THE SOLAR WIND IS a flow of ionized solar plasma and a remnant of the solar magnetic field that pervades interplanetary space. It is a result of the huge difference in gas pressure between the solar corona and interstellar space. This pressure difference drives the plasma outward, despite the restraining influence of solar gravity. The existence of a solar wind was surmised in the 1950s on the basis of evidence that small variations in the earth's magnetic field (geomagnetic activity) were produced by observable phenomena on the sun (solar activity), as well as from theoretical models for the equilibrium state of the solar corona. It was first observed directly and definitively by space probes in the mid-1960s.
Measurements taken by spacecraft-borne instruments since that time (and until recently) have yielded a detailed description of the solar wind across an area from inside the orbit of Mercury to well beyond the orbit of Saturn. Solar-wind observations and theory have been the subjects of several books (Parker, 1963; Brandt, 1970; Hundhausen, 1972) and a continuing series of conferences summarizing new research (e.g., Pizzo et aI., 1987). Why has this distant and tenuous plasma been the subject of sustained interest in the scientific community? The answer to this question probably stems from two important aspects of solar-wind research.
The first of these concerns the role of the solar wind in the interdisciplinary subject known as solar-terrestrial relations. As we shall see later in this chapter, the solar wind is significantly influenced by solar activity (or, in physical terms, by changes in the solar magnetic field) and transmits that influence to planets, comets, dust particles, and cosmic rays that are immersed in the wind. Some of these effects will be described more extensively in later chapters. The origin of the solar influence through interaction of the solar magnetic field with the expanding coronal plasma has become a major topic in present-day solar-wind research and will be heavily emphasized in this chapter.
The second important aspect of solar-wind research that helps to explain the sustained interest in the subject concerns the physical processes that occur in its formation and expansion from the hot solar corona to the cool and far more tenuous regions of the outer solar system.