Discovery of the auroral soft radiation

As already mentioned, the most significant single new result from the early rockoon flights was the discovery and early characterization of the auroral soft radiation. That discovery was completely unexpected and turned out to have important impli­cations.

Following the initial detection of the extra radiation during two rockoon flights during the summer of 1953, Meredith, Gottlieb, and Van Allen tentatively hypothe­sized that the GM counters had registered the high-energy tail of the primary auroral particles.36 They stated that the observed particles were most likely electrons having energies in the neighborhood of 1 MeV that were directly penetrating the residual atmosphere above the rocket, the sheet metal of the nose cone, and the wall of the GM counter. They eliminated protons as the cause by reasoning that, if the particles were protons, then they must have possessed energies up to 35 MeV and beyond in order to penetrate the various absorbing materials. They then pointed out that protons of that energy would have too large a radius of curvature as they spiraled around lines of the Earth’s magnetic field to produce the observed spatial inhomogeneity. They also stated that the observed energy spectrum was low enough that most of the particles could not be coming along Stormer-type trajectories directly from external sources such as the Sun.

The auroral soft radiation was seen again during three of the summer 1954 rockoon flights. Two flights of the first type of instrument containing paired GM counters with different absorber thickness had been ballistically successful, and the second one dramatically revealed the auroral soft radiation superimposed on the primary cosmic ray background. Based on those data, Ellis, Gottlieb, and Meredith reported in an abstract in July 1955 that the ratio of counting rate for the counter without the absorber to the rate for the counter with the added absorber was about three for the

CHAPTER 2 • THE EARLY YEARS 55

upper part of the flight (55 to 60 miles). It was about two lower in the flight where the radiation was first encountered.37

Data from the flights of the second instrument type containing McDonald’s new scintillation detector-GM counter instruments were somewhat confusing.38 That con­fusion was partly resolved by the summer of 1955, when McDonald, Ellis, and Gottlieb published an abstract stating that, of three successful flights of that instrument, two revealed the soft radiation.39 It was seen as an elevated counting rate in the single GM counter mounted ahead of the scintillation detector. They concluded that the radiation had not been energetic enough to activate the telescope by traversing the combined absorbing materials ahead of and in the instrument. Those abstracts of the preliminary analyses did not offer further speculation about the particle species.

Van Allen also published a brief abstract at that time, in which he stated that the average density of material penetrated by the particles was of the order of 180 milli­grams per square centimeter in aluminum and 220 mg/cm2 in the atmosphere.40 He alluded to possible interpretations in terms of gamma rays having energies of about 20 KeV, electrons of energy about 1 MeV, or protons of energy about 15 MeV. Thus, although the possibility that the detectors were directly detecting elec­trons had not yet been entirely discounted, other possibilities were being seriously considered.

The situation was finally resolved following analysis of the aggregate of all rockoon data following completion of our 1955 expedition. The results were promptly reported at the spring 1956 meeting of the American Physical Society.4142 Van Allen and Joe Kasper’s summary paper asserted that (1) the auroral primary radiation consisted of electrons with energies of the order of tens of KeV and (2) the GM counters were actually registering X-rays (referred to as bremsstrahlung, or braking radiation) produced in the nose cone by bombardment by the electrons.

Those early assessments were expanded upon and summarized by Van Allen in a classic paper published in early 1957 by the National Academy of Sciences. He summarized the salient features of the radiation:

a) The latitude distribution and the temporal variability of the effect [the soft radiation] strongly suggest that it is to be associated with aurorae.

b) The radiation is quite soft (by cosmic ray standards), being completely or nearly completely absorbed… by amounts of material ranging from several gm/cm2 to several hundred mg/cm2 of air and/or aluminum and being attenuated by a factor ranging from 3 to greater than 50 by 150 mg/cm2 of lead.

c) Referring to the crystal measurements which give absolute energies dissipated in the crystal, we have observed no case in which resolved pulses corresponding to greater than 200 keV occur (except for the expected number of cosmic ray pulses), even though there is a simultaneous occurrence of a very large counting rate in the more heavily shielded Geiger tubes. . .

d) The “wings” of the counting rate versus time curves are in all cases “regular” in character and are believed attributable to atmospheric absorption.43

OPENING SPACE RESEARCH

In that paper, Van reaffirmed the conclusion that the detectors could not have been directly registering protons or electrons. He further asserted that X-rays having energies in the range 10-100 keV were consistent with all observed data. He provided an estimate that the X-ray intensity was of the order of magnitude 103-105 photons per square centimeter per second. It was believed that the X-rays seen at relatively low altitudes (25-45 miles) were bremsstrahlung from electrons that were stopped at 55 miles or above in the atmosphere, and that when the rockets were at higher altitudes (say, above 65 miles), the primary auroral electrons were striking the walls of the apparatus and creating the bremsstrahlung locally.

The locations of ships at the times of launching all of the 55 rockoon flights during the 1952, 1953, 1954, and 1955 expeditions are indicated in Figure 2.14. The flights clustered near Thule, Greenland, were made during the 1952 expedition when the rockoon technique was being initially tested. The flights extending from Boston, up the Nova Scotia coast, and around Newfoundland were largely shake­down flights, although several were fully successful and yielded data for the latitude survey, the original program objective. The rest of the flights, those off the coast of Labrador, up Davis Strait, and across Baffin Bay, represent attempts either to obtain data points for the latitude survey or to investigate the auroral soft radiation after it was initially discovered in 1953. The initial discovery of the soft radiation by SUI flight 13 on 28 July 1953 is indicated by the star located just north of Resolution Island.44

Van Allen summarized the results of many of those flights in another form in a figure in his 1957 paper, reproduced here as Figure 2.15.45 In this figure, the peak counting rates from the 10 flights represented by the stars in Figure 2.14 are plotted as a function of geomagnetic latitude.

The peak occurrence of visible auroras occurs near the center of the shaded region in Figure 2.14, and near the 68 degree geomagnetic latitude region in Figure 2.15. Taken together, these two figures dramatically illustrate the close association of the auroral soft radiation observations with the visible aurorae. Those results constituted the first in situ detection and measurements of the presence and composition of the radiation responsible for the visible aurorae. Further rockoon observations by Van Allen and colleagues during 1957 (described in Chapter 4) helped to further define the characteristics of that phenomenon.46

Had more been known about magnetospheric physics in 1956-1957, the Iowa group might have deduced that some substantial portion of the X-rays were being produced by charged particles mirroring in the northern cusp of the later-discovered outer region of high-intensity trapped radiation. Postulation that huge populations of charged particles were durably trapped in the Earth’s magnetic field was not made, however, until after the initial Explorer I and III data were examined in 1958.

CHAPTER 2 • THE EARLY YEARS

FIGURE 2.14 The approximate locations of all rockoon flights during the 1952,1953,1954, and 1955 expeditions are indicated by circles (unsuccessful), plus signs (instruments reached a height of 120 miles or more but did not observe the auroral soft radiation), and stars (instruments reached a height of 120 miles or more and detected the auroral radiation). The shaded oval indicates the approximate location of the region where visible auroras are most frequently seen.

Anderson’s Canadian balloon flights in early 1956

Kinsey Anderson became another highly productive member of the Iowa cosmic ray group when he joined it in the fall of 1955.

OPENING SPACE RESEARCH

Kinsey A. Anderson

Kinsey A. Anderson was bom on 18 September 1926 at Preston, Minnesota, and grew up there. He received his B. S. degree in physics from Carleton College in 1949, and went on to the University of Minnesota, where he received his Ph. D. degree after the spring semester in 1955. He stayed on there for the summer as a research associate to work with John Winckler and colleagues on a survey of cosmic ray intensity over the range 51 degrees to 65 degrees north geomagnetic latitude, using a triple-coincidence Geiger counter lofted by small latex balloons.

During Kinsey’s work that summer, one of their balloon flights, made from Flin Flon, Manitoba, on 26 August 1955, at a geomagnetic latitude of 65 degrees, revealed a dramatic increase in the counting rate of their counter telescope. This was quite unlike anything they had seen during the earlier, more southerly flights. They were aware of the discovery and study by our Iowa group of the auroral soft radiation during the 1953 and 1954 rockoon expeditions, and of the further studies being conducted during the 1955 expedition. Kinsey and Winckler were fully aware that we were beginning to think seriously about X-rays as the possible cause of the anomalous high counting rates at the rockoon altitudes. But they could not understand how X-rays might penetrate to the lower balloon altitude to produce the result they had seen. This mystery served as a major motivation for Kinsey’s later research program at Iowa.

Kinsey joined the Iowa Physics Department in September 1955 as a research associate, advancing to assistant professor in 1958. He left Iowa City in November 1959, spending the next several months at the Royal Institute of Technology in Stockholm, Sweden. He moved to the University of California at Berkeley (UCB) in the autumn of 1960 to join the space research program there. During a long and distinguished career at UCB, he advanced to full

CHAPTER 2 • THE EARLY YEARS 59

professor in 1966, contributed substantially to the U. S. space research program, and served as director of the Space Science Laboratory for many years. He is currently a Research Physicist Professor Emeritus at Berkeley.

Kinsey’s initial undertaking at Iowa was to continue the theme of his Minnesota research, but with the use of larger Skyhook balloons. His first instrument, a GM counter telescope for measuring cosmic ray protons and helium nuclei, was ready by early 1956. He carried his flight instruments to Goodfellow AFB as the sole Iowa participant on a third ONR-sponsored field exercise.47

Experimenters from other laboratories on that Goodfellow expedition were from the universities of Chicago and Minnesota. An ONR field representative, R. C. Cochran, was in overall charge, and General Mills again handled the balloons. Notices were placed on all flight packages to facilitate their quick recovery but, acting on previous experience, in this case, advance notices were also sent to Texas ranchers and cattlemen so they could be on the lookout for equipment landing in their areas.

That expedition saw the launch of 10 balloon flights between 25 January and 15 February 1956, several of which carried Kinsey’s instruments.48