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Published online by Cambridge University Press: 25 April 2016
It is now generally recognized that the solar wind represents that part of the solar corona which is not confined by the solar magnetic field, and therefore escapes into interplanetary space. The escaping gas is heated by sources of solar origin (presumably low frequency waves) to about 2 x 106 K within a distance less than (1/20) R from the sun’s surface; although the solar wind temperature decreases thereafter, heating sources may continue to act through 1 a.u. heliocentric distance. A transition from subsonic to supersonic flow occurs within a few solar radii of the sun’s surface, heat conduction representing the principal energy supply for the acceleration of the solar wind. However, additional accelerating processes may also be active.