SPOT OF BOTHER The same complicated sunspot spat out seven flares from September 4 to 10. This image shows in ultraviolet light the second-largest flare (bright flash at right), which was emitted on September 10.
A series of rapid-fire solar flares is providing the first chance to test a new theory of why the sun releases its biggest outbursts when its activity is ramping down. Migrating bands of magnetism that meet at the sun's equator may cause the biggest flares, even as the sun is going to sleep.

A single complex sunspot called Active Region 2673 emitted seven bright flares - powerful bursts of radiation triggered by magnetic activity - from September 4 to September 10. Four were X-class solar flares, the most intense kind. The strongest, released at 8:02 a.m. EDT on September 6, was an X9.3. The most powerful flare since 2006 (and the eighth largest since monitoring started in June 1996), it disrupted shortwave radio communication over Africa for up to an hour. It also flung a blob of energetic plasma, called a coronal mass ejection, speeding toward Earth, which sparked auroras the night of September 7 that were visible as far south as Arkansas.

All that activity is counterintuitive, as the sun is near the end of an unusually weak solar cycle, which began in 2008 (SN: 11/2/13, p. 22). The sun's magnetic activity waxes and wanes roughly every 11 years, generating more dark sunspots at the peak of the cycle and fewer at the trough.

DOUBLE WHAMMY This animation shows two bright flares that the sun emitted September 6, as observed by NASA’s Solar Dynamics Observatory in ultraviolet light. The second is the strongest solar flare since 2006.
"This cycle's a wiener," says solar physicist Scott McIntosh, director of the High Altitude Observatory at the National Center for Atmospheric Research in Boulder, Colo. When the cycle peaked in 2013, it was already looking like the weakest in a century.

Solar physicists realized in the 1960s that the peak flare rate comes a few years after the sunspot maximum. Even stranger, the strongest flares tend to occur on the cycle's downslope. The quietest cycles may even produce the biggest flares. The biggest solar storm in recorded history, called the Carrington event, occurred at the end of another especially weak cycle in early September 1859. Modern simulations estimate that flare may have been an X45.

"When you're descending to a quiescent phase of the cycle and things are getting more organized and simplified, how is it we are getting things this complex?" asks solar physicist Madhulika Guhathakurta, spokesperson for NASA's Heliophysics Division. "It still remains an interesting question."

McIntosh has an idea why. In a series of papers, including a 2015 paper in Nature Communications, he and colleagues argued that complex sunspots like AR 2673 and their forceful flares are the result of opposing bands of magnetism vying for supremacy.

These bands are like magnetic jet streams, McIntosh says. But unlike jet streams on Earth which generally stay anchored at certain latitudes, the bands migrate over the course of the solar cycle. They begin closer to the sun's poles, about 55° N and 55° S. Over time, the bands move toward the equator, possibly by pulling on each other with tremendous magnetic force.

The northern band and the southern band twist in opposite directions, so when they finally meet at the equator, it's chaos. Their magnetic lines tangle and twist. McIntosh thinks those warring bands may create more complicated sunspots, called deltas, which appear as a mottled mess of light and dark representing different magnetic poles. McIntosh could tell that AR 2673 was a delta as soon as he saw it. These deltas "comprise about 5 percent of the total number of sunspots, but contribute almost 100 percent of the trouble," he says.

Over the next year and a half or so, the bands will cancel each other out completely, marking the end of the solar cycle. "This is their last hurrah," McIntosh says.

During weak solar cycles, McIntosh and colleagues suggest, this process takes longer. That lets the bands spend more time wrestling with each other and merging, creating complex spots that build up massive amounts of energy to release in flares.

Monitoring the sun for more deltas and large flares will help test if the theory is right. "If this delta is joined by further deltas before the end of the cycle, that's more empirical proof that our idea of band interaction really is steering the game," he says.

Proving the idea will require a better understanding of what's going on deep in the interior of the sun, which computer simulations are not up to yet, Guhathakurta says. But she likes McIntosh's approach of merging observations and theory. Predicting the most powerful flares is important for protecting communications as well as satellites and power grids from the surge in energetic particles flares can cause.

"If we can actually figure this out, really associate it to something in the solar cycle, then it certainly helps us with long-term forecasting," she says.