Image courtesy of SwRI

Minimum and maximum. A 10-year prediction has the sun devoid of sunspots when it should be covered with them.

End of the Sunspot Cycle?

Dick writes about Earth and planetary science for Science magazine.

Things may be about to get very dull on the sun. Three different measurements of solar activity, reported by scientists at a press conference today, suggest that the next 11-year-long solar cycle will be far quieter than the current one. In fact, it may not happen at all: Sunspots, the enormous magnetic storms that erupt on the sun's surface as the cycle builds, might disappear entirely for the first time in approximately 400 years.

If the reported trends continue—a big if, other researchers note—a hibernating sun would have only a slight cooling effect on climate. But solar storms hurtling toward Earth that can disrupt satellites, power grids, and other electronics, would be much subdued, giving scientists a chance to study the sun in a phase unseen in modern times. For centuries, solar activity has been swinging from solar maximum (lots of dark sunspots, solar flares, and massive ejections of plasma, some aimed at Earth) to a far quieter solar minimum every 11 years or so. The current solar cycle, dubbed number 24 (it's the 24th solar cycle since 1755, when sunspot activity began being recorded), has just gotten off to a late, slow start in the past year as more sunspots appear.

At the press conference, held at the annual meeting of the Solar Physics Division of the American Astronomical Society in Las Cruces, New Mexico, three scientists gave a forecast of sorts for the next solar cycle, number 25. "Cycle 24 may be the last normal one for some time," said solar physicist Frank Hill of the National Solar Observatory (NSO) in Tucson, Arizona, "and the next one, cycle 25, may not happen. The solar cycle may be going into hiatus, like a TV show." Hill and colleagues reported on a jet-stream-like flow within the sun that they have been monitoring since 1995 using "helioseismology," the study of sun-wide oscillations of the solar surface. They expected the next cycle's jet to appear in 2008 or 2009, but it's still a no-show.

Another still-missing harbinger of the next solar cycle is the rapid march of magnetic activity toward the poles in the sun's very hot but faint gaseous corona high above the visible surface. Richard Altrock of NSO in Sunspot, New Mexico, showed a 40-year record that suggested this "rush to the poles" is far behind schedule in the current cycle. That might mean that this cycle will not clear the decks, magnetically speaking, to make room for the next cycle. In that case, "it's not clear what would happen" in the next cycle, Altrock said.

And Matthew Penn of NSO in Tucson and colleagues reported a trend in the intensity of the magnetic field of sunspots as gauged using a ground-based telescope during 13 years. The stronger a spot's magnetic field, the darker the spot. Below a certain field strength, a spot will fade away. Penn finds that the typical field strength of spots began declining in the past cycle and continues to decline in this cycle. Assuming the trend continues, the maximum of the current cycle would have half as many sunspots as the previous cycle did, and the next cycle would have no spots at all, he said.

Taken together, the scientists say, the three trends suggest that no visible solar cycle will begin at the next expected start time, around 2020. Such a gap last happened during the Maunder Minimum 400 years ago. But other researchers are cool to the idea. Solar physicist Mausumi Dikpati of the National Center for Atmospheric Research in Boulder, Colorado, notes that success forecasting solar activity a few years out has been modest at best; forecasting a decade or two out would be even trickier. "The data is very limited as yet, only one or two cycles," she says, making prediction difficult.

Dikpati and space physicist Yi-Ming Wang of the Naval Research Laboratory (NRL) in Washington, D.C., also interpret some of the physics underlying the three observed trends differently from the three forecasters. In their alternative interpretations, the trends are of little help in forecasting. All in all, writes space physicist Judith Lean of NRL in an e-mail, the understanding of the sun's behavior "is so uncertain that projections far into the future are more or less speculation."

Posted in Space, Physics