A solar tsunami can be seen as a dark wave spreading across the surface of the Sun (small sphere on left). The greyed-out band has been enhanced for contrast. The green shows the solar flare or CME that has caused the tsunami.
Credit: NASA
SYDNEY: Observations from NASA's STEREO space probes have confirmed that vast 'solar tsunamis', taller than the Earth itself, ripple across the Sun for millions of kilometres.
The technical name is 'fast-mode magneto -hydrodynamical wave (MHD)'. The one the STEREO probes recorded reared up to 100,000 km in height, and raced outward at 900 km/h packing as much energy as 2,400 megatons of TNT.
The findings are reported in the Astrophysical Journal Letters. See a video here of a solar tsunami as seen from different angles by the STEREO spacecraft.
Waves of plasma
In the 1990s when astronomers first witnessed a towering wave of hot plasma racing along the Sun's surface, they were perplexed. The scale was staggering; it rose up higher than Earth itself and rippled out from a central point in a circular pattern millions of kilometres in circumference.
Sceptical observers suggested it might be a shadow of some kind - a trick of the eye - but surely not a real wave. "Now we know... solar tsunamis are real," said Joe Gurman of the Solar Physics Lab at the U.S. space agency's Goddard Space Flight Centre in Greenbelt, Maryland.
The twin STEREO spacecraft confirmed the existence of solar tsunamis in February 2009 when a sunspot 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°, giving researchers an unprecedented view of the event.
"Definitely a wave"
"It was definitely a wave," said Spiros Patsourakos of George Mason University in Fairfax, Virginia, and lead author of the paper reporting the finding. "Not a wave of water... a giant wave of hot plasma and magnetism."
Solar tsunamis were discovered back in 1997 by NASA's Solar and Heliospheric Observatory (SOHO). In May of that year, a CME came blasting up from an active region on the Sun's surface, and SOHO recorded a tsunami rippling away from the blast site. "We wondered," recalled Gurman, "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.
The question remained open until after the launch of STEREO in 2006. At the time of the February 2009 eruption, STEREO-B was directly over the blast site while STEREO-A was stationed at right angles - "perfect geometry for cracking the mystery," said co-author Angelos Vourlidas of the Naval Research Lab in Washington DC.
The physical nature of the waves has been further confirmed by movies of the waves crashing into things. "We've seen the waves reflected by coronal holes," said Vourlidas. "And there is a wonderful movie of a solar prominence oscillating after it gets hit by a wave. We call it the 'dancing prominence.'"
No threat to Earth
Solar tsunamis pose no direct threat to Earth. Nevertheless, they are important to study. "We can use them to diagnose conditions on the Sun," said Gurman. "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," added Vourlidas, "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."
Follow Cosmos on Twitter!
twitter.com/cosmosmagazine
with Science@NASA.
