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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.

  1. Abstract
  2. Introduction
  3. Early Work on Geomagnetism
  4. The Sunspot Cycle
  5. Electron Beams from the Sun?
  6. The Chapman-Ferraro Cavity
  7. The Ring Current
  8. Alfvén's Theory and Electric Fields
  9. Interplanetary Plasma
  10. Polar Magnetic Storms
  11. Assessment
References: A-H
References: I-Z

The Chapman-Ferraro Cavity

    Sidney Chapman, a relative newcomer to the field of geomagnetism who was apparently unaware of Schuster's work, again raised the idea of solar electron streams in a 1918 paper on magnetic storms [Chapman, 1918; Akasofu et al. 1969]. He was pounced upon by Frederick Lindemann, Oxford professor of physics (Lord Cherwell, Winston Churchill's controversial World War II science adviser), who pointed out that the negative charge accumulated on the Earth would disrupt the process [Lindemann, 1919]. Lindemann then suggested that any cloud or stream expelled from the Sun would have to be electrically neutral, containing equal charge from ions and electrons.

    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.

    The image dipole 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.


Chapman-Ferraro cavity 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.

    Solar flares were one obvious source of plasma clouds. However, as Maunder had already noted, many storms could not be traced to any clear source, not even to definite sunspots. This held especially for moderate storms with a 27-day recurrence, extensively studied by Bartels [1932] and Newton [1932], who named their elusive sources "M-regions." The mystery deepened with the realization [Bartels 1934a] that in one carefully studied solar cycle, recurrent storms tended to cluster around solar minimum, including periods when no sunspots were visible at all. The answer was delayed until Mariner 2 detected high speed streams in the solar wind [Snyder et al. 1963; Neugebauer and Snyder, 1966], which seem associated with recurrent storms. Still later it was shown by solar observations from space and especially those of Skylab [Bohlin, 1977, section la] that such streams came from "coronal holes," most prevalent near solar minimum [Zirker, 1977; Hundhausen, 1979].

    An additional problem was posed by the imperfect correlation between sudden commencements (sc) and magnetic storms: often an sc is followed by no storm, and storms often occur without any sc. This problem, too, required space data for its resolution: it was only explained in the 1960s with the realization of the major role played by the interplanetary magnetic field (IMF) in solar-terrestrial interactions, in particular by the north-south component of the IMF. When the direction of the IMF is not favorable, the arrival of a plasma cloud may well produce no storm. Nowadays a sudden steplike rise of the magnetic field is termed an ssc (storm sudden commencement) if it is followed by a storm and an si (sudden impulse) if not.

The Ring Current

    If the cloud advances at 1000 km/s, the cavity will be fully formed in a few minutes. A typical magnetic storm, however, lasts much longer. Its main features are a "main phase" in which the north-south component of the Earth's field (at low and middle latitudes) gradually weakens over 6-12 hours, followed by a slow recovery of the field lasting 1-3 days. This part of the storm disturbance can be far more intense than the "sudden commencement," yet the Chapman-Ferraro cavity provided no good explanation.

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.
    Other evidence for plasma in the distant geomagnetic field came from low-frequency radio emissions and especially from whistlers [Helliwell, 1965, chapter 2; Alpert, 1980]. Starting with the work of Preece [1884] note was taken of clicks and whistles on long telephone lines: the cause was later identified as electromagnetic waves in the audio frequency range, picked up by the lines, which acted as antennas. Such sounds were also noted on field telephone lines during World War I and included a sound like "piou," descending in frequency. The phenomenon was studied by Barkhausen [1919, 1930] and later by Eckersley [1925], and the descending tones were named "whistlers." Owen Storey [Storey, 1953, 1956] definitely identified their source as lightning, sometimes occurring in the opposite hemisphere: the waves were guided along magnetic field lines and often oscillated several times between hemispheres before decaying. Their dispersion suggested an appreciable plasma density even in the most distant portions of the field lines, and that subject was widely studied by 1957, the year the first artificial satellites were orbited. However, it should be realized that the plasma involved here was mostly thermal, more related to the ionospheric plasma than to the more energetic particles of the ring current. Ring-current drift

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.

  Figure 4.   Schematic drift path of an equatorial ring current proton around the Earth, viewed from north of the equator.

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?

Last updated 17 October 2005