Some further measurements of upper atmospheric ionization

In a previous communication an account was given of some measurements of upper atmospheric ionization in south-east England made by the critical frequency method. These measurements were confined principally to the Kennelly-Heaviside region (region E) and showed that the maximum ionization at that level in the ionosphere exhibits a diurnal and seasonal variation such as may be explained in terms of a solar ionizing agency travelling rectilinearly. It was found, for example, that the ionization reaches a maximum about noon, falls off rapidly as sunset approaches, and continues to decrease less rapidly during the night. A short time before ground sunrise a minimum of ionization is attained, after which the value increases rapidly until the noon maximum is again reached. In the same communication reasons were advanced, based on the radio measurements made by one of us during the solar eclipse of 1927, for believing the ionizing agency for this region to be ultra-violet light from the sun, as opposed to the view then prevailing that a neutral stream of corpuscles was the effective agent, and it was pointed out that the solar eclipse of August 31, 1932, visible in Canada, provided an excellent opportunity for deciding the matter. Using the same critical-frequency method, Dr. J. T. Henderson working at Vankleek Hill, Ontario, was able to set the question beyond doubt and it may now be regarded as established that ultra-violet light is the principal ionizing agency for the Kennelly-Heaviside region. The present communication deals with further measurements made in south-east England by the critical-frequency method in which it is applied to the study of diurnal and seasonal variations in ionization in the upper ionized region (region F) as well as in region E. Since 1932, many workers in different parts of the world have adopted the critical-frequency method for the measurement of maximum ionization content and it is hoped that, in due course, the results of a world-wide survey of the diurnal and seasonal variations in ionospheric conditions will be available. Meanwhile, we are able to compare our present results with those we have previously obtained and also with those obtained in America at places of lower latitude.