Ion dynamics in the Venus ionosphere

Dynamics play an important role in defining the characteristics of the Venus ionosphere. The absence of a significant internal magnetic field at Venus allows the ionization to respond freely to gradients in the plasma pressure. The primary response to a gradient in plasma pressure is the nightward flow of the ionization away from a photoionization source on the dayside. The flow is approximately symmetric about the Sun-Venus axis and provides the source of O⁺ that maintains the nightside ionosphere during solar maximum. Modelling efforts have generally been successful in describing the average nightward ion velocity. Asymmetric and temporally-variable flow is measured, but is not well described by the models. Departures from axially-symmetric flow described in this paper include ionospheric superrotation at low altitudes and an enhanced flow at high altitude at the dawn terminator. Variability that is the result of changes in the ionopause height induced by changes in solar wind dynamic pressure is especially strong on the nightside. Ion flow to the nightside is also reduced during solar minimum because of a depressed ionopause.     

The structure of the Venus ionosphere

Our current knowledge of the spatial structure of the Venus ionosphere and its temporal behavior is reviewed, with emphasis on the more recent Pioneer Venus measurements and analysis not covered in earlier reviews. We will stress the ionosphere structure, since other papers in this issue deal with its dynamics, and its magnetic properties. We also discuss some of the limitations that the orbit has placed on the spatial and temporal coverage of the ionosphere. For the benefit of future users of the data some of the factors which affect the measurement accuracies are discussed in an Appendix.Currently at Space Physics Research Laboratory, University of Michigan, Ann Arbor, MI 48109, U.S.A.     

Electric fields in the ionosphere and magnetosphere

In this paper we review low altitude observations of the high latitude convection electric field as obtained with a variety of instruments including polar orbiting spacecraft, barium, incoherent and coherent scatter radars, and ground-based magnetometers. There still appears to be some contradiction in the observations particularly with regard to plasma flow into and out of the polar cap. Also, there does not appear to be any simple relationship between the sign of B _y and the local time location of the throat region. Rather, under active conditions, it appears that the plasma entry and exit regions rotate towards earlier times and there is a significant component of dawn-dusk flow across the polar cap. Superimposed on this may be some B _y-dependence of the plasma entry region.     

Electric fields and currents in the ionosphere

In this paper some theories and experimental data on the electric fields and currents in the ionosphere are reviewed. Electric fields originating in the polarization of the ionosphere as well as in local irregularities are considered. Special attention is paid to field-aligned currents as a regulator of the intensity and configuration of the ionospheric polarization field, the anomalous resistivity being one of the most important characteristics of the magnetospheric plasma. Present-day models of the magnetosphere and corresponding electric field generation mechanisms are discussed. Various models of the DP1 current system are considered and the main characteristics that allow us to distinguish between them are listed. Experimental data on the ionospheric electric field are considered; a modified model of Silsbee and Vestine is shown to fit these data reasonably well.     

Measurement of electric fields in the ionosphere and magnetosphere

Space Science Reviews -     

Electrodynamics of the ionosphere

We review various important studies in the field of electrodynamics of the ionosphere. Four topics are presented; (1) conductivity, (2) wind and the dynamo theory, (3) drift and its effect on the ionosphere formation and (4) interaction between wind and electromagnetic field.We point out some important future problems. They are: (1) We need to consider in the dynamo theory of the geomagnetic daily variation the connection of the ionosphere of both hemispheres by lines of force of the geomagnetic field. (2) Non-periodic wind may be important for producing electric field. (3) Drift to cause interchange of ionization contained in tubes of the geomagnetic field lines, and diffusion of ionization in these tubes control dynamic behaviours of the F2 region. (4) Interaction between wind and electric current presents a new problem. (5) The ionosphere and the magnetosphere react to each other.     

Source processes in the high-latitude ionosphere

Space Science Reviews -     

Dynamics of the disturbed ionosphere

The ionospheric storm process at F layer heights is reviewed and an explanation in terms of wind-induced diffusion of atomic oxygen is given.     

Electromagnetic resonances in the polar ionosphere

The utilization of resonant oscillations for ionospheric modification experiments and ionospheric diagnosis, based on plasma physics and non-linear electrodynamics, is discussed. The spectra of resonant oscillations of natural and artificial origin in the polar ionosphere and the excitation of these oscillations and their development are analysed. The thermal instabilities in weakly ionized heterogeneous magnetoactive ionospheric plasma, due to plasma heating, are illustrated. The circle of resonant situations in the polar area is expanded essentially due to intense current systems in this region.     

Electromagnetic resonances in the equatorial ionosphere

The diagnostic and communication possibilities, based on resonant properties of ionospheric magnetoplasma near the Equator, are discussed for a wide spectral range of electromagnetic waves. The utilization of geometrical resonances of both natural and artificial origin and plasma eigenmodes are considered for a model of anisotropic heterogeneous collisional magnetoplasma at low latitudes. The possibilities of linear and non-linear wave transformation and scattering, together with the orthogonality of ionospheric gradients and geomagnetic fields, are illustrated. The threshold and self-oscillation phenomena, stimulated by heating, produced by an equatorial electrojet, photo-electrons and powerful transmitters, are examined.Non-stationary electromagnetic processes, accompanied by the transversal transport phenomena in a magnetoplasma near the equatorial plane, are discussed and the tendencies of resonant effects employment for active diagnostic and remote communication at low latitudes are also considered.     

Theory of formation of the ionosphere

Current knowledge about the solar radiation and absorption and ionization cross sections of atmospheric gases is reviewed. Next the main observed features of ionospheric layers are summarized. Using CIRA 1965 model atmospheres the heights of the peak of the ionization rate are calculated for a number of solar emission lines and it is made clear which of these lines are responsible for the formation of E and F1 layers. The mechanism of electron removal in the F and upper E regions as well as in the lower regions is considered, and the mechanism of formation and some behaviours of each ionospheric layer is discussed. In particular, the equatorial F2 layer is briefly considered. Discrepancies are pointed out between the values of the recombination coefficient and the rate constant for ion-atom interchange reaction obtained from ionospheric observations and from laboratory experiments. Inconsistency of the values of the intensity of solar radiation measured by rocket techniques and inferred from ionospheric considerations is also noted. Some evidence is presented suggesting that corpuscular radiation may be responsible for part of the ionization in the ionosphere even in temperate latitudes.     

Electron and ion temperatures in the ionosphere

Many measurements of ionospheric temperature have been made during the last decade and have shown a considerable departure from thermal equilibrium there. Theoretical work has led to a general understanding of the processes which determine the energy balance of the charged particles, and most features of the experimental results are well understood in the F-region. The position in the E-region where different methods of measurement, known to agree fairly well at greater heights, appear to disagree is less clear, whilst in the D-region no direct observations of charged particle temperature exist. The theoretical expectation here is that, on account of the rapid exchange of energy amongst the particles by collisional processes, temperature equality will prevail.This review is based on material published up to January, 1970.On leave of absence from Mullard Space Science Laboratory, University College London.     

Elementary processes involving electrons in the ionosphere

The production, slowing-down and disappearance processes of the ionospheric electrons are discussed. Emphasis is placed on the individual elementary processes, especially on electron collisions with other atmospheric constituents, rather than on other topics involving the gross structure of the real ionosphere.     

The atmosphere of Venus

The investigations of Venus take a special position in planetary researches. It was just the atmosphere of Venus where first measurements in situ were carried out by means of the equipment delivered by a space probe (Venera 4, 1967). Venus appeared to be the first neighbor planet whose surface had been seen by us in the direct nearness made possible by means of the phototelevision device (Venera 9 and Venera 10, 1975). The reasons for the high interest in this planet are very simple. This planet is like the Earth by its mass, size and amount of energy obtained from the Sun and at the same time it differs sharply by the character of its atmosphere and climate. We hope that the investigations of Venus will lead us to define more precisely the idea of complex physical and physical-chemical processes which rule the evolution of planetary atmospheres. We hope to learn to forecast this evolution and maybe, in the far future, to control it. The last expeditions to Venus carried out in 1978 — American (Pioneer-Venus) and Soviet (Venera 11 and 12) — brought much news and it is interesting to sum up the results just now. The contents of this review are:

1. The planet Venus — basic astronomical data.
2. Chemical composition.
3. Temperature, pressure, density (from 0 to 100 km).
4. Clouds.
5. Thermal regime and greenhouse effect.
6. Dynamics.
7. Chemical processes.
8. Upper atmosphere.
9. Origin and evolution.
10. Problems for future studies

Here we have attempted to review the data published up to 1979 and partly in 1980. The list of references is not exhaustive. Publications of special issues of magazines and collected articles concerning separate space expeditions became traditional last time. The results obtained on the Soviet space probes Venera 9, 10 (the first publications) are collected in the special issues of Kosmicheskie issledovanija (14, Nos. 5, 6, 1975), analogous material about Venera 11, 12 is given at Pis'ma Astron. Zh. (5, Nos. 1 and 5, 1978), and in Kosmicheskie issledovanija (16, No. 5, 1979). The results of Pioneer-Venus mission are represented in two Science issues (203, No. 4382; 205, No. 4401) and special issue of J. Geophys. Res. (1980). We shall mention some articles to the same topic among previous surveys: (Moroz, 1971; Sagan, 1971; Marov, 1972; Hunten et al., 1977; Hoffman et al., 1977) and also the books by Kuzmin and Marov (1974) and Kondrat'ev (1977). Some useful information in the part of ground-based observations may be found in the older sources (for example, Sharonov, 1965; Moroz, 1967). For briefness we shall use as a rule the abbreviations of space missions names: V4 instead of Venera 4, M10 instead of Mariner 10 and so on. The first artificial satellites of Venus in the world (orbiters Venera 9 and 10) we shall mark as V9-O, V10-O unlike the descent probes V9, V10. Fly-by modules of Venera 11 and Venera 12 we shall mark as V11-F and V12-F. Pioneers descent probes — Large (Sounder), Day, Night and North — will be marked as P-L, P-D, P-Ni, P-No, orbiter as P-O, and bus as P-B.     

The clouds of Venus

The current state of knowledge of the Venusian clouds is reviewed. The visible clouds of Venus are shown to be quite similar to low level terrestrial hazes of strong anthropogenic influence. Possible nucleation and particle growth mechanisms are presented. The Pioneer Venus experiments that emphasize cloud measurements are described and their expected findings are discussed in detail. The results of these experiments should define the cloud particle composition, microphysics, thermal and radiative heat budget, rough dynamical features and horizontal and vertical variations in these and other parameters. This information should be sufficient to initialize cloud models which can be used to explain the cloud formation, decay, and particle life cycle.     

Lightning induced optical emissions in the ionosphere

A discussion of lightning induced optical emissions in the ionosphere is presented. Emphasis is placed on accounting for the puzzling observation of the spatial structure in the optical emissions and the Sprite ‘seeding’ before the development of the ‘tendrils’ (or streamers). In this context we discuss the generation of spatial brightness variations, within the required lightning parameter thresholds, due to spatio-temporal electric fields and spatial neutral density perturbations. This revised version was published online in August 2006 with corrections to the Cover Date.     

The exploration of Venus

This paper is an introduction to a special issue ofSpace Science Reviews dedicated to the exploration of Venus and the role played by the Pioneer Venus program. The Pioneer Venus program consists of a Multiprobe and Orbiter mission, both to be launched and to encounter Venus in 1978. The evolution of the program is traced from its conception in 1968 as the Goddard Space Flight Center Planetary Explorer Program through its transfer to Ames Research Center in 1971 as Pioneer Venus to the present.     

Magnetic field variations in the polar region during magnetically quiet periods and interplanetary magnetic fields

The concepts of near-pole magnetic field variations during magnetically quiet periods are explored, with special emphasis on the relationships of these variations to interplanetary magnetic field components. Methods are proposed for relating the variations which have been observed to the fields from the various sources, based on a thorough selection of reference levels. We assume that the field variations in the summer polar cap during magnetically quiet periods consist of the following components: (i) the middle-latitude S _qvariation extended to the polar region; (ii) the DPC(B _y) single-cell current system with a polar electrojet in day-side cusp latitudes; (iii) the DMC(B _z) two-cell current system of magnetospheric convection, in the form of a homogeneous current sheet in the polar cap towards the sun, with return currents through lower latitudes; (iv) the DPC(B _z) single-cell counterclockwise current system with a focus in the day-side cusp region. Quantitative relations between the near-pole variation intensities and the value and sign of the IMF azimuthal component, with a 1 hr time resolution, have been obtained and used to suggest ways of diagnosing the interplanetary magnetic field on the basis of ground observations.