Making waves: Flare leaves Sun quaking
BY DAVID F. SALISBURY
Scientists have shown for the first time that solar flares produce seismic waves in the Sun's interior that closely resemble those created by terrestrial earthquakes. The researchers observed a flare-generated solar quake that contained about 40,000 times the energy released in the great earthquake that devastated San Francisco in 1906.
Alexander G. Kosovichev, a senior research scientist from Stanford University, and Valentina V. Zharkova from Glasgow University found the telltale seismic signature in data on the Sun's surface collected by the Michelson Doppler Imager on board the European Space Agency/NASA Solar and Heliospheric Observatory (SOHO) spacecraft immediately following a moderate-sized flare on July 9, 1996.
The finding is reported in the May 28 issue of the journal Nature, and will be the subject of a 9 a.m. press conference at the Spring meeting of the American Geophysical Union in Boston on May 27.
The solar quake that the scientists recorded looks much like ripples spreading from a rock dropped into a pool of water. But the solar waves were nearly two miles high and, over the course of an hour, traveled a distance equal to 10 Earth diameters before fading into the fiery background of the Sun's photosphere. Unlike water ripples that travel outward at a constant velocity, the solar waves accelerated from an initial speed of 22,000 miles per hour to a maximum of 250,000 miles per hour before disappearing.
It would take a magnitude 11.3 quake on Earth to unleash an amount of energy equivalent to that released by the solar quake. That is enough energy to power the United States for 20 years at its current level.
"People have looked for evidence of seismic waves from flares before, but they didn't have a theory so they didn't know where to look," says Kosovichev.
The intense, bright region in this X-ray image of the Sun shows a moderately sized solar flare that occurred July 9, 1996.
Several years ago he and Zharkova developed a theory that can explain how a flare, which explodes in space above the Sun's surface, can generate a major seismic wave in the Sun's interior. According to the currently accepted model of solar flares, the primary explosion creates high-energy electrons. These are a funneled down into a magnetic flux tube, an invisible tube of magnetic energy, and produce X-rays, microwaves and shock waves that heat the solar surface. Kosovichev and Zharkova developed a theory that predicts the nature and magnitude of the shock waves that this beam of energetic electrons should create when they slam down into the solar atmosphere.
Although their theory directed them to the right area to search for the seismic waves, the waves that they found were 10 times stronger than they had predicted. "They were so strong that you can see them in the raw data," Kosovichev says.
The solar seismic waves appear to be compression waves like the "P" waves generated by an earthquake. They travel throughout the Sun's interior. In fact, the waves should recombine on the opposite side of the Sun from the location of the flare to create a faint duplicate of the original ripple pattern, Kosovichev predicts.
Now that they know how to find them, the SOHO scientists say that the seismic waves generated by solar flares should allow them to verify independently some of the conditions in the solar interior that they have inferred from studying the pattern of waves that are continually ruffling the Sun's surface.
SOHO is a joint project of the
European Space Agency and the National Aeronautics and Space