May 27, 2010
Solar Scientists Agree That the Sun's Recent Behavior Is Odd, but the Explanation Remains Elusive
The most recent solar minimum was both long and pronounced. But why?
By John Matson
MIAMI—In very 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 to mark the cycle's evolution, appearing in droves at maximum and all but disappearing at minimum. But the behavior of our host star is not as predictable as all that—the most recent solar minimum was surprisingly deep and long, finally bottoming out around late 2008 or so.
Solar physicists here at the semiannual meeting of the American Astronomical Society this week offered a number of mechanisms to shed light on what has been happening on the sun of late, but conceded that the final answer—or more likely answers—remains opaque. Beyond scientific understanding, motivations for better solar weather forecasts include hopes to use them to safeguard against electrical grid disruptions, damage to Earth-orbiting satellites and threats to the health of space travelers posed by solar radiation flare-ups.
One researcher has looked for clues to solar weather in the meridional flow, which moves from the solar equator toward the poles, and which seems to change speed during the shifting solar cycle. Another looked at the solar "jet stream," a slow current that originates at solar mid-latitudes and pushes in a bifurcated stream toward both the equator and the poles. Another scientist examined the inner workings of the sun through the oscillation of sound waves propagating through the solar interior; yet another looked at magnetic maps to chart the shifting flux across the sun.
"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—geomagnetic activity, weakness of polar magnetic fields, flagging solar deflection of galactic cosmic rays—the minimum was the deepest on record, Hathaway said, although some of those records contain just a few cycles. Hathaway focused on shifting speeds of the meridional flow, finding that the flow was anomalously fast at the most recent minimum. But, speaking of heliophysics forecasting techniques in general, he cautioned against leaping to any conclusions based on small-number statistics. "We need to be careful about extending what we've seen in one or two cycles to all of them," he said.
Frank Hill of the National Solar Observatory (NSO) instead examined the jet stream, a periodic east–west flow of material that corresponds with the onset and end of the solar cycle. With helioseismology data, which track acoustic oscillations on the sun, researchers can check in on the progress of the jet stream at depths of roughly 1,000 kilometers, potentially allowing for better forecasts of the timing of the solar cycle. But it is "still too early to tell" if the jet stream can robustly predict solar activity, Hill acknowledged, noting that the stream could be a cause or an effect of the cycle.
Hill's NSO colleague Sushanta Tripathy also turned to helioseismology to investigate the recent solar minimum, finding that in acoustic oscillations deep within the sun there were in fact two separate minima—one in late 2007 that did not correspond to the sunspot minimum, and one around late 2008 that did. In prior data, from 1995 to 2007, the frequency shifts in the oscillations had matched up well with the sunspot counts. And at shallower depths within the sun, the seismic and sunspot activity were in phase for the most recent solar minimum as well. All in all, the cycle was definitely unusual, Tripathy said.
Julia Saba of SP Systems, Inc., and the NASA Goddard Space Flight Center in Greenbelt, Md., had yet another approach, turning to magnetic maps to track regional differences on the sun. Her approach accurately forecast the timing of the 2008 solar minimum 18 months in advance, she said, but acknowledged that the forecast had been revised from an earlier prediction. Based on current data, Saba said, the next solar cycle looks like it will be weak and prolonged. But that could all change—her predictions assume "that the sun doesn't change on us again."
After hearing his colleagues' various approaches to investigating the sun's behavior, 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," Hill said. "What's not so gratifying is we have no clue why any of these effects are happening."