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 perturba- tions in the
geomagnetic field. Geomagnetic data have therefore pro- vided 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 perspec-
tive. 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.