Research Profile

by Luisa Bonolis

Hannes Alfvén

Nobel Prize in Physics 1970 together with Louis Néel
"for fundamental work and discoveries in magnetohydro-dynamics with fruitful applications in different parts of plasma physics"


The Birth of Magnetohydrodynamics
Hannes Olof Gösta Alfvén was the founder of the modern field of plasma physics, the study of electrically conducting gases, and the father of the branch of plasma physics known as magnetohydrodynamics, the study of plasmas in magnetic fields. He applied his theories to a wide range of physical and astrophysical phenomena, including charged particle beams in accelerators and interplanetary and magnetospheric physics. As space plasma physics has evolved, the significance of his discoveries became increasingly evident. The number of concepts bearing his name indicates the significance of his contributions to science: Alfvén waves, electromagnetic waves that propagate through a highly conducting medium, such as the ionised gas of the Sun, or also Alfvén number, Alfvén layer, Alfvén velocity, and so on, have come to be among the most frequently used terms in this field of science.
Alfvén was born in Norrköping, Sweden, in 1908 and grew up in a socially and financially secure family with a strong intellectual tradition. According to Alfvén's own account, two experiences in his youth had great importance in his intellectual development and his scientific career: the popular book of astronomy by Camille Flammarion, that kindled his lifelong fascination with astronomy and astrophysics, and his activity in the school's radio club, where he learned how to build radio receivers and came to know how radio waves behave within Earth's varying ionospheric plasma. These interests led him to study mathematics and physics at the University of Uppsala, where he received his doctor's degree in 1934 for a dissertation entitled “Investigations of the ultra-short electromagnetic waves,” a direct continuation of his radio club activities. In that same year he was appointed lecturer in physics at Uppsala University, and there he continued to cultivate his interest and expertise in electronics, sustained by the application of instrumentation to nuclear physics and to experiments on cosmic rays, two research fields in full bloom and still strongly connected at that time. He became more and more interested in the acceleration of charged particles to very high energies, especially in the extreme energies of cosmic rays and, since 1933, he attempted to develop a theory of the origin of cosmic radiation based on the view that they were “very fast particles, coming from world space.” Up to that time most physicists believed that cosmic rays were gamma rays that permeated the whole universe. But in 1933, experiments conducted in different parts of the world had definitely established their corpuscular nature through experiments showing that cosmic rays fluxes arriving from space were influenced by the Earth's magnetic field, thus showing that they were charged particles.

In 1937, Alfvén became research physicist at the newly inaugurated Nobel Institute of Experimental Physics in Stockholm, where he began his groundbreaking work in plasma physics. Plasmas are highly ionised gases containing both free positive ions and free electrons. The dominant state of matter in the universe, they are rare on Earth, but abundant in stars, galaxies, and intergalactic space. In 1937, he conceived the theorem of frozen-in flux wherein a plasma is regarded as frozen onto or tied onto the magnetic lines of flux that pass through the plasma. Thus when the plasma moves, the magnetic field moves with it. On the basis of his theory of magnetic fields frozen in plasmas, Alfvén proposed that if the galaxy contained a large-scale magnetic field, then cosmic rays could move in spiral orbits within the galaxy, because of the forces exerted by the magnetic field. He argued that there could be a magnetic field pervading the entire galaxy if plasma were spread throughout the galaxy. This plasma could carry the electrical currents that would then create the galactic magnetic field. At that time interstellar space was held to be a vacuum, because the magnetic field of individual stars, declining in a vacuum as 1/r3, would be too weak to fill interstellar space, so that Alfvén's intuitive hypothesis was first dismissed. But Alfvén proposed that interstellar space could contain sufficient plasma to carry electric currents that would produce the required field locally.
Up to the space era, interplanetary space was generally regarded to be a good vacuum, disturbed only by occasional comets. This opinion was broadly accepted because space had been viewed only with telescopes at optical wavelengths. The electrical currents proposed by Alfvén, however, created a telltale signature only in the radio portions of the electromagnetic spectrum, so they had not yet been observed at the time he stated his view that such currents existed in space. Therefore, Alfvén’s proposition met with great scepticism. Only much later was the existence of large-scale galactic magnetic fields confirmed.
In Stockholm, Alfvén had also regularly attended the seminars of Oscar Klein at the University of Stockholm. The seminars dealt with a multitude of topics, including cosmology, a topic that later came to occupy a great deal of Alfvén's professional interest.

Alfvén's interest in the physics of the space between the Sun and the Earth also began at this time, with a paper on the aurora borealis. The electrical nature of the aurora and the major role played by the Earth's magnetic field had already been established by the Norwegian physicist Kristian Birkeland, who first elucidated the nature of the aurora borealis during expeditions to high-latitude regions, organised in 1902-1903. He established a network of observatories to collect magnetic field data and at the beginning of the 20th century proposed that aurora and polar electric disturbances were caused by a system of currents that flowed along geomagnetic field lines into and away from the polar regions into the atmosphere. Such field-aligned currents are known today as Birkeland currents. However, their existence could not be confirmed from ground-based measurements alone, and in the decades after Birkeland's death in 1917 his theory was disputed, especially by the eminent British geophysicist and mathematician Sydney Chapman, who had become the recognised leader in interplanetary and magnetospheric physics. Chapman argued that currents could not cross the vacuum of space and therefore the currents had to be generated by the Earth, flowing only in the ionosphere, with no down-flowing currents. Chapman's theory was so mathematically elegant that it won wide acceptance. Birkeland’s work might have disappeared completely except for Alfvén, who became involved long after Chapman’s ideas had gained predominance. Following Birkeland's ideas, in 1939 Alfvén wrote a remarkable paper in which he proposed a theory for magnetic storms and auroras. He was the first to devise the technique that enables the complex spiral movement of a charged particle in a magnetic field to be calculated with relative ease. In considering the complex motion of a charged particle in a magnetic field, Alfvén introduced the simplifying approximation of circular rotation about a ‘guiding centre' which was itself drifting along magnetic lines. He applied this principle to the study of magnetic storms and auroras, finding that particles in the earth's magetic field should move back and forth along the field lines, reflected from regions of increasing field strength. Alfvén insisted that Birkeland’s current system made more sense because down-flowing currents that followed Earth’s magnetic field lines were necessary for driving most of the ionospheric currents. Alfvén's 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 along the Earth's magnetic fields into the atmosphere, was rejected by the Journal of Geophysical Research because it disagreed with Chapman's theories. For much of his career Alfvén's ideas were dismissed or treated with condescension. He was often forced to publish his papers in obscure journals. The dispute was finally settled in 1974, four years after Chapman’s death, when Earth satellites measured down-flowing currents for the first time. The concept of a magnetic mirror became important in work on controlled thermonuclear fusion requiring the confinement of hot plasmas whose contact would destroy the walls of any container. These ideas were later useful also in interpreting such phenomena as the Van Allen radiation belt currents of electrons circulating in the Earth's magnetic field during magnetic storms.
These papers, and all of his later work, was based on the principle that theories of cosmic phenomena must agree with known results from laboratory experiments on Earth, because the same laws of Nature must apply everywhere.

In 1940, Alfvén was appointed professor of Electromagnetic theory and electrical measurements at the Royal Institute of Technology in Stockholm. This appointment was a major step in his career. He expanded the departmental activities into electronics, plasma physics, magnetohydrodynamics, fusion plasma physics and accelerator technology, thus developing an international reputation for himself and his department.
In 1942, Alfvén put forth a theory of the origin of the planets in the solar system, which hypothesised that planets were formed from the material captured by the Sun from an interstellar cloud of gas and dust. As atoms were drawn toward the Sun, they became ionised and influenced by the Sun's magnetic field, thus condensing into small particles in turn coalescing to form the planets. Although the theory did not adequately explain the formation of the inner planets, it was important in suggesting the role of magnetohydrodinamics in the origin of the solar system. He advocated sending spacecrafts to asteroids and comets rather than planets and moons, believing that information about primordial conditions has almost entirely been lost on the larger bodies due to internal mixing and surface erosion. Comets are actually time capsules containing primitive material left over from the epoch when the Sun and its planets formed. From 1978, many missions have achieved encounters with comets and asteroids, and in November 2014, Rosetta, was the first mission in history to rendezvous with a comet. The Rosetta probe Philae landed on the comet on 12 November 2014. Studying the comet, via both remote and in situ observations, the Rosetta mission could become a key to unlocking the history and evolution of our Solar System.

Alfvén Waves and Plasma Physics
In the early 1940s, Alfvén got interested in the origin of sunspots and the sunspot cycle. It had long been known that sunspots are regions of localised high magnetic field, that solar prominences appear to follow the lines of force emanating from sunspots, and that there exists a weak general dipole field. He reasoned that the enhanced magnetic fields observed in sunspots must be caused by electrical currents in the solar plasma, and that currents and magnetic fields together must give rise to forces that affect the motion of this plasma, which, in turn, induces electric fields in a cyclic manner. In considering the motion of a highly conducting gas in a uniform magnetic field, Alfvén combined the equations of hydrodynamics with Maxwell's equations of electromagnetism, and thought through the entire phenomenon on purely physical grounds. He arrived at the conclusion that, in the simplest case of uniform fields and uniform conducting fluid, transverse waves (now called the Alfvén wave) could be propagated with no attenuation or reflection along the lines of force through a highly conducting medium such as the ionised gas of the sun or through ionised gas anywhere. He then worked out the mathematical theory, which was published in 1942 in the open letters section of Nature: “Existence of Electromagnetic-Hydrodynamic Waves”. Until this time, electromagnetic theory and hydrodynamics had been well-established but separate fields of physics. In this paper, Alfvén combined them. Since his hypothesis contradicted Maxwell's theory of electromagnetism, initially no one took it seriously. It was known that electromagnetic waves could penetrate only a very short distance into a conductor and that as the resistance of a conductor became smaller and smaller, the depth of penetration of an electromagnetic wave would approach zero. Plasma, like metals, is regarded as having high electrical conductivity. Thus, with an ideal electrical conductor, there could be no penetration of electromagnetic radiation. This erroneous conclusion was reached by thinking that the current in a plasma is directly proportional to the strength of the electric field. While this is generally true for metals, it is not true for a plasma. In a metal, only electrons move to carry the current, while ions are locked in place in order to make the metal a solid. In a plasma, however, the ions are part of a gaseous medium, so ions, like electrons, are free to move in response to the combination of magnetic fields and electric currents. Because ions constitute nearly the entire mass of a plasma (at least 99.95% of the mass of a given volume of plasma), plasma motions are important. This simple understanding made all the difference.

Alfven's discovery was generally dismissed with such remarks as “If such waves were possible, Maxwell himself would have discovered them some hundred years earlier.” It was not until 6 years later, during his first visit to the United States, when he gave several lectures on hydromagnetic waves, that his work was recognised as both correct and significant. Alfvén himself has told the story of when Enrico Fermi, who had got interested in the origin of cosmic rays, asked him to explain what the magnetohydrodinamic waves were, after Alfvén seminar in Chicago: “Since I published my first paper in 1942 very few people - with Lyman Spitzer and Martin Schwarzschild as the most prominent exceptions - had believed in them. I got letters from colleagues who asked me whether I had not understood that this was nonsense... Fermi listened to what I said about them for five or ten minutes, and then he said: «Of course such waves could exist.» Fermi has such an authority that if he said «of course» today, every physicist said «of course» tomorrow.” Fermi immediately applied Alfvén's ideas about the probable existence of extended magnetic fields in our galactic system in his most cited paper of 1949 on the mechanisms accelerating cosmic ray particles in space, in which the main process of acceleration was due “to the interaction of cosmic particles with wandering magnetic fields which, according to Alfvén, occupy the interstellar space.” Since this field would necessarily be dragged along by the moving and ionised interstellar material, here was an excellent way to obtain the acceleration mechanism for which he was looking. Fermi's outline of a method of accelerating cosmic ray particles served as a basis for most discussions on the subject.
Another reason for the period of non-acceptance was that the Alfvén waves were not demonstrated experimentally for several years. At that time, the technical means of producing high-temperature plasma in laboratories on the Earth did not yet exist, and plasma in outer space was still out of reach. The first experimental evidence for these waves was obtained using mercury by Alfvén’s student S. Lundquist. The situation changed in the 1950s, when the beginning of thermonuclear research led to technological developments that made it possible to generate high temperature plasmas artificially in laboratories on the Earth. Waves were observed in plasmas by fusion researchers in 1959. At about the same time, Owen Storey, using ground-based instrumentation and brilliant theoretical analysis, discovered that, instead of a vacuum, Earth is surrounded by plasma out to a distance of several Earth radii. Later, instruments sent into space quickly confirmed Storey's discovery. Because gases beyond the atmosphere (above 400 km) are magnetised plasma, magnetohydrodynamics became an indispensable tool in the emerging field of space physics.
The discovery in the early 1940s of the lines of multiply ionised iron in the solar corona spectrum posed an important problem for the fundamental physics: What is the mechanism of heating the solar corona and why is the temperature of the corona 100 times larger then the temperature of the photosphere? In 1947, Alfvén advocated the idea that the absorption of Alfvén waves generated by the turbulence in the convection zone and propagating along the magnetic field lines is the mechanism for heating the solar corona. The mechanism of wave damping in the solar corona is now widely accepted as the basic source of heating.
Alfvén stressed the importance of the high magnetic field observed in some stars and in 1950, together with his colleague N. Herlofson, he was the first to identify non-thermal radiation from astronomical sources as synchrotron radiation, which is produced by fast moving electrons about the lines of force of magnetic fields. Further radio astronomical observations provided convincing evidence that very-high-energy electrons and magnetic fields are present throughout the plane of our Galaxy. The recognition that the synchrotron mechanism of radiation is important in celestial objects proved extremely productive in astrophysics, since nearly all the radiation recorded by radio telescopes derives from this mechanism. The study of the radio emission of Cassiopeia A, for example, provided direct evidence for the acceleration of huge fluxes of very-high-energy electrons in supernova remnants. Further evidence showed that the energies present in the source regions had to be enormous, in order to account for the radio emission. These results, at the end of the 1950s, marked the beginning of high-energy astrophysics in its modern form and later had a major role in the astrophysical discovery of quasars, which capped an exciting period in which the distance scale of the universe was expanded nearly tenfold, adding another important piece of evidence in favour of the Big Bang theory.

Most of Alfvén's ideas were made known to the scientific community through his famous book Cosmical Electrodynamics, a collection of his early papers, printed in 1950. Distinguished scientists like Subrahmanyan Chandrasekhar and Lyman Spitzer, who gave Alfvén full credit for laying out the basic physics of cosmic plasma, considered this book a standard example for the presentation of plasma physics.
Since 1945, Alfvén was a member of the Swedish Atomic Commission, set up a few weeks after Hiroshima. At that time he was a supporter of the development of nuclear power to provide an autonomous source of energy for Sweden. Already in 1955, Alfvén visited the Kurchatov Institute in Moscow to meet Lev A. Artsimovich, head of the nuclear fusion programme in the Soviet Union. In 1956, when research on the control of high-temperature plasma by magnetic fields became known, he invited a number of workers in the field to join the IUPAP symposium in Stockholm on cosmical electrodynamics, where some papers from Russian and UK fusion research were given. The subject was declassified at the second conference on the Peaceful Uses of Atomic Energy in 1958, held in Geneva. Alfvén led the presentations with a review of magnetohydrodynamics and its application to the thermonuclear problem. He was particularly concerned with nuclear proliferation and the easy availability of military nuclear materials from civil nuclear fission programmes. He actually feared that the primary goal of civil nuclear programmes was to find a civil application that maintained a basic military capability.
Since 1964, Alfvén attended the Pugwash Conferences on Science and World affairs, founded in 1957 to provide a forum for influential scientists and public figures concerned with reducing the danger of armed conflict and seeking cooperative solutions to global problems. The movement had been initiated by Albert Einstein and Bertrand Russell, who named it after the small Canadian fishing village where the first meeting was held and where the Einstein-Russell Manifesto was generated. Alfvén served as President of Pugwash from 1970 to 1975 and was a prime mover in founding the Swedish Pugwash Group in 1964.
As explained by his biographer, Anthony L. Peratt (http://tmgnow.com/repository/cosmology/alfven.html), “In 1967 Alfvén issued a stinging condemnation of Sweden's nuclear research program, protesting what he considered to be insufficient funds for projects on peaceful uses of thermonuclear energy, and he left, saying, «My work is no longer desired in this country.» He actually claimed in the Swedish press that his criticism had led to a severe cut in his departmental research funding at the Royal Institute of Technology. He was immediately offered chairs in both the Soviet Union and the United States. After two months in the Soviet Union, he moved to America and from 1967 to 1988 he held a professorship at the University of California, San Diego, but remained in close contact with the Royal Institute of Technology, where he usually spent six months of the year, the time “from the Vernal Equinox until the Autumnal Equinox”. This arrangement survived until 1988 when he returned full-time to Sweden.

In 1970, Alfvén was awarded the Nobel Prize for Physics “for his contributions and fundamental discoveries in magneto-hydrodynamics and their fruitful applications in different areas of plasma physics.” He was the first space scientist to have received the Nobel Prize. In an article commenting the 1970 Nobel Prize in Physics, A. J. Dessler recalled how “for much of Alfvén's career, his ideas were dismissed or treated with condescension; he was often forced to publish his papers in obscure journals; and he was continually disputed by the most renowned senior scientist working in the field of space physics. Even today there is a rather pervasive unawareness of Alfvén's multifaceted contributions to fields of physics where his ideas are used with apparently little appreciation of who originated them.” With his many original ideas Alfvén had founded the science of magnetohydrodynamics, deeply transforming much of the thinking in the space sciences. For this reason, remarked Dessler, it was definitely fitting that the “massive contributions of Hannes Alfvén to the fields of plasma physics, space physics, and astrophysics be recognised with a Nobel Prize.” But actually he did more than that; his most important heritage is represented by the vision of a new paradigm in cosmic physics - the Plasma Universe.

Bibliography
Alfvén, H. O. G (1988) Memoirs of a Dissident Scientist. American Scientist 76.
Dessler A. J., W. C. Koehler and E Cabib (1970) Nobel Prizes: 1970 Awards Honor Three in Physics and Chemistry. Science 170 (3958): 604-609
Fälthammar C.-G. (1996) Hannes Alfvén. Q. J. R. astr. Soc. 37: 259-260
Hannes Alfvén, 30 May 1908 •2 April 1995
Carl-Gunne Fälthammar and Alexander J. Dessler. 2006. Proceedings of the American Philosophical Society 150 (4): 649-662
Leiter D. J. (2003) Alfvén, Hannes Olof G\"osta, in A to Z of Physicists, American Library Association, pp. 3-6
R. S. Pease and S. Lindqvist: (1998) Hannes Olof Gösta Alfvén. 30 May 1908-2 April 1995. Biographical Memoirs of Fellows of the Royal Society 44 : 2-19
Peratt A. 2008. L. Alfvén, Hannes Olof Gosta. Complete Dictionary of Scientific Biography. Vol. 19. Detroit: Charles Scribner's Sons, 2008. 40-45.
http://go.galegroup.com/ps/i.do?id=GALE%7CCX2830905433&v=2.1&u=mpi_vb&it=r&p=GVRL&sw=w&asid=252def4020038e48cd95cb61cf470052


Cite


Specify width: px

Share

Cite


Specify width: px

Share