11

Deepening Perspective:

A New Look at the Old World

Among the most important contributions rockets, satellites, and space probes made to science was the new perspective they afforded in many areas, particularly in the earth and planetary sciences. Earth scientists, of course, had always enjoyed an advantage in being close to the object of study, living on the earth and immersed in its atmosphere, where the investigator could collect great quantities of data in situ. This was the very advantage that scientists seized upon when sounding rockets made it possible at long last to get on-the-spot measurements in the upper atmosphere. But a certain myopia was also associated with being too close to the object of study.

One of the tasks facing the researcher on the ground was developing an integrated picture of what was often a very large-scale, as well as complex, system. The meteorologist, for example, in spite of the enormous quantities of data he gathered on the weather, still found them too sparse. Even on land they came from rather widely separated stations, and there were none at all from vast stretches of the oceans. As a consequence the investigator was hard pressed to describe with any confidence the huge cyclonic systems and their interrelationships that characterized the general circulations of the earth's lower atmosphere, let alone tell what the weather was like in remote unobserved regions. But when the first weather satellite pictures became available, showing cloud patterns over both continents and oceans, the meteorologist had at hand one of the integrating factors that he needed. For, clouds, being intimately associated with pressure patterns and air circulations, showed by their distributions the major weather systems. Most of what was seen in the early cloud pictures was expected, but there were also surprises. The author can recall hearing Dr. Harry Wexler, director of research for the U.S. Weather Bureau and strong proponent of weather satellites years before any satellite had flown, exclaim that he had never expected the large-scale patterns of atmospheric vortices that stood out in many satellite photographs. When in the course of time satel

lite cloud imaging was improved in resolution and supplemented by techniques for measuring cloud heights, the vertical distribution of atmospheric temperatures, and local winds, meteorology became not merely local, not merely regional, but the global science it had always aspired— and needed-to be.1

Meteorologists were among the most ready to take advantage of the new approach and in short order used the satellite pictures in making weather forecasts. But such pictures also showed complete ice fields, total watersheds, entire geological provinces such as volcanic fields or geosyncline basins, varying patterns of land use, and vast expanses of ocean. To many it was clear from the start that the perspective afforded by satellite observations would in time prove of immense value in these and other areas. Such has proved true.2

After more than a decade of rocket sounding of the upper atmosphere, space science was quite ready to benefit from the new perspective. In the first half dozen years following the formation of NASA, especially rapid progress was made in the continued study of the upper atmosphere and ionosphere, solar physics, rocket astronomy, geodesy, and the magnetosphere. Accomplishments in the last two areas provide good illustrations of the power of space techniques for scientific research and are the subject of this chapter. The contributions to geodesy were anticipated, causing a number of researchers to give serious attention to the possibilities during the 1950s, years before Sputnik went aloft.3 In contrast, the magnetosphere emerged as something of a surprise from the early rocket and satellite work on particles and fields.

THE MAGNETOSPHERE

For want of a more appealing name the phrase particles and fields came into early use in the space program to denote the study of magnetic and electric fields in space and a variety of particle radiations. Among the last named were the extremely energetic cosmic rays, plasma radiations from the sun, and the electrons, protons, or whatever they were that were thought to cause the auroras. (Gravitational fields were not included, falling rather under geodesy, relativity, and cosmology, with which gravity studies were naturally associated.) The term magnetosphere denotes the region of space surrounding the earth where the earth's magnetic field plays a prominent, often controlling, role relative to various particle radiations found there. As will be seen, magnetospheric physics constituted an important aspect of the discipline of particles and fields.

The discovery of the magnetosphere began with Van Allen's discovery of the earth's radiation belt. At White Sands, New Mexico, Van Allen had traced the curve of cosmic ray intensity through the Pfotzer maximum to a more or less steady value at heights greater than 55 km that looked very

much as though it might be the free space value of the cosmic ray intensity. Cosmic rays, being charged particles, were affected by the earth's magnetic field, and fewer of them were able to get in over the geomagnetic equator than in the polar regions. The less energetic rays were the most affected by the magnetic field, making it difficult to determine what the lower end of the cosmic ray spectrum might be in interplanetary or interstellar space. Since the total energy spectrum of the cosmic radiation in space would be an important factor in trying to figure out how and where cosmic rays were generated, Van Allen took a special interest in investigating the variation of the high-altitude cosmic ray intensity with geomagnetic latitude. For this purpose he took Aerobee rockets to sea aboard the U.S. Navy's seaplane tender Norton Sound, which had to be specially outfitted with an Aerobee launching tower. Van Allen's sounding ranged from the geomagnetic equator off the coast of Peru to Alaskan waters.5 The measured variations were sufficiently intriguing that Van Allen pursued the subject further with Rockoons-the small sounding rockets that he launched from Skyhook balloons in the stratosphere. These Rockoon experiments turned up a most interesting and puzzling phenomenon. In the auroral regions above 60 km was a rather soft-i.e., moderately penetrating-radiation that could be a mixture of charged particles and x-ray photons. This radiation was assumed to be in some way connected with the auroras, and efforts were begun to explore the connection.

At about this time the appearance of the International Geophysical Year satellite program gave Van Allen the chance to extend these investigations to even higher altitudes. When the first Explorer was launched (31 January 1958), Van Allen's counters appeared to show a zero counting rate at certain locations, which didn't seem to make sense. Further study showed, however, that actually the counters were saturating because of ambient radiations far exceeding intensities with which the counters had been expected to cope. Explorer 3 (26 March 1958) pursued the question.

Soon Van Allen decided that he was observing a region of intense radiation surrounding the earth at high altitude, and on 1 May 1958 he announced his discovery.' The region at once became known as the Van Allen Radiation Belt. Soviet measurements in Sputnik 3 (15 May 1958) confirmed the discovery.

An explanation was quickly forthcoming. The radiations were attributed to charged particles caught in the earth's magnetic field, unable to escape because their energies were too low to allow them to cross the surrounding field lines. One thus visualized trapping regions within the earth's field and spoke of trapped radiations. Suddenly it was crystal clear that the earth's magnetic field, which could prevent some charged particles in interplanetary space from ever reaching the earth, could also prevent other particles already near the earth from leaving.

In retrospect it seemed remarkable that the existence of the radiation belt had not been anticipated long before its discovery. Workers concerned with the problem of how gases escaped from the atmosphere understood that the magnetic field would hinder the escape of ions. More significantly, the experiments of K. Birkeland and E. Brüche with cathode rays aimed at small magnetized spheres and the half century of theoretical work by Carl Størmer and others on the influence of the earth's magnetic field on auroral particles and cosmic rays provided a substantial basis for predicting the existence of trapped radiations near the earth. Seeking an explanation for the auroras, Størmer had developed a theory of the motion of an electron approaching the earth's dipole magnetic field from the sun. He showed that such an electron would be deflected by the earth's field away from the equator to the polar regions, an action that appeared to him to explain the existence of auroral regions or zones at high latitude.

Størmer's calculations showed that there were regions inside the earth's magnetic field which such solar electrons could not reach, to which he gave the name "forbidden regions." Birkeland, with whom the theory had originated, had already demonstrated in the laboratory that electrons would be deflected to the polar regions, a fact Stormer's calculations nicely brought out.

Later, in the 1930s and after, theorists interested in explaining the geomagnetic-latitude effect observed in cosmic ray intensities, extended Størmer's work to much higher energy relativistic particles-i.e., particles approaching the speed of light-such as were to be found in the cosmic rays. 10 Their calculations also revealed forbidden regions toward the geomagnetic equator and served to explain why cosmic rays increased in intensity with increasing geomagnetic latitude.

These investigations furnish an excellent example of how initial orientation can markedly bias an investigator's conclusions. To those seeking explanations of the auroras or the cosmic-ray-latitude effect, the orientation was from outside in. Their particles were approaching the earth from great distances. It was natural, then, that the regions which the earth's magnetic field prevented those particles from entering should be named forbidden regions. While the point was not missed, still the investigators did not focus on the fact that for a particle already within one of those regions, it could be the outside that was forbidden-in other words, a particle of too low an energy already within one of those regions couldn't get out. What were forbidden regions for particles approaching from the outside were trapping regions for some particles already there.

It was only a tiny step from this realization to the idea that these trapping regions might well be filled with trapped radiations forming a radiation belt around the earth. But no one paid any attention to this possibility until, on the eve of Van Allen's discovery, S. Fred Singer in discuss