Incredibly powerful waves of plasma rippling across the surface of the sun and dubbed "solar tsunamis" were first observed years ago, but were thought to be an optical illusion. Scientists have now confirmed, though, that they are really real.
When scientists first saw the phenomenon, it was hard to believe that a towering wave of hot plasma was actually racing along the sun's surface. One of the waves rose up higher than the diameter of Earth and rippled out from a central point in a circular pattern millions of miles wide, like a gargantuan pattern of waves created by a pebble dropped in a pond.
Skeptical observers suggested it might be a shadow of some kind — a trick of the eye. But new observations from NASA's STEREO (Solar Terrestrial Relations Observatory) spacecraft are telling researchers that this controversial phenomenon isn't an illusion.
This week, NASA released a remarkable video of a solar tsunami.
"Now we know," said Joe Gurman of the Solar Physics Lab at NASA's Goddard Space Flight Center in Greenbelt, Md. "Solar tsunamis are real."
The twin STEREO spacecraft confirmed their reality in images captured in February when sunspot 11012 unexpectedly erupted. The blast hurled a billion-ton cloud of gas (a coronal mass ejection, or "CME") into space and sent a tsunami racing along the sun's surface.
STEREO recorded the wave from two positions separated by 90 degrees, giving researchers an unprecedented view of the event.
"It was definitely a wave," said Spiros Patsourakos of George Mason University in Virginia and lead author of a paper reporting the finding in the Astrophysical Journal Letters. "Not a wave of water," he adds, "but a giant wave of hot plasma and magnetism."
The technical name is "fast-mode magnetohydrodynamical wave" – or "MHD wave" for short. The one STEREO saw reared up about 62,000 miles (100,000 km) high, and raced outward at 560,000 mph (250 km/s) packing as much energy as 2,400 megatons of TNT.
Solar tsunamis were discovered back in 1997 by the Solar and Heliospheric Observatory (SOHO). In May of that year, a CME blasted up from an active region on the sun's surface, and SOHO recorded a tsunami rippling away from the blast site.
"We wondered," Gurman recalled, "is that a wave – or just a shadow of the CME overhead?"
SOHO's single point of view was not enough to answer the question–neither for that first wave nor for many similar events recorded by SOHO in years that followed, until STEREO launched in 2006. The mission uses two spacecraft — one orbiting the sun ahead of the Earth, the other behind it — to get, literally, a stereo view of the sun.
"We've seen the waves reflected by coronal holes (magnetic holes in the sun's atmosphere)," Vourlidas said. "And there is a wonderful movie of a solar prominence oscillating after it gets hit by a wave. We call it the 'dancing prominence.'"
Solar tsunamis pose no direct threat to Earth. Nevertheless, they are important to study, scientists say.
"We can use them to diagnose conditions on the sun," Gurman said. "By watching how the waves propagate and bounce off things, we can gather information about the sun's lower atmosphere available in no other way."
"Tsunami waves can also improve our forecasting of space weather," Vourlidas added, "Like a bull-eye, they 'mark the spot' where an eruption takes place. Pinpointing the blast site can help us anticipate when a CME or radiation storm will reach Earth."