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.

The European Space Research Organisation put its first satellite into orbit in March 1968 and was successful with two more before the end of that year. It was not entirely surprising therefore that the third annual ESLAB/ESRIN Symposium should in some way deal with the results obtained. This book is the Proceedings of that Sym posium which, for reasons which Dr. Trendelenburg explains in the introductory talk, concentrated on intercorrelating phenomena occurring during solar events and in particular during the event of 25 February 1969. It is generally acknowledged that space data could yield a much more fruitful harvest if measurements taken simulta neously in different regions of space could be compared and the detectors inter calibrated. ESRO therefore sought right from the start to encourage this comparison of data. The first two days of the Symposium (16 and 17 September 1969) were devoted to review lectures on inter-related phenomena occurring on the sun, through inter planetary space and the magnetosphere right down to the earth's ionosphere. The last two days were used to hear papers presenting results obtained from the ESRO and certain U. S. S. R. and U. S. A. satellites during the solar events around 25 February 1969. The Proceedings published here follow the same sequence as the Symposium presentations. Because of the short time interval between the solar event and the Symposium, some of the data presented in the second part must be regarded as preliminary.

The magnetotelluric method is a technique for imaging the electrical conductivity and structure of the Earth, from the near surface down to the 410 km transition zone and beyond. This book forms the first comprehensive overview of magnetotellurics, from the salient physics and its mathematical representation to practical implementation in the field, data processing, modeling and geological interpretation. Electromagnetic induction in 1-D, 2-D and 3-D media is explored, building from first principles, and with thorough coverage of the practical techniques of time series processing, distortion, numerical modeling and inversion. The fundamental principles are illustrated with a series of case histories describing geological applications. Technical issues, instrumentation and field practices are described for both land and marine surveys. This book provides a rigorous introduction to magnetotellurics for academic researchers and advanced students, and will be of interest to industrial practitioners and geoscientists wanting to incorporate rock conductivity into their interpretations.

The Earth's Magnetic Field : Its History, Origin, and Planetary Perspective