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Brief History Introduction

A Brief History of Magnetospheric
Physics During the Space Age

Reviews of Geophysics, 34, 1-31, 1996
David P. Stern, Laboratory for Extraterrestrial Physics
Goddard Space Flight Center, Greenbelt, MD 20771

Table of Contents

Clicking on any marked section on the list below brings up a file containing it and all unmarked sections immediately following it on the list. This list is repeated at the beginning of each file.

  1. Introduction
  2. Discovery of the Radiation Belts
  3. Artificial Belts and Early Studies
  4. The Large Scale Structure
  5. Convection
  6. Reconnection
  7. The Open Magnetosphere
  8. Observational Tests
  9. The Polar Aurora
  10. Field Aligned Voltage Drops
  11. Birkeland Currents
  12. Substorms: Early Observations
  13. Substorms: The Satellite Era
  14. Substorms: Theory
  15. Convection in the Geotail
  16. Planetary Magnetospheres
  17. Other Areas
  18. Assessment
References: A-G
References: H-P
References: Q-Z
Back to "Exploration"


2. Discovery of the Radiation Belts

    Early US magnetospheric research was mainly focused on the Earth's radiation belts, discovered by Van Allen and his colleagues in the spring of 1958. It drew from four sources. The first source was laboratory plasma physics, aimed at achieving nuclear fusion. Its early focus was in Princeton, where a number of "stellarator" confinement machines were built [Bishop, 1958], and it is interesting to note that James Van Allen, too, worked at the Princeton Plasma Lab in 1953-4 [Van Allen, 1983a, 1990]. Early plasma research provided an understanding of particle confinement in magnetic fields and of adiabatic invariants [Spitzer, 1956; Rosenbluth and Longmire, 1957; Northrop and Teller, 1960], essential to the theory of the radiation belts.

    The second source was high altitude research on radiation in space, mainly cosmic rays, using balloons and rockets [Friedman, 1994]. Balloon studies of the primary cosmic radiation started shortly after World War II [Simpson, 1994]; they led among other things to the discovery of the pion (pi-meson), and they were greatly expanded towards the IGY, the International Geophysical Year [Van Allen, 1983b; Odishaw and Ruttenberg, 1958]. Rocket-borne studies began shortly after the end of WW-II, when captured German V-2 rockets were brought to the US and were used there for high-altitude research [DeVorkin, 1992], and they continued with vehicles specifically designed for science, in particular the "Aerobee" [Newell, 1959]. Rocket instruments of the University of Iowa were launched in 1954 towards the aurora and their particle counters registered the presence of radiation [Meredith et al., 1955], later credited to x-rays produced by the electrons in the rocket shell or the atmosphere [e.g. Van Allen, 1995]. On the first day of the IGY balloon-borne instruments of the University of Minnesota also observed X-rays produced by auroral electrons, which penetrated deeper into the atmosphere than the electrons themselves [Winckler et al., 1957, 1958].

    The third source was interest in high-energy particles originating at the sun. This interest was given a great boost by the large solar particle event of February 23, 1956, which registered on cosmic ray detectors around the world. Solar particle research was also one of the active foci of the IGY (7.1.57-12.13.58), which coincided with a peak in the sunspot cycle. It was believed at the time that such particles were energized in solar flares by processes involving magnetic fields [Giovanelli, 1947; Hones, 1984b], and this stimulated the developments of theories of particle acceleration. Many of the results developed in this context, especially those of "reconnection theory" (see below), were later applied to the magnetosphere.

    A fourth source was the study from the ground of the aurora and of magnetic variations. Its community included Sydney Chapman, James Heppner, Ernest Vestine and Neil Davis, and was initially rather loosely coupled with the space observervations. Soon, however, it contributed in major ways to their interpretation.

    Sputnik 1 was launched October 4, 1957, followed on November 3 by Sputnik 2; Explorer 1, the first successful US spacecraft, was launched January 31, 1958, and Explorer 3, by which the radiation belt was discovered, was orbited March 26.

    Sputnik 1 carried no radiation detector, but Sputnik 2 did so, rising to an altitude of 1680 km. S.N. Vernov actually reported a significant (though not overwhelming) increase of the radiation rate between 500 and 700 km and in hindsight, this apparently marked the fringes of the radiation belt; however, he did not realize the implications [Singer, 1962; p. 249-258]. The apogee of Sputnik 2 was above Australia; the Australians who tracked it there asked the USSR for the key to its signals but were refused, and hence the data were not analyzed at that time [Hess, 1968, p.11; Dessler, 1984].

    The mission of Explorer 1 was assembled in a hurry. The US was trailing the USSR in space exploration, and the first attempted launch of its official entry, the Vanguard, ended in flames on the launching pad, and a launch vehicle was therefore improvised from available components, a back-up plan devised earlier by Von Braun [1964; Pickering, 1963]. The resulting orbit was rather non-circular and rose to an apogee of 2500 km, deep inside the radiation belt. Explorer 1 carried a radiation detector, a Geiger counter provided by Van Allen's team at the University of Iowa (Figure 1), which also included Carl McIlwain, Ernie Ray and George Ludwig [Van Allen, 1981, 1983a]. The counter was meant to measure the overall cosmic ray intensity, which by Stoermer's theory [see BH-1] was expected to increase with magnetic latitude, and its predicted counting rate was about 30 counts per second. The experimental package was a modified version of one designed for a later launch in the Vanguard series, and included a tape recorder designed to store the data for retransmission when the spacecraft passed over tracking stations. However, it was decided to provide this feature only on later missions and to omit it from Explorer 1, so that stations were only able to collect a few minutes' worth of data whenever Explorer 1 passed within range.

    On passes below 600 km the counting rate was nominal. Near apogee, however, no counts at all were detected, and on one pass, with the spacecraft around 1200 km and rising, counts were received but suddenly stopped [Van Allen, 1981, Fig.8]. Actually, much of this was only noted later: what was mainly noted was that sometimes the counter on the spacecraft operated normally and at other times it seemed dead. McIlwain showed experimentally that very high particle fluxes would overwhelm the counter and produce zero counts, and there is little doubt that further analysis of Explorer 1 data would probably have led to the discovery of the radiation belt. Before that could happen, however, much less ambiguous data were obtained from another experiment.

    Figure 2. Counting rate of
Explorer 3 during a pass
through the radiation belt.
The highest rate was encoun-
tered during the segment
of zero counts.

Explorer 2 failed to orbit, but Explorer 3 was successful. Unlike Explorer 1, it carried a tape recorder, and its continuous record of data (Figure 2) made clear what was happening. At low altitudes, only cosmic rays were detected; then as the satellite rose the recorded counting rate increased up to the highest it could record, and it stayed pegged there for a while. At a still higher altitude it abruptly fell to zero, and during descent the same transitions occurred in reverse order. The periods of zero counts near apogee clearly marked not the absence of radiation but a very high radiation flux: the Geiger counter was discharged so frequently that it did not recover between pulses and its output signals decreased until they no longer triggered the counting circuit.

    That was how the radiation belt was discovered [Van Allen et al., 1958; Van Allen, 1981]. Ernie Ray's comment was: "My God, space is radioactive!" [Hess, 1968]. The identity of the particles was quite uncertain. Auroral electrons had been observed in near-Earth space, but they lacked the energy to penetrate the counter walls. They could trigger a count by means of secondary X-rays, with a probability of the order of 10-5 [Frank, 1962], an explanation which had been previously used to explain rocket observations in the auroral zone [Van Allen, 1995]. Such an interpretation would have implied a huge flux of electrons, of order 100 million per cm-sq. sec, a figure which proponents of manned space flight viewed with justified alarm. Sputnik 3, launched May 12 to an apogee of 1880 km, carried scintillation detectors and confirmed the existence of Van Allen's belt, but it did not resolve the identity of the particles, and neither did Explorer 4, discussed in the next section [Van Allen et al., 1959]. As noted further below, the particles of the intense innermost part of the belt were soon accounted for. It was later pointed out by Dessler [1960; also Dessler and Vestine, 1960] that the extremely high fluxes, implied by the X-ray interpretation, would have modified the Earth's field above and beyond the changes actually observed. However, the uncertainty about the outer belt persisted until the work of Davis and Williamson [1963, 1966] and Frank's electron measurements with OGO 3 [Frank, 1967].

    In conclusion, it should be stressed here that the above is a very abbreviated summary of a complex period of discovery, and a much more detailed picture is available from Van Allen [1983a].

On to Section 3: Artificial Belts and Early Studies

Last updated 25 November 2001
Re-formatted 9-28-2004