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.
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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