A dramatic view of our Sun. Photo: NASA

By compiling all the solar wind data gathered in the space age, NASA scientists have concluded that even though the solar magnetic field is constantly changing, it always returns to its original shape and position.

"We now know that the Sun's magnetic field has a memory and returns to approximately the same configuration in each 11-year solar cycle," said Dr. Marcia Neugebauer, a Distinguished Visiting Scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Current theories imply that the field is generated by random, churning motions within the Sun and should have no long- term memory. Despite this expectation, the underlying magnetic structure remains fixed at the same solar longitude."

"It's interesting that the solar magnetic field varies in strength and direction, but not in longitude," said Dr. Edward Smith, senior research scientist at JPL.

The solar wind is composed of charged particles ejected from the Sun that flow continuously through interplanetary space. The solar wind carries part of the Sun's magnetic field into space. Before completing this research, scientists knew that features of the solar wind reaching the Earth tended to repeat about every 27 days, said Neugebauer. The new information pinpoints the repetition interval at 27 days and 43 minutes and shows that the Sun has kept this steady rhythm, much like a metronome, for at least 38 years.

This pattern escaped previous detection because it is a very subtle statistical effect. There are many larger variations in the solar wind that come and go, which largely mask the underlying pattern. This repetitive behavior can't be seen if these data are examined for only a few months or years, but it was revealed in this 38-year database.

"Why the Sun's magnetic field behaves in this way is a puzzle, but the answer must lie deep within the Sun," Smith said.

"We're trying to understand how magnetic fields are generated in the Sun, the planets and the stars," said Neugebauer. "A better understanding of how the Sun generates its magnetic field will help us better understand the solar wind and space weather."

Fluids conducting electricity under the Sun's surface generate the magnetic field, Neugebauer explained, and the field's apparent memory is most likely caused by a structure and process occurring deeper inside the Sun than previously believed. "There may be something asymmetric about the Sun's interior, perhaps a deep-seated lump of old magnetic field," she said.

The findings, published in the February 1 issue of the Journal of Geophysical Research, are based on all the solar wind data collected from the dawn of space exploration through 1998, both by Earth-orbiting satellites and interplanetary spacecraft. This includes about 335,000 hours of solar wind speed data and 250,000 hours of magnetic field data. Co-authors of the article, in addition to Neugebauer and Smith, are Drs. Alexander Ruzmaikin, Joan Feynman and Arthur Vaughn, all of JPL.

This study was funded under the Supporting Research Program of NASA's Office of Space Science, Washington, D.C. JPL is a NASA center managed by the California Institute of Technology, Pasadena.