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A solar eruption. Credit: NASA/LMSAL |
The sun's temper ebbs and flows on what scientists had thought was a pretty predictable cycle, but lately our closest star has been acting up.
Typically, a few stormy years would knock out a satellite or two and maybe trip a power grid on Earth. Then a few years of quiet, and then back to the bad behavior. But an extremely long stretch of low activity in recent years has scientists baffled and scrambling for better forecasting models.
An expected minimum of solar activity, between 2008 and 2009, was unusually deep. And while the sun would normally ramp up activity by now, heading into its next cycle, the sun may be on the verge of a weak solar cycle instead, astronomers said at the 216th meeting of the American Astronomical Society in Miami last month.
"We're witnessing something unlike anything we've seen in 100 years," said David Hathaway of NASA's Marshall Space Flight Center in Huntsville, Ala.
The sun's constant interaction with Earth makes it important for solar physicists to keep track of solar activity. Stormy periods can force special safety precautions by satellite operators and power grid managers, and astronauts can be put at risk from bursts of radiation spat out by solar storm. Scientists need to more reliably predict what's in store.
At the conference, four solar physicists presented four very different methods of measuring and tracking solar cycles. ?
The sun has spots
Sunspots are areas of concentrated magnetic activity that appear as dark dots on the solar surface. The ebb and flow of the sun's magnetic activity, manifested in the appearance of sunspots, make up the solar cycle.
Typically, a cycle lasts about 11 years, taking roughly 5.5 years to move from a solar minimum, a period of time when there are few sunspots, to peak at the solar maximum, during which sunspot activity is amplified.
The previous cycle 23's extraordinary minimum recorded the highest number of days without sunspots that researchers had seen since 1913, said Hathaway.
Hathaway and his team of researchers measured what is called the meridional flow, which is the circulation of stellar material from the sun's equator toward the poles and back again. This flow can often influence a cycle's strength.
The scientists examined the changes in the structure of the flow, and the levels of geomagnetic activity, as they corresponded to the minimums and maximums of the previous solar cycles.
"We found that there were variations in the strength of that flow," Hathaway said. "The last minimum in 1996, that velocity was about 11 meters per second (about 22 miles an hour), which is pretty slow for an object as big as the sun. That flow slowed down as we went to maximum in 2001."
The meridional flow then quickly increased again, and by 2004, it was faster than it was at the last maximum, said Hathaway. This flow continued to stay fast on the approach to this most recent minimum.
"My suspicion is that this sunspot cycle 23 was a weaker cycle than the last two, with fewer sunspots and weaker magnetic fields. These may feed into what happens with the meridional flow that is going to lead to another weak cycle."
Hathaway predicts that cycle 24 should reach its peak in mid-2013 at about half the size of the last three cycles.
The sun's out of sync
In a different approach, Sushanta Tripathy of the National Solar Observatory used the frequencies of acoustic oscillations to look for signatures of changes in the solar activity cycle.
Tripathy found that changes in acoustic frequencies were, for the most part, in phase with solar activity. But, during the extended minimum, he noticed that the frequencies of waves that cover a large portion of the solar interior became out of sync with solar activity.
"We find that the frequencies of sound waves that travel to the deep interior show an early minimum during late 2007, while the waves that are confined to near the surface show the signature of minimum in late 2008, nearly coinciding with solar activity minimum."
The two seismic lulls detected using acoustic oscillation have not been seen before in previous cycles, said Tripathy, leading researchers to conclude that the extended minimum between cycles 23 and 24 is quite unusual.
Jet streams on the sun
Frank Hill, also of the National Solar Observatory, took a separate approach, attempting to predict the sunspot cycle based on a phenomena on the sun that can be likened to solar jet streams.
This east-west flow on the surface of the sun was first discovered in 1980, and is known as "torsional oscillation."
The jet stream exists at a depth of at least 65,000 miles (about 105,000 kilometers) below the solar surface, and Hill and his team of researchers were able to examine its behavior at a depth of 600 miles (966 km).
"The position of the magnetic field is very highly correlated with the position of this flow," Hill said. "From helioseismology, we see the flows for two prominent cycles ? Cycle 23, the cycle that we're coming out of, and Cycle 24, the cycle that we're in now."
It turns out that the flow appears well before the level that solar activity spikes. This led the researchers to conclude that there is some sort of triggering mechanism that appears before the onset of activity.
While observations of the solar jet could one day be useful for predicting the timing of the solar cycles, a larger data set is still required to ensure the method's accuracy.
"We're definitely going to need several cycles to improve the predictions," Hill said.
Further investigation will also be needed to determine whether the jet stream is a cause or effect of the solar cycle.
Our magnetic star
In yet another approach, Julia Saba of SP Systems and NASA's Goddard Space Flight Center in Greenbelt, Md., used X-ray and magnetic field strength indicators in order to predict the precise time mark for the onset of solar cycles.
Saba used magnetic maps of the sun, called synoptic charts, to observe solar cycles 21 through 23 and into 24. By evaluating trends in X-ray activity, Saba was able to predict the onset approximately 18 months ahead of time, and was accurate to within two months.
"By May of 2010, we see that cycle 24 is clearly underway, though things are still pretty quiet in the southern hemisphere in general," Saba said.
This method of determining a solar cycle's onset could be a valuable way to compare the different phases in solar activity because it can be observed in near real-time, Saba explained.
"It's a little easier to tell in real time than by solar maximum or solar minimum," she said.
While the four ways of monitoring solar activity take different approaches, the researchers are all in agreement that we are witnessing an interesting minimum. And while these methods could be useful for future studies of solar cycles, they all require further research.
"One problem we have with all solar cycle studies is the statistics of small numbers," Hathaway said. "Even with 23 sunspot cycles, it's not enough. What we've seen today are some newer measurements that weren't available even two cycles ago that are shedding new light. We need to be careful with using what we've seen from one or two cycles to make inferences for all of them."
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