A Brief History of Magnetospheric Physics
Before the Spaceflight Era

David P. Stern Laboratory for Extraterrestrial Physics NASA
Goddard Space Flight Center Greenbelt, Maryland

Appeared in Reviews of Geophysics, 27, 1989, p. 103-114.

Abstract
Introduction
Early Work on Geomagnetism
The Sunspot Cycle
Electron Beams from the Sun?
The Chapman-Ferraro Cavity
The Ring Current
Alfvén's Theory and Electric Fields
Interplanetary Plasma
Polar Magnetic Storms
Assessment

Alfvén's Theory and Electric Fields

Hannes Alfvén in Sweden was an early investigator of plasmas in space. In the last years before World War II he proposed that ring current effects (and auroras as well) were due to the entry into the Earth's field of particles from the solar plasma cloud, convected there by an electric field due to the cloud's motion. If the cloud has a high electric conductivity, then the local electric field E* inside it vanishes,

E* = E + v × B = 0 (3)

yielding the so-called MHD condition. The electric field E enables particles to flow perpendicular to magnetic field lines, imparting to their average position ("guiding center") the velocity

v = E × B/B² (4)

Alfvén did not believe in the Chapman-Ferraro theory, which treated the cloud as a continuous fluid [Alfvén, 1951] but rather viewed the cloud as a collection of individually moving particles. Those particles would flow together with the above bulk velocity v until they came close to the Earth's dipole; there the guiding center motion (in a manner somewhat similar to what was later invoked by Singer [1957] for trapped particles) would move ions and electrons in opposite directions, creating a cavity around the Earth and also leading to the ring current field (Figure 5).

Figure 5. The motion of ions (dark lines) and electrons (light lines) according to Alfvén's theory [after Cowling, 1942]. The Sun is to the left.

Alfvén's argument was somewhat more involved and also included an explanation of the aurora. Chapman, as might be expected, strongly disagreed [Dessler, 1970; Akasofu. 1970], and in the end, Alfvén's article on his theory was not accepted by any major journal but appeared in Sweden in a relatively obscure format at the end of 1939 [Alfvén, 1939; Cowling, 1942] (see also Stoermer [1955, section 61] and Stern [1977]). Though it made some important points, in 1957 it was still poorly known and appreciated outside Scandinavia.

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.