For 200 years scientists have known that they occasional disappear but no one was sure exactly why. A trio writing in Nature say they have solved the mystery and now can even predict the next time. Piet Martens, Dibyendu Nandi and Andres Munoz-Jaramillo say they have discovered why sunspots were missing from 2008 to 2010, which coincided with an extra-long "solar minimum" and unusually weak magnetic fields at the sun's poles. The fields are ordinarily much stronger when solar activity is minimal.
Sunspots normally go through 11-year cycles. Sometimes sunspots are so abundant that they cover one percent of the sun's surface. Sometimes they disappear. But the recent lull lasted twice as long as usual, an unusual occurrence that last happened around 1913. It happened at least once in the 19th century as well, in 1810. Understanding sunspots is important because solar activities influence space weather, which affects technology in space and on the Earth, Martens said.
Sunspots and solar flares shoot radiation and highly energetic particles toward Earth, which can interfere with airline travel, astronauts in space, sensitive equipment on satellites, short-wave communication and power grids, Martens explained. If scientists can predict space weather, people might minimize the damage by powering down satellites and diverting planes away from the North and South Poles, which are vulnerable to the extra radiation. Astronauts can hide behind lead shields.
Four days after a solar eruption takes place, its effects are felt on Earth, Martens said.
Recent solar activity. Credit:NASA
Madhulika Guhathakurta, lead scientist for NASA's Living with a Star program, which funds this MSU research, said that for decades, scientists focused on maximum solar activity. Predicting minimum solar activity is new, however, and might be even more significant.
An extended lack of solar activity allows the accumulation of space junk, for example, because the normal rate of orbital decay slows down and almost stops. Space junk can include dead satellites orbiting the Earth. A dearth of sunspots also makes space exploration more dangerous for humans – like those astronauts aboard the International Space Station -- because planners are uncertain about exposure to galactic cosmic rays. Galactic cosmic rays intensify when solar activity dies down. In 2009 during the extended period without sunspots, cosmic ray intensities increased 19 percent beyond anything seen in the past 50 years.
"It's easier to shield against energetic protons from solar flares than from high-energy nuclei that make up galactic cosmic rays," Guhathakurta said. "Astronauts prefer solar maximum."
Martens said solar physicists everywhere noticed that the sun was quiet for an unusually long time during the last sunspot cycle. They wondered how long it would last and and tried to model the phenomenon on their computers. Martens said he, Munoz-Jaramillo and Nandi succeeded because of Munoz-Jaramillo's work as an MSU graduate student and Nandi's leadership. Martens added that they were inspired by the prospect of being able to verify their results with data from the Solar Dynamics Observatory satellite, which was launched last year and involved MSU in a major way.
When Munoz-Jaramillo was earning his doctorate at MSU, he wrote a new computer program that made it possible to model the emergence of sunspots in ways that hadn't been done before, Martens said.
Nandi, a former assistant research professor at MSU who spent eight years in Bozeman, suggested an idea that was explored using Munoz-Jaramillo's program to study the lack of sunspots. With it, the scientists simulated 210 sunspot cycles and discovered the relationship between extended spotless periods and weak magnetic fields at the solar poles. They identified that variations in a hot plasma flow in the sun's interior, known as the meridional circulation, is the likely reason behind the lack of sunspots and a weak magnetic field at the sun's poles.
Martens and associate research professor David McKenzie at MSU helped design and calibrate four telescopes that were launched on the Solar Dynamics Observatory satellite. Munoz-Jaramillo wrote the programs that carry out simulations of solar magnetic fields that are featured on the SDO website at http://www.youtube.com/watch?v=2ifkdLpWYqA and will be tested with SDO observations. The SDO is the first mission in NASA's Living with a Star program and the most advanced spacecraft ever designed to study the sun.
Martens said he hopes he, Nandi and Munoz-Jaramillo – natives of three different countries -- will continue to collaborate on projects even though Nandi and Munoz-Jaramillo are no longer at MSU. Nandi has returned to his native India where he is an assistant professor at the new Indian Institute of Science Education and Research in Kolkata. Munoz-Jaramillo, a native of Colombia, defended his thesis in July 2010. He is now a visiting fellow at the Harvard-Smithsonian Center for Astrophysics where he studies solar and stellar magnetic cycles.
Nandi, Munoz-Jaramillo and Martens correspond regularly by e-mail, however, and Nandi continues to visit MSU in the summers. Martens and Munoz-Jaramillo both traveled to India in January, and they meet frequently at the Harvard-Smithsonian, as well.
Martens said he and Nandi plan to encourage the space agencies in their respective countries to study stars that are like the sun. Learning how magnetic cycles work on multiple stars will teach them more than they could learn by studying the sun only, Martens said. Scientists already know that most stars have magnetic fields, and their "starspot" cycles can last from three to 25 years.
Whether or not the three-way collaborations continue, Munoz-Jaramillo said that attending graduate school at MSU was probably the best decision he ever made. He commented by e-mail that MSU didn't just do something for his career. MSU defined his career.
"MSU has one of the best solar physics groups in the USA, which is at the same time one of the nicest in terms of the human quality of its members," Munoz-Jaramillo said.
"Additionally I had the great fortune of working with Piet and Dibyendu, who are the best advisers a graduate student can ask for," Munoz-Jaramillo continued. "They not only have been incredibly supportive of my ideas and generous with theirs, but they also involved me from the beginning in those activities, apart from research, that make up the life of a scientist: writing grant proposals, attending scientific conferences, assembling scientific collaborations, etc."