Peering deep into the sun's churning plasma, solar physicists have discovered a way to forecast the emergence of sunspots before they reach the solar surface.

Sunspots are dark patches marking magnetically active regions that often host solar flares and violent belches called coronal mass ejections. Trimming lead times on sunspot detection would allow for better forecasts of space weather—bursts of radiation and charged particles from the sun that can cause real problems on and around Earth. Bad spells of space weather can damage power grids, endanger satellites and aircraft, and pose radiation threats to astronauts in orbit.

A group of Stanford University researchers has found that they can identify the signature of sunspots forming at depths of 60,000 kilometers or so, a full day or two before the sunspots bloom on the surface. The researchers reported their findings in the August 19 issue of Science.

"This is the first time that we have detected sunspots before they appear on the solar disk," says Stathis Ilonidis, a Stanford graduate student who co-authored the new study with physicists Junwei Zhao and Alexander Kosovichev.

Ilonidis and his colleagues plumbed the sun's inner workings with a method called time–distance helioseismology. Working from archival data from the sun-orbiting SOHO spacecraft, the researchers measured how long it takes sound waves to travel from one point on the solar surface to another, some 100,000 to 200,000 kilometers away, along a refracted, or bent, path through the interior. In four locations where a sunspot was soon to emerge, acoustic waves returned to the solar surface with anomalous rapidity—more than a dozen seconds faster than normal. "At these distances, the travel time [through the sun's interior] is about one hour," Ilonidis says. "If there is a sunspot region along one of these paths, the travel time will be a little bit shorter." The emerging sunspot region seems to boost the local speed of sound, thereby hastening the refracted return of sound waves passing through that part of the solar interior.

The researchers also charted sound waves passing through nine quiet regions of the sun, where no sunspots were developing, and found no significant anomalies in the waves' travel time. By looking for sound waves propagating with anomalous speed, solar physicists going forward might be able to predict where sunspots are going to appear and provide longer lead-time forecasts back on Earth.

But Ilonidis acknowledges that more research is needed to test the predictive power of the new method. "What we need to do in the future is have more statistics, to look at more regions—both regions with sunspots and without sunspots—and check the statistics, whether we have false positives or false negatives," he says. "We need to find what is the success rate with our technique."

The strong signature of sunspots rising from the depths highlights how poorly understood is the sun's inner structure. The emerging sunspots speed up sound waves far more than had been expected; a recent analysis had predicted that sunspot regions would hasten the arrival time of sound waves by only one second or thereabouts. But Ilonidis and his colleagues found anomalies of 12 to 16 seconds for sound waves passing through a sunspot region. "That was a big surprise, because it's much higher than what we expected from the current theoretical models," he says.

What is more, the researchers found a sort of sweet spot for their soundings, a depth where the sunspot signal is strongest. But why emerging sunspots should reveal themselves clearly at one depth but not at others is a mystery. "We can detect sunspots at a depth of 60,000 kilometers, but if we try to detect them deeper or closer to the surface, the travel-time shift becomes weaker," Ilonidis says. "We don't understand why, only at a specific depth, the detection of sunspots is easier."

Solar physicist Philip Scherrer, who is also based at Stanford, notes that theoretical models of how sunspots appear are relatively successful at describing surficial solar activity but much less true to reality at depth. "So what happens deeper, how the dynamo is really generated from the interplay of rotation and convection and existing fields, isn't really understood," he says. "For all of these things you can tune up a model and make it work for a little while, but some basics are still missing."

Theoretical explanation or no, the new finding may soon afford airlines, satellite operators and other watchers of space weather advance warning of impending solar activity. "It will probably help us to make better forecasts for what the sun is going to do," Scherrer says. "Up until now we've waited until we see the magnetic field erupting."