ORIGINS OF MAGNETOSPHERIC PLASMA

A review is given of recent (1987-1990) progress in understanding of the origins of plasmas in the Earth's magnetosphere. In counterpoint to the early supposition that geomagnetic phenomena are produced by energetic plasmas of solar origin, 1987 saw the publication of a provocative argument that accelerated ionospheric plasma could supply all magnetospheric auroral and ring current particles. Significant new developments of existing data sets, as well as the establishment of entirely new data sets, have improved our ability to identify plasma source regions and to track plasma through the magnetospheric system of boundary layers and reservoirs. New computing capabilities have likewise improved our ability to plausibly model the magnetosphere so that new interpretative hypotheses may be tested. These developments suggest that the boundary between ionospheric and solar plasmas, once taken to lie at the plasmapause, actually lies much nearer to the magnetopause. Defining this boundary as the surface where solar wind and ionosphere contribute equally to the plasma, it is referred to herein as the "geopause." It is now well established that the an infusion of ionospheric O+ plays a major role in the storm-time distention of the magnetotail and inflation of the inner magnetosphere. This role is often dominant in the larger storms. Smaller quantities of particles with high charge states and very energetic ionospheric molecules are observed as well. However, after more than two decades of observation and debate, the question remains: Are magnetospheric protons of solar or terrestrial origin?