The accurate determination of ionospheric equivalent heights

A method has been described for measuring the equivalent height of an ionized region of the upper atmosphere with an accuracy of ? 0.5 km. It has been shown that though the accuracy is of this magnitude the resolving-power of the apparatus, which is fixed by the width of the pulse used, is not so great, there being difficulties in determining the height difference of two pulses less than 5 km. apart. However, when certain precautions are taken it is possible to obtain height separations of the order of 5 km. with a fair degree of accuracy. With the aid of the apparatus described it has been shown that there are no sudden changes of height in either the E1 or the F2 region greater than those which might be expected from errors of measurement. The equivalent height of the so-called abnormal E region has been compared with that of E1 and is shown to be not more than 3 or 4 km. greater. The frequency separation of the two critical frequencies for the ordinary and the extraordinary rays returned from the abnormal E region has been measured and is shown to be in agreement with the value to be expected on the magneto-ionic theory of the propagation of waves through an ionized medium. A discontinuity in the curve of height against frequency has been shown to exist at a frequency just less than 2 Mc./sec. This phenomenon, which was first noticed by Naismith at Slough two years ago, is shown to have all the characteristics of penetration and leads to the inference that there is a secondary maximum of ionization a few kilometres below the maximum of the E1 region, somewhat similar to the ledge on the F region discovered by Appleton. For purposes of reference in the paper this maximum is called Es. It has been shown that the height of the E1 region is less in winter than in summer but that the diurnal variation of height shows a minimum of height several hours after midday. It is suggested that the winter minimum is due to a general increase in molecular density in the atmosphere caused by the lower temperatures of winter time.