Satellite beacon contributions to studies of the structure of the ionosphere

Abstract

The signals transmitted by radio beacons on board artificial earth satellites have been widely used for studies of the earth's ionosphere. Following the launch of Sputnik 1, two lines of investigation quickly emerged, the study of the total columnar content of the ionosphere (up to the height of the satellite) and the study of the fine‐scale irregularities within the layer responsible for causing rapid fading of the received signals. Early studies of total content employed either the Faraday rotation or the differential Doppler method and suffered because of an ambiguity in the results. This was overcome when satellites were launched that carried beacons transmitting on closely spaced frequencies. The measurements then provided information on the diurnal, seasonal, and sunspot cycle variations of the total content. In addition, by comparing the measurements with true height analyses of ionosonde records, useful results were obtained concerning the ratio of the number of electrons lying above the peak of the F layer to the number below and the layer thickness. Presently, the most useful product of the low‐altitude satellite measurements of ionospheric total content is in revealing latitudinal irregularities produced, for example, by traveling ionospheric disturbances or by auroral zone processes. The advent of geostationary satellites has made possible long continuous records of total content for many fixed locations on the earth, and by using multiple frequencies it has also been possible to study the exchange of plasma between the ionosphere and the magnetosphere. An extension of these observations to higher latitudes could resolve the question of whether the shrinkage of the plasmasphere during magnetically disturbed periods is accomplished by ‘peeling away’ the outer shells or by an inward compression of the plasma into the ionosphere. The morphology of ionospheric scintillation has been studied extensively by using beacon signals. In addition, these studies have shown that the spectrum of irregularities is power law (rather than Gaussian) and at the equator can extend to very small spatial scales. It is generally agreed that the irregularities responsibile for scintillation are created by some form of plasma instability; there seems to be a good candidate in the case of the equatorial irregularities but not in the case of irregularities at mid‐and auroral latitudes. It may be that different instability mechanisms operate in the auroral zone and at mid‐latitudes and/or at different times.