โ Scribed by H. Rishbeth
No coin nor oath required. For personal study only.
This paper discusses the bearing that eclipse observations have on contemporary theories of the ionospheric E and F regions; in particular, on the determination of production and loss rates, and on the role of diffusion and temperature changes. Various complicating factors that arise are discussed, but the paper is not intended as a comprehensive survey of results obtained.
I. Introduction
From the earliest days of ionospheric research, great interest has been taken in the effects of solar eclipses. This is shown by the bibliography, 'Literature on Solar Eclipses and the Ionosphere', containing about 200 items dated from 1912 to 1955, which is appended to the book Solar Eclipses and the Ionosphere . The book is a report on the symposium held in London in 1955, and contains a very full record of the subject as it existed at the time. Since then there have been many new developments.
Observations during an eclipse offer a special opportunity for studying both the solar ionizing radiations and the earth's ionosphere. They are not ideal for this purpose. The ionospheric physicist might wish that the sun could be regarded as a constant, uniform source of ionizing radiation; but investigations of the sun show that it is not. The solar physicist would like to regard the ionosphere as a detector for ionizing radiation. But the ionosphere does not meet the basic requirements of a good detector: straightforward operation, reproduceability, and a linear or other convenient type of response.
These remarks must be qualified. The high angular resolution provided by the moon's limb does make it possible to locate active regions on the solar disk. In this regard eclipse observations certainly contribute to solar physics. Also, the lower ionosphere -the D region below 90 km-seems to be quite a good detector for X-rays.
If, on the other hand, eclipse observations are to be used to study the ionosphere itself, detailed electron density information is desirable. For the E and F layers, this can be obtained by means of ionosondes; but for the D region, the ground-based techniques are more difficult, and there is not the wealth of data that exists for the other layers. Although some striking results have been obtained from rocket observations of the eclipsed D region (SMITr~ et al., 1965; are not yet sufficient to permit a detailed discussion of the very complex physical situation which must then exist. Consequently the D region is not included in the scope of this article.
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