A 'sunquake' is caused by enormous amounts of energy released by an exploding solar flare

ATLANTA (CNN) -- A tremor recently exploded and rippled for thousands of miles, but no one on earth felt the slightest vibration.

The reason is because the tremor occurred on the surface of the sun. A solar flare spewing a huge bubble of superheated gas accompanied the so-called "sunquake," which at an unheard-of magnitude of 11.3 was 40 times stronger than the devastating San Francisco earthquake in 1906.

"(It's like spreading) dynamite over the surface of the earth about a yard thick over all the land masses of the earth and then setting it off all at the same time, that would be about the same as the energy released in the flare," said Craig DeForest, a solar research scientist at Stanford University.

A 'sunquake'
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DeForest said the solar flare came at the sun with such force that it caused waves two miles high on the sun's surface which spread out for tens of thousands of miles before gradually dropping off in size.

The quake caused enough energy to power the United States for 20 years, based on current needs.

Experts hope this first-ever observation and its research will help predict solar flares, which can affect electronic equipment on Earth.

"We've seen ripples spreading out across the surface of the sun in response to an explosion or solar flare up in the corona, but this is the first time people have seen how the surface of the sun reacts to the much more tenuous gases higher up," DeForest said.

Alexander Kosovichev, a senior research scientist from Stanford University in Palo Alto, California, and Valentina Zharkova of Glasgow University in Scotland, found signs of the sunquake in data collected by the SOHO spacecraft during a solar flare on July 9, 1996.

A solar flare is an enormous explosion of hydrogen and helium above the sun's surface. The July 9 flare was considered fairly moderate.

Twenty minutes after the flare, ripples appeared on the sun's surface, looking like ripples in a pool of water. In one hour, sound waves from the quake traveled 74,567 miles, a distance equivalent to 10 Earth diameters.

Unlike watery ripples that move at a constant speed, the waves sped up, from 22,000 mph to 250,000 mph, before disappearing.

Image from an animation of the Solar and Heliospheric Observatory (SOHO) spacecraft, a joint European Space Agency/NASA project

Kosovichev and Zharkova developed a theory several years ago to explain how a flare at the surface can produce a quake in the sun's interior. The sound waves they detected were 10 times stronger than they predicted.

Solar flares create a massive electromagnetic explosion in the upper solar atmosphere. The heat from the explosion generates a shock wave. When that shock wave hits the lower atmosphere or so-called surface of the sun 1,200 miles to 1,800 miles below, it creates the sound waves, Kosovichev said.

"What's remarkable about this in my view is that it's some manifestation of the flare occurring much lower in the atmosphere than people had ever thought," said George Fisher, a senior fellow at the Space Sciences Laboratory at the University of California, Berkeley.

Fisher said the sunquake will provide scientists with clues about how solar flares release energy and "will help us to predict them better."

Prediction is important because strong solar flares can disrupt radio and telephone communications on Earth, cause power surges and blackouts and damage satellites.

Related stories:

• Satellite sheds light on solar storms - December 9, 1997
• Solar storms could put a jolt into earthly affairs - November 6, 1997
• Giant rivers of plasma seen on the sun - August 28, 1997
• SOHO gives scientists a better look at the sun - July 31, 1997
• Solar flare heading toward Earth - April 8, 1997

Related sites:

• NASA Homepage
  • SOHO: The Solar and Heliospheric Observatory
• European Space Agency
  • SOHO