Hannes Alfvén (1908 - 1995) Hannes Alfvén was born on May 30, 1908, in Norrköping, Sweden. After a fulfilling life and career containing remarkable achievements, he died in his home in Djursholm, Sweden, April 2,1995, a few weeks before his 87th birthday. During his career, Alfvén made a number of fundamental theoretical discoveries. The one for which he is best known is the magnetohydrodynamic (hydromagnetic) wave, commonly called the Alfvén wave. But he invented a number of other fundamental concepts that are not so closely associated with his name. These include simplifications in the concepts with which we treat the behavior of ionized gases (plasmas). He found the established way of calculating particle orbits (Störmer orbit theory) to be impractical, especially in the energy range relevant to auroras. This led him to develop, as a tool, the guiding-center approximation for the motion of charged particles in electric and magnetic fields. He also discovered the first adiabatic invariant of charged particle motion, and he invented the concept of frozen-in magnetic flux. Together, these tools established magnetohydrodynamics as a resource and as a field of research. It is hard to imagine working today in plasma physics without using the tools he provided us. This work, cited as "contributions and fundamental discoveries in magnetohydrodynamics" earned him a Nobel Prize for Physics. The son of Johannes Alfvén and his wife Anna-Clara Romanus, Hannes had an exceptional family background. His mother was one of the first female physicians in Sweden, a remarkable achievement at that time. His father, also a practicing physician, had a strong interest in science. One of his uncles, Hugo Alfvén, was a famous composer, another was an inventor, and a third, an agronomist by profession, was very interested in astronomy and was far ahead of his contemporaries in formulating ideas about the environment and its problems. According to Alfvén's own account, two experiences in his youth, one at home and one in school, influenced his intellectual development and his professional career. One was the gift at an early age of a popular book on astronomy, written by the French astronomer Camille Flammarion. This he read passionately, and it kindled a lifelong interest in astronomy and astrophysics. The other was his membership in the school's radio club where he built radio receivers. There was no nearby radio station, and the one in Stockholm was too weak to be received with primitive equipment in Norrköping. The most promising was the strong station in Aberdeen, Scotland. Alfvén has described, with some passion, the thrill he felt when some faint notes of music emerged out of the atmospheric noise and could be identified as coming from Aberdeen. After high school, he entered the University of Uppsala where he studied mathematics, experimental, and theoretical physics. Working in a physics department that was focused on spectroscopy, he demonstrated his characteristic intellectual independence by choosing topics that we would now classify as nuclear physics and electronics. The title of his doctoral thesis (1932), which he said was a direct continuation of his radio club activities, was "Ultra-Short Electromagnetic Waves". Alfvén, again following his own visions, moved into electronics and astronomy just when "everyone else" was moving into nuclear physics. For the next eight years, he worked first at the University of Uppsala and later at the Nobel Institute in Stockholm. During this period, he spent two relatively brief periods abroad: a few months with Lise Meitner and Otto Hahn in Berlin and a half year or so in Cambridge with Rutherford. While at the Nobel Institute, Alfvén became more and more interested in the acceleration of charged particles to very high energies, and especially to the extreme energies of cosmic rays. His early attempt to develop a theory of the origin of cosmic radiation was published in Nature in 1933. The paper reflects an important aspect of Hannes Alfvén's approach to science that he maintained throughout his career. It criticized earlier speculations concerning cosmic radiation because they did not "seem to be in accordance with the latest experimental results", and he stated that it should be possible to "explain the origin of the cosmic rays, introducing no new hypotheses, and only applying the kinetic gas theory to the conditions of world space". In 1940, only 32 of age, Hannes Alfvén was appointed Professor of Electromagnetic Theory and Electric Measurements at the Royal Institute of Technology, Stockholm. He once said that, at the time, he attached more significance to joining the Faculty of the Royal Institute than to the award of the Nobel Prize some 30 years later. As a result of the rapid evolution of his interests, his own professorship changed, first to Electronics in 1946, and then to Plasma Physics in 1963. His vigorous leadership led to a rapid expansion with the creation of a number of new professorships and departments. The three departments that directly trace their origin to his work now form a separate entity within the Royal Institute of Technology - the Alfvén Laboratory, founded in 1990. In 1967 he accepted a professorship at the University of California, San Diego, and he divided his time so that he was at the Royal Institute "from the Vernal Equinox until the Autumnal Equinox" and at UCSD from fall until spring. His intent was to avoid crossing the Atlantic at other times; he yielded only twice - once for the birth of a grandchild and once to receive his Nobel Prize. His scientific activity continued well beyond his formal retirement in 1973. In 1988 he stopped his seasonal shifts and settled in Sweden. Using the magnetic-moment invariant and the guiding-center approximation, Alfvén gave a simple, physically intuitive demonstration of how a belt of energetic charged particles can maintain stable circulation around a magnetized planet such as the Earth. Before Alfvén's discoveries, particle trajectories had to be calculated by numerical integration, and without the benefit of digital computers! We were told by one of Störmer's "computers" (Nicholai Herlofson) that the numerical integration for a simple trapped-particle orbit required "a fortnight". The tools Alfvén introduced brought physical imagery and simplified mathematical effort into what had been a specialized field of mathematical drudgery. Thus, he developed, two decades before its discovery, the basic tools we use today to describe the Van Allen radiation belt. He proposed a cosmic-ray acceleration mechanism that is now known as the Fermi Mechanism (although Alfvén did it before Fermi). And he fought for years to make us aware of the existence and importance of electric fields and currents in space. He was always in the lead. When his ideas on the existence of electric fields that were perpendicular to magnetic fields were still being questioned, if not attacked, he added the "double layer", regions of strong electric fields parallel to the local magnetic field in the rarefied plasma of space. His magnetic-field-aligned electric field, in combination with field-aligned currents (originating in what is now called the Alfvén layer), is now accepted as crucially important for the acceleration of the charged particles that cause the polar aurora. He also proposed astrophysical applications for his double layers. He proposed a new mechanism for the interaction of plasma and un-ionized gas in relative motion - the Alfvén critical-velocity ionization mechanism. In an entirely different arena, he was first to offer a plasma-physics theory for the formation of comet tails in the expanding solar plasma (the solar wind). Alfvén's impact is such that if one attends a meeting on magnetospheric physics, solar physics, or space plasma physics, it is rare to sit through more than a few papers before one hears his name mentioned. Besides Alfvén waves and the associated Alfvén speed, his name has become attached to Alfvén layers, Alfvén critical points, several different kinds of Alfvén radii, and Alfvén distances. Hannes Alfvén possessed a gift that allowed him to extract results of great importance and generality from specific problems. It is a mark of his genius that his initial understanding came primarily from physical reasoning; the mathematical demonstrations came only after he had, in his mind's eye, determined the physical process. The discovery of Alfvén waves is, in many ways, representative of his approach. It grew out of a specific problem, namely that of sunspots. He first determined that it was possible to propagate electromagnetic waves in a highly conducting plasma. (This he claimed was the easy part.) Only then did he develop the mathematical demonstrations. The idea that such waves were possible ran contrary to the conventional thought of the time because it was taught that electromagnetic waves could propagate no more than a skin depth (about a wavelength) into a good conductor. But Alfvén had found, by pure power of intellect, an entirely new propagation mode. He discovered how electromagnetic waves can propagate without damping in a plasma of arbitrarily high conductivity. His work on the cosmic-ray problem led him to propose in 1937 the existence of a galactic magnetic field. Because interstellar space was, then, held to be a vacuum, it was widely believed that there could be no interstellar magnetic field because the magnetic field of individual stars, declining in a vacuum as 1/r3, would be too weak to fill interstellar space. Alfvén proposed that interstellar space could contain sufficient plasma to carry electric currents that would produce the required field locally. Only much later was the existence of the galactic magnetic field confirmed, and, as is typical of many of his contributions, without formal recognition of his original proposal. He abided by the principle that theories of cosmical phenomena must agree with laboratory experiments. (The definition of "laboratory" was later enlarged to include experiments in space.) He accepted the proposition that the laws of nature apply everywhere. A key to his success seems to be the fresh perspective that came from applying laboratory results to problems in space physics and astrophysics. For example, his proposal of the double layer was, in part, based on his experience with mercury-vapor rectifiers used in commercial, high-voltage DC transmission. Hannes Alfvén received the Nobel Prize in Physics in 1970 from the Swedish King Gustavus Adolphus VI. Considering his many fundamental accomplishments, it now seems bizarre that, until he was awarded the Nobel Prize in 1970, he was not well regarded by leading members of the scientific establishment (and, of course, a substantial segment of the scientific community that constituted their followers). For example, in 1939 he wrote a remarkable paper in which he proposed a theory for magnetic storms and auroras. This paper, which lays out presently accepted basic ideas on how plasma flows around a dipole magnetic field to create Birkeland currents that flow in and out of the auroral zone, was rejected by the predecessor of the Journal of Geophysical Research because it disagreed with the theories of Sydney Chapman and his colleagues. To get this and others of his important early papers published, he eventually turned to journals that did not enjoy international readership. Most of his ideas were finally made known to the scientific community through his marvelous book, Cosmical Electrodynamics, published by Oxford University Press in 1950. This book has been the inspiration for a number of books by others having similar approaches, similar contents, and, in some cases, even similar titles. It usually took years for his ideas to be accepted. For example, his discovery of hydromagnetic waves was presented in an admirably simple and clear mathematical form in a Letter to Nature published in 1942. Acceptance came suddenly some six years later when, as Alfvén recounted, at the end of a seminar he gave at the University of Chicago in 1948, the famous physicist Enrico Fermi nodded his head and said "of course" and, according to Alfvén's account, the prestige of Fermi was such that "the next day, everyone nodded and said, 'of course'". Some of Hannes Alfvén's ideas are still controversial. One example is symmetric cosmology, which implies that the Universe consists of equal amounts of matter and anti-matter separated by thin boundary layers. The AGU is holding a medal he created - The Alpha-Centauri Medal - to be given to the first person who determines whether that star is made of matter or anti-matter. This was typical of the spirit of Hannes Alfvén; when the Bowie Medal was bestowed upon him, he responded by giving the Alpha-Centauri Medal to the AGU for safekeeping and eventual placement. A picture of the medal can be seen in EOS [EOS Trans. AGU, vol. 70, p. 10, 1989]. Besides the Nobel Prize and the AGU's Bowie Medal, Alfvén received other honors. He was one of the few who were members of both the American and the Soviet Academies of Sciences. Hannes Alfvén, with his wife Kerstin, also took an active interest in important matters outside science, especially those related to environment, population growth, and disarmament. One result of these interests was a series of books that Hannes wrote, some together with Kerstin. When nuclear power first became a possibility, Alfvén supported its development for commercial use. However, within a few years, when disadvantages emerged, Alfvén became a vigorous opponent. In Sweden, his persuasive support of the anti-nuclear position is acknowledged as an important element in Sweden's eventual decision to abandon its nuclear power program. Alfvén contributed to the progress of science not only by his own work but also by the inspiration he gave to his many students as well as to colleagues all over the world. His death has left many of us with a feeling of great loss but also of deep gratitude for all that he has meant as a scientist and as a friend. C.-G. Fälthammar, Royal Institute of Technology, Stockholm A. J. Dessler, University of Arizona, Tucson