Solar flare triggers energetic sunquake.

Exploding a few thousand kilometers above the sun's surface, solar flares generate a tumult of X rays, beams of charged particles, and high-speed streams of ionized gas. A new study reveals that these eruptions can produce seismic waves akin to titanic earthquakes beneath the solar surface.

Although the finding comes from a study of only one flare, it suggests that these atmospheric explosions may offer a new way to probe the sun's interior.

Alexander G. Kosovichev of Stanford University and Valentina V. Zharkova of Glasgow University in Scotland examined a medium-energy solar flare that erupted on July 9, 1996. One minute after X-ray emissions from the flare had reached their peak, the Solar and Heliospheric Observatory (SOHO) found that a shock wave from the explosion had begun moving toward the sun. The impact of that shock--the first to be observed in association with a flare--helped generate the seismic waves that the SOHO craft detected about 20 minutes later, the researchers assert.

The waves, which resemble ripples created by a pebble cast into a pond, lasted for more than 35 minutes and were recorded by a SOHO instrument that measures the velocity of gas rising and falling at the sun's surface. Roughly 3 km in height, the waves spread about 100,000 km across the surface and penetrated an estimated 40,000 km into the sun, report Kosovichev and Zharkova in the May 28 Nature.

They also presented the findings at a meeting of the American Geophysical Union in Boston this week.

"There's no question about the [detection]--it's there," says astronomer George H. Fisher of the University of California, Berkeley. The puzzle, he adds, is how the seismic waves were triggered.

According to previous models, energetic electrons and protons created by a flare heat the sun's atmosphere. Some of that heat excites the Corona, or outer atmosphere of the sun, but a portion generates a shock wave that hammers the surface and produces a sunquake.

Kosovichev and Zharkova note, however, that the shock wave deduced from the SOHO data had too little energy and momentum to generate the seismic waves, which unleashed 40,000 times as much energy as the infamous 1906 San Francisco earthquake. Instead, they propose, electrons and protons from the flare struck the surface directly, contributing to the formation of the waves.

Alternatively, says Fisher, there may be some "new phenomenon associated with solar flares that we don't know about." He adds that not all flares may produce a sunquake. As the sun nears the peak of its 11-year activity cycle and eruptions become more frequent, researchers will have an opportunity to find out.

Kosovichev and Zharkova note that astronomers may want to take a closer look at other stars that have powerful flares. Such flares may trigger quakes big enough to be observed from Earth.

In a separate presentation at the geophysics meeting, Alan M. Title of the Stanford-Lockheed Institute for Space Research in Palo Alto, Calif., unveiled the latest ultraviolet images of the sun taken by the recently launched Transition Region and Coronal Explorer (TRACE) satellite. The images reveal that energy associated with magnetic fields in the sun's atmosphere is unleashed over periods of less than 20 seconds and distances of less than 700 km. Theorists will have to take these scales into account as they perfect models of how the sun's Corona attains its million-degree temperature.