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.