Short-period oscillations of the earth's electromagnetic field with periods of 1-500 sec are of two basic types: regular, almost-sinusoidal oscillations of a persistent nature, and irregular, erratic oscillations. Some regularities are established for changes in the period spectrum of these oscillations as a function of storm intensity. Short-period oscillations connected with bremsstrahlung from energetic electrons incident in the upper atmosphere are described. The mean regularity of excitation of the small-period oscillations is compared with regularities in the occurrence of luminous atmospherics. It is found that (in the case of similar diurnal, annual and cyclic variations) small-period oscillations and F-scattering appeared simultaneously in only 40-50% of the cases at the middle latitudes.

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This note concerns variations of the two most frequently encountered types of oscillations. Type one consists of long-continuous steady oscillations, which have their diurnal maximum of occurrence about 4 hr GMT. Type two oscillations occur in short bursts or trains. Their maximum of occurrence is about 19 hr GMT. These variations are illustrated along with portions of ordinary earth-current recordings. Maximum occurrence of the oscillations was during the spring and autumn months.

The subject of geomagnetic micropulsations has developed extremely rapidly and it is difficult to know when is an appropriate time to pause and assess the sum total of our knowledge-both observational and theoretical. There has in recent years been a tremendous increase in both the quantity and quality of data and also many theoretical ad vances in our understanding of the phenomenon. Undoubtedly there will be further progress in both areas but it seems worthwhile now to review both our knowledge and our ignorance. This book was essen tially completed by the end of April 1969 and tries to give a summary of the subject up to that time. The Earth is enclosed in the magnetosphere, a hollow carved out of the solar wind by the Earth's magnetic field. Above the ionosphere there is a very tenuous thermal plasma of partially ionized hydrogen in diffusive equilibrium with magnetic and gravitational forces, and ener getic protons and electrons that constitute the trapped Van Allen ra diation belts. Throughout this anisotropic and inhomogeneous plasma, natural and man-made electromagnetic energy propagates in a wide variety of modes and frequency bands. This book is concerned with that class of natural signals called geomagnetic micropulsations-short period (usually of the order of seconds or minutes) fluctuations of the Earth's magnetic field.