The geomagnetic field observed on the surface of the earth has been an important source of information on the dynamic behavior of the magnetosphere. Because the· magnetosphere and its environment are filled with plasma in which electric current can easily flow, dynamic processes that occur in the magnetosphere tend to produce perturbations in the geomagnetic field. Geomagnetic data have therefore provided valuable means for sensing the processes taking place at remote locations, and such basic concepts as the magnetosphere, solar wind, and trapped radiation were derived in early, presatellite days from geomagnetic analyses. Because of this advantage, geomagnetic observations have been widely utilized for monitoring the overall condition of the magneto­ sphere. Although the advent of space vehides has made it possible to observe magnetospheric processes in situ, supplementary information on the overall magnetospheric condition is frequently found to be indispensable for interpreting these observations in the proper perspective. Hence for magnetospheric physicists involved in various branches of the field it has become a common practice to employ geomagnetic data as a basic diagnostic tool.