A Brief History of Magnetospheric Physics
Before the Spaceflight Era

    It took more than 10 years before Chapman figured out how a neutral beam could cause magnetic disturbances. In 1927 he was joined in his quest by Vincent C. A. Ferraro, newly graduated [Cowling, 1975].

    The two had realized that an electrically neutral mixture of ions and electrons--what would nowadays be called a plasma--would be a very good conductor of electricity. Therefore, when a cloud of such matter approached the Earth, electric currents would be induced in it, creating a magnetic disturbance. But how could such currents be calculated? Chapman felt that as an approximation to a three-dimensional cloud one might start with a two-dimensional conducting sheet, approaching the Earth in its equatorial plane [Ferraro, 1969]. He knew that Maxwell had calculated currents induced in conducting sheets and advised Ferraro to look up that work. However, when Ferraro saw Maxwell's calculation, he realized that a different sheet approximation would be even better.

    If a large plasma cloud nears the Earth, its front boundary appears like an approaching wall--as the elephant did to one of the blind men in the parable [Saxe, 1936]. Furthermore, if the cloud is a perfect electrical conductor, all induced currents flow on the surface of that "wall." Maxwell had shown that when a perfectly conducting flat plane approached a dipole, its externally induced field was the same as the field of an equal "image dipole" located symmetrically on the other side of the plane. Thus the initial magnetic disturbance caused by the cloud should resemble the field of an image dipole at twice the distance of the cloud, rushing toward Earth at twice the cloud's speed (Figure 2).

Figure 2.   The Earth's dipole field (left), flattened by the addition of the field of an image dipole (right), as proposed by Chapman and Ferraro.

    That was how Chapman explained the "sudden commencement," a rapid steplike increase in the magnetic field heralding the onset of many (though not all) magnetic storms [Chapman and Ferraro, 1930, 1931, 1932]. There was a postscript [Dungey, 1979]: much later, Gold [1955] pointed out that the fact that the cloud maintained a sharply defined front boundary long after it had left the Sun suggested that this boundary was a collision-free shock and that therefore a sufficient density of interplanetary plasma existed for such a shock to form.

The Earth's magnetic field also exerts a force on the induced currents, and that force grows stronger as the cloud draws nearer. Ultimately, Chapman and Ferraro argued, it became strong enough to stop any further frontal advance of the cloud toward Earth; however, the flanks continued to advance, so that soon a cavity was formed, enveloping the Earth. That was known for many years as the "Chapman-Ferraro cavity," the region from which the plasma of the cloud was excluded by the action of the Earth's magnetic field (Figure 3).

Figure 3.   The formation of the Chapman-Ferraro cavity. Arrows trace the paths of ions and electrons by which Chapman and Ferraro proposed to account for ring current effects.

In the early 1900s the idea arose that a "ring current" of trapped particles might exist around the Earth's equatorial plane. Electrons and positive ions of sufficiently high energy could circle the Earth's equatorial plane in opposite directions, each contributing an electric current in the same sense, which always weakens the Earth's main field as observed.

    Carl Stoermer proposed such a ring current [Stoermer, 1910, 1911, 1912] to overcome a discrepancy in his theory, which predicted the aurora far closer to the magnetic pole than where it was observed [Smith, 1963; Chapman and Bartels, 1940, section 24.13]. Soon afterward, however, Adolf Schmidt suggested that a ring current was also the cause of the main phase of magnetic storms [Schmidt, 1924].

    The main problem was that the energy required for motions like those suggested by Stoermer was rather high: such orbits, when close to the Earth (distant orbits have other problems) are now recognized as appropriate for cosmic ray particles. As part of their theory, Chapman and Ferraro also proposed their own version of the ring current concept, set up (somehow) inside the Chapman-Ferraro cavity [Chapman and Ferraro, 1933; Smith, 1963]; the curved arrows in Figure 3 are related to their theory. This was later expanded by Martyn [1951] and Stoermer, [1955, section 60]. But as Chapman remarked [cited by Hulburt, 1937],

The whole theory is necessarily both speculative and difficult; probably the most doubtful feature is that relating to the ring current, the existence and formation of which are still very uncertain.

   
Shortly before the discovery of the radiation belt, Singer [1957] pointed out that trapped particles of low energy could also carry a ring current, even though their motion was more complex. An ion confined to the equatorial plane, for instance, tends to circle locally around field lines, but its circle will be slightly tighter where it comes closest to Earth, because the field there is slightly stronger. This causes the mean position of the ion to drift slowly in longitude, gradually carrying it around the Earth (Figure 4); ions and electrons drift in opposite directions, and therefore a neutral plasma yields a net circulating current.

The concept of such drift motion was first noted by Gunn [1929] and was described by Alfvén [1950] in Cosmical Electrodynamics, where he presented the equations of guiding center motion. Slow motion around the Earth was also found in Stoermer's numerically integrated orbits [Stoermer, 1955]. Singer [1957] noted that owing to this motion, a ring current could also be carried by a belt of trapped particles of relatively low energy; he suggested that such belts were formed in magnetic storms and lasted up to a few days before decaying.

    And yet, if those orbits were truly trapped, both entry and escape would be impossible. If they became populated during storms, how did particles reach them?