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Flares Touch Off Sunquakes

Quakes constantly rock the sun, triggered as convecting gas patches rise and shake the solar surface. Now astronomers report in tomorrow's issue of Nature that huge flares of x-rays can also trigger solar quakes. "This is the first time that a flare is related to a seismic event on the solar surface," says Valentina Zharkova of Glasgow University in the United Kingdom. The solar flares could provide a new tool for helioseismologists, who study the sun's interior via quakes.

A solar flare, caused by a disturbance in the sun's magnetic field, accelerates electrons from the solar corona down into the sun's lower atmosphere, where they give off x-rays. In 1995, astronomers Zharkova and Alexander Kosovichev of Stanford University calculated that the impact of the electrons would also cause the gas to expand suddenly as a shock wave. When the shock wave hits the solar surface, they proposed, it could create seismic waves.

Now Zharkova and Kosovichev have caught one of these waves emanating from a flare. In 1996 they observed a large x-ray flare that lasted a few minutes using an x-ray detector aboard the Compton Gamma Ray Observatory satellite. At the same time, they monitored the sun from the orbiting Solar and Heliospheric Observatory (SOHO) using the Michelson-Doppler Imager, an instrument that watches for tiny shifts in wavelength due to the movements of the solar surface. The data indicated strong upward and downward motions--up to 3 kilometers--of gas over an area 3000 to 5000 kilometers wide around the flare site. "We were surprised; the phenomenon was stronger than we expected," says Kosovichev. For a few hours, the waves propagate outward over the solar surface at a speed of up to 100 kilometers per second, "like ripples from a pebble thrown in a pond," says Zharkova.

SOHO's ultraviolet (UV) imager has also revealed possible ripples, says Allan Gabriel of France's Institute of Space Astrophysics near Paris. However, these images did not reveal motion; the processes producing changes in UV intensity are too complex, he says. The new Doppler observations show that the flash and the shake are linked. "This is quite exciting," Gabriel says.