Iowa’s cosmic ray experiment

The cosmic ray experiment that led to the radiation belt discovery was the one that Van Allen first proposed in November 1954.6 Its objectives were “(a) To measure total cosmic ray intensity above the atmosphere as a function of geomagnetic latitude and (b) To measure fluctuations in such intensity and their correlation with solar activity.”

Less than a year later, on 25 September 1955, and less than two months after Eisenhower’s announced decision to include a satellite program as a part of the U. S. contribution to the International Geophysical Year (IGY), Van Allen submitted a revised and extended version of that proposal to Joseph Kaplan, chairman of the U. S. National Committee for the IGY. The first paragraph of that letter read, “There is enclosed a ‘Proposal for Cosmic Ray Observations in Earth Satellites.’ Recent discussion with Dr. G. F. Schilling has indicated that it is appropriate to submit definite proposals at this time.”7 He followed that letter with a further-expanded version that he presented at the forty-third meeting of the Upper Atmosphere Rocket Research Panel at Ann Arbor, Michigan, on 26-27 January 1956.8 The latter proposal was eventually accepted as the basis for our development of the Vanguard cosmic ray instrument.

The January 1956 proposal stated its general objective as a “study of the cosmic-ray intensity above the atmosphere on comprehensive geographical and temporal bases for the first time.” It included extended discussions of the interpretation of expected data with respect to (1) the effective geomagnetic field, (2) the magnetic rigidity spectrum of the primary radiation, (3) time variations of intensity, and (4) cosmic ray albedo of the atmosphere.

Cosmic ray albedo refers to particles that leave (splash out from) the Earth’s atmo­sphere as a result of nuclear interactions caused by primary cosmic rays crashing into the atmosphere from above. Van Allen’s paper included a figure that plotted

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lune-shaped regions in the Earth’s vicinity within which particles of particular mag­netic rigidities and traveling in certain directions might be trapped.

That drawing and its discussion reflected the fact that there had already been a substantial body of earlier study into the behavior of charged particles in the Earth’s magnetic field.9 Sightings of the aurora Polaris (aurora borealis, popularly the northern lights, in the north polar region and aurora Australis in the southern hemisphere) had been recorded for centuries. A substantial amount of theoretical and experimental work was done during the first half of the twentieth century in attempting to explain those aurorae. Many of those early studies were conducted in Scandinavia, quite naturally, since populated portions of those countries lie well within the northern auroral zone. Kristian Olaf Bernhard Birkeland (1867-1917) was one of the leading early auroral researchers and, even today, is considered one of Norway’s greatest scientists. He published the first realistic theory of the north­ern lights, including his belief that they resulted from charged particles ejected from the Sun that were somehow captured or focused by the Earth’s magnetic field.

To help prove his theory, Birkeland performed his famous torella experiment. He directed an electron beam toward a conducting sphere that had a dipole magnetic field. The sphere’s surface was sensitized, and the experiment was conducted in near­vacuum. Electrons were seen to hit the sphere primarily in two rings that suggested auroral ovals similar to those seen on Earth.

With that finding, Birkeland asked his former teacher, Jules Henri Poincare (1854­1912), to examine the motion of electrons in magnetic fields. Poincare was able to solve mathematically the problem of the motion of charged particles near a magnetic monopole. Although magnetic monopoles have not been seen in nature, his work showed convincingly that the electrons were guided toward the poles of a real dipole magnet, thus preparing the way for later work. Birkeland suggested this problem to a mathematician friend, Carl Fredrik Mulertz Stormer (1874-1957), who devoted much of his career to its further study.10

One of Stormer’s most important contributions was to show that, for electrically charged particles of various combinations of mass, charge, and vector velocity, two dynamical regions exist within a dipolar magnetic field such as that of the Earth. One is of unbounded motion, and helps to account, for example, for the arrival of particles from outside the Earth’s immediate neighborhood (from the Sun, for example) into the Polar Regions.

The second region is one containing bounded trajectories. Stormer showed that certain classes of charged particles can spiral around the magnetic lines of force and that, as their centers of motion move north or south, they are reflected by the converging magnetic field lines. Moving then toward the opposite pole, the same action takes place, and the particles continue to mirror back and forth between the poles until

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they are scattered by irregularities in the magnetic field or interactions with other particles.

It is clear that the early researchers tended to view the region from the outside in. That is, they envisioned the particles as approaching the Earth from the Sun and beyond, and they referred to the region of the magnetic field that we now refer to as the trapping region as the forbidden zone, i. e., a region within which particles from the outside could not enter. Although they certainly realized that a particle injected by some mechanism into that zone with the proper rigidity and direction could be reflected back and forth by the action of the magnetic field, they did not appear to harbor any expectation that there might be a substantial reservoir of particles durably trapped there.

We at the Iowa campus enjoyed a special treat during the first semester of the 1954— 1955 school year, when Sydney Chapman joined us as a visiting distinguished pro­fessor. During that semester, Chapman taught a course titled Physics and Chemistry of the Upper Atmosphere. Among other things, he included extended discussions of the aurorae, and of theories that attempted to describe them, including the works of Birkeland and Stormer. Detailed notes from his lectures were assembled by Ernie Ray as a mimeographed, unpublished compendium.11 The formal course was accom­panied by many stimulating informal discussions by Chapman, the faculty members, and us students.

Interest in the trajectories of charged particles in the Earth’s geomagnetic field, especially after the interaction with Chapman, resulted in a flurry of activity within the State University of Iowa (SUI) Physics Department. Ernie Ray and Joe Kasper undertook concentrated studies of that phenomenon. With Van Allen and others, they began to apply that knowledge to help explain the auroral soft radiation that had been detected, first on the 1953, 1954, and 1955 rockoon expeditions, and then by Carl McIlwain with his rocket shots at Fort Churchill, Canada, in 1957-1958.

In their studies, which involved tracing the charged particle motions near the Earth, Ernie made some of his earliest attempts to program the newly evolving digital computers to solve the differential equations involved. Joe configured the analog differential analyzer that he had developed for his master’s thesis for a similar purpose.

During that period, there were many spirited discussions of Stormer trajectories, cosmic ray motion, auroral mechanisms, and other related topics, both on our campus and within the larger research community. The field was abuzz with activity, both experimental and theoretical.

S. Fred Singer, as early as April 1956, suggested that the motions of charged particles in the Earth’s magnetic field, by the process hypothesized by Stormer many years

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earlier, might account for the Earth’s ring current. The ring current is an electric cur­rent, predominantly consisting of protons and heavier negative ions drifting westward around the Earth, which slightly perturbs the magnetic field at its location. Studies of the ring current had occupied Singer’s attention for some time, and that was the object of Laurence Cahill’s rockoon flights during the fall of 1957, as recounted in an earlier chapter.

Singer further elaborated on his ideas related to particle trapping in April 1957:

The [magnetic] storm decrease is produced by the high-velocity particles following the shock wave (up to nine hours later) which enter because of field perturbations into the normally inaccessible St0rmer regions around the dipole. Here they are trapped and will drift, producing the ring current which gives rise to the storm decrease. Particles with a small pitch angle, however, can reach the Earth’s atmosphere and contribute to aurora, the airglow, and ionospheric ionization.12

However, before the discovery of the high-intensity radiation by Explorers I and III, no one within the worldwide community of researchers, including Singer, had made the intellectual leap to suggest that a huge population of particles might be trapped there to form a durable region of intense radiation surrounding the Earth.