Sun storms spawn magnetic reversal.

Every 11 years, the sun reaches a peak in its turbulent activity, sporting huge numbers of sunspots and hurling many billion-ton clouds of charged particles into space. At about the same time, the sun's magnetic poles flip: North becomes south, and south becomes north (SN: 3/3/01, p. 139).

Solar physicists have had scant clues about why this switch happens, but a new study suggests that the clouds, known as coronal mass ejections (CMEs), play a central role. By prying loose magnetic-field loops anchored to the visible solar surface, coronal mass ejections may sweep the surface clean of old magnetic fields and prepare the sun for its magnetic reversal.

The gradual process requires more than 1,000 CMEs erupting from the polar regions over several years, notes study coauthor Nat Gopalswamy of NASA's Goddard Space Flight Center in Greenbelt, Md. "When it's all over, the sun's magnetic stripes run in the apposite direction," Gopalswamy says.

In the Nov. 20 Astrophysical Journal Letters, Gopalswamy and his colleagues consider the rate and timing of polar CMEs and the direction of the polar magnetic field. For the current cycle, the team analyzed CME data collected by the orbiting Solar and Heliospheric Observatory from 1996 to 2002. For the previous cycle, they examined data recorded by a U.S. Air Force satellite, P78-1, from 1979 to 1985. Ground-based telescopes provided data on the sun's polar magnetic field during these years.

The analysis reveals "an important connection" between the reversal of the sun's magnetic poles and polar CMEs, the researchers note. Although polar CMEs make up only 16 percent of all CMEs on the sun, they are the ones most important for the magnetic reversals.

At each pole and for both solar cycles the team examined, the number of polar CMEs and their speeds peaked just before the magnetic reversals occurred. Remnants of the sun's polar magnetic field were often found within these CMEs. The findings indicate that high-latitude CMEs rid the polar regions of their old magnetic field by literally lifting them off the sun's surface.

Solar physicist David H. Hathaway of NASA's Marshall Space Flight Center in Huntsville, Ala, notes that getting rid of the sun's old polar magnetic field is only part of the story in reversing polarity.

The sun's surface layer is divided into alternating bands of electric charge, either positive or negative, and each band has a different magnetic polarity. When the magnetic field at the north pole is removed by CMEs, a band with opposite polarity moves up, as if on a conveyor belt, to replace it. This flow of material from the equator is also crucial to flipping the magnetic field, says Hathaway.

Scientists don't yet know what drives the 11-year solar cycle. The answer to that question, Gopalswamy says, lies deep within the sun's churning interior.