#9H. Trapped Radiation -- History

The orbits Stoermer was most concerned with belonged to particles with rather high energies. In general they did not resemble tidy spirals, because they covered large sections of the magnetic field, and in the course of each excursion around the field, the intensity and direction of the magnetic field did not stay the same but varied appreciably. Later, when cosmic rays were discovered, it turned out that Stoermer's theory applied quite well to their motion: but it did not solve the mystery of the polar aurora, as Stoermer had hoped.

Scientists in those days also wondered whether trapped particles were the cause of the mysterious "ring current" responsible for magnetic storms, but it was hard to imagine a process which created enough ions or electrons of cosmic ray energies to sustain them. Only in 1957 did S. Fred Singer (U. of Maryland) propose that the ring current may be carried by particles of much lower energies, injected somehow into trapped orbits during magnetic storms. In the following year the radiation belt was discovered--existing all the time, not just during storms--and gradually the details fell into place.

Meanwhile in the 1950s efforts began to confine plasma in laboratory magnetic fields for producing controlled nuclear fusion, and in the process the theory of trapped particles was greatly developed and expanded.

Project Argus

Perhaps the most interesting story of that era concerns Nicholas Christofilos, a Greek elevator engineer whose avocation was the study of particle motion in magnetic fields. Pursuing those studies he discovered "strong focusing," now a widely-used method of controlling beams of energetic ions in accelerators. He communicated this to scientists in the US, but received little attention until the principle was independently re-discovered. He then came to the US and worked for the rest of his career on various aspects of magnetic trapping of particles, in particular the trapping of plasma in devices intended to release fusion energy (see plasma).

As a grand experiment he proposed in October 1957 to the US Air Force to launch rockets with small atomic bombs, and detonate them in space. Atomic bombs produce large numbers of energetic electrons, and Christofilos hoped that such electrons would become trapped in the magnetic field as an artificial radiation belt.

That project, conducted in secrecy and code-named Argus, received a great boost when Van Allen's Explorers 1 and 3 discovered the natural radiation belt, and it was carried out above the Southern Atlantic in August and September 1958. Three bombs were exploded outside the atmosphere, above a deserted stretch of ocean, and the public only learned about it the following year, when many related scientific studies were published.

The bombs indeed produced many high-energy electrons. Some of these were guided upwards along magnetic field lines, followed those lines across the equator and came down again near the Azores islands, where a remarkable artificial aurora was seen, in a region where no auroras had ever been observed before. Other electrons mirrored above the atmosphere and stayed trapped, creating artificial radiation belts which gradually decayed in the matter of weeks. The new belts were studied by the satellite Explorer 4, built for this purpose by Van Allen's group at the University of Iowa.

Van Allen kissing
Explorer 4 good-bye.

#9H. Trapped Radiation -- History

Stoermer's Calculations

Project Argus

Further Reading: