After a perplexing quiet spell, the sun appears to be stirring-but astrophysicists remain divided about what our star is going to do next. The sun was expected to hit a low in 2008 as part of its normal 11-year cycle of activity.
But it stayed quiet until very recently, confounding scientists and sparking speculation of a sun-triggered "little ice age."
Solar physicists have denied that potential, saying that today's greenhouse gases have much more influence on global temperatures than the sun. (Watch video about how greenhouse gases are affecting Earth.)
Now the sun appears to be waking up, and the latest prediction from a panel convened by NASA and the National Oceanic and Atmospheric Administration says that the sun is simply a year late.
Solar activity will peak in 2013, the experts say, with 90 sunspots predicted that year.
Still, this would be the lowest peak recorded since the 1920s, and the experts are cautious about their own predictions.
"Go ahead and mark your calendar for [a peak in] May 2013," panel member Dean Pesnell of the Goddard Space Flight Center said in a press statement.
"But use a pencil."
Back to Normal?
Sunspots, solar flares, and so-called zonal flows-streams of plasma akin to Earth's jet streams-are all tracked as signs of magnetic activity on the sun. (See solar activity pictures.)
When the sun is very active, solar storms can disrupt satellites, endanger astronauts, and knock out power grids on Earth.
Recent data show that the sun's activity is slowly ratcheting back up. Most experts, including panel member and solar researcher Leif Svalgaard, are taking this as a sign that the sun is back on track and headed toward a solar maximum.
Svalgaard notes, however, that current predictions are based more on long-term statistics than the sun's recent behavior. A peak of 90 sunspots, he said, may be optimistic.
Meanwhile, other experts are suggesting that this year's low may not be so unusual.
In a paper in the June issue of the Astrophysical Journal, Ilya Usoskin of the Solankyla Geophysical Observatory in Finland suggests that the past 50 years represent a so-called grand maximum in solar activity.
During this period, Usoskin says, the sun's average magnetic activity was unusually high.
Mike Lockwood, a solar terrestrial physicist at the University of Southampton in the U.K., agrees. The sun may now be returning to the quieter times of the 1920s, which were closer to normal, he said.
Astrophysicists over the past few decades didn't recognize the grand maximum, he suggests, because scientists back then had incomplete data.
Sunspots have been tracked since the invention of the telescope, for example. But zonal flows were first studied only 30 years ago, and the sun's radio emissions were first observed in the 1940s.
"If the ground rules have changed underneath you, then the prediction could be completely wrong," he said.
"It is quite possible that the solar activity will be even lower than the panel is estimating," Lockwood said. "I have a suspicion ... this will be a yet weaker cycle" than the one before.
NASA panel member Svalgaard argues that ice-core evidence from Greenland "does not indicate unusually high recent solar activity compared to the last 600 years."
And no matter the numbers, he said, the risk remains that any single solar storm could be strong enough to cause billions of dollars in damage to communications systems, including satellites.
"The frequency of storms does depend on the solar cycle," Svalgaard said. "But the strength of an individual storm isn't related."
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