Absence of sunspots make scientists wonder if they're seeing a calm before a storm of energy
Tuesday, June 22, 2010
Sunspots come and go, but recently they have mostly gone. For centuries, astronomers have recorded when these dark blemishes on the solar surface emerge, only to fade away after a few days, weeks or months. Thanks to their efforts, we know that sunspot numbers ebb and flow in cycles lasting about 11 years.
But for the past two years, the sunspots have mostly been missing. Their absence, the most prolonged in nearly 100 years, has taken even seasoned sun watchers by surprise. "This is solar behavior we haven't seen in living memory," says David Hathaway, a physicist at NASA's Marshall Space Flight Center in Huntsville, Ala.
The sun is under scrutiny as never before, thanks to an armada of space telescopes. The results they beam back are portraying our nearest star, and its influence on Earth, in a new light. Sunspots and other clues indicate that the sun's magnetic activity is diminishing and that the sun may even be shrinking. Together, the results hint that something profound is happening inside the sun. The big question is: What?
Groups of sunspots forewarn of gigantic solar storms that can unleash a billion times more energy than an atomic bomb. Fears that these giant eruptions could create havoc on Earth and disputes over the sun's role in climate change are adding urgency to these studies. When NASA and the European Space Agency launched the Solar and Heliospheric Observatory almost 15 years ago, "understanding the solar cycle was not one of its scientific objectives," says Bernhard Fleck, the mission's project scientist. "Now it is one of the key questions."
Sunspots are windows into the sun's magnetic soul. They form where giant loops of magnetism, generated deep inside the sun, well up and burst through the surface, leading to a localized drop in temperature that we see as a dark patch. Any changes in sunspot numbers reflect changes inside the sun. "During this transition, the sun is giving us a real glimpse into its interior," says Hathaway.
When sunspot numbers drop at the end of each 11-year cycle, solar storms die down and all becomes much calmer. This "solar minimum" doesn't last long. Within a year, the spots and storms begin to build toward a new crescendo, the next solar maximum.
What's special about this latest dip is that the sun is having trouble starting the next solar cycle. The sun began to calm down in late 2007, so no one expected many sunspots in 2008. But computer models predicted that when the spots did return, they would do so in force. Hathaway was reported as thinking the next solar cycle would be a doozy: more sunspots, more solar storms and more energy blasted into space. Others predicted that it would be the most active solar cycle on record.
The trouble was, no one told the sun.
The first sign that the prediction was wrong came when 2008 turned out to be even calmer than expected. That year, the sun was spot-free 73 percent of the time, an extreme dip even for a solar minimum. Only the minimum of 1913 was more pronounced, with 85 percent of that year clear.