This book contains the lectures presented at the Summer Advanced Institute and Ninth ESRO Summer School which was held in Cortina, Italy, during the period August 30 through September 10, 1971. One hundred seventy-nine persons from eight een different countries attended. The authors and the publisher have made a special effort for rapid publication of an up-to-date status of the particles, fields, and processes in the earth's magnetosphere, which is an ever changing area. Special thanks are due to the lecturers for their diligent preparation and excellent presentations. The individual lectures and the published papers were deliberately limited; the author's cooperation in conforming to these specifications is greatly appreciated. The contents of the book are organized by sub ject area rather than in the order in which papers were presented during the Institute/ School. Many thanks are due to Drs J. Ronald Burrows, James W. Dungey, Harry Elliot, Roger Gendrin, Edward W. Hones, Jr. , Reimar Liist, and J. Ortner who served as session chairmen during the Institute and contributed greatly to its success by skill fully directing the discussion period in a stimulating manner after each lecture. Many persons contributed to the success of the Institute/School. The co-chairman, Dr Reimar Liist, was most helpful during all phases of the preparation and planning. Drs J. Ronald Burrows, Harry Elliot, Carl-Gunne Fiilthammar, M. Giorgi, J. Ortner, J. R. U. Page, Alois Schardt, James A. Van Allen, and Martin Walt were especially helpful in preparing the technical program.