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Gamma ray image
A superposition of RHESSI images of gamma-ray and X-ray emissions with a TRACE satellite extreme ultraviolet image taken 90 minutes later of the July 23, 2002, solar flare. The superposition clearly shows the large separation between the high-energy emissions. Solar physicists expected to see X-rays and gamma rays emerging from the same spots at the base of the flare loops. (Credit: RHESSI and TRACE)

RHESSI satellite offers clues about how solar explosions act as particle accelerators

– The first gamma-ray image of a massive explosion on the sun called a solar flare held a big surprise for solar physicists and has overturned a long-standing assumption about the way flares accelerate particles.

The discovery "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 Craig DeForest, a solar researcher at the South West Research Inst. in Boulder, Colo.

NASA solar flare animation
RHESSI observations overlay a black-and-white movie of July 23, 2002 solar flare.
NASA has used images captured by RHESSI to post animations of solar flare activity

A team of researchers used NASA's orbiting Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), built and operated by the University of California, Berkeley, to take pictures of a large solar flare on July 23, 2002. While RHESSI had been snapping X-ray pictures of flares since the satellite's launch in February 2002, this was the first flare large enough to allow images to be made from high-energy gamma-ray emissions.

According to Robert Lin, professor of physics at UC Berkeley and principal investigator for RHESSI, scientists had assumed that gamma rays, produced by ionized particles slamming into denser gas on the sun, originate in the same place as X-rays, which are produced in a similar way by high-speed electrons. Both should issue from the feet of the glowing loops that arc hundreds of thousands of miles across the face of the sun and accelerate ions and electrons to very high speeds before they crash into surrounding gas.

The observations showed, however, that gamma rays come from a spot far from the source of X-rays, separated by about the diameter of the Earth. Apparently, the sun's magnetic processes are able to separate the electrons from the ions, sorting them either by mass or electric charge, as they blast them to almost the speed of light. The gamma rays seem to be coming from the feet of the large magnetic field loops, while the X-rays come from the feet of the small magnetic loops nested inside the larger ones.

"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 Lin, who is director of UC Berkeley's Space Sciences Laboratory and co-author of many of the 16 papers analyzing RHESSI data scheduled to appear in the Oct. 1 print issue of Astrophysical Journal Letters. The papers will appear online during the week of Sept. 8.

Solar flares are among the largest explosions in the solar system, releasing as much energy as a billion one-megaton nuclear bombs. They are often closely associated with fast coronal mass ejections (CMEs), which propel energetic particles into space that impinge on the Earth's magnetic field, igniting the auroras and generating magnetic storms that can interfere with radio communications and satellites.

Flares arise in the solar atmosphere, which is a gas of electrically charged particles - a plasma - comprised of negatively charged electrons and positively charged ions. Because these particles "feel" magnetic forces, they are forced to flow along magnetic field lines that permeate the sun's atmosphere. Solar flares happen when magnetic fields in the sun's atmosphere become twisted and suddenly break, like an overstretched rubber band. Unlike a rubber band, though, the magnetic field lines reconnect to form a new configuration. This is called magnetic reconnection.

Up until now, scientists have believed that particles in the solar atmosphere accelerate when they are flung by the magnetic field as it snaps to a new shape, like a stone in a slingshot. However, if it were this simple, all the particles would be shot in the same direction, Lin said. The new observations from RHESSI show that this is not so: the heavier ions end up in a different location than the lighter electrons.

There are many possible mechanisms by which flares could sort particles by mass, according to the team. The particles could be sorted by their electric charge, since ions are positively charged and electrons negatively charged. If this is so, an electric field would have to be generated in the flare, since particles move in different directions in an electric field according to their charge. In either case, magnetic reconnection provides the energy, though the acceleration process is more complex.

RHESSI's images of the July 23 flare showed two X-ray emitting regions, one at each foot of the flare's inner loop, or arch, as expected. However, the satellite detected only a diffuse gamma-ray glow about 15,000 kilometers (approximately 9,300 miles) south of the X-ray sites, around the foot of a larger loop arcing over the inner one.

The RHESSI observation may also upset theories about how solar flares create and destroy antimatter, rare counterparts of normal matter that usually last only a short time before they encounter normal matter and together annihilate one another. On Earth, antimatter is seen only in the interactions inside particle accelerators and cyclotrons.

The July 23 flare generated half a kilogram - about one pound - of antimatter, enough to power the entire United States for two days. While it is known that large flares are antimatter factories, the RHESSI images and data indicate that antimatter generated in the July 23 flare was not destroyed where expected.

Antimatter is generated by the fast-moving particles from a flare when they collide with slower particles in the sun's atmosphere. Flare theory says these collisions happen in denser 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 its production site, since there are so many particles of ordinary matter to run into. However, RHESSI data indicate 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.

"Antimatter shouldn't get far," said Gerald Share, a scientist at the Naval Research Laboratory in Washington, D.C., who is lead author of a paper on RHESSI's observations of antimatter destruction in the July 23 flare. Scientists do not yet have an explanation for the surprising finding.

For a complete list of the articles to appear in Astrophysical Journal Letters, link to http://sprg.ssl.berkeley.edu/~hhudson/rhessi/.