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See Inside August 2010

An Extra Quiet Sun

A long and pronounced solar minimum befuddles astronomers

Miami—In rough terms, the sun’s activity ebbs and flows in an 11-year cycle, with flares, coronal mass ejections, and other energetic phenomena peaking at what is called solar maximum and bottoming out at solar minimum. Sunspots, markers of magnetic activity on the sun’s surface, provide a visual proxy for the cycle’s evolution; they appear in droves at maximum and all but disappear at minimum. But the behavior of our host star is not as predictable as all that—the most recent solar minimum of late 2008 was surprisingly quiet and prolonged.

Solar physicists have offered a number of mechanisms to shed light on the solar cycle. Beyond improving fundamental scientific understanding, better predictions of solar behavior would help safeguard against electrical grid disruptions, damage to Earth-orbiting satellites and radiation threats to astronauts. In a press conference at the semiannual meeting of the American Astronomical Society in May, researchers laid out different approaches to tracking and predicting the sun’s activity, but the final explanation—or, more likely, explanations—for the sun’s curious recent lull remained opaque. “I think we’re almost in violent agreement that this is an interesting minimum,” said David Hathaway of the NASA Marshall Space Flight Center in Huntsville, Ala.

By several measures the minimum was the deepest on record, although some of those records contain just a few cycles. Hathaway’s research focused on shifting speeds of the meridional flow, which moves from the solar equator toward the poles, finding that the flow was anomalously fast at the most recent minimum. But he cautioned against leaping to any conclusions based on a small number of cycles.

The solar jet stream, a slow current orig­inating at solar midlatitudes that pushes toward both the equator and the poles, provides a different window into the sun’s roiling innards. Frank Hill of the National Solar Observatory in Tucson examined the periodic nature of the jet stream, which seems to correspond to the onset and end of the solar cycle. With helioseismology data, which track acoustic oscillations on the sun, researchers can follow the development of the jet stream thousands of kilometers below the sun’s surface, potentially allowing for better forecasts of the timing of the solar cycle. But it is still too early to tell if the technique can robustly predict solar activity, Hill acknowledged, noting that the stream could be a cause or an effect of the cycle.

A third scientist used helioseismology to look at minima in acoustic oscillations that often correspond with sunspot minima. Yet another turned to magnetic maps to chart the shifting flux across the sun.

After hearing of all these approaches, Hill took stock of a field with many open questions. “My main impression of all this is I’m gratified to see that we all agree that this is an interesting minimum,” he said. “What’s not so gratifying is we have no clue why any of these effects are happening.”

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