Early unclassified Explorer IV results

As in the case of the earlier Explorers, paper strip-charts were produced as a first data reduction step for Explorer IV, using the equipment setup shown earlier in Figure 11.4. But for Explorer IV, the process was a bit more complicated because of the highly classified nature of the Argus Project. For the initial month (before the first Argus detonation), during which all of the data were unclassified, data reduction was much as it had been for Explorers I and III.

CHAPTER 13 • ARGUS AND EXPLORERS IV AND V 377

However, during the month following the first nuclear burst, i. e. during the times that portions of the data showed the effects of the tests, Carl McIlwain served as a data screener. He diverted the charts containing indications of the Argus tests for special handling, where he served as the primary data reader.

Explorer IV results were disclosed in three steps: (1) an initial public release of the unclassified results, (2) an exchange of classified Argus data and results within a small circle of appropriately cleared personnel, and (3) a public release of the Argus results sometime later.

Work on the unclassified portion of the Explorer IV data was straightforward, even though rushed. The first tentative written expression of results was on 2 August 1958 in the form of a telegram from Van Allen.19 On the same date, the same information, with the addition of a block diagram of the instrument package, was recorded as a report in the Physics Department’s serial report series.20

Those two documents provided a very sketchy report based on the examination of only 15 station recordings from 26 through 29 July in the northern hemisphere and covering the altitude range from about 165 to 1000 miles. They reported that the instruments were operating properly and gave some very tentative information on the rapid increase in radiation intensity, as the satellite climbed above 250 miles height. Some first-ever information from Carl’s detector about the total energy density was also included.

Due to Van Allen’s, Mcllwain’s, and my heavy involvement in the Explorer IV effort, Ernie Ray was the only one from our laboratory to attend the Fifth General Assembly of the IGY Committee in Moscow on 30 July through 9 August 1958. While there, he received a telegram from us that conveyed a summary of the information from the two documents mentioned above. He presented that information at the conference.21

Additional releases quickly followed. On 20 August, a slightly expanded report based on a larger collection of data from the first two weeks of Explorer IV opera­tion was released.22 Among other things, it reported fluxes of both penetrating and nonpenetrating components, with the penetrating particles predominating at lower latitudes (in what came to be known as the inner radiation belt) and the nonpenetrat­ing particles predominating at the higher latitudes (at the horns of the outer radiation belt).

By late November 1958, our analysis had progressed far enough for us to issue a substantially expanded Department of Physics report.23 That paper included a number of interesting figures, one of which is reproduced here as Figure 13.3. The shape of the contours around 30 degrees north latitude provided a first hint of what was later recognized as the gap between the inner and outer radiation belts, with the cusp north of 30 degrees being the lower tail of the outer belt.

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FIGURE 13.3 A sketch from the 20 August Physics Department report showing initial results from Explorer IV. This is a meridional section through the Earth showing counting rates from the relatively unshielded GM counter. The data were taken between 26 July and 26 August 1958 within the longitude range west 60 to 100 degrees. (Courtesy of the University Archives, Papers of James A. Van Allen, Department of Special Collections, University of Iowa Libraries.)

The contours in that set of figures led to a speculative extrapolation of the radiation levels farther into space, reproduced as Figure 13.4. Actually, that particular model of the high-intensity radiation was one of two being considered when the paper was prepared. A second model, suggested and particularly advocated by Carl McIlwain, regarded the high-latitude cusps appearing consistently in the set of data plots to be the lower ends of a second distinct region of high-intensity radiation. Later observations from Pioneer 3 (described in the next chapter) showed Carl’s model to be the correct one.24 Since those Pioneer data were not available when the November 1958 report was prepared, the “simpler” of the two models, i. e., the one showing a single region of trapping, was chosen for publication.

The November report was presented at a meeting of the American Physical Society in Chicago in early 1959 and was published in March in the Journal of Geophysical Research}5 Since it had been actually mailed for publication late in 1958, well before the Pioneer 3 results were available, it still included this Figure 13.4, showing the single donut-shaped region of high-intensity radiation.

In addition to the variation in intensity with altitude and latitude provided in the first reports, the new paper provided information on the intensity variation with longitude, the angle of arrival of particles, and the nature of the radiation. In summary, the in­tensity varied with longitude in the way that one might expect from knowledge of the actual shape of the Earth’s magnetic field. Second, there was a strong dependence of radiation intensity on detector pointing angle. That was interpreted as indicating that the particles were moving predominantly in discs lying nearly perpendicular to the

CHAPTER 13 • ARGUS AND EXPLORERS IV AND V FIGURE 13.4 A combination of the data from Explorer IV served as the basis for this conjectural diagram. The solid portions of the intensity contours near the Earth show actual data from the same counter and longitude as the plot of Figure 13.3.The dashed portions indicate a speculative extrapolation of intensities, based on a simple model ofthe Earth’s magnetic field, to form a donut­shaped region of high-intensity radiation circling the Earth. (Courtesy ofthe University Archives, Papers of James A. Van Allen, Department of Special Collections, University of Iowa Libraries.)

lines of the magnetic field, thus helping to substantiate the model of particle trapping by spiral movement along the field lines. Third, although information on the compo­sition of the trapped radiation was very sketchy, electrons seemed to predominate at the higher latitudes, and there was a major proton component at the lower latitudes.

The last section in that paper, in both its November 1958 and March 1959 forms, was devoted to extended remarks on the interpretation of the data. It considered as well established that the “great radiation belt” around the Earth (by then the singular term belt was still being widely used) consisted of charged particles, temporarily trapped in the Earth’s magnetic field in Stormer-Treiman lunes. The paper went on to state that the overall decrease in intensity at the lower altitudes was almost certainly due to atmospheric scattering and collisional energy loss. Scattering would predominate for electrons and collisional loss for protons.

As to the injection rate, which would have to equal the loss rate in order to maintain a stable belt intensity, the paper stated that the decay of neutrons moving out from the atmosphere as a result of cosmic ray collisions with atmospheric molecules might

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help feed the belt, but that that source was inadequate by a large factor to produce the observed intensity. The paper asserted that solar plasma must replenish the reservoir of stored particles from time to time, working its way into the outer reaches of the Earth’s magnetic field under some conditions, and then being trapped in the magnetic field.

Finally, the paper suggested that the leakage of electrons from the trapping region at high latitudes might be the direct cause of the aurorae.