The IGY Program at Iowa

T

he program of the International Geophysical Year—1957-1958 (IGY) provided a unique opportunity for cosmic ray research in general, and for us at Iowa City. As an active leader in the overall planning, James Van Allen had helped to shape both the general and specific character of the IGY program. In that role, he provided a great service to the research community.

In our Physics Department, Van Allen set the stage for the next few years of research. Acting to take advantage of the tremendous opportunity, he met in early 1956 with his graduate students to discuss possibilities for projects that they might undertake for their thesis work. By Carl McIlwain and Larry Cahill’s recollections, the list of possible projects that he placed on the table in that session included six cosmic ray, two auroral soft radiation, and two magnetic field studies.12 He envisioned that they could use a variety of balloons, ground-launched rockets, rockoons, and satellites. Those suggestions were in addition to the cosmic ray satellite experiment which he had proposed, and on which I had already begun work.

Following that session, decisions were made quickly, and specific proposals for IGY projects were submitted to Washington and funded. The work was undertaken with great enthusiasm and energy by the department’s graduate students, faculty, and staff.

Ground-launched rockets

The Iowa IGY research program, as it evolved, included experiments employing a mix of ground-launched rocket, balloon, rockoon, and satellite instruments. Carl McIlwain initiated the rocket program with an ambitious plan to probe the northern auroral zone.

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Mcllwain’s Fort Churchill flights At the time of the graduate student meeting, Carl was completing the work for his master’s degree. He received that in June 1956, with a thesis based upon the data from his Loki rockoon flights during the summer 1955 Davis Strait expedition.3

Van Allen’s suggestion that Carl might take advantage of the IGY program to fly some Nike-Cajun rockets at Fort Churchill, Canada, caught his attention, and he began thinking about various possibilities. He was captivated by the possibility that those rockets might be able to directly detect the particles that created the aurora, and thus shed light on the particle composition and energy spectra. It was known at the time, based on the alignment of the visible auroral features with the Earth’s magnetic field, that the auroral light was due to charged particles entering the Earth’s atmosphere. It was also known that at least some of those particles were protons in the 100 keV energy range, based on the presence of Doppler-shifted H„ and Hg spectral lines. The earlier direct detection of the soft auroral radiation by State University of Iowa (SUI) rockoons, however, indicated the presence of electrons, and Carl was eager to follow up on that new information.

To emphasize once again the trust placed by Van Allen in his graduate students, he assigned projects to them and then gave them tremendous freedom in seeing them through to completion. In this case, he designated Carl as the SUI chief scientist, and Carl bore full responsibility for preparing the instruments and conducting the field operation.

Carl’s first major technical challenge was to devise detectors that could detect and measure the very soft radiation that was capable of penetrating only very small amounts of material. Developing those instruments occupied much of 1957. He was joined in that work by Donald (Don) Enemark, a second-year electrical engineering student, and Donald (Don) Stilwell, an undergraduate physics student. They developed and built instruments capable of detecting the low-energy electrons in time for two Nike-Cajun flights that were scheduled for late summer 1957. Carl is shown working on one of his instruments in Figure 4.1.

Carl’s apparatus included three charged-particle detectors, a photometer, and a magnetometer. The first particle detector employed a thin thallium-doped cesium iodide (CsI [Tl]) scintillator crystal mounted on the face of a photomultiplier tube. Apertures, plus a 400 gauss permanent magnet, prevented electrons with less than 1 MeV energy from reaching the scintillator. The light sensitivity of the scintilla­tor crystal was reduced by a coating of 40 microgram per square centimeter alu­minum that was evaporated on its surface. Although the detector was sensitive to both protons and heavier ions, such as alpha particles, it was referred to as the proton detector.

CHAPTER 4 • THE IGY PROGRAM AT IOWA FIGURE 4.1 Carl Mcllwain at the Fort Churchill launch site, checking out his nearly complete third payload during his February 1958 expedition. The top plate contains part of the system of apertures and magnets. Two more plates were added at the top after this picture was taken to complete the package. The next deck down housed the proton and electron detectors and the GM counter. The photometer can be seen pointing to the right near the center of the payload. Finally, the flux-gate magnetometer was located on the lower deck behind Carl’s right hand. The other decks were crammed with electronics and batteries. (Courtesy of Carl E. McIlwain.)

The second particle detector was designed to characterize electrons. It, too, used a CsI (Tl) scintillator on the face of a photomultiplier tube. This one was annular in shape, with a thick plastic baffle filling the center opening. A ring aperture was located ahead of the detector, and an electromagnet between the aperture and scintil­lator focused electrons having specific energy ranges onto the scintillator. The pulses produced in the photomultiplier tube were integrated, and the resulting current was passed through a nonlinear network to produce a current that was roughly propor­tional to the logarithm of the electron energy flux over the range from 10-2 to 10+2 ergs per second per square centimeter per steradian. The magnet current was se­quenced through seven steps to make the detector sensitive to electrons in various energy ranges. The highest magnet current focused electrons with energies in the neighborhood of 100 keV The crystal on that second detector, too, was covered by a thin coat of aluminum to reduce its light sensitivity. That coating set the lower energy sensitivity of the detector at about 3 keV

A Geiger-Muller (GM) counter was included in the particle detector complement. It was surrounded by a one-sixteenth inch thick lead shield, except for a slit located

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next to a thin aluminum window in the side of the instrument package. It was sensitive to directly impinging electrons having energies in the range of 1 to 5 MeV In addition, it served as a low-efficiency detector for lower energy electrons through the process of converting electrons to X-ray photons (bremsstrahlung) in the atmosphere and the mass of the instrument. That arrangement provided a way to relate the new measurements to those made earlier with GM counters on the rockoon flights.

Carl included a photometer to measure the total directional intensity of visible auroral light. And a flux-gate magnetometer assisted in determining the pointing direction of the detectors during flight. Finally, a narrowly focused photometer was used on the ground during the flight to measure the auroral intensity straight above the launch site. In addition to being used to interpret the data, its output was used to help determine the best times to fire the rockets.

Carl’s two inaugural flights ran into difficulties. In the first firing (rocket number II 6.22F on 27 August 1957),4 the Nike first stage had not quite completed its burn when the two stages separated, and the Nike nudged the Cajun enough to break off the instrumented nose cone. Carl later reported that he found the payload in the muskeg the next day, and some of the electronics were still operable. His second flight (rocket II 6.23F on 30 August 1957 to a height of 70 miles) was somewhat more successful. The payload reached an aurora, but the Cajun, by chance, was pointing the instruments downward during the critical portion of the flight. Thus, he was not able to observe the incoming auroral particles.

Early the next year, Carl was ready with four more instruments of the same design. When he arrived at Fort Churchill, he found Les Meredith, who was by then working at the Naval Research Laboratory (NRL), with Leo Davis, also from NRL, already there with low-energy detectors that they had developed for flight on Aerobee-Hi rockets. They had made a successful flight (rocket NN 3.03F on 20 January 1958 to a height of 112 miles) by the time that Carl arrived and triumphantly announced that they had “already found what causes the aurora—low energy electrons, and that Carl might as well go home.”5

Believing that there was still new information to be gained, Carl proceeded, never­theless, with the checkout of his instruments (Figure 4.2). He made four flights during a 13-day period in February (II 6.24F on 13 February 1958 to 80 miles, II 6.25F on 16 February 1958 to 75 miles, II 6.26F at 05:34 UT on 22 February 1958 to 80 miles,6 and II 6.27F at 05:48 UT on 25 February 1958 to 80 miles). Fortuitously, a large solar flare occurred during the night of 11 February, a few days before his launches. It was quickly followed by a severe magnetic storm at the Earth, marked by decreases in the cosmic ray and neutron monitor intensities. That night was also marked by a bright red aurora that was observed over a large range of latitudes and longitudes. The following two weeks were characterized by moderate magnetic activity and frequent

CHAPTER 4 • THE IGY PROGRAM AT IOWA FIGURE 4.2 Preparing one of Carl Mcllwain’s Nike-Cajun rockets for firing from the facility lo­cated on the shore of Hudson Bay, Fort Churchill, Canada. (Courtesy of Carl E. McIlwain.)

occurrences of high-latitude visible auroras. The weather conditions at Fort Churchill were good throughout most of that period, permitting excellent observations.

Carl’s first three flights were technically successful, but data were obtained from only relatively dim and quiescent auroras. The flight on 22 February, for example, produced excellent data on the flux and energy spectra of protons and electrons but was not located in a very active region.

Those results were tantalizing, but not really what Carl was looking for. He de­cided to make a special effort to place his last rocket into a more active aurora and played the waiting game during the next several nights. His account of that launch read:

So, just visualize the scientists who were waiting around for me to get my last rocket off so they could fire theirs, and the impatience of the range safety people. Even though a graduate student, I still had control of when to launch. I told them, “Things are still not quite right.”

We waited at T minus 5 minutes night after night, and they said, “Come on, there is some aurora up there. Fire the thing,” but I insisted on waiting, and was very lucky. Upon seeing an auroral breakup just to the south of Churchill, I finally decided it was the time to finish the countdown.7

That launch (II 6.27F) finally took place in the early morning of 25 February 1958, and the Cajun reached observing altitude just as the aurora appeared over­head. The instrument remained pointed upward during the time of peak interest and

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produced the first-ever direct measurements of particles producing a bright auroral display.8

Enormous fluxes of low-energy electrons were detected, but they had an energy spectrum substantially different than had been seen earlier by both Les Meredith and Carl in the more quiescent auroras. Whereas the earlier measurements had revealed a broadly spread spectrum, the spectrum from this flight showed a strong peak at about 6 keV Carl concluded that the electrons had fallen through an electric potential that must have had a component aligned parallel to the magnetic field lines. That finding was highly controversial, as most of the theoreticians were quite convinced that it was impossible to have an electric field aligned parallel with magnetic field lines in a plasma. So he was hesitant about putting that conclusion into print, but did so somewhat later. Although still not universally accepted, the strong preponderance of belief today is that parallel electric fields do, in fact, exist and that they serve as a prime driver for the auroral particles.

I have tremendous admiration for Carl and his work. Following his arrival at Iowa, he very rapidly gained a deep understanding of physical processes when photons and charged particles move through space and interact with matter. The accomplished musician had become a virtuoso physicist!