ray and extreme ultraviolet (EUV) radiation (λ = 0.1 − 102.7 nm and can be characterized by the 10.7 cm solar radio flux). Previous estimations of the average Martian dayside exospheric temperature inferred from topside plasma scale heights, UV airglow and Lyman-a dayglow observations of up to ∼500 K imply a stronger dependence on solar activity than that found on Venus by the Pioneer Venus Orbiter (PVO) and Magellan spacecraft. However, this dependence appears to be inconsistent with exospheric temperatures (<250 K) inferred from aerobraking maneuvers of recent spacecraft like Mars Pathfinder, Mars Global Surveyor and Mars Odyssey during different solar activity periods and at different orbital locations of the planet. In a similar way, early Lyman-α dayglow and UV airglow observations by Venera 4, Mariner 5 and 10, and Venera 9–12 at Venus also suggested much higher exospheric temperatures of up to 1000 K as compared with the average dayside exospheric temperature of about 270 K inferred from neutral gas mass spectrometry data obtained by PVO. In order to compare Venus and Mars, we estimated the dayside exobase temperature of Venus by using electron density profiles obtained from the PVO radio science experiment during the solar cycle and found the Venusian temperature to vary between 250–300 K, being in reasonable agreement with the exospheric temperatures inferred from Magellan aerobraking data and PVO mass spectrometer measurements. The same method has been applied to Mars by studying the solar cycle Variation of the ionospheric peak plasma density observed by Mars Global Surveyor during both solar minimum and maximum conditions, yielding a temperature range between 190–220 K. This result clearly indicates that the average Martian dayside temperature at the exobase does not exceed a value of about 240 K during high solar activity conditions and that the response of the Upper atmosphere temperature on Mars to solar activity near the ionization maximum is essentially the same as on Venus. The reason for this discrepancy between exospheric temperature determinations from topside plasma scale heights and electron distributions near the ionospheric maximum seems to lie in the fact that thermal and photochemical equilibrium applies only at altitudes below 170 km, whereas topside scale heights are derived for much higher altitudes where they are modified by transport processes and where local thermodynamic equilibrium (LTE) conditions are violated. Moreover, from simulating the energy density distribution of photochemically produced moderately energetic H, C and O atoms, as well as CO molecules, we argue that exospheric temperatures inferred from Lyman-α dayglow and UV airglow observations result in too high values, because these particles, as well as energetic neutral atoms, transformed from solar wind protons into hydrogen atoms via Charge exchange, may contribute to the observed planetary hot neutral gas coronae. Because the low exospheric temperatures inferred from neutral gas mass spectrometer and aerobraking data, as well as from CO 2 + UV doublet emissions near 180–260 nm obtained from the Mars Express SPICAM UV spectrograph suggest rather low heating efficiencies, some hitherto unidentified additional IR-cooling mechanism in the thermospheres of both Venus and Mars is likely to exist.

The interaction of the shocked solar wind with the ionosphere of Venus (with a glimpse to Titan)

In this review, we attempt to show that investigators in the early stage of the data analysis of the Pioneer Venus Orbiter, and previous missions reached a physical picture which is still valid for the description of the interaction of the shocked solar wind with a non-magnetized, conductive obstacle like Venus. It was concluded that behind the bow shock an extended layer (the so-called “mantle) separated the shocked plasma flow and the ionosphere. Elements of this picture were forgotten, reinvented then under new names. We summarize these early results; and we show that current results do validate this early picture, while its details have been much enhanced. We also present a model that describes wave excitations, attempting to explain how events on microphysical scale can be connected to the more general macrophysical experimental results. The results of the Cassini mission near Titan are used as examples.

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.

Loss of hydrogen and oxygen from the upper atmosphere of Venus

Atmospheric escape from the upper atmosphere of Venus is mainly influenced by the loss of hydrogen and oxygen caused by the interaction of solar radiation and particle flux with the unprotected planetary environment. Because one main aim of the ASPERA-4 particle/plasma and VEX-MAG magnetic field experiments on board of ESA's forthcoming Venus Express mission is the investigation of atmospheric erosion processes from the planet's ionosphere–exosphere environment, we study the total loss of hydrogen and oxygen and identified the efficiency of several escape mechanisms involved. For the estimation of pick up loss rates we use a gas dynamic test particle model and obtained average loss rates for H + , and O + pick up ions of about 1 × 10 25 s - 1 and about 1.6 × 10 25 s - 1 , respectively. Further, we estimate ion loss rates due to detached plasma clouds, which were observed by the pioneer Venus orbiter and may be triggered by the Kelvin–Helmholtz instability of about 0.5 – 1 × 10 25 s - 1 . Thermal atmospheric escape processes and atmospheric loss by photo-chemically produced oxygen atoms yield negligible loss rates. Sputtering by incident pick up O + ions give O atom loss rates in the order of about 6 × 10 24 s - 1 . On the other hand, photo-chemically produced hot hydrogen atoms are a very efficient loss mechanism for hydrogen on Venus with a global average total loss rate of about 3.8 × 10 25 s - 1 , which is in agreement with Donahue and Hartle [1992. Solar cycle variations in H + and D + densities in the Venus ionosphere: implications for escape. Geophys. Res. Lett. 12, 2449–2452] and of the same order but less than the estimated H + ion outflow on the Venus nightside of about 7.0 × 10 25 s - 1 due to acceleration by an outward electric polarization force related to ionospheric holes by Hartle and Grebowsky [1993. Light ion flow in the nightside ionosphere of Venus. J. Geophys. Res. 98, 7437–7445]. Our study indicates that on Venus, due to its larger mass and size compared to Mars, the most relevant atmospheric escape processes of oxygen involve ions and are caused by the interaction with the solar wind. The obtained results indicate that the ratio between H/O escape to space from the Venusian upper atmosphere is about 4, and is in a much better agreement with the stoichiometrically H/O escape ratio of 2:1, which is not the case on Mars. However, a detailed analysis of the outflow of ions from the Venus upper atmosphere by the ASPERA-4 and VEX-MAG instruments aboard Venus Express will lead to more accurate atmospheric loss estimations and a better understanding of the planet's water inventory.

Dynamics of the Venus upper atmosphere: Outstanding problems and new constraints expected from Venus Express

A consistent picture of the dynamics of the Venus upper atmosphere from ∼ 90 to 200 km has begun to emerge [e.g., Bougher, S.W., Alexander, M.J., Mayr, H.G., 1997. Upper Atmosphere Dynamics: Global Circulation and Gravity Waves. Venus II, CH. 2.4. University of Arizona Press, Tucson, pp. 259–292; Lellouch, E., Clancy, T., Crisp, D., Kliore, A., Titov, D., Bougher, S.W., 1997. Monitoring of Mesospheric Structure and Dynamics. Venus II, CH. 3.1. University of Arizona Press, Tucson, pp. 295–324]. The large-scale circulation of the Venus upper atmosphere (upper mesosphere and thermosphere) can be decomposed into two distinct flow patterns: (1) a relatively stable subsolar-to-antisolar (SS–AS) circulation cell driven by solar heating, and (2) a highly variable retrograde superrotating zonal (RSZ) flow. Wave-like perturbations have also been observed. However, the processes responsible for maintaining (and driving variations in) these SS–AS and RSZ winds are not well understood. Variations in winds are thought to result from gravity wave breaking and subsequent momentum and energy deposition in the upper atmosphere [Alexander, M.J., 1992. A mechanism for the Venus thermospheric superrotation. Geophys. Res. Lett. 19, 2207–2210; Zhang, S., Bougher, S.W., Alexander, M.J., 1996. The impact of gravity waves on the Venus thermosphere and O 2 IR nightglow. J. Geophys. Res. 101, 23195–23205]. However, existing data sets are limited in their spatial and temporal coverage, thereby restricting our understanding of these changing circulation patterns. One of the major goals of the Venus Express (VEX) mission is focused upon increasing our understanding of the circulation and dynamical processes of the Venus atmosphere up to the exobase [Titov, D.V., Lellouch, E., Taylor, F.W., 2001. Venus Express: Response to ESA's call for ideas for the re-use of the Mars Express platform. Proposal to European Space Agency, 1–74]. Several VEX instruments are slated to obtain remote measurements (2006–2008) that will complement those obtained earlier by the Pioneer Venus Orbiter (PVO) between 1978 and 1992. These VEX measurements will provide a more comprehensive investigation of the Venus upper atmosphere (90–200 km) structure and dynamics over another period in the solar cycle and for variable lower atmosphere conditions. An expanded climatology of Venus upper atmosphere structure and wind components will be developed. In addition, gravity wave parameters above the cloud tops will be measured (or inferred), and used to constrain gravity wave breaking models. In this manner, the gravity wave breaking mechanism (thought to regulate highly variable RSZ winds) can be tested using Venus general circulation models (GCMs).

Magnetic field investigation of the Venus plasma environment: Expected new results from Venus Express

The Venus Express mission is scheduled for launch in 2005. Among many other instruments, it carries a magnetometer to investigate the Venus plasma environment. Although Venus has no intrinsic magnetic moment, magnetic field measurements are essential in studying the solar wind interaction with Venus. Our current understanding of the solar wind interaction with Venus is mainly from the long lasting Pioneer Venus Orbiter (PVO) observations. In this paper, we briefly describe the magnetic field experiment of the Venus Express mission. We compare Venus Express mission with PVO mission with respect to the solar wind interaction with Venus. Then we discuss what we will achieve with the upcoming Venus Express mission.

The solar wind interaction with Venus through the eyes of the Pioneer Venus Orbiter

Venus has no internal magnetic dynamo and thus its ionosphere and hot oxygen exosphere dominate the interaction with the solar wind. The solar wind at 0.72 AU has a dynamic pressure that ranges from 4.5 nPa (at solar max) to 6.6 nPa (at solar min), and its flow past the planet produces a shock of typical magnetosonic Mach number 5 at the subsolar point. At solar maximum the pressure in the ionospheric plasma is sufficient to hold off the solar wind at an altitude of 400 km above the surface at the subsolar point, and 1000 km above the terminators. The deflection of the solar wind occurs through the formation of a magnetic barrier on the inner edge of the magnetosheath, or shocked solar wind. Under typical solar wind conditions the time scale for diffusion of the magnetic field into the ionosphere is so long that the ionosphere remains field free and the barrier deflects almost all the incoming solar wind. Any neutral atoms of the hot oxygen exosphere that reach the altitude of the magnetosheath are accelerated by the electric field of the flowing magnetized plasma and swept along cycloidal paths in the antisolar direction. This pickup process, while important for the loss of the Venus atmosphere, plays a minor role in the deceleration and deflection of the solar wind. Like at magnetized planets, the Venus shock and magnetosheath generate hot electrons and ions that flow back along magnetic field lines into the solar wind to form a foreshock. A magnetic tail is created by the magnetic flux that is slowed in the interaction and becomes mass-loaded with thermal ions. The structure of the ionosphere is very much dependent on solar activity and the dynamic pressure of the solar wind. At solar maximum under typical solar wind conditions, the ionosphere is unmagnetized except for the presence of thin magnetic flux ropes. The ionospheric plasma flows freely to the nightside forming a well-developed night ionosphere. When the solar wind pressure dominates over the ionospheric pressure the ionosphere becomes completely magnetized, the flow to the nightside diminishes, and the night ionosphere weakens. Even at solar maximum the night ionosphere has a very irregular density structure. The electromagnetic environment of Venus has not been well surveyed. At ELF and VLF frequencies there is noise generated in the foreshock and shock. At low altitude in the night ionosphere noise, presumably generated by lightning, can be detected. This paper reviews the plasma environment at Venus and the physics of the solar wind interaction on the threshold of a new series of Venus exploration missions.

Effect of the motional electric field on the Venus nightside ionopause

The purpose of this paper is to clarify how the average structure of the Venus nightside ionopause for solar zenith angles (SZA) greater than 90° depends on (1) the direction and (2) the magnitude of the motional electric field of the solar wind. Plasma density structure in the Venus nightside ionosphere has been investigated by using data sets of the Pioneer Venus Orbiter observations. It is found that the distribution of the nightside ionopause locations is asymmetric with respect to the direction of the solar wind electric field, leaning opposite to the electric field vector. It is also found that the asymmetry is increasingly prominent as the magnitude of the motional electric field increases, while not so prominent for small field magnitude. This result suggests that the asymmetric ionopause location in the nightside is related to the acceleration of pickup ions.

Venus O + pickup ions: Collected PVO results and expectations for Venus Express

Observations of oxygen pickup ions by the plasma analyzer on the Pioneer Venus Orbiter (PVO) Mission arguably launched broad interest in solar wind erosion of unmagnetized planet atmospheres, and its potential evolutionary effects. Oxygen pickup ions may play key roles in the removal of the oxygen excess left behind from the photodissociation of water vapor by enabling direct escape, additional sputtering of oxygen when they impact the exobase, and escape as energetic neutrals produced in charge exchange reactions with the ambient exospheric oxygen and hydrogen. Although the PVO observations were compromised by an ∼8 keV energy limit for O + detection, a lack of ion composition capability, and the limited sampling and data rate of the plasma analyzer which was designed for solar wind monitoring, these measurements provide our best information about the extended O + exosphere and wake at Venus. Here we show the full picture of the spatial distribution and energies of the O + ion observations collected by the plasma analyzer during PVO's ∼5000 orbit tour. A model of O + test particles launched in the circum-Venus fields described by an MHD simulation of the solar wind interaction is used to help interpret the PVO observations and to anticipate the expanded view of Venus O + escape that will be provided by the ASPERA-4 experiment on Venus Express.

Magnetic field configuration in the Venus ionosphere

We investigated the magnetic field configuration in the Venus ionosphere, from the data (orbit 1–600) obtained by Pioneer Venus Orbiter (PVO). The magnetic field lines were draping the Venus ionosphere, but the magnetic field lines were opened to the interplanetary space in the nightside ionosphere. We found the closed magnetic field line around Venus near the magnetic equator at 600–800 km altitudes during the high solar wind dynamic pressure. The result shows that magnetic reconnection occurs in the Venus nightside ionosphere.

Comparative outline of the region of interaction of the solar wind with the Ionosphere of Venus and Mars

The general features of the region of interaction of the solar wind with the ionosphere of Venus and Mars are compared using data obtained with the Mariner 5 and the Pioneer Venus Orbiter (PVO) spacecraft for Venus and with the Phobos II, the Mars Global Surveyor (MGS) and the Mars Express spacecraft for Mars. Despite the overall weak intrinsic global magnetic field that is present in both planets there are significant differences in the manner in which the interplanetary magnetic field accumulates and is organized around and within their ionosphere. Such differences are unrelated to the crustal magnetic field remnants inferred from the MGS measurements around Mars. In fact, while in Venus and Mars there is a region in which the magnetic field becomes enhanced as it piles up in their plasma environment it is shown that such a region exhibits different regimes with respect to changes in the ion composition measured outside and within the ionosphere. At Venus the region of enhanced magnetic field intensity occurs in general above the ionopause which represents the boundary across which there is a change in the ion composition with dominant solar wind protons above and planetary O + ions below. At Mars the region of enhanced magnetic field is located below a magnetic pileup boundary across which there is also a comparable change in the ion composition (solar wind protons above and planetary O + ions below). It is argued that this difference in the relative position of the region of enhanced magnetic field with respect to that of a plasma boundary that separates different ion populations results from the peculiar response of the ionosphere of each planet to the oncoming solar wind dynamic pressure. While at Venus the peak ionospheric thermal pressure is in general sufficient to withhold the incident solar wind kinetic pressure there is a different response in Mars where the peak ionospheric thermal pressure is in general not large enough to deviate the solar wind. In this latter case the ionosphere is unable to force the solar wind to move around the ionosphere and as a result the oncoming electron population can reach low altitudes where it is influenced by neutral atmospheric particles (the solar wind proton population is replaced at the magnetic pileup boundary which marks the upper extent of the region where the interplanetary magnetic field becomes enhanced). Peculiar conditions are expected near the magnetic polar regions and over the terminator plane where the solar wind is directed along the sides of the planet.