Some of the energy transported outward from the solar interior goes into heating the Sun's outer atmosphere, called the corona, to over a million degrees by processes that are currently the target of intense study. The hot corona in turn becomes the source of the solar wind, but there are still major questions about how that occurs. Almost certainly more than one acceleration mechanism is required to explain the different types of solar wind: the high-speed, nearly steady wind from features called coronal holes; the more highly structured low-speed wind associated with long-lived coronal streamers and perhaps also with the quiet corona; and the transient solar wind arising from explosive events called coronal mass ejections. This campaign will determine the different ways in which the solar wind is accelerated and the will follow its evolution to the outer boundary of the heliosphere.

As the solar wind flows out through the solar system, it carves out a large cavity called the heliosphere, in the interstellar medium. Because of its very large scale (~100 AU), the heliosphere provides a unique laboratory for studying plasma processes in relative isolation from boundary effects; from heliospheric studies it is possible to learn much about instabilities in expanding plasmas, the interactions of colliding plasmas, the generation and evolution of plasma waves and turbulence, and the acceleration and propagation of energetic particles in turbulent magnetic fields.

Among the objectives of this campaign are the study of the acceleration of the solar wind and of the latitudinal variations of heliospheric particles and fields over the full range of solar activity; testing of theories about the interaction of the solar wind with the interstellar gas and plasma; and the determination of the properties of that medium which surrounds the heliosphere.

The results of this campaign will be important to understanding a broad class of stars with winds and how they interact with their surroundings.