THE CALM BEFORE THE STORMS

Scientists studying the 11-year solar cycle are trying to predict when a quiet sun will next turn violent. Below the surface of the sun, a radical transition is afoot. In 5 years or so, the sun will be awash in sunspots and more prone to violent bursts of magnetic activity. Then, about 11 years from now, the solar cycle will conclude: Sunspots will fade away and the sun will again grow quiet. In early March, a dozen scientists, part of the Solar Cycle 25 Prediction Panel, descend on the National Center for Atmospheric Research (NCAR) here to predict how unruly the sun will become at its peak. NASA and the National Oceanic and Atmospheric Administration have sponsored these panels since 1989, aiming to understand what drives the sun's 11-year cycles and assess methods for predicting them. But the exercise is not just academic: the military, satellite operators, and electric utilities all want to know what the sun has in store, because of its tendency to flare up and send dangerous charged particles crashing into Earth as it approaches solar max. Sunspots can be seen with the naked eye, but it wasn't until the mid-1800s that astronomers realized they come and go on a rough schedule. They first appear at midlatitudes and then proliferate, migrating toward the equator over about 11 years. In 1848, German astronomer Johann Rudolph Wolf published an account of the sunspot record, identifying 1755–66 as "Cycle 1," the first period when counts were reliable. He then created a formula for counting the number of daily sunspots—a somewhat subjective technique that has evolved into a counting method still used today to marry data sets across the centuries. The cycles are capricious, however. Sometimes, the sun goes quiet for decades, with anemic sunspot counts across several cycles—as occurred during the 19th century's so-called Dalton minimum. Such variations are what the scientists at NCAR have gathered to forecast. The problem is that no one—in this room or elsewhere—really knows how the sun works.