Eugene N. Parker deduced the existence of the solar wind—the stream of charged particles and radiation constantly emanating from the Sun. His deduction was confirmed by a Soviet satellite in 1959.
The solar wind is one of the most fascinating interplanetary phenomena to be found in Earth’s solar system. Its very name summons up visions of the kind of wild and primitive natural violence that characterizes the life of any star, including the sun. When it first was proposed, however, the notion of a steady stream of charged particles emanating from the sun at supersonic speeds was not universally accepted. Even when the solar wind was confirmed by satellite instrument packages, it continued to be dismissed by some as impossible.
Events that now are known to be caused by the solar wind were quite familiar by the 1950’s. The auroras, for example, both at the North Pole (aurora borealis) and at the South Pole (aurora australis), had been observed for centuries. The fact that the tail of a comet always points away from the sun, no matter in which direction the comet is moving, was known. Magnetic storms, which cause fluctuations in Earth’s magnetic field
In 1957, Eugene N. Parker was an assistant professor at the Institute for Nuclear Studies and the Department of Physics at the University of Chicago. At that time, he had been working on the origin of Earth’s magnetic field, thermal instability of the gas in the atmosphere of the sun, and cosmic-ray modulation. Ludwig Biermann, Director of the Max Planck Institute for Astrophysics in Munich, was visiting the University of Chicago that same year. Biermann told Parker of his studies of comet tails. Comets
Biermann was interested in why comet tails always point away from the sun, even when that means that the tail is pointing in the direction in which the comet is moving. That behavior had been attributed to radiation pressure, which is the pressure exerted by any electromagnetic field, such as that from a light source, on whatever that field strikes. The pressure of sunlight, although very small, is substantial so far as the tenuous tail of a comet is concerned. Nevertheless, calculations by Biermann involving the absorption cross section of the atoms in a comet tail showed that they did not present sufficient surface area for solar radiation to have the effect that was observed: The dust and gas in the tail are not merely being carried away from the comet; they are being blown away with great force.
Given that a single atom can be pushed only insofar as it casts a shadow, and that the shadow cast by an individual atom is inadequate, radiation pressure could not be the answer. The classical explanation simply did not work, Biermann believed, and there was only one other explanation: solar corpuscular radiation. Solar corpuscular radiation is the discharge of particles from the sun at the time of a solar flare. Such bursts were known to cause the auroras and magnetic storms. The rest of the time, however, interplanetary space was thought to be empty.
The idea was interesting, and although it was not taken seriously by most workers in the field, it appeared to be inescapable. The corpuscular radiation evidently was shot out from the sun with average velocities on the order of 500 kilometers per second. Biermann’s findings also fitted in well with work being done at the University of Chicago by John A. Simpson, a colleague of Parker. Simpson was one of the first to recognize the importance of the variations in cosmic-ray intensity. He made great strides in the study of cosmic-ray modulation and its implications for the active conditions in space.
Parker was influenced by Biermann’s theories, because although they are often obscured by clouds, there are always auroras somewhere around the auroral zone of Earth. There are always small fluctuations in Earth’s magnetic field. The tail of a comet always points away from the sun. Interplanetary space, therefore, always must be filled with solar corpuscular radiation. Parker needed to determine why it was always there and why it was moving so forcefully out from the sun even between the occurrences of solar flares.
Shortly after a discussion with Biermann, Parker was in Boulder, Colorado, where he had been invited to give a lecture at the High Altitude Observatory. He had an opportunity to learn of the work of Sydney Chapman on the solar corona. The corona is an envelope of thin, hot gas that surrounds the sun. Chapman showed Parker some calculations indicating that the outer atmosphere of the sun, the corona, extended out into space, past the orbit of Earth, because of its high temperature. (At such high temperatures, even a very tenuous gas transmits a substantial amount of heat.) The high temperature of the corona was causing it to expand slowly upward against the gravitational field of the sun. It gradually increased in speed until at 2 to 5 solar radii, it reached supersonic velocity. The farther away from the sun it got, the thinner it became.
Chapman’s conclusion not only was novel and interesting but also appeared to be inescapable. A little more thought, however, seemed to indicate a conflict with Biermann’s equally inescapable conclusion that solar corpuscular radiation continually fills interplanetary space. Both Chapman’s static corona and Biermann’s solar corpuscular radiation must be composed of solar material, ionized hydrogen, that is, a plasma of protons and electrons. Both plasmas would be very tenuous so that the individual protons and electrons rarely would collide among themselves. It is not possible to have one tenuous plasma streaming at high velocity through another. The two would interact electrostatically with each other to produce ripples and electrostatic fields that would stop the streaming, locking the two plasmas together to form a single tenuous plasma.
Parker believed that solar corpuscular radiation and the extended solar corona must be the same. They both were, as he came to name it, the “solar wind.” The auroras always were there, to a greater or lesser extent; the earth’s magnetic field always was fluctuating somewhat; comet tails always were being blown very strongly away from the sun; and interplanetary space always was filled with the constantly expanding solar corona, moving outward from the sun.
An examination of the equations describing the dynamical behavior of the hot solar atmosphere in the presence of the powerful gravitational field of the sun showed that, with the extended high temperature deduced by Chapman, the corona is approximately static near the sun, but gradually expands with velocities that increase to 500 to 1,000 kilometers per second at large distances from the sun. The expansion reaches supersonic speeds at a distance of several solar radii and continues to accelerate for as far out as the temperature is maintained.
Parker realized that to develop the theory of the solar wind, even with fully supporting mathematical equations, and to have it accepted by the scientific community would be quite a task. In 1958, Parker wrote a paper titled “Dynamics of the Interplanetary Gas and Magnetic Fields.” He reconciled Chapman’s work on the expansion of the solar corona with Biermann’s findings on solar corpuscular radiation. The paper contained equations developed by Parker showing supersonic velocities for the solar wind of several hundred kilometers per second. When the paper reached the Astrophysical Journal,
The rejection represented a major setback, but a colleague, Subrahmanyan Chandrasekhar, who was the editor of Astrophysical Journal, informed Parker that the referees of his paper were experts in the field, and they asserted that the outward acceleration of the corona to supersonic speeds was scientifically absurd, and in view of this expert opinion, it might be reasonable to withdraw the paper. Parker pointed out to Chandrasekhar that the referees had failed to find any error in the calculations and that he wished to go ahead with publication. Chandrasekhar believed he had given adequate warning, and since no mathematical error was evident, he accepted the paper.
With the November, 1958, publication of Parker’s paper in the Astrophysical Journal and its mathematical prediction of the solar wind, all that remained to be accomplished was an actual “sighting.” Instruments designed by Soviet scientist K. I. Gringauz and carried aboard a Luna satellite were the first to detect a gas moving past the earth at some speed in excess of 60 kilometers per second—a supersonic velocity that confirmed Parker’s prediction. It would be several more years before the phenomenon of the solar wind was accepted as a fact by the scientific community. Subsequently, the actual velocity was determined by several U.S. satellites to vary from 300 to 1,000 kilometers per second.
The discovery of the solar wind has added to the knowledge and understanding of the origin, evolution, and the continuing operation of the universe. Besides giving the correct explanations for the auroras, magnetic disturbances, and the behavior of comet tails, understanding the solar wind gives scientists a perspective on the activity of the solar system and adds to the knowledge of stars. Most other stars have their own stellar winds along the same general lines as the solar wind.
The traditional view of stars saw them as tranquil objects, shining away for billions of years and eventually going out. It has been learned that a star is very active, yet very mysterious. For example, the origin of the magnetic field inside the sun is not understood yet. Although it has been established that the luminosity of the sun varies by one part in six hundred, there is evidence from other sun-type stars that it could vary at times by one part in one hundred, in the space of a few years; that would be sufficient to cause substantial changes in the earth’s climate. A decrease in luminosity of that magnitude would cause the polar ice caps to advance, thus producing a small ice age. In the absence of any other explanation, the substantial variations in climate recorded over the past thousand years suggest strongly that there are substantial variations in the luminosity of the sun.
There is no basis for thinking that a change of 1 percent would create the same conditions that caused any of the several major periods of glaciation dating as far back as 570 million years ago. Nevertheless, it easily could be equivalent to the Little Ice Age, which began in the thirteenth century and reached its maximum around the middle of the eighteenth century, at which point glaciers were more extensive on Earth than at any other time. The Little Ice Age affected global agricultural output, leading to hardship in China and in Europe. Killing frosts in the North American Great Plains were commonplace each summer.
Even if the luminous intensity of the sun does vary, it is an occurrence that takes place only over a period of centuries, making it impossible to measure over any useful human period of time and difficult to predict and adjust. To overcome that difficulty, a group of Harvard University researchers is using the 152-centimeter telescope at Mount Wilson to conduct a ten-year study of about fifty sun-type stars. By being able to detect the warning signs of a major climate-altering change early enough, preparations could be made to accommodate a significant change in the earth’s climate.
Understanding the solar wind shows scientists that the sun is losing mass at the rate of about one million tons per second. That, however, is not a problem. Because of the immense size of the sun, the loss it has experienced has amounted to only one-ten-thousandth of its original mass.
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Menzel, Donald H., and Jay M. Pasachoff. Stars and Planets. Boston: Houghton Mifflin, 1983. An excellent book for the novice astronomer/astrophysicist who wishes to become familiar with the night sky, the backdrop of comet shows. Provides the reader with understandable explanations of the “operation” of the sun and other stars. -
Mitton, Simon, ed. The Cambridge Encyclopaedia of Astronomy. New York: Crown, 1977. The book carries the distinction of having an internationally eminent advisory board. They have succeeded in creating not only a reference work of the highest scientific standard and of lasting value but also one that is a challenge and a pleasure to read. A broadly based survey of the field of astronomy, with descriptive sections on planets, Earth’s sun, other stars, and cosmology. -
Muirden, James. The Amateur Astronomer’s Handbook. 3d ed. New York: Harper & Row, 1987. Recognized as a standard work, this book has all the information necessary for establishing an amateur observatory to examine “the place of business” of the astrophysicist. Muirden explains the ways many observers have overcome the drawbacks of modest equipment by teaching themselves to excel in technique. -
The Rand McNally New Concise Atlas of the Universe. New York: Rand McNally, 1989. Contains an atlas of Earth from space, an atlas of the Moon, an atlas of the solar system, and an atlas of the stars. Much of the material in this book has been made available by the U.S. Geological Survey and the National Aeronautics and Space Administration (NASA). -
Zeilik, Michael. Astronomy: The Evolving Universe. 9th ed. New York: Cambridge University Press, 2002. Clear, simple writing is used. The evolution of the ideas about the universe—from superstition and myth to contemporary science—is traced. Zeilik introduces the idea of the scientific model, a simple concept that need not scare the nonscientist.
Von Weizsäcker Forms His Quantitative Theory of Planetary Formation
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