Site Map
Quest. & Ans.
Glossary
Timeline
For Teachers
Review (1)
Review (2)
Central link




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


Early Work on Geomagnetism

    The history of geomagnetism begins with the magnetic compass, invented in China around the year 1000 and quickly adopted by Arabs and Europeans [Mitchell, 1932]. Gradually, it was realized that the magnetic needle did not point to true north; Columbus observed during his crossing of the Atlantic that it shifted from one side of true north to the other [Mitchell, 1937]. Magnetism was the avocation of William Gilbert, Queen Elizabeth I's personal physician. Gilbert gave a convincing explanation of the action of the compass: the Earth was a great magnet. He reached his conclusion with the help of a spherical magnet, a model of the Earth which he named the "terrella," or "little Earth." Moving a compass over the surface of the terrella, he observed that its needle pointed toward the magnetic poles, and he also demonstrated this before the queen. Gilbert's book De Magnete appeared in 1600 and described all that was then known about magnetism and electricity [Gilbert, 1958]. It was one of the important scientific books of the age of Galileo and among other things contained the first use of the term "electric force" which led to the later term "electricity."

Important advances in geomagnetism followed in the next two centuries [Chapman and Bartels, 1940, volume 2, chapter 26; Nelson et al. 1962]:

  1. The discovery by Gellibrand in 1635 of the slow variation of the Earth's field [Malin and Bullard., 1981; Brush and Banerjee, 1988]. Graham [1724] (see Chapman and Bartels, [1940, section 26.9]) of "magnetic storms" (later term), large irregular disturbances of the compass needle.
  2. The first magnetic survey of the Atlantic Ocean by Halley, in 1699 [Bullard, 1956; Ronan, 1969; Evans, 1988].
  3. The discovery by Oersted in 1820 that electric currents produced magnetic forces [Shamos, 1959; Dibner, 1962].
  4. The laws of electromagnetism, by Ampére in 1821 [Williams, 1965, 1989].
  5. Electromagnetic induction, by Faraday in 1831 [Faraday, 1952; Williams, 1963, 1965].

    In 1839 Carl Friedrich Gauss [Gauss, 1839, 1877; Dunnington, 1955] published a method for mathematically describing the Earth's field B by means of a scalar potential γ

B = –∇γ                     (1)

expanded at any point (r, θ, φ) in spherical harmonics:

γ =   a   Σ (a/r) n+1 Pnm (θ) [gnm sin mφ + hnm cos mφ]

                +   a'   Σ (r/a) n Pnm (θ) [Gnm sin mφ + Hnm cos mφ]         (2)

    The first sum represents sources inside the Earth, and the second one external sources. Gauss and his associate Wilhelm Weber then went on to found a network of observatories, greatly expanded by British and Russian help [Cawood, 1979; Malin, 1969]. From data thus obtained, the coefficients due to sources inside the Earth were derived [see Barraclough, 1978]; as for the external coefficients, the calculation gradually confirmed what had been suspected, that better than 99% of the field originated inside the Earth. However, as Graham's work suggested, some magnetic effects did originate on the outside. Observations of such effects were advanced by the work of Charles Coulomb, who in 1777 greatly increased the sensitivity of magnetic measurements by suspending a magnetic needle from a fine string [Gillmor, 1971; Shamos, 1959]. Such instruments could be made even more sensitive by attaching a small mirror which moved a spot of light, and this type dominated geomagnetism for close to 200 years [Nelson et al. 1962; Multhauf and Good, 1987].

    With such tools it was observed that the Earth's field was occasionally disturbed for a day or so: these events were termed "magnetic storms," but no one knew their cause. Celsius found that the large magnetic disturbance of April 5, 1741, was detected simultaneously by him in Uppsala and by Graham in London [Chapman and Bartels, 1940, section 26.10], demonstrating the nonlocal nature of magnetic storms. The magnetic network started by Gauss' and Weber later showed the storms to be a worldwide phenomenon.

The Sunspot Cycle

    Enter the Sun. In the first half of the nineteenth century there lived in the German town of Dessau a pharmacist named Samuel Heinrich Schwabe whose hobby was astronomy [Newton, 1958]. Every day when the Sun was not obscured, Schwabe observed it, paying attention to sunspots, noting their numbers, and keeping a tally of days when they were absent [Meadows 1970]. He started observing in 1826 and 10 years later published a report of his results: no one seemed to pay attention. In 1843 he published a more complete account, suggesting a l0-year cycle: at first, again, no response. Eventually, however, Schwabe's work caught the eye of Alexander von Humboldt, naturalist and promoter of the sciences, who in 1851 included Schwabe's results in his third volume of Kosmos, an encyclopaedic compilation of information about the physical world [Schwabe, 1851]. Suddenly, sunspots and their cycle became a hot topic: astronomers began counting sunspots and studying them, earlier cycles were reconstructed from old observations, and searches began for terrestrial effects which correlated with the sunspot cycle.

    Very soon such a correlation was found. Edward Sabine, a British scientist and the main architect of a worldwide network of magnetic observatories (an expansion of an earlier effort by Gauss and Weber), announced in 1852 that the frequency of magnetic storms rose and fell with the number of sunspots [Sabine, 1852; Meadows and Kennedy, 1982] (see also Lamont [1852]).

    Evidence was soon also found that the polar aurora was more frequently seen (at relatively low latitudes) near the' peak of the sunspot cycle. Here too a magnetic connection existed: as early as 1741 the Swedish scientist Celsius. reported that during auroral displays the magnetic needle was disturbed [Stoermer, 1955, section 6; Eather, 1980]. The actual discovery may have been due to Hiorter, a student of Celsius who later wrote that when he reported the magnetic effect of the aurora to his mentor, Celsius said that he too had observed the phenomenon but had not mentioned it in order to see whether his student would find it independently.

    How did sunspots exert their influence? The first clue came on September 1, 1859, in an unexpected observation by the distinguished British astronomer Richard Carrington [Meadows, 1970, p. 181]. Carrington was in the middle of an 8-year study of sunspots and was observing a large sunspot group when "two patches of intensely bright and white light broke out. . . the brilliancy was fully equal to that of direct sunlight" Noting that the spot was rapidly brightening, Carrington rushed off to find a witness, but coming back only 60 seconds later he found the spot of light "much changed and enfeebled" and soon afterward it faded altogether [Carrington, 1860].

    As luck had it, the astronomer Hodgson [1860] (see Meadows [1970, p. 187]) observed the same event from another part of England. An unusually intense magnetic storm followed 17 hours afterward, accompanied by polar aurora that could be seen far from the polar regions (another such storm had occurred a few days earlier, probably from the same sunspot group). Carrington noted the coincidence but added "one swallow does not make a summer."

    We now know that Carrington had seen a solar flare, a rapid release of energy probably drawn from the sunspot's magnetic field, capable of accelerating electrons and ions to high energies. Flares rank among the most rapid of the Sun's observed phenomena: they can extend over tens of thousands of kilometers, and their fastest features have time scales of seconds, though the whole sequence usually lasts tens of minutes to an hour.

    Only rarely do flares emit intense white light, as Carrington's did, but they are readily observable through filters which isolate the red Hα brightenings near sunspots, and in 1892 George Ellery Hale [Wright, 1966] devised the spectroheliograph, which produced images of entire areas on the Sun using only a single spectral wavelength. On July 15 of that year, Hale produced a series of photographs documenting the evolution of a large flare, which was followed 19 hours later by a large magnetic storm [Hale, 1892].

    More such correlations soon followed, leaving no doubt that something was propagating from the Sun to the Earth at about 1000 km/s (or faster, as in the two events cited here), causing a magnetic disturbance upon its arrival [Fitzgerald, 1892].

Electron Beams from the Sun?

    What was it? One clue seemed to come from discharges in low-pressure gases and from beams of "cathode rays" propagating between electrodes in evacuated vessels. Laboratory studies showed that these "rays" consisted of electrically charged particles whose properties were measured by J. J. Thomson and which were eventually named electrons [Thomson, 1967; Shamos, 1959]. Electron beams propagated at great speed, which led to the plausible suggestion that the source of observed disturbances was streams of electrons emitted from sunspot regions.

    The first serious study of this phenomenon was performed by the Norwegian Kristian Birkeland [Birkeland, 1901, 1908; Egeland, 1984,1986; Devik, 1968; Boström, 1968]. In 1896 Birkeland aimed cathode rays at a magnet and found that the magnet apparently "sucked in" cathode rays: he suggested that the Earth's field did the same to beams from the Sun. He communicated his findings to his former teacher, the French mathematical physicist Henri Poincaré, who showed that rather than being attracted, charged particles were guided by magnetic field lines [Poincaré, 1896]. Poincaré calculated the motion of an electron in the field of a magnetic monopole, a completely soluble problem, and found that the electron spiraled around a cone bounded by field lines, gradually losing headway until at a certain distance it was reflected backward [Rossi and Olbert, 1970, section 2.5; Mitchell and Burns, 1968].

Birkeland's Terrella
 Figure 1.   Birkeland's terrella

    Birkeland then built a large vacuum tank, placed in it a spherical magnet--like Gilbert he called it a terrella--and aimed at it beams of cathode rays (Figure 1). Bright spots appeared where the beams hit the terrella, generally in the polar regions. In some experiments there were even bright rings around the magnetic poles.

    By that time, appreciable information had accumulated about the aurora. Elias Loomis of Yale published a map of contours of equal auroral frequency in the northern hemisphere [Loomis, 1860], showing that they centered on the magnetic pole (rather than the geographic one) and that their frequency was highest in an oval band about 200 from the pole [Eather, 1980]. Hermann Fritz conducted a similar study with far greater precision [Fritz, 1881]. Following Birkeland's work the pieces suddenly seemed to fall into place: flares (or sunspots) apparently emitted electron streams, which were steered by the Earth's field toward the auroral zones--and since a stream of electrons carried an electric current, a magnetic disturbance would also be produced [Stoermer, 1917].

    This view was supported by Maunder [1904], who had deduced a tendency of storms to recur at 27-day intervals, the rotation period (relative to Earth) of low solar latitudes where sunspots tend to occur. He noted that recurrent storms were hard to correlate with solar phenomena (see further discussion below), but he still believed that "solar streams" were responsible and wrote as follows:

    That, therefore, which Lord Kelvin spoke of twelve years ago as "the fifty years' outstanding difficulty" is now rendered clear. Our magnetic disturbances have their origin in the Sun. The solar action which gives rise to them does not act equally in all directions, but along narrow, well-defined streams, not necessarily truly radial. These streams rise from active areas of limited extent. These active areas are not only the source of our magnetic disturbances but are also the seats of the formation of sun-spots. . . .
    Birkeland certainly did his best to promote the notion of solar electron streams. He also asked a colleague, the young mathematician Carl Stoermer, to calculate the motion of electrons in a dipole field, and Stoermer spent a large part of his career attacking that problem [Stoermer, 1955; Nutting, 1908]. Unfortunately, motion in a dipole field (unlike the monopole problem) has no analytical solution but is beset by pathologies resembling those of the notorious three-body problem of celestial mechanics [Dragt and Finn, 1976], so that Stoermer never achieved what he had sought, though he did integrate many orbits numerically.

    He did, however, manage to prove that a wide class of orbits existed in the dipole field that were trapped and did not extend to infinity. He furthermore showed that for sufficiently low particle energies all orbits hitting the terrella at low and middle latitudes were trapped, so that particles arriving from a distant source, like the electrons in Birkeland's experiment, never reached those latitudes but were always steered to the polar regions or turned away, in full accord with Birkeland's observations. In the Earth's field, Stoermer's theory worked well for cosmic ray particles in the Gev range, but not for auroral electrons. It was realized quite early that the atmospheric density at 100 km, where auroral electrons generally stopped, was so low that the energy of such electrons had to be low too. For instance, Harang [1951, table 31] estimated their speed at 0.3c, corresponding to about 23 keV. At such low energies, Stoermer's theory predicted impacts very close to the magnetic poles, contrary to observations that showed the midnight aurora peaked around magnetic latitude 68. Neither could it explain the observation that auroras were scarce near the magnetic pole itself.

    The theory of solar electron streams soon hit another snag: Arthur Schuster [Schuster, 1911; Chapman, 1934; Bartels 1934b] showed that electrostatic repulsion would quickly disperse any stream of solar electrons.


Last updated 17 October 2005