For unknown reasons, the sun goes through cycles of activity that last about 11 years. During this time, the number of sunspots on its surface increases to a point called a solar maximum, in which the star's magnetic field grows strongest, and then gradually drops, with a corresponding weakening of its magnetism. These dark, Earth-sized zones of cooler temperatures and powerful magnetic forces sometimes disappear altogether. However, sunspots haven't stayed away this long--2 years--in nearly a century.

Scientists had been at a loss to explain the lull, as ScienceNOW reported last month (ScienceNOW, 8 May), but now a group from the National Solar Observatory (NSO) in Tucson, Arizona, thinks it has found the reason. It has to do with a magnetic phenomenon called solar jet streams. Every 11 years, the sun simultaneously generates twin streams of plasma at each of its poles. Unlike the jet streams on Earth, the solar versions are magnetized and travel only toward the equator. This migration takes place very slowly--at about 10 kilometers per hour. For reasons still not understood, when the streams reach 22 degrees of latitude, north and south, they touch off a new solar cycle, and the sunspots reappear.

That is what has just happened on the sun, but with a twist, says NSO scientist Frank Hill. He and colleague Rachel Howe have been tracking the solar jet streams since the mid-1990s using a technique called helioseismology. The method is similar to that used by seismologists to detect and evaluate earthquakes, and it's necessary because the solar streams occur several thousand kilometers beneath the sun's surface. Hill and Howe discovered that the jet streams generated in 1996 have migrated more slowly than normal, taking 13 years to reach the critical 22-degree latitude instead of the usual 11 years.