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Antimatter factory on Sun yields clues to solar explosions

September 4, 2003

The best look yet at how a solar explosion becomes an antimatter factory gave unexpected insights into how the tremendous explosions work. The observation may upset theories about how the explosions, called solar flares, create and destroy antimatter. It also gave surprising details about how they blast subatomic particles to almost the speed of light.

Solar flares are among the most powerful explosions in the solar system; the largest can release as much energy as a billion one-megaton nuclear bombs. A team of researchers used NASA's Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft to take pictures of a solar flare on July 23, 2002, using the flare's high-energy X-rays and gamma radiation.

"We are taking pictures of flares in an entirely new color, one invisible to the human eye, so we expect surprises, and RHESSI gave us a couple already," said Dr. Robert Lin, a faculty member in the Dept. of Physics, University of California, Berkeley, who is the Principal Investigator for RHESSI.

Gamma-rays and X-rays are the most energetic forms of light, with a particle of gamma ray light at the top of the scale carrying millions to billions of times more energy than a particle of visible light. The results are part of a series of papers about the RHESSI observation to be published in Astrophysical Journal Letters October 1.

Antimatter annihilates normal matter in a burst of energy, inspiring science fiction writers to use it as a supremely powerful source to propel starships. Current technology only creates minute quantities, usually in miles-long machines employed to smash atoms together, but scientists discovered the July 2002 flare created a half-kilo (about one pound) of antimatter, enough to power the entire United States for two days. According to the RHESSI images and data, this antimatter was not destroyed where expected.

Antimatter is rare in the present-day universe. However, it can be created in high-speed collisions between particles of ordinary matter, when some of the energy from the collision goes into the production of antimatter. Antimatter is created in flares when the fast-moving particles accelerated during the flare collide with slower particles in the Sun's atmosphere.

According to flare theory, these collisions happen in relatively dense regions of the solar atmosphere, because many collisions are required to produce significant amounts of antimatter. Scientists expected that the antimatter would be annihilated near the same places, since there are so many particles of ordinary matter to run into. "Antimatter shouldn't get far," said Dr. Gerald Share of the Naval Research Laboratory, Washington, D.C., lead author of a paper on RHESSI's observations of the antimatter destruction in the July 23 flare.

However, in a cosmic version of the shell game, it appears that this flare might have shuffled antimatter around, producing it in one location and destroying it in another. RHESSI allowed the most detailed analysis to date of the gamma rays emitted when antimatter annihilates ordinary matter in the solar atmosphere. The analysis indicates that the flare's antimatter might have been destroyed in regions where high temperatures made the particle density 1,000 times lower than where the antimatter should have been created.

Alternatively, perhaps there is no "shell game" at all, and flares are able to create significant amounts of antimatter in less dense regions, or flares somehow may be able to maintain dense regions despite high temperatures, or the antimatter was created "on the run" at high speeds, and the high-speed creation gave the appearance of a high-temperature region, according to the team.

Solar flares are also capable of blasting electrically charged particles in the Sun's atmosphere (electrons and ions) to almost the speed of light (about 186,000 miles per second or 300,000 km/sec.). The new RHESSI observation revealed that solar flares somehow sort particles, either by their masses or their electric charge, as they propel them to ultra-high speeds.

"The result is as surprising as gold miners blasting a cliff face and discovering that the explosion threw all the dirt in one direction and all the gold in another direction," said Dr. Craig DeForest, a solar researcher at the South West Research Inst. Boulder, Colo. The means by which flares sort particles by mass is unknown; there are many possible mechanisms, according to the team. Alternatively, the particles could be sorted by their electric charge, since ions are positively charged and electrons negatively charged. For images, animation, and the complete article, refer to:

http://www.gsfc.nasa.gov/topstory/2003/0903rhessi.html

NASA Goddard Space Flight Center