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.     

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.     

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.     

Transterminator ion flow in the Martian ionosphere

The upper ionospheres of Mars and Venus are permeated by the magnetic fields induced by the solar wind. It is a long-standing question whether these fields can put the dense ionospheric plasma into motion. If so, the transterminator flow of the upper ionosphere could explain a significant part of the ion escape from the planets atmospheres. But it has been technically very challenging to measure the ion flow at energies below 20 eV. The only such measurements have been made by the ORPA instrument of the Pioneer Venus Orbiter reporting speeds of 1-5 km/s for O⁺ ions at Venus above 300 km altitude at the terminator ( [Knudsen et al., 1980] and [Knudsen et al., 1982]). At Venus the transterminator flow is sufficient to sustain a permanent nightside ionosphere, at Mars a nightside ionosphere is observed only sporadically. We here report on new measurements of the transterminator ion flow at Mars by the ASPERA-3 experiment on board Mars Express with support from the MARSIS radar experiment for some orbits with fortunate observation geometry. We observe a transterminator flow of O⁺ and O₂⁺ ions with a super-sonic velocity of around 5 km/s and fluxes of 0.8×10⁹/cm² s. If we assume a symmetric flux around the terminator this corresponds to an ion flow of 3.1±0.5×10²⁵/s half of which is expected to escape from the planet. This escape flux is significantly higher than previously observed on the tailside of Mars. A possible mechanism to generate this flux can be the ionospheric pressure gradient between dayside and nightside or momentum transfer from the solar wind via the induced magnetic field since the flow velocity is in the Alfvénic regime. We discuss the implication of these new observations for ion escape and possible extensions of the analysis to dayside observations which may allow us to infer the flow structure imposed by the induced magnetic field.     

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.     

Non-thermal water loss of the early Mars: 3D multi-ion hybrid simulations

In this study we analyze the non-thermal loss rates of O⁺, O₂⁺ and CO₂⁺ ions over the last 4.5 billion years (Gyr) in the Martian history by using a 3D hybrid model. For this reason we derived the past solar wind conditions in detail. We take into account the intensified particle flux of the early Sun as well as an Martian atmosphere, which was exposed to a sun's extreme ultraviolet (EUV) radiation flux 4.5 Gyr ago that was 100 times stronger than today. Furthermore, we model the evolution of the interplanetary magnetic field by a Weber & Davis solar wind model. The ‘external’ influences of the Sun's radiation flux and solar wind flux lead to the formation of an ionospheric obstacle by photoionization, charge exchange and electron impact. For the early Martian conditions we could show that charge exchange was the dominant ionization mechanism. Several hybrid simulations for different stages in the evolution of the Martian atmosphere, at 1, 2, 5, 10, 30 and 100 EUV, were performed to analyze the non-thermal escape processes by ion pick-up, momentum transfer from the solar wind to the ionosphere and detached ionospheric plasma clouds. Our results show a non-linear evolution of the loss rates. Using mean solar wind parameters the simulations result in an oxygen loss equivalent to the depth of a global Martian ocean of about 2.6 m over the last 4.5 Gyr. The induced magnetic field strength could be increased up to about 2000 nT. A simulation run with high solar wind density results in an oxygen loss of a Martian ocean up to 205 m depth during 150 million years after the sun reached the zero age mean sequence (ZAMS).     

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.     

Ionospheric plasma acceleration at Mars: ASPERA-3 results

The Analyzer of Space Plasma and Energetic Atoms (ASPERA) on-board the Mars Express spacecraft (MEX) measured penetrating solar wind plasma and escaping/accelerated ionospheric plasma at very low altitudes (250 km) in the dayside subsolar region. This implies a direct exposure of the martian topside atmosphere to solar wind plasma forcing leading to energization of ionospheric plasma. The ion and electron energization and the ion outflow from Mars is surprisingly similar to that over the magnetized Earth. Narrow “monoenergetic” cold ion beams, ion beams with broad energy distributions, sharply peaked electron energy spectra, and bidirectional streaming electrons are particle features also observed near Mars. Energized martian ionospheric ions (O⁺, O⁺₂, CO⁺₂, etc.) flow in essentially the same direction as the external sheath flow. This suggests that the planetary ion energization couples directly to processes in the magnetosheath/solar wind. On the other hand, the beam-like distribution of the energized plasma implies more indirect energization processes like those near the Earth, i.e., energization in a magnetized environment by waves and/or parallel (to B) electric fields. The general conditions for martian plasma energization are, however, different from those in the Earth's magnetosphere. Mars has a weak intrinsic magnetic field and solar wind plasma may therefore penetrate deep into the dense ionospheric plasma. Local crustal magnetization, discovered by Acuña et al. [Acuña, M.J., Connerey, J., Ness, N., Lin, R., Mitchell, D., Carlsson, C., McFadden, J., Anderson, K., Rème, H., Mazelle, C., Vignes, D., Wasilewski, P., Cloutier, P., 1999. Science 284, 790-793], provide some dayside shielding against the solar wind. On the other hand, multiple magnetic anomalies may also lead to “hot spots” facilitating ionospheric plasma energization. We discuss the ASPERA-3 findings of martian ionospheric ion energization and present evidences for two types of plasma energization processes responsible for the low- and mid-altitude plasma energization near Mars: magnetic field-aligned acceleration by parallel electric fields and plasma energization by low frequency waves.     

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.     

The Venus ionosphere at grazing incidence of solar radiation: Transport of plasma to the night ionosphere

Using a quasi-two-dimensional model of the Venus ionosphere, we calculated the ion number densities and horizontal ion bulk velocities expected for a range of solar zenith angles near the terminator (80 to 100°), and compared them with data obtained from the Pioneer Venus Orbiter retarding potential analyzer. The calculated ion bulk velocity arises entirely from the solar EUV-induced plasma pressure gradient and has a magnitude consistent with observations; ionization by suprathermal electrons is neglected in those computations. We find that while photoionization is the dominant source of ionospheric plasma for solar zenith angles less than 92°, plasma transport from the dayside is the dominant plasma source for solar zenith angles greater than 95°. We also show that the main nightside plasma peak at approximately 140 km altitude is of the F₂ type (i.e., is diffusion controlled). Its altitude and shape are thus quite insensitive to the altitude of the ion source.     

Ion flow and momentum transfer in the Venus plasma environment

An analysis of ion data from 390 Venus Express, VEX, orbits demonstrates that the flow of solar wind- and ionospheric ions near Venus is characterized by a marked asymmetry. The flow asymmetry of solar wind H⁺ and ionospheric O⁺ points steadily in the opposite direction to the planet’s orbital motion, and is most pronounced near the Pole and in the tail/nightside region. The flow asymmetry is consistent with aberration forcing, here defined as lateral forcing induced by the planet’s orbital motion. In addition to solar wind forcing by the radial solar wind expansion, Venus is also subject a lateral/aberration forcing induced by the planet’s orbital motion transverse to the solar wind flow.The ionospheric response to lateral solar wind forcing is analyzed from altitude profiles of the ion density, ion velocity and ion mass-flux. The close connection between decreasing solar wind H⁺ mass-flux and increasing ionospheric O⁺ mass-flux, is suggestive of a direct/local solar wind energy and momentum transfer to ionospheric plasma. The bulk O⁺ ion flow is accelerated to velocities less than 10 km/s inside the dayside/flank Ionopause, and up to 6000 km in the tail. Consequently, the bulk O⁺ outflow does not escape, but remains near Venus as a fast (km/s) O⁺ zonal wind in the Venus polar and nightside upper ionosphere. Furthermore, the total O⁺ mass-flux in the Venus induced magnetosphere, increases steadily downward to a maximum of 2 × 10⁻¹⁴ kg/(m² s) at ≈400 km altitude, suggesting a downward transport of energy and momentum. The O⁺, and total mass-flux, decay rapidly below 400 km. With no other plasma mass-flux as replacement, we argue that the reduction of ion mass-flux is caused by ion-neutral drag, a transfer of ion energy and momentum to neutrals, implying that the O⁺ plasma wind is converted to a neutral (thermosphere) wind at Venus. Incidentally, such a neutral wind would go in the same direction as the Venus atmosphere superrotation.     

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.     

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.     

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.     

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.     

The international planetary patrol program: An assessment of the first three years

Day-to-day and hour-to-hour changes in the large-scale atmospheric and surface features of the planets can now be studied more effectively than previously possible. Since 1969 a network of observatories has obtained almost uninterrupted photographic coverage during all apparitions of Jupiter and Mars, plus some of Venus. Patrol films and catalogues are available to the scientific community. Recent or current analyses include the distribution and motion of clouds on Mars, the development and decay of Martian dust storms, the seasonal, diurnal and random fluctuations in contrast between adjacent light and dark regions on Mars, the detection of vertical shear in the Jovian atmosphere, the longitudinal oscillation of the Red Spot, the dependence of rotation period on xenographic latitude and on time, the eruption and spread of SEB disturbances, and the retrograde circulation of the Venus cloud deck.     

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.     

Suprathermal oxygen and hydrogen atoms in the upper Martian atmosphere

The processes of kinetics and transport of hot oxygen and hydrogen atoms in the transition (from the thermosphere to the exosphere) region of the upper Martian atmosphere are studied. The reaction of dissociative recombination of the principal ionospheric ion O ₂ ⁺ with thermal electrons in the ionosphere of Mars served as the source of hot oxygen atoms. The process of momentum and energy transfer in elastic collisions between hot oxygen atoms and atmospheric hydrogen atoms with thermal energies was regarded as the source of hot hydrogen atoms. The kinetic energy distribution functions are determined for suprathermal oxygen and hydrogen atoms. It is shown that the exosphere is populated with a significant number of suprathermal oxygen atoms with kinetic energies ranging up to the escape energy of 2 eV (i.e., the hot oxygen Martian corona is formed). The transfer of energy from hot oxygen atoms to thermal hydrogen atoms creates an additional nonthermal flux of atomic hydrogen escaping from the Martian atmosphere.     

Mars: Satellite origin and angular momentum

The origin of Phobos and Deimos is considered with a view to accounting for the existence of very small satellites with circular orbits in the Martian equatorial plane, and simultaneously for the suspected angular momentum deficiency of the Mars system. All models considered failed to satisfy at least one requirement, and the problem is considered more puzzling than is at first apparent. The Martian angular momentum deficiency, if physically significant, may be unrelated to the present satellites' origin, but might relate to a large ancient satellite, long ago destroyed. Accretion onto Mars of large amounts of asteroidal dust brought in by Poynting-Robertson drag may have some bearing on the angular momentum problem.