Astrophysicist Recognized for Discovery of Solar Wind

John Roach
for National Geographic News
August 27, 2003
In 1958 Eugene Parker discovered that a stiff wind blows incessantly from the sun, filling local interstellar space with ionized gas. The discovery forever changed how scientists perceive space and helped explain many phenomena, from geomagnetic storms that knock out power grids on Earth to the formation of distant stars.

Now, for his groundbreaking discovery more than four decades ago, Parker, a professor emeritus of physics, astronomy, and astrophysics at the University of Chicago, will receive the 2003 Kyoto Prize for Lifetime Achievement for Basic Science on November 10 in Japan. The award, which comes with a gold medallion and a check for about U.S. $400,000, is one of three annual Kyoto Prizes that recognize significant contributions to the scientific, cultural, and spiritual development of humankind.

"It is very nice coming late in retirement to know that my work is recognized," said Parker during a break from tending his summer garden to speak with National Geographic News.

Scientists who study the sun and its interactions with interstellar space, the planets, and distant stars say that Parker's discovery of the solar wind opened the door to a new field of astrophysics.

"The existence of the solar wind is a fundamental physical fact, like the atomic nature of matter, and it eluded science until the 1950s and Gene Parker," said Paal Brekke, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, on behalf of the scientists working on the Solar and Heliospheric Observatory (SOHO) project.

The SOHO spacecraft and mission is a joint venture between the European Space Agency and NASA to study the internal structure of the sun, its outer atmosphere, and the origin of the solar wind. The project carries forward the field of research elucidated by Parker.

In addition to the solar wind, Parker's achievements include many contributions to the study of the large-scale dynamics of gas and magnetic fields in space.

He explained the process that creates the solar magnetic field—dynamo theory—and many terms in the area of research known as "cosmical magnetohydrodynamics" bear his name: the Sweet-Parker model of magnetic reconnection, the Parker instability in the interstellar medium, the Parker limit on magnetic monopoles, and the Parker equation describing the propagation and acceleration of energetic particles in space.

Solar Wind

But Parker's most well-known discovery was the solar wind: a flow of ionized gas—plasma—expanding away from the sun's surface into space, ultimately reaching speeds of 200 to 600 miles per second (300 to 1,000 kilometers per second) at large distance, which is 1,000 to 3,000 times the speed of sound in the air on Earth. Before the discovery scientists regarded interstellar space as a vacuum.

The beginning of the wind is best "seen," said Parker, in pictures of a total solar eclipse. During a total solar eclipse the moon passes between the Earth and sun, blocking sunlight from reaching Earth. In pictures of the events, bits of the sun's corona—the outer edge of the sun—can be seen extending out into space.

"Around the 1930s scientists determined the temperature [of the corona] must be a million degrees [Celsius] because of the way it stood out into space," said Parker. "If it weren't so hot, it wouldn't be puffed out so much. Then some very clever spectroscopic detective work confirmed this extraordinary temperature."

In the mid-1950s the British mathematician Sydney Chapman calculated the properties of a gas at a million degrees Celsius (1,800,000 degrees Fahrenheit) and determined it is such a superb conductor of heat that the million-degree gas at the sun must extend way out into space, beyond the orbit of Earth.

Also in the 1950s a German scientist named Ludwig Biermann got interested in the fact that no matter whether a comet is headed towards or away from the sun, the comet's tail always points away from the sun. Biermann postulated that this happens because the sun emits a steady stream of particles that push the comet tail away.

Parker realized that the heat flowing from the sun in Chapman's model and the comet tail blowing away from the sun in Biermann's theory had to be the result of the same phenomenon.

"It occurred to me that Chapman must be right near the sun and Biermann must be right far from the sun," said Parker. "That suggested a gradual outward expansion of the corona of the sun."

Parker performed the calculations to show that even though the sun's corona is strongly attracted to the sun by solar gravity, it is such a good conductor of heat that it is still very hot at large distances from the sun. Since gravity weakens as distance from the sun increases, the outer coronal atmosphere escapes into interstellar space.

Joseph Hollweg, an astrophysicist at the University of New Hampshire in Durham, said Parker had the insight to calculate what the pressure does inside the corona as it gets farther and farther from the Sun and discovered a pressure gap between the sun and interstellar space. To fill the pressure mismatch, something had to move.

"I like to think of it as interstellar space sucking out the corona like a vacuum cleaner," said Hollweg. "Most people would say it is the pressure of the corona that is pushing the coronal atmosphere out."

Opposition to Parker's theory on the solar wind was strong. The paper on it he submitted to the Astrophysical Journal in 1958 was rejected by two reviewers. It was saved by then editor Subrahmanyan Chandrasekhar, who received the 1983 Nobel Prize in physics.

In the 1960s the theory was confirmed through direct satellite observations of the solar wind, which also made it possible to explain magnetic storms, auroras, and other solar-terrestrial phenomena.

"It solidified a whole slough of observations of things happening on the sun and happening on Earth a few days later," said Hollweg.

Fertile Field

Parker said his general curiosity about why things do what they do captured his interest as a young child. By the time he was in high school, he found physics class so interesting that it sealed his career path. Astrophysics, he said, is simply applying the laws of physics to large-scale phenomena that cannot unfold inside a laboratory.

"It is a wide open field of new things to puzzle over and figure out and that is why I pursued it," he said. "It is a fertile field, so why not?"

Parker acknowledges that his elucidation of the solar wind changed the perception of outer space, but he said there is still much to learn. As an example, he said that while scientists know the corona is heated to one to two million degrees Celsius, they are uncertain as to what heats the corona.

One suggestion is small flares on the sun. "I think that is probably true," said Parker. "But if you said 'could you prove it in a court of law,' I'd say 'no way.'"

Parker said that he hopes the next round of space- and ground-based telescopes will have the ability to resolve such questions. In the meantime, scientists such as Brekke and his colleagues on the SOHO team continue to advance the study of the space between the sun and the edge of the solar system, a field opened by Parker.

"We live on the edge of it, we seek to travel through it, and its variability affects life here on Earth. Without his life's work, we would have only the most confusing of clues of how to understand, predict, or mitigate those effects," said Brekke.

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