Initial Venus Express magnetic field observations of the magnetic barrier at solar minimum

Although there is no intrinsic magnetic field at Venus, the convected interplanetary magnetic field piles up to form a magnetic barrier in the dayside inner magnetosheath. In analogy to the Earth's magnetosphere, the magnetic barrier acts as an induced magnetosphere on the dayside and hence as the obstacle to the solar wind. It consists of regions near the planet and its wake for which the magnetic pressure dominates all other pressure contributions. The initial survey performed with the Venus Express magnetic field data indicates a well-defined boundary at the top of the magnetic barrier region. It is clearly identified by a sudden drop in magnetosheath wave activity, and an abrupt and pronounced field draping. It marks the outer boundary of the induced magnetosphere at Venus, and we adopt the name “magnetopause” to address it. The magnitude of the draped field in the inner magnetosheath gradually increases and the magnetopause appears to show no signature in the field strength. This is consistent with PVO observations at solar maximum. A preliminary survey of the 2006 magnetic field data confirms the early PVO radio occultation observations that the ionopause stands at ∼250 km altitude across the entire dayside at solar minimum. The altitude of the magnetopause is much lower than at solar maximum, due to the reduced altitude of the ionopause at large solar zenith angles and the magnetization of the ionosphere. The position of the magnetopause at solar minimum is coincident with the ionopause in the subsolar region. This indicates a sinking of the magnetic barrier into the ionosphere. Nevertheless, it appears that the thickness of the magnetic barrier remains the same at both solar minimum and maximum. We have found that the ionosphere is magnetized ∼95% of the time at solar minimum, compared with 15% at solar maximum. For the 5% when the ionosphere is un-magnetized at solar minimum, the ionopause occurs at a higher location typically only seen during solar maximum conditions. These have all occurred during extreme solar conditions.     

Ionospheric currents induced by solar wind interaction with planetary atmospheres

In a steady-state model for the interaction of the solar wind with the atmosphere of a non-magnetic planet, the magnetized solar wind acts as a dynamo over the dayside of the planet and induces Ohmic currents in the planet's ionosphere. A model for the dynamo mechanism and for the induced current configuration is developed. Based on this model and assumed model atmospheres of Mars and Venus, the distribution of currents entering the ionosphere through the ionopause is calculated. The requirement that the total current be of such a magnitude as to cancel the shock-compressed interplanetary magnetic field fixes the ionopause altitude. The calculations for Venus are in reasonable agreement with observations. The calculations for Mars indicate the possibility of an observable ionopause in the altitude range from 325 to 425 km.     

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.     

Magnetic states of the ionosphere of Venus observed by Venus Express

Strong ultraviolet radiation from the Sun ionizes the upper atmosphere of Venus, creating a dense ionosphere on the dayside of the planet. In contrast to Earth, the ionosphere of Venus is not protected against the solar wind by a magnetic field. However, the interaction between charged ionospheric particles and the solar wind dynamic and magnetic pressure creates a pseudo-magnetosphere which deflects the solar wind flow around the planet (Schunk and Nagy, 1980). The combination of changing solar radiation and solar wind intensities leads to a highly variable structure and plasma composition of the ionosphere. The instrumentation of the Venus Express spacecraft allows to measure the magnetic field (MAG experiment) as well as the electron energy spectrum and the ion composition (ASPERA-4 experiment) of the upper ionosphere and ionopause. In contrast to the earlier Pioneer Venus Orbiter (PVO) measurements which were conducted during solar maximum, the solar activity was very low in the period 2006-2009. A comparison with PVO allows for an investigation of ionospheric properties under different solar wind and EUV radiation conditions. Observations of MAG and ASPERA have been analyzed to determine the positions of the photoelectron boundary (PEB) and the “magnetopause” and their dependence on the solar zenith angle (SZA). The PEB was determined using the ELS observations of ionospheric photoelectrons, which can be identified by their specific energy range. It is of particular interest to explore the different magnetic states of the ionosphere, since these influence the local plasma conductivity, currents and probably the escape of electrons and ions. The penetration of magnetic fields into the ionosphere depends on the external conditions as well as on the ionospheric properties. By analyzing a large number of orbits, using a combination of two different methods, we define criteria to distinguish between the so-called magnetized and unmagnetized ionospheric states. Furthermore, we confirm that the average magnetic field inside the ionosphere shows a linear dependence on the magnetic field in the region directly above the PEB.     

Planetary ENA Imaging: Venus and a comparison with Mars

We present simulated images of energetic neutral atoms (ENAs) produced in charge exchange collisions between solar wind protons and neutral atoms in the exosphere of Venus, and make a comparison with earlier results for Mars. The images are found to be dominated by two local maxima. One produced by charge exchange collisions in the solar wind, upstream of the bow shock, and the other close to the dayside ionopause. The simulated ENA fluxes at Venus are lower than those obtained in similar simulations of ENA images at Mars at solar minimum conditions, and close to the fluxes at Mars at solar maximum. Our numerical study shows that the ENA flux decreases with an increasing ionopause altitude. The influence of the Venus nighttime hydrogen bulge on the ENA emission is small.     

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 × 10²⁶ 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.     

Solar wind flow past Venus and its implications for the occurrence of the Kelvin–Helmholtz instability

In this paper, the solar wind flow around Venus is modeled as a nondissipative fluid which obeys the ideal magnetohydrodynamic equations extended for mass loading processes. The mass loading parameter is calculated for four different cases, corresponding to solar minimum and maximum XUV flux and to nominal and low solar wind velocity. We get smooth profiles of the field and plasma parameters in the magnetosheath. Based on the results of this flow model, we investigate the occurrence of the Kelvin–Helmholtz (K–H) instability at the equatorial flanks of the ionopause of Venus. By comparing the instability growth time with the propagation time of the K–H wave, we find that the K–H instability can evolve at the ionopause for all four solar wind conditions.     

Comparison study of magnetic flux ropes in the ionospheres of Venus, Mars and Titan

Magnetic flux ropes are created in the ionosphere of Venus and Mars during the interaction of the solar wind with their ionospheres and also at Titan during the interaction of the Saturnian magnetospheric plasma flow with Titan’s ionosphere. The flux ropes at Venus and Mars were extensively studied from Pioneer Venus Orbiter and Mars Global Surveyor observations respectively during solar maximum. Based on the statistical properties of the observed flux ropes at Venus and Mars, the formation of a flux rope in the ionosphere is thought first to arise near the boundary between the magnetic barrier and the ionosphere and later to sink into the lower ionosphere. Venus flux ropes are also observed during solar minimum by Venus Express and the observations of developing and mature flux ropes are consistent with the proposed mechanism. With the knowledge of flux rope structure in the Venus ionosphere, the twisted fields in the lower ionosphere of Titan from Cassini observations are studied and are found to resemble the Venus flux ropes.     

Disappearing ionospheres on the nightside of Venus

Instruments on the Pioneer Venus Orbiter have detected a substantial ionosphere on the nightside of Venus during most orbits. However, during some orbits the nightside ionosphere seems to have almost disappeared, existing only as irregular patches of low-density plasma. The solar wind dynamic pressure on these occasions is greater than average. We have correlated data from several instruments (Langmuir probe, ion mass spectrometer, retarding potential analyzer, magnetometer, and plasma analyzer) for a number of orbits during which the nightside ionosphere had disappeared. The magnetic field tends to be coherent, horizontal, and larger than usual, and the electron and ion temperatures are much larger than they usually are on the nightside. We suggest mechanisms which might explain the reasons for the disappearance of the ionosphere when the solar wind dynamic pressure is large.     

Formation of the plasma mantle in the Venusian magnetosphere

A model of the interaction of the solar wind with Venus is proposed including magnetic barrier formation, ionopause structure, plasma dynamics in the magnetic barrier, and the formation of the Venusian tail (wake). It is shown that under stationary conditions the ionopause is practically an equipotential boundary and its current is determined by a diamagnetic drift. The source of the plasma mantle can be provided by photoions appearing in the magnetic barrier and convecting toward the wake as a result of both magnetic pressure gradient and magnetic tension. The formation of the magnetic tail is determined by convection of magnetic barrier flux tubes in which the solar-wind plasma is replaced by ions of planetary origin. Compared to observational data the proposed model gives somewhat overestimated values of ion convective velocity and magnetic barrier thickness near the terminator and underestimated values of number density and magnetic field strength in the tail. Accordingly this suggests the possible influence of the anomalous ionization effects in the solar wind—Venus interaction.     

Ion exchange with the solar wind for planets with negligible intrinsic magnetic fields

The exchange of ions between the ionosphere of a planet with negligible intrinsic magnetic field, and the solar wind is examined. It is suggested that a balance exists between the outflow of ionospheric ions at the plasmapause and ions from the solar wind in a restricted region close to the subsolar point. This results in a current system towards the subsolar point on the surface of the ionopause and a toroidal magnetic field. Simple calculations are made of the current and field configuration that might result from the system for conditions similar to those encountered on the Viking 1 and 2 transits of the Mars ionosphere.     

The Kelvin–Helmholtz instability at Venus: What is the unstable boundary?

The Kelvin–Helmholtz instability gained scientific attention after observations at Venus by the spacecraft Pioneer Venus Orbiter gave rise to speculations that the instability contributes to the loss of planetary ions through the formation of plasma clouds. Since then, a handful of studies were devoted to the Kelvin–Helmholtz instability at the ionopause and its implications for Venus. The aim of this study is to investigate the stability of the two instability-relevant boundary layers around Venus: the induced magnetopause and the ionopause. We solve the 2D magnetohydrodynamic equations with the total variation diminishing Lax–Friedrichs algorithm and perform simulation runs with different initial conditions representing the situation at the boundary layers around Venus. Our results show that the Kelvin–Helmholtz instability does not seem to be able to reach its nonlinear vortex phase at the ionopause due to the very effective stabilizing effect of a large density jump across this boundary layer. This seems also to be true for the induced magnetopause for low solar activity. During high solar activity, however, there could occur conditions at the induced magnetopause which are in favour of the nonlinear evolution of the instability. For this situation, we estimated roughly a growth rate for planetary oxygen ions of about 7.6 × 10²⁵ s⁻¹, which should be regarded as an upper limit for loss due to the Kelvin–Helmholtz instability.     

Helium on Mars and Venus: EUVE observations and modeling

Long-exposure spectroscopy of Mars and Venus with the Extreme Ultraviolet Explorer (EUVE) has revealed emissions of He 584 Å on both planets and He 537 Å/O⁺ 539 Å and He⁺ 304 Å on Venus. Our knowledge of the solar emission at 584 Å, eddy diffusion in Mars' upper atmosphere, electron energy distributions above Mars' ionopause, and hot oxygen densities in Mars' exosphere has been significantly improved since our analysis of the first EUVE observation of Mars [Krasnopolsky, Gladstone, 1996, Helium on Mars: EUVE and Phobos data and implications for Mars' evolution, J. Geophys. Res. 101, 15,765-15,772]. These new results and a more recent EUVE observation of Mars are the motivation for us to revisit the problem in this paper. We find that the abundance of helium in the upper atmosphere, where the main loss processes occur, is similar to that in the previous paper, though the mixing ratio in the lower and middle atmosphere is now better estimated at 10±6 ppm. Our estimate of the total loss of helium is almost unchanged at 8×¹⁰²³ s⁻¹, because a significant decrease in the loss by electron impact ionization above the ionopause is compensated by a higher loss in collisions with hot oxygen. We neglect the outgassing of helium produced by radioactive decay of U and Th because of the absence of current volcanism and a very low upper limit to the seepage of volcanic gases. The capture of solar wind α-particles is currently the only substantial source of helium on Mars, and its efficiency remains at 0.3. A similar analysis of EUV emissions from Venus results in a helium abundance in the upper atmosphere which is equal to the mean of the abundances measured previously with two optical and two mass spectrometers, and a derived helium mixing ratio in the middle and lower atmosphere of 9±6 ppm. Helium escape by ionization and sweeping out of helium ions by the solar wind above the ionopause is smaller than that calculated by Prather and McElroy [1983, Helium on Venus: implications for uranium and thorium, Science 220, 410-411] by a factor of 3. However, charge exchange of He⁺ ions with CO₂ and N₂ between the exobase and ionopause and collisions with hot oxygen ignored previously add to the total loss which appears to be at the level of 10⁶ cm⁻² s⁻¹ predicted by Prather and McElroy [1983, Science 220, 410-411]. The loss of helium is compensated by outgassing of helium produced by radioactive decay of U and Th and by the capture of the solar wind α-particles with an efficiency of 0.1. We also compare our derived α-particle capture efficiencies for Mars and Venus with observed X-ray emissions resulting from the charge exchange of solar wind heavy ions with the extended atmospheres on both planets [Dennerl et al., 2002, Discovery of X-rays from Venus with Chandra, Astron. Astrophys. 386, 319-330; Dennerl, 2002, Discovery of X-rays from Mars with Chandra, Astron. Astrophys. 394, 1119-1128]. The emissions from both disk and halo on Mars agree with our calculated values; however, we do not see a reasonable explanation for the X-ray halo emission on Venus. The ratio of the charge exchange efficiencies derived from the disk X-ray emissions of Mars and Venus is similar to the ratio of the capture efficiencies for these planets. The surprisingly bright emission of He⁺ at 304 Å observed by EUVE and Venera 11 and 12 suggests that charge exchange in the flow of the solar wind α-particles around the ionopause is much stronger than in the flow of α-particles into the ionosphere.     

Viscous flow properties in the transport of solar wind momentum to the Venus upper ionosphere

A comparative study of the viscous transport of solar wind momentum to the upper layers of the Venus ionosphere with that occurring within the trans-terminator flow leads to estimates of the ratio of the viscosity coefficients that are applicable to both cases. Support for viscous forces between the solar wind and the ionospheric plasma in the trans-terminator flow derives from the momentum flux balance between the momentum flux in the latter flow and the deficiency of solar wind momentum along the flanks of the ionosheath. By comparing the relative width of the viscous boundary layer in the Venus ionosheath and the width of the trans-terminator flow we find that the transport of momentum within the upper ionosphere proceeds at a rate similar to that at which momentum is delivered to the upper ionosphere from the solar wind. Comparable values are obtained for the viscosity coefficient of the solar wind that streams over the ionosphere and that implied from momentum transport within the ionospheric trans-terminator flow. It is further suggested that despite the different nature of the processes that give place to the viscous transport of the solar wind momentum to the upper ionosphere (wave-particle interactions) and those responsible for its distribution within the ionosphere (through coulombian collisions) there is a similar response in the behavior of both plasmas to momentum transport. Calculations show that with comparable values of the viscosity coefficient in the ionosheath and in the upper ionospheric plasma the mean free path suitable to wave-particle interactions in the ionosheath is of the same order of magnitude as the mean free path of the planetary O⁺ ions that interact through coulombian collisions in the upper ionosphere. The effects of this similarity are considered in the discussion.     

Dynamics of the Venus ionosphere: A two-dimensional model study

A two-dimensional model of the ionosphere of Venus which simulates ionospheric dynamics by self-consistently solving the plasma equations of motion, including the inertial term, in finite difference form has been constructed. The model, which is applied over the solar zenith angle range extending from 60 to 140° and the altitude range 100 to 480 km, simulates the measured horizontal velocity field quite satisfactorily. The ion density field is somewhat overestimated on the dayside because of the choice model neutral atmosphere and underestimated on the nightside because of setting the ionopause height at too low an altitude. It is concluded that solar photoionization on the dayside and ion recombination on the nightside are the processes mainly responsible for accelerating the plasma to the observed velocities. The plasma flow appears to be sufficient to maintain the nightside ionosphere at or near the observed median level of ion densities.     

An electrodynamic model of electric currents and magnetic fields in the dayside ionosphere of Venus

The electric current configuration induced in the ionosphere of Venus by the interaction of the solar wind has been calculated in previous papers (Cloutier and Daniell, Planet. Space Sci. 21, 463, 1973; Daniell and Cloutier. Planet. Space Sci.25, 621, 1977; Cloutier and Daniell, Planet. Space Sci.27, 1111, 1979) for average steady-state solar wind conditions and interplanetary magnetic field. This model is generalized to include the effects of (a) plasma depletion and magnetic field enhancement near the ionopause, (b) velocity-shear-induced MHD instabilities of the Kelvin-Helmholtz type within the ionosphere, and (c) variations in solar wind parameters and interplanetary magnetic field. It is shown that the magnetic field configuration resulting from the model varies in response to changes in solar wind and interplanetary field conditions, and that these variations produce magnetic field profiles in excellent agreement with those seen by the PIONEER-VENUS Orbiter. The formation of “flux-ropes” by the Kelvin-Helmholtz instability is shown to be a natural consequence of the model, with the spatial distribution and size of the flux-ropes determined by the magnetic Reynolds number.     

Observations of energetic ions near the Venus ionopause

Ions (primarily O⁺) with spacecraft rest frame energies >40 eV have been observed by the Pioneer Venus Neutral Mass Spectrometer. The signature occurs in about 13% of the 700 orbits examined, primarily near the ionopause and at all solar zenith angles. The energetic ions coincide in location with superthermal ions observed by the Ion Mass Spectrometer and more rarely occur in some of the plasma clouds observed by the Electron Temperature Probe. These observations in conjunction with measurements by the Plasma Wave Instrument near the ionopause suggest that the ions are accelerated out of ionospheric plasma by the shocked solar wind through plasma waveparticle interactions.     

Magnetic fields of mars and venus: Solar wind interactions

Recent U.S.S.R. studies of the magnetic field and solar wind flow in the vicinity of Mars and Venus confirm earlier U.S.A. reports of a bow shock wave developed as the solar wind interacts with these planets. Mars 2 and 3 magnetometer experiments report the existence of an intrinsic planetary magnetic field, sufficiently strong to form a magnetopause, deflecting the solar wind around the planet and its ionosphere. This is in contrast to the case for Venus, where it is assumed to be the ionosphere and processes therein which are responsible for the solar wind deflection. An empirical relationship appears to exist between planetary dipole magnetic moments and their angular momentum for Moon, Mars, Venus, Earth and Jupiter. Implications for the magnetic fields of Mercury and Saturn are discussed.Paper presented at the Lunar Science Institute Conference on Geophysical and Geochemical Exploration of the Moon and Planets, January 10–12, 1973     

Effectivity of the modified two stream instability operating in the vicinity of Venus

This paper is devoted to the application of the modified two stream or cross current instability (MTSI) to the interaction of the solar wind and Venus. Two scenarios are presented providing favorable conditions for the excitation of the instability. For the first scenario, the free energy source of the MTSI is a significant gradient drift of the solar wind protons near the subsolar ionopause. The corresponding growth rates and frequencies of the MTSI are calculated within a full electromagnetic approach for a two-component plasma. The driving source of the second considered scenario consists in the relative drift velocity between solar wind and planetary particles. For modelling this situation, the dispersion equation for a four-component plasma is solved numerically.     

Dynamics of the Venus ionosphere revisited

A dynamical model of the Venus ionosphere reported in an earlier paper has been modified to yield better agreement between predictions and observations. The results are interpreted with respect to changes in the ionopause height and neutral atmosphere structure.