Burning questions. Recent gamma ray observations raise new questions about solar flares.

Solar Flare Puzzler

DENVER--When the sun belches, scientists listen--and watch. At a meeting of the American Physical Society here this week, a satellite with gamma ray eyes that watch solar flares erupt in extraordinary detail presented a new picture of flares that is leaving solar physicists sunstruck.

Most scientists believe that solar flares, huge explosions on the sun, occur when the sun's magnetic field lines snap and then reconnect. The magnetic fields accelerate charged particles--electrons and ions--in the solar atmosphere and slam them back into the surface of the sun. The process sends torrents of gamma rays and x-rays shooting out into space. The 2-year-old Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite is designed to spot those gamma and x-rays, which reveal how charged particles in a flare behave.

In 2002, RHESSI glimpsed a solar flare on the east part of the sun--the first gamma ray picture of a solar flare in action. What the satellite saw, though, was unexpected, says Robert Lin, principal investigator of the RHESSI mission. "It was thought that both ions and electrons were accelerated at the same time and should be in the same place," he says, but the electrons' gamma rays and the ions' gamma rays came from spots on the sun thousands of kilometers apart. "That was a big surprise to us."Enormous solar flares that erupted in October and November 2003 seem to show the same trend. "There's a suggestion that [ions and electrons] are not coincident," Lin announced at the meeting. "The jury's really out; we don't know why they end up in different places."Even though the reasons for the separation are still obscure, the observations are likely to help physicists unravel what's going on in a solar flare. "That's interesting," says Gerald Share, a physicist at the Naval Research Laboratory in Washington, D.C. "Maybe the ions are traveling down a different magnetic loop than the electrons [are]." Share joined the RHESSI team 7 months ago to work on slightly different aspects of the gamma ray data--and he's extremely excited about what the satellite's observations will reveal about how solar flares work.Lin shares Share's enthusiasm. "We'd like to understand how the sun releases its energy," he says. "We're just getting gamma ray measurements that bear on this."Related site
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