About twenty years ago, the so‐called “infall” theory had been thoroughly worked out by Hoyle and by Alfvén. They reasoned that, since the sun moves through clouds of interstellar hydrogen gas, the neutral hydrogen must fall in toward the sun under the influence of the gravitational field. On its way toward the surface of the sun, the hydrogen encounters a strong flux of ultraviolet radiation and, because of the small rate of recombination, it becomes completely ionized. Thus the inner solar system is filled with a dilute, highly conducting plasma. At about the same time the infall theory was worked out, it became rather generally accepted that the corona of the sun has a temperature of 1 or 2 million degrees. In fact, Hoyle tried to account for the heating of the corona in terms of the energy released by the hydrogen falling into the sun.
Plasmas in space
Herbert S. Bridge; Plasmas in space. Physics Today 1 March 1963; 16 (3): 31–37. https://doi.org/10.1063/1.3050802
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