Venusian Ionosphere Research Articles

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

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

The nightward ion flow scenario at Venus revisited

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

Solar cycle variations of electron density and temperature in the Venusian nightside ionosphere

The return of periapsis to low altitudes during the Summer and Fall of 1992 provided a unique opportunity for the Pioneer Venus Orbiter (PVO) to make in situ measurements in the Venusian ionosphere at much lower levels of solar activity (F10.7 ≈120) than existed when periapsis was at low altitudes in 1979 and 1980 (F10.7≈220). We present the observations of electron density (Ne) and temperature (Te) made by the Orbiter Electron Temperature Probe (OETP) during the Entry Period. Empirical models of the Ne and Te height variations are presented and compared with similar models based on OETP measurements made at solar maximum. The median Ne at the ionospheric peak (∼140 km) was essentially unchanged from its solar maximum value, but the ionosphere was increasingly depleted at higher altitudes, reaching a factor of 7 lower densities at 200 km. Te was lower by almost a factor of 2 at 140 km but was rather significantly enhanced at higher altitudes; exceeding its solar maximum values by a factor of 1.3 at 200 km and a factor of 2 at 500 km. In general these results support the earlier conclusions that the nightside upper ionosphere is depleted at lower levels of solar activity by a reduction of the nightward ion flow. The lack of Ne variation near the peak (between solar maximum and entry) suggests that nightward ion transport does not play as large a role in the peak formation as does local ion production by energetic particles. The decrease in Te at low altitudes suggests that the low densities of the upper ionosphere at the time of PVO entry could no longer support the conduction of heat from the dayside ionosphere, thus allowing the lower nightside ionosphere to cool by collisions with ions and neutrals, and by heat conduction to the cooler regions below.

Physics of magnetic flux ropes

Magnetic flux ropes are tubes of magnetic flux that contain field lines twisted about the axis of the rope. Such tubes may occur in isolation, as in the ionosphere of Venus, or surrounded by equally strong magnetic fields, as in the terrestrial magentosphere. Magnetic flux ropes appear to play a critical role in solar physics, magnetospheric physics, in the Venusian ionosphere, and in interplanetary space. More than 100 scientists gathered the week of March 27 at an AGU Chapman Conference held at the Princess Hotel, Hamilton, Bermuda, to discuss the physics of magnetic flux ropes. Conference convenors were E. R. Priest, L. C. Lee and C. T. Russell.

The vibrational distribution of O +2 in the dayside ionosphere

The vibrational distributions of O2(+) in the X2Pi(g), A2Pi(u), a4Pi(u), and b4Sigma(-)g states in the dayside terrestrial ionosphere are calculated for both low and high solar activity models. The distributions are found to be significantly different from the O2(+) vibrational distributions found by Fox (1985) for the Venusian ionosphere. The sources and sinks of vibrational excitation and the implications for the chemistry, dayglow, and hot oxygen coronas are discussed. Finally, intensities of the first negative and second negative band systems of O2(+) are presented.

The O 2+ vibrational distribution in the Venusian ionosphere

Calculations are presented of the vibrational distribution of O 2 + in the Venusian ionosphere for a model atmosphere based on Pioneer Venus data. At 100 km, quenching precludes the survival of vibrationally excited O 2 +. At the exobase, near 200 km, more than half are vibrationally excited. The effects of vibrationally excited O 2 + on the hot oxygen corona and the airglow are discussed.

Compression of the Venusian ionosphere on May 10, 1979, by the interplanetary shock generated by the solar eruption of May 8, 1979

An interplanetary shock wave that was produced by a solar eruption and its associated coronal transient on May 8, 1979, has been ‘tracked’ through interplanetary space to a rendezvous 2 days later with Venus. The interaction of the shock wave with the ionospheric obstacle at Venus produced a significant compression of the dayside ionosphere. It is believed that the tracking, as it were, was accomplished for the first time via the diagnostic observations provided by Hα and white light imagery near the sun and the plasma and field measurements of two, nearly radially aligned, spacecraft.

The chemistry of metastable species in the Venusian ionosphere

Reactions of metastable species are important in determining the densities of minor ions in the Venusian ionosphere. Calculations are carried out in which the coupled continuity and momentum equations are solved for twelve ions and four neutral species in the dayside ionosphere, including O +( 2D), O +( 2P), N( 2D), and N( 2P) . Altitude profiles of these metastable species are presented. Their reactions are shown to be a significant source of several minor ions, especially N 2 +, CO +, and N +. The discrepancies which existed between model and measured densities of these ions are resolved.

Solar wind governing the response of Venusian atmosphere and ionosphere

The ionosphere of Venus is primarily formed by photoionization of a gaseous blanket around Venus. The impact ionization by energetic solar charged particles also plays an important role in the variability of Venusian ionospheric ion, electron density and their temperature profiles. The microscopic variations in the solar wind velocity, particle flux and orientations of frozen-in interplanetary magnetic field determine the solar wind interaction with the Venusian ionosphere. The ion and electron density profiles obtained by Pioneer Venus Orbiter and Pioneer Venus Entry Probes have been analysed in the light of simultaneous solar wind velocity and particle flux. Marked changes in height profiles of ion, electron densities and their temperatures have been found to correlate with the simultaneous changes in the solar wind velocity and particle flux. It is shown that the solar wind plays a more important role in controlling the physical properties and behavior of daytime as well as nighttime ionosphere of Venus, whereas the solar xuv sustains the primary ionization process.

Comparative analysis of Venusian ionosphere dual-frequency radio soundings with the satellites Venera-9, 10 and Pioneer-Venus

A comparison of the Venusian ionosphere electron density profiles obtained by the dual-frequency radio occultation method for the satellites Venera-9,10 (1975) and the Pioneer-Venus orbiter (1978–1979) has been carried out. It is shown that the general nature of the profiles, the main maximum heights and electron densities, the ionopause height positions determined with the satellites Venera-9, 10 on the one hand and Pioneer-Venus orbiter on the other hand are in good agreement. This fact testifies to the reliability of the experimental dual-frequency radio occultation data. An attempt to compare the radio occultation data with the direct measurements points out the essential contradictions between them, which need detailed analysis and discussion.

Observation of the Venus mantle, the boundary region between solar wind and ionosphere

For three orbit paths of the Pioneer Venus orbiter the interaction between the solar wind and the Venusian ionosphere has been studied. Results of the retarding potential analyzer and the magnetometer are described for the boundary region between the solar wind and the planetary ionosphere. These are the first measurements that show that a transition region exists between the two plasmas of different origin. The observed magnetic field and current system producing it appear strong enough to stop the solar wind flow in front of the ionosphere and to separate the shocked solar wind from the ionosphere. The transition region between the ionosheath and the ionosphere is called the ‘mantle.’ The observed mantle electron energy spectra close to the ionopause show ionospheric character. With increasing height the number of electrons that have ionospheric energies decreases, and the number of electrons that have solar wind energies gradually increases toward the ionosheath boundary, where only solar wind energy spectra are observed. The mantle surrounds the frontside of the ionosphere and extends probably more than eight Venus radii downstream.

Simple models of the thermal structure of the Venusian ionosphere

Simple models, analytical and numerical, of plasma temperatures in the Venusian ionosphere are developed. They are applied to thermal-structure data obtained by the Pioneer Venus Orbiter to estimate the magnitudes of important plasma thermal parameters.

Pioneer Venus Orbiter Planar Retarding Potential Analyzer Plasma Experiment

The retarding potential analyzer (RPA) on the Pioneer Venus Orbiter Mission measures most of the thermal plasma parameters within and near the Venusian ionosphere. Parameters include total ion concentration, concentrations of the more abundant ions, ion temperatures, ion drift velocity, electron temperature, and low-energy (0-50 eV) electron distribution function. Several functions not previously used in RPA's were developed and incorporated into this instrument to accomplish these measurements on a spinning spacecraft with a small bit rate. The more significant functions include automatic electrometer ranging with background current compensation; digital, quadratic retarding potential step generation for the ion and low-energy electron scans; a current sampling interval of 2 ms throughout all scans; digital logic inflection point detection and data selection; and automatic ram direction detection.

Daytime ionosphere of Venus as studied with Veneras 9 and 10 dual-frequency radio occultation experiments

We present results of the dual-frequency radio sounding of the Venusian ionosphere carried out by the Venera 9 and 10 satellites in 1975. Thirteen height profiles of electron density for different solar zenith angles varying from 10 to 87° have been obtained by analyzing the refraction bending of radiorays in the sounded ionssphere. The main maximum of electron density at a height of 140–150 km depends on the solar zenith angle and is 1.4 to 5 × 10 5 cm −3. The lower maximum is determined definitely to be at ∼130 km high. In the main and lower maxima the electron density variations with solar zenith angle are in good agreement with the Chapman layer theory. For the first time it is found that the height of the upper boundary for the daytime ionosphere ( h i) depends regularly on the solar zenith angle. At Z ⊙ < 60°, h i does not exceed 300 km while at Z ⊙ > 60°, it increases with Z ⊙ and comes up to ∼ 600 km at Z ⊙ ∼ 80°.

Viscous flow in the near-Venusian plasma wake

The acquisition of the Venera 9 and 10 plasma probe data has provided further support for the notion that the shocked solar wind in the flank regions of the Venusian ionosheath interacts viscously with the local ionospheric plasma and that the resulting mixing region is rapidly forced into the planetary umbra. The measured velocities and flow directions suggest that the deflection of the mixing layer downstream of the terminator proceeds at a fast pace, as was predicted from the analysis of a suitable MHD analog proposed by Perez-de-Tejada and Dryer (1976). By measuring the apparent displacement of the mixing region within the planetary umbra with respect to the position of the undeviated ionopause (calculated from the inviscid flow model of Spreiter et al. (1970)) it is possible to obtain an order-of-magnitude estimate of the equivalent kinematic viscosity coefficient of the flow in the region. Some speculative comments are also given in regard to the behavior of the flow in the Venusian plasma wake, in particular, to the implications that the penetration of the mixing layer into the umbra may have for the formation of the nighttime Venusian ionosphere.

The ionospheres of Mars and Venus

Observations made by spacecraft in the Mariner and Venera series have provided considerable knowledge of the structure of the ionospheres and atmospheres of Mars and Venus. This paper begins with brief but complete discussions of these measurements and their interpretations. Specifically, we summarize the characteristics and use of UV radiometry, magnetometers, and ion probes for determining solar wind properties near planets, as well as the application of the occultation experiment. Some of the significant results were the very small, perhaps zero, magnetic moment of Venus and the possibly small but nonzero magnetic moment of Mars (Mars 2 and Mars 3 spacecraft); observation of daytime ionospheres on both Mars and Venus with peak number densities of 105 and 6 × 105 cm−3, respectively; observation of the top side scale heights for both planets; observation of a ledge of ionization with a large scale height and the anemopause above Venus; revelation by the two‐frequency radar occultation experiment carried by Mariner 5 of the existence of a nighttime Venusian ionosphere with a peak number density of 10³ cm−3 and a very large plasma scale height above the peak; and measurement by Mariner 6, 7, and 9 and Mars 2 and 3 of EUV radiation intensities (scattered solar radiation and dayglow emission), which yielded data for reasonably precise estimates of the concentration profiles of CO2, CO, O, C, and H as well as the thermospheric temperature (350°K for Mariner 6 and 7) on Mars. Corresponding observations by Mariner 5 and Venera spacecraft have not yielded as much information about Venus; Mariner 5 did measure the atomic hydrogen profile, whereas Venera 4 established an upper limit on atomic oxygen on the night side. The structure of the neutral atmosphere at high altitudes is of key significance to ionospheric structure. Although CO2 is the major constituent up to and including the main layer of the ionosphere, lighter gases such as O, He, and H probably play important roles at heights between the main layer and the anemopause. The occurrence of these gases, their high‐altitude profiles, and the closely related problem of the photolytic stability of the Martian and Venusian atmospheres are discussed in considerable detail. The anemopause also has an important bearing on the structure of the upper ionosphere. Unlike the earth the solar wind impinges directly on the ionospheres of Mars and Venus, thus acting as a possible heat source. However, no adequate theory has yet been developed to predict details of the interaction. The crude method of computing the day side boundary is discussed: it requires the balancing of the solar wind dynamic pressure by the ionospheric static pressure. The ‘downwind’ side of the boundary is also discussed, but mainly from the standpoint of our ignorance of the wake structure. The principles for constructing models of ionospheres are presented from the standpoint of mass, momentum, and energy conservation. The relevant ion chemistry is presented, and the meaning and use of plasma scale heights are discussed. The general principles in varying degrees of simplification have been employed by several investigators to model the upper ionospheres of Mars and Venus. The models discussed are the model of Cloutier and co‐workers, who suggested that direct hydrodynamic pressure of the solar wind on the top side Martian ionosphere depresses the ionospheric scale height to a value equal to the neutral atmosphere scale height; the models of Banks and Axford, of Whitten, and of Herman and co‐workers, who attempted to account for the ionospheric ledge above the main layer of the Venusian ionosphere by proposing the ionization of light constituents; the model developed by McElroy and Strobel to account for the nighttime ionosphere of Venus; and the models developed by a number of investigators for the thermal structure of the Martian and Venusian ionospheres. In presenting the essentials of each model, their strengths and weaknesses are also discussed. The paper concludes with a summary of current knowledge of the upper ionospheres of Mars and Venus and the information needed to understand fully their structures.

Difficulties with thermal protons in the Venusian topside ionosphere

An analysis of recent findings and studies is presented which shows that thermal protons of planetary origin are rapidly destroyed in the Venusian ionosphere, and thus cannot be the principal topside ions. He(+) is conjectural, whereas there might be a possibility that O(+) is present in significant density.

Origin and Dynamical Behaviour of Thermal Protons in the Venusian Ionosphere

An important source of thermal H(+) and/or D(+) has been overlooked in the ionosphere of Venus which supports the choice of H(+) and/or D(+) as the dominant light ion at high altitudes.