Our sun is a tangled mess of magnetic fields, but it knows how to do spring cleaning. A new study shows that thousands of titanic eruptions of gas, called coronal mass ejections (CMEs), sweep away the stubborn remnants of old magnetic fields near the sun's poles. The storms play the key role in completing the sun's regular cycle of reversing its magnetic fields.
Every 11 years or so, the sun flips the north and south poles of its magnetic field. As the switch nears completion, the magnetic field lines--marked by churning loops of charged gas--crowd together at the poles. Anchored to the sun, they form the last stand of the old magnetic alignment.
The new research demonstrates that CMEs uproot those magnetic anchors as they expel billions of tons of charged gas from the surface and atmosphere, called the corona. The evidence comes from analysis of images taken by the Solar and Heliospheric Observatory (SOHO) satellite. A team led by solar physicist Nat Gopalswamy of NASA's Goddard Space Flight Center in Greenbelt, Maryland, studied nearly 7000 CMEs caught by SOHO between 1996 and 2002. Most eruptions spewed from sunspots near the equator, but Gopalswamy's team focused on 1200 CMEs closer to the poles.
In the 20 November Astrophysical Journal Letters, the team describes a striking correlation between the timing of the eruptions and ground-based records of magnetic field patterns at the poles. The polar eruptions climaxed for about 18 months as the field directions flip-flopped in chaotic fits and starts. But as soon as the new magnetic field was in place, the CMEs stopped. Gopalswamy concludes that the CMEs are blasting loose the anchored fields. "They stretch and change the magnetic field conformation in such a way that the polarity ultimately changes," he says.
That clinches a new player in the field reversals. "This is the first clear evidence that CMEs are related to the sun's polarity reversal," comments solar theorist Boon Chye Low of the National Center for Atmospheric Research in Boulder, Colorado. Low also thinks that the study will compel solar physicists to consider the combined effects of CMEs, not just individual eruptions. "They have a major global influence," he notes.