Symposium on the upper atmosphere
Authors: Appleton Sir Edward 
Published: February 2007

Sir Edward Appleton gave a survey of the present state of our knowledge of the structure of the upper atmosphere, indicating the regions that are the seats of the various phenomena which were to be discussed by subsequent speakers in the symposium. He likened the upper atmosphere to a vast laboratory in which the sun daily actuates a series of experiments at low pressures, in conditions and on a scale which we cannot hope to reproduce experimentally. Solar ultra‐violet radiation creates, at different levels, the ozonosphere and the ionosphere by dissociating molecules and by ionizing both atoms and molecules. Further, the gravitational attraction of the sun and moon causes marked atmospheric tides at high levels, and the influence of the earth's magnetic field comes into action as soon as any charged atmospheric particle begins to move. Meteors, sporadic and in showers, produce streaks of ionization which can be observed by eye if clouds are absent and always — even in Manchester — by radio. It may be that fine meteoric dust causes a permanent part of the ionosphere.

Sir Edward then discussed some ionospheric problems which he and his research assistants in Edinburgh, Mrs. Pritchard and A. J. Lyon, have recently examined. Considering first the world morphology of the F2 layer, they have investigated the variation of fF2 (the critical frequency for the F2 layer and an indication of the maximum electron density) at noon at the equinoxes with both geographical and geomagnetic latitude and have found the closer fit with the latter. The curve shows a trough in the region of the equator, where fF2 is a minimum. When curves are drawn for other times of the day, it is found that in the late evening hours the trough becomes a crest. No such effect appears in similar curves drawn for the E and F1 layers. This difference between the F2 layer and the lower E and F1 layers is not thought to indicate that the F2 layer is caused by electrified corpuscles. Ultra‐violet radiation, certainly the cause of the E and F1 layers, is also probably responsible for the F2 layer and the effect found is due to geomagnetic distortion. The movements associated with the diurnal oscillation, tidal or thermal in origin, which these observations disclose, would be subject to strong control by the geomagnetic field at the low densities obtaining at the F2 level. But, though theories have been proposed by Martyn, Maeda and Weiss, there is as yet no satisfactory theoretical explanation. The investigation of the F2 layer has further revealed that, during an ionospheric storm, the value of noon fF2 is reduced in high latitudes and increased at the equator.

While this examination of the world morphology of the E and F1 layers has shown no strikingly abnormal effects, even here there are small departures from simple theory. It was well worth while investigating such discrepancies, for there is always the possibility that the investigation may yield a method of measuring the magnitude of the factor which itself causes the discrepancy. In illustration, some features of the E layer were considered; from the continuity equation, in an ionized medium it follows that: where N is the ion (or electron) concentration, x the zenith distance of the sun and α the coefficient of recombination. If q0 and α are constant, then N is uniquely determined by cos x. A curve showing the annual variation of NE for constant x may be drawn using experimental data, since x is the same at noon in winter and in the early morning or late evening in summer. It is found, in fact, that NE for constant x has a minimum in June, so that either q0 or α, or both, varies; conditions are not the same in summer and winter at the same height.

Finally, Sir Edward discussed the methods being used in an attempt to determine accurate values of the recombination coefficient at low pressures at all times of the day throughout the year and showed how the coefficient may be determined from observation of the time‐lag between the maximum of the ionizing process (at noon) and the ionization maximum; values of the order of 10−9 and 10−8 have been obtained. The opportunity, afforded by the recent eclipse, of making such measurements at a minimum of solar influence has been utilised; Mr. Lyon has successfully made radio observations on the ionosphere at Slough, and these are at present being analysed.