Coupling between the outer magnetosphere and the high-latitude ionosphere

The coupling between the ionosphere and the outer magnetosphere depends on the topology of the geomagnetic field. Some aspects of the closed and open magnetospheric models are briefly discussed.The assumption that the geomagnetic field lines are equipotentials is critisized both on observational and on theoretical grounds. Measurements of H Doppler profiles, of precipitating particles above the ionosphere, and of charged particle densities in the magnetosphere indicate the existence of electric fields, E_\\, parallel with the magnetic field.Two different models of E_\\ are considered. Both models violate the condition of frozen-in magnetic fields. In one of them there are occasional transient electric field impulses along the field lines which cause precipitation splashes. The other model invokes electrostatic fields which vanish occasionally due to instabilities. This gives rise to precipitation splashes of about equal numbers of ions and electrons.The latter model seems to be favoured by known satellite data concerning the pitch angle distributions of electrons above the ionosphere.It is suggested that electric fields in space should be measured by satellites and rockets. Expected values of the fields in different regions of space are given.     

On the outer ionosphere (and its transition into interplanetary space)

Some properties of the outer ionosphere and its boundary region are discussed on the basis of recent experimental results.The analysis of the new data has shown that the outer ionosphere, a plasma above the ionospheric main maximum, extends to a distance of 3 to 3.5 earth radii from the earth's surface, that is, up to the region of the so-called knee, detected and observed by means of whistlers. During periods of relatively weak magnetic storms, from time to time the electron concentration at this ionospheric boundary jumps downward by factors of 10 to 100, over a height range of only a few hundred kilometres. The inflow of charged particles into the ionosphere apparently takes place through the boundary region. Sometimes these particles are swept into it from the overlying regions.There is a great number of names for the outer ionosphere. Some of these terms, for instance the geocorona, are not at all applicable to the outer ionosphere.From the new experimental results it can be inferred that in a great part of the outer ionosphere there is no quasineutrality, that there are rather strong electric fields, and that the Maxwell ion distribution law of particle velocities breaks down. Therefore, to analyze the ionization balance one should know the particles' velocity distribution functions. Otherwise it would hardly be possible to solve the problem of the formation of the ionosphere.It is shown that within the limits of uncertainty all experimental results are in good agreement and produce a single, comprehensive picture of the structure of the outer ionosphere. Only some data, deduced from measurements of particle streams by means of ion traps, are an exception. They contradict the numerous experimental results. This discrepancy is in particular due to the difficulties of determining the plasma concentration from current density measurements.Some methods are discussed briefly. For instance, the analysis of low-frequency waves, in particular the so-called whistler and the low-frequency plasma radiation, represents a physically adequate and fruitful method for investigating the outer ionosphere.For a theoretical analysis of the above-mentioned data, it is in some cases required to take into account the effect of kinetic corrections to the refraction coefficient, of cyclotron and erenkov attenuation and radiation, etc. Over the next few years this method will come to play a great part in the exploration of the outer ionosphere, interplanetary space, and planets.Measurements of the energy spectra of incoherent back scattering of radio waves on the electron fluctuations will make another very interesting source for studying the outer ionosphere. This method is based on the interaction phenomena of radio waves with the plasma. Therefore, the scattering spectra are functions of the oscillating properties of the plasma. However, these data should be subjected to a thorough theoretical treatment on the basis of a complete theory of scattering.Up till now a sufficiently complete probe theory has not been evolved due to essential theoretical difficulties. Often this does not allow one to interpret adequately the results of measurements and considerably limits the possibilities of these methods.     

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.     

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.     

9. The venus ionosphere and solar wind interaction

The current state of knowledge of the chemistry, dynamics and energetics of the upper atmosphere and ionosphere of Venus is reviewed together with the nature of the solar wind-Venus interaction. Because of the weak, though perhaps not negligible, intrinsic magnetic field of Venus, the mutual effects between these regions are probably strong and unique in the solar system. The ability of the Pioneer Venus Bus and Orbiter experiments to provide the required data to answer the questions outstanding is discussed in detail.     

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.     

Source processes in the high-latitude ionosphere

Space Science Reviews -     

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.     

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.     

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.     

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.     

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.     

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.     

Magnetic fields in the ionosphere of Venus

This review surveys the observations of the ionospheric magnetic fields of Venus as observed on the Pioneer Venus Orbiter and the models that have been developed to describe them over the last decade. The models for the large-scale ionospheric field have developed to the advanced stage of one-dimensional, self-consistent, multi-fluid MHD models which provide a detailed picture of the field in the subsolar region for specific upper boundary conditions. In contrast, the models for the small-scale fields and the nightside fields have only reached a rudimentary stage. Much challenging work remains to be done on the origin of the ionospheric flux ropes and nightside ionospheric hole fields. On the whole, the subject of the ionospheric fields would greatly benefit from 3-dimensional global MHD models with self-consistent treatments of the ionosphere.     

Electrodynamic coupling of the magnetosphere and ionosphere

The auroral zone ionosphere is coupled to the outer magnetosphere by means of field-aligned currents. Parallel electric fields associated with these currents are now widely accepted to be responsible for the acceleration of auroral particles. This paper will review the theoretical concepts and models describing this coupling. The dynamics of auroral zone particles will be described, beginning with the adiabatic motions of particles in the converging geomagnetic field in the presence of parallel potential drops and then considering the modifications to these adiabatic trajectories due to wave-particle interactions. The formation of parallel electric fields can be viewed both from microscopic and macroscopic viewpoints. The presence of a current carrying plasma can give rise to plasma instabilities which in a weakly turbulent situation can affect the particle motions, giving rise to an effective resistivity in the plasma. Recent satellite observations, however, indicate that the parallel electric field is organized into discrete potential jumps, known as double layers. From a macroscopic viewpoint, the response of the particles to a parallel potential drop leads to an approximately linear relationship between the current density and the potential drop.The currents flowing in the auroral circuit must close in the ionosphere. To a first approximation, the ionospheric conductivity can be considered to be constant, and in this case combining the ionospheric Ohm's Law with the linear current-voltage relation for parallel currents leads to an outer scale length, above which electric fields can map down to the ionosphere and below which parallel electric fields become important. The effects of particle precipitation make the picture more complex, leading to enhanced ionization in upward current regions and to the possibility of feedback interactions with the magnetosphere.Determining adiabatic particle orbits in steady-state electric and magnetic fields can be used to determine the self-consistent particle and field distributions on auroral field lines. However, it is difficult to pursue this approach when the fields are varying with time. Magnetohydrodynamic (MHD) models deal with these time-dependent situations by treating the particles as a fluid. This class of model, however, cannot treat kinetic effects in detail. Such effects can in some cases be modeled by effective transport coefficients inserted into the MHD equations. Intrinsically time-dependent processes such as the development of magnetic micropulsations and the response of the magnetosphere to ionospheric fluctuations can be readily treated in this framework.The response of the lower altitude auroral zone depends in part on how the system is driven. Currents are generated in the outer parts of the magnetosphere as a result of the plasma convection. The dynamics of this region is in turn affected by the coupling to the ionosphere. Since dissipation rates are very low in the outer magnetosphere, the convection may become turbulent, implying that nonlinear effects such as spectral transfer of energy to different scales become important. MHD turbulence theory, modified by the ionospheric coupling, can describe the dynamics of the boundary-layer region. Turbulent MHD fluids can give rise to the generation of field-aligned currents through the so-called -effect, which is utilized in the theory of the generation of the Earth's magnetic field. It is suggested that similar processes acting in the boundary-layer plasma may be ultimately responsible for the generation of auroral currents.     

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.