Solar flare leaves sun quaking

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. They observed a flare-generated solar quake that
contained about 30 billion 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
tell-tale seismic signature in data on the Sun's surface collected by the
Michelson Doppler Imager onboard 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.

The solar quake that they recorded looks much like the ripples that spread
when a rock is dropped into a pool of water. Over the course of an hour,
however, the solar waves traveled for 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.

The waves contained enough energy to make the solar quake a 15 on the
Richter scale, a scale that measures the total energy released by an
earthquake and that, for practical reasons, is limited to magnitude 10 on
Earth. The amount of energy released was enough 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.

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 Administration.