Earth's Radiation Belts.

This report reviews the status of radiation-belt science at the close of the data-acquisition phase 1976-79 of the International Magnetospheric Study IMS. The purpose is to place recent discoveries in context with respect to long-standing problems, and to indicate possible directions for future research in radiation-belt physics. The review includes a synopsis of results achieved on topics related to the source, energization, transport, and loss processes that affect geomagnetically trapped radiation, as well as results concerning certain subtle consequences of adiabatic charged-particle motion. Major areas of interest include the effects of charge exchange and radial transport on the ion composition of the ring current and radiation belts, the measurement of ion distributions in all three dimensions of velocity space, and the evolution of radiation-belt particle intensities as a consequence of temporal variations of transport coefficients and boundary conditions over the solar cycle and over the course of a magnetic storm. Moreover, the realization is developing that the major uncertainties in radiation-belt physics lie beyond the radiation belts themselves.