Interplanetary Plasma
Chapman and Ferraro had assumed that except for their plasma clouds, interplanetary space was relatively empty, but evidence to the contrary came from observations of comet tails. For many years it was held that the long tails of comets were adequately explained by the pressure of sunlight, but Hoffmeister [1943, 1944] found that many comet tails deviated by several degrees from the radial direction, in a way suggesting that they were shaped not by sunlight but by solar particles propagating at a lower velocity. After World War II this was picked up by Biermann [1951], who noted that dust tails, whose spectra resembled scattered sunlight, could be explained by light pressure, but that the distinct ion tails often showed huge accelerations which could only be accounted for by a "solar corpuscular radiation." For a long time, h owever, more direct evidence was lacking.
When it was discovered, from spectra of highly ionized species [Grotrian, 1939; Edlén, 1941, 1942, 1945; Shapley, 1960; Billings, 1966, chapter 1; Lang and Gingerich, 1979] that the Sun's corona had a temperature around 106 K°, the question arose of how the Sun's gravity could keep such a hot atmosphere attached [see Lüst, 1962; Parker, 1964]. Coronal temperature near the Sun was observed not to decrease with height, and this was explained by the high heat conductivity of the plasma, which seemed to preclude a stratified atmosphere like the Earth's, with temperature decreasing with height. Chapman proposed a theory in which a static equilibrium was still possible, yielding moderately lower temperatures at the Earth's orbit. Eugene Parker, however, derived an alternative solution in which the corona was not in equilibrium but instead continually streamed away from the Sun to form a high-speed "solar wind" [Parker, 1958; Dessler, 1967; Brandt, 1970], The process converted heat to kinetic energy rather efficiently.
The debate between proponents of a static corona, Parker's solar wind theory, and an alternative "solar breeze" theory of Chamberlain [1960, 1961; Dessler 1967] was only settled by observations from space. Gringauz et al. [1960] (see also Gringauz [1961]) mounted charged particle traps on Lunik 2 (September 1959) and later on Lunik 3 (October 1959), and they detected far from Earth a flow of energetic positive charges, consistent with solar wind ions and also displaying appropriate modulation due to spin of the spacecraft. In 1961 the Massachusetts Institute of Technology particle trap aboard Explorer 10 obtained more detailed evidence for the solar wind [Rossi, 1984; Bonetti et al. 1963], and information concerning the continuous nature of the solar wind came in 1962 from the flight of Mariner 2 to Venus [Snyder et al.. 1963; Neugebauer and Snyder, 1966]. It then became clear that the Chapman-Ferraro cavity was not a temporary feature but existed at all times, and it received the name "magnetosphere," coined by Gold [1959]. Rapidly spreading plasma clouds produced by solar flares, like those envisioned by Chapman and Ferraro, are sometimes superposed on the solar wind flow. We now know that when the expansion velocity of such clouds greatly exceeds that of the solar wind, they are indeed preceded by collision-free shocks.
Last updated 17 October 2005
