Origins of Magnetospheric Physics: An Expanded Edition

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Origins of Magnetospheric Physics
By James A. Van Allen
UNIVERSITY OF IOWA PRESS Copyright © 2004 University of Iowa Press
All right reserved.
ISBN: 978-0-87745-921-7



Chapter One Scientific Heritage

The scientific heritage of magnetospheric physics lies principally in studies of geomagnetism, aurorae, and the geophysical aspects of cosmic radiation and solar corpuscular streams. The external magnetic field of the earth plays a central role in the phenomena of all of these subjects. It was shown by Gilbert [1600] that the magnetic field on the surface of the earth is similar to that on the surface of a uniformly magnetized sphere (terrella) of magnetite, or lodestone. Alternatively, such a magnetic field can be attributed [Maxwell 1891] to a point magnetic dipole at the center of a nonmagnetic sphere or to a small current carrying loop of wire there. The magnetic field external to the sphere (fig. 1) is identical to all three cases and does not distinguish among them Geophysical evidence, however, shows conclusively that the general magnetic field of the earth must be attributed to a system of electrical currents in its deep interior [Chapman and Barrels 1940: Rikitake 1966]

By the use of networks of fixed and portable magnetometers, refined studies of the geomagnetic field as a function of time and as a function of latitude and longitude over the surface of the earth reveal an immense richness of detail The simplest approximate representation of the general field is that of a point magnetic dipole of moment 8.0 x [10.sup.25] gauss [cm.sup.3], located at the geometrical center of the earth with its axis tilted to the rotational axis of the earth by 11�.4 such that it pierces the earth's surface at latitude 78�.6 N, longitude 69�.8 W in the northern hemisphere, and at latitude 78�.6 S, longitude 110�.2 E in the southern hemisphere. In the conventional nomenclature of magnetic poles, the north pole of the dipole lies in the southern hemisphere of the earth and vice versa. In a next higher ardor representation, the point dipole retains the same orientation but is displaced from the geometrical center of the earth by 450 km toward latitude 17�.2 N, longitude 148�.8 E. The latter (eccentric dipole) representation is reasonably adequate for most magnetospheric purposes at low latitudes (� 45�) and at radial distances less than about 5 [R.sub.E] (1 [R.sub.E] = 6378 km, the equatorial radius of the body of the earth), though the addition of higher order terms is required to reproduce the effects of local anomalies. At large radii a much more elaborate description of the external magnetic field of the earth is required.

Superimposed on the average field at the surface of the earth, relatively small (typically [??] 1% in magnitude and [??] 1� in direction) but highly significant temporal variations in the magnetic vector occur. From the point of view of magnetospheric physics, the most important temporal variations {magnetic "storms") occur sporadically but also exhibit a quasi-persistent periodic variation with the synodic period of rotation of the sun (~27 days) The onsets of magnetic storms are identified with flares in localized, disturbed regions on the sun and are a delayed effect, the delay typically being two days. During the first half of the twentieth century [Chapman and Bartels 1940], this evidence was taken to imply the sporadic emission of solar corpuscular streams (streams of ions and electrons. or neutral but ionized gas) traveling radially outward through the vacuum of interplanetary space at a velocity of as much as 900 kilometers per second, as calculated from the time delay. The arrival of such a stream at the earth was visualized as being the cause of a magnetic storm by first compressing the terrestrial field (initial phase) and by then generating a westward-flowing, equatorial ring current encircling the earth (main phase). The twenty-seven-day recurrence of magnetic disturbances was taken to signify the quasi-persistence of such a corpuscular stream, having a localized spread in solar longitude and latitude and an intensity that decayed with a lifetime of the order of several months. Often, two or more such hypothetical streams were present simultaneously, causing a superposition of geomagnetic effects. By virtue of the rotation of the sun and the assumed radial motion of the stream, an individual stream would have the instantaneous shape of an archimedean spiral, as viewed in an inertial coordinate system centered on the sun (fig. 2). The quantitative aspects of this theory were developed most notably by Chapman and Ferraro [[1931, 1932] and, from a different point of view, by Alfvén [1950, 1955]. Much of this early work continues to be a valid and basic part of contemporary concepts.

Another important body of relevant knowledge came from systematic study of the polar aurora, the common and often spectacular display of luminous emissions in the upper atmosphere at high latitudes, both north and south. The occurrence of overhead auroral displays is observed to be most probable, not at the magnetic poles, hut within two halo-shaped strips (the auroral zones or auroral ovals) encircling the earth, one in the northern hemisphere and one in the southern hemisphere. These two strips are approximate mirror images of each other with respect to the geomagnetic equator; they have a latitudinal width of about 10� and are centered on the average at geomagnetic latitudes 67� north and south (fig. 3). Auroral emissions occur most prominently at altitudes of 60 to 200 km. The spectrum of auroral light is principally that of excited neutral and partially ionized atoms of the major atmospheric constituents, nitrogen and oxygen. In early work the exciting agent was usually assumed to be downward streaming electrons having such energies as would permit them to penetrate the atmosphere to an altitude of 60 km, namely about 10 keV. Birkeland [1908, 1913] conducted a major observational program on the aurora polaris and then a series of laboratory investigations with electron beams in the held of a small magnetized sphere (terrella) placed in an imperfect vacuum. In these laboratory studies he demonstrated the scaled occurrence of auroral ovals, resembling those of the earth. Later investigations of this nature are exemplified by the work of Malmfors [1945] and Block [1955]. Birkeland's experiments stimulated Størmer [1955] to embark on a long career devoted to the theory of the motion of electrically charged particles in the field of a magnetic dipole. This theory never succeeded in explaining the observed auroral ovals as being caused by the arrival of either electrons or ions directly from the sun, but it provided a great stimulus to the subject. Perhaps even more important, it laid the basis for understanding two related but different bodies of phenomena: (a) the role of the earth's dipolar field as a huge magnetic spectrometer for cosmic rays and for energetic particles from the sun and (b) the durable trapping of charged particles within the geomagnetic field.

Størmer's theory of the allowed cone for the arrival of energetic charged particles from infinity at a given latitude in a given direction (later refined by Lemaitre, Vallarta, and many others) is basic to deriving the energy spectrum of the primary cosmic radiation from the dependence of intensity on latitude, to determining the algebraic sign of the electrical charge on the arriving particles, and hence to distinguishing electrons from ions.

Størmer's work has been the foundation of much of my own research efforts over a period of thirty-seven years. In its simplest form his theory shows that, for an isolated, electrically charged particle of specified magnetic rigidity R [equivalent or equal to] pc/Ze (where p is its momentum; Ze, its electrical charge; and c, the speed of light), there are, subject to certain restrictions on the magnitude of R, two dynamical regions within a dipolar magnetic field-one of unbounded motion and one of bounded motion The first is the one of interest fur the arrival of particles from infinity; the second, the one that is basic to the physics of geomagnetically trapped particles (fig. 4). In the simple case of the motion of isolated, noninteracting particles in a vacuum, there is no connection between the two regions-i.e., a particle arriving near the earth from infinity must either pass by the earth or strike the atmosphere but can never become trapped whereas one injected into the trapping region can never escape therefrom, though in the real case it might collide with the atmosphere and be lost. As will be shown later, much of the physics of the magnetosphere results from departures from the idealized Størmerian case. Nonetheless, the simple theory provides a point of departure for all more elaborate treatments of the subject.

Beginning in the 1930s, Jacob Clay, Arthur H. Compton, Robert A. Millikan, Ira S. Bowen, H. Victor Neher, William H. Pickering, Erich Regener, Georg Pfotzer, Hugh Carmichael, and others undertook latitude surveys of cosmic ray intensity using ionization chambers and Geiger tube detectors at ground level and carried by balloons in altitudes up to about 30 km [Bowen, Millikan, and Neber 1938]. Their measurements (fig. 5) were of basic importance, but they left the nagging question of how to extrapolate reliably the highest altitude measurements, still beneath some 10 grams per square centimeter of atmosphere, to free space, i.e., above the appreciable atmosphere. For many years the desire to answer this question was the central motivation for much of my scientific work, leading fortuitously to the first direct observations of the primary auroral radiation and to the discovery of the radiation belts of the earth.

Chapter Two The U. S. Program of Rocket Flights of Scientific Equipment

The preceding pages give a brief sketch of certain elements of the solar-geophysical knowledge that was available before the start of high-altitude observations with rocket-borne equipment in 1946.

As early as 1943 Erich Regener and Ernst Stuhfinger in Germany planned investigations of the cosmic-ray intensity above the atmosphere and of the solar ultraviolet spectrum with equipment carried on test flights of V-2 military rockets. But such flights were never conducted because of war time operations, including Allied air raids on the German rocket base at Peenemünde that destroyed some of the instruments already built lot the purpose.

The post-World War II period was characterized by intensive efforts within the United States and the Soviet Union to develop high performance rockets for military purposes. At a less well-known level it was also characterized by the aspirations of scientists to use such rockets as vehicles for carrying scientific equipment to high altitudes. On September 26, 1945, the Jet Propulsion Laboratory (JPL) successfully flew a small sounding rocket, called a WAC Corporal, from the newly established White Sands Proving Ground (WSPG) in New Mexico. This rocket, a reduced scale version of the JPL military rocket Corporal, reached an altitude of 70 km with a potential payload of 5 kg. In some sense, this flight advanced the much earlier plans of Robert Goddard to conduct scientific work with rockets, flown to high altitudes for sounding, or investigating, the upper atmosphere but the WAC Corporal was not applied in any significant manner to scientific work.

The first major development in the history of the rocket flight of scientific equipment came a few months later. In late 1945 the U.S. Army Ordnance Department transferred a group of German rocket engineers and a large stock of V-2 rocket components from Peenemünde to the United States. It planned to assemble and fire a number of V-2s for the purpose of technical assessment and experience as part of the then embryonic effort of the United States to develop ballistic missiles In response in the expressed interest of Ernst H. Krause of the Naval Research Laboratory (NRL), Col. Holger N. Toftoy and Lt. Col James G. Bain invited scientists from universities and government laboratories to formulate a program for the utilization of the payload capacity (~1,000 kg) of the V-2 test flights for the conduct of scientific investigations [Newell 1953].

At that time I was seeking to return to civilian employment at the Applied Physics Laboratory (APL) at Johns Hopkins University after service as an ordnance and gunnery officer in the U.S. Navy since November 1942. In 1940 and 1941 I had worked at the Department of Terrestrial Magnetism (DTM) of the Carnegie Institution of Washington in helping develop rugged vacuum tubes and photoelectric and radio proximity fuzes for gun-fired projectiles. The director of DTM, Merle A. "Rave, had established APL in early 1942 and then served as its wartime director. Along with other colleagues in the proximity fuze group at DTM, I was transferred to APL at the time of its creation. At Tuve's request I was commissioned directly from my post at APL into the U.S. Naval Reserve as a lieutenant (junior grade) in November 1942 as one of three individuals to take the first issue of secret radio-proximity fuzed 5"/38 projectiles to the South Pacific Fleet. My functions were to introduce this new type of anti-aircraft ammunition into service in the fleet, to instruct gunnery officers in its use, to observe and report its effectiveness in combat, and later to set up re-batterying facilities in Australia, New Caledonia, Espíritu Santo, Tulagi, Eniwetok, and Manus. After my return to the United States and the end of the war in the Pacific, I had a number of conversations with Tuve on resuming peacetime research. My 1939 Ph.D. from the University of Iowa had been in experimental nuclear physics, and I had continued in this field at DTM as a research fellow of the Carnegie Institution in 1939-40. During this period I had acquired an interest in cosmic-ray research from Scott Forbush and others at DTM. Tuve encouraged me to pursue this interest and invited me to return to APL to conduct research in this field.

Henry H. Porter of APL told me about the Army Ordnance Department's plans for the V-2 firings, and on January 16, 1946, I joined with many other interested scientists at a meeting at the Naval Research Laboratory for a briefing on the possibilities. One tangible result of this meeting was the formation of an unofficial group of scientists who had a realistic expectation of preparing equipment for flight. I was fortunate enough to be a member of this group, a circumstance that was pivotal to my subsequent career. My gunnery experience and my earlier intensive experience in developing vacuum tubes and associated circuits that survived linear accelerations of up to 20,000 g while being fired from guns led me to think that building electronics and scientific instruments for rocket borne payloads would be easy-an expectation that later proved to be only partially true.

With Tuve's support I organized a high altitude research group of kindred spirits: Howard E. Tatel, John J. Hopfield, Robert Peterson, Lawrence W. Fraser, Russell S. Ostrander, Clyde T. Holliday, and, later, Jeffrey F. R. Floyd, S. Fred Singer, Gone M. Melton, Albert V. Gangnes, James F. Jenkins, Jr., Harold E. Clearman, and others. We very quickly developed plans for a comprehensive program of measurements of primary cosmic rays, the ultraviolet spectrum of the sun, and the geomagnetic field in the ionosphere. Ernest H. Vestine of DTM and Allen Maxwell of the Naval Ordnance Laboratory were important collaborators in the high altitude magnetometer program for the direct measurement of the magnetic effects of ionospheric currents. Later, the solar ultraviolet work led Hopfield and me into a determination of the altitude distribution of ozone in the upper atmosphere. Also, Holliday began the development of cameras with recoverable film casettes for photographing the earth from high altitudes, in both the visible and photographic infrared. In addition to general super vision of this work, my own special interest was investigating the nature and absolute intensity of the primary cosmic radiation before it encountered the earth's atmosphere.

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