Characteristics of the upper region of the ionosphere

Abstract

Operation of stations making observations of the ionosphere in widely separated locations over the Earth's surface by the Department of Terrestrial Magnetism, Carnegie Institution of Washington, has produced data of unusual interest. Values of critical frequency of penetration and minimum virtual‐height for theF2‐region are now available from Washington, D.C., U.S.A., Huancayo, Peru, and Watheroo, Western Australia, for periods ranging from one to six years. These data are presented in the form of monthly averages of noon values. It is shown that noon values are a measure of conditions during the daytime hours. The average noon values of critical frequency for theF2‐region are found to have similar characteristics in both hemispheres, reaching a maximum from about November to February and a minimum from about May to August, at stations in both the Northern and Southern hemispheres simultaneously. This leads to a strong presumption that this variation is, predominantly, an annual effect. The simple hypothesis of thermal expansion of the upper atmosphere, which has been evolved by other investigators to account for the effects observed in the Northern Hemisphere alone, will not explain these critical‐frequency phenomena, observed in both hemispheres simultaneously. In this respect, therefore, it must be radically revised or discarded.Average minimum virtual‐heights of this region observed at noon show that at Washington and at Watheroo these heights change inversely with the noon zenith‐angle of the Sun, while at Huancayo, they change directly with the noon zenith‐angle. This reversal in character in equatorial regions is considered. It is possible that these effects may be explained by a new hypothesis of thermal movement of the upper atmosphere, considering the forces imposed upon the motions of the ions in the Earth's magnetic field.