19 April 1991
Vol 252, Issue 5004

About The Cover

Earth's magnetosphere, created by the interaction of the supersonic solar-wind plasma (blue) with the geomagnetic field. The plasma sheet (red) is a reservoir of hot (˜1 kiloelectron volt) plasma originating in the solar wind and in the ionosphere. The solar wind-magnetosphere interaction drives a circulation of plasma that populates the Van Allen radiation belts (red dots) and powers auroral optical and radio emissions. See page 410. [Source, T. W. Hill; illustration by Susan Nowoslawski]