Craft finds where sun’s corona gets its hots

New findings may help solve one of the biggest puzzles about the sun: Although its outer atmosphere, or corona, lies thousands of kilometers above the sun’s visible surface, this distant region is about 1,000 times hotter than the surface. It’s as if the air high above a candle were more blistering than the flame that heats it.

False-color image of the loops of hot gas that make up the sun’s outer atmosphere, or corona. White indicates the highest-intensity radiation, observed in ultraviolet light. Aschwanden et al./NASA

Scientists trying to explain this riddle haven’t lacked for theories, but they’ve been stymied by a scarcity of data to test them. Observations from a NASA satellite called TRACE (Transition Region and Coronal Explorer) reveal for the first time that the energy for the heating is dumped in at the corona’s base. The source of the heat remains uncertain.

The corona consists of millions of giant arches of gas—some high enough to span 30 Earths—that move along looping magnetic field lines. The TRACE measurements indicate that the heating occurs near the bottom of these arches, about 16,000 kilometers above the sun’s surface.

“The mysterious energy source that makes the sun’s atmosphere so incredibly hot has been an enigma for more than 70 years, and before we discover what it is, we needed to learn where it is,” notes Markus J. Aschwanden of the Lockheed Martin Advanced Technology Center in Palo Alto, Calif. He and his Lockheed colleagues, Carolus J. Schrijver, and David Alexander, describe their work in the Oct. 10 Astrophysical Journal.

A popular theory had assumed that the arches were heated throughout their height by waves generated by the turbulent ocean of gases at the solar surface. As these waves travel along the arches, they would gradually transfer their energy. If that theory were correct, the tops of the arches would be hottest, because the gas there is thinnest and can’t dissipate the heat it absorbs as quickly as can the denser gas at the corona’s base.

Instead, TRACE found that the temperature is uniform from the top of the corona to the bottom. That can only occur if some energy source is continually reheating the corona’s base, which leaks heat quickly to higher regions, the Lockheed team asserts.

“I’m hoping these observations will lead to a breakthrough,” says Spiro K. Antiochus of the Naval Research Laboratory in Washington, D.C. Any mechanism now proposed to heat the corona “had better be able to dissipate all of its energy within the first 10,000 km or so [of the corona],” he notes.

Earlier observations by the Solar and Heliospheric Observatory (SOHO) spacecraft suggest such a mechanism, several solar physicists told Science News. The craft found that tens of thousands of magnetic field bundles carpet the sun’s surface and loop up into the corona (SN: 11/08/97, p. 295). Each bundle is composed of pairs of oppositely directed field lines that merge and annihilate each other, releasing vast reserves of energy every 40 hours or so.

Like thousands of pilot lights, this energy could heat the corona’s base. The magnetic carpet “is the most likely source of the energy . . . although we still don’t know the mechanism of energy release,” says Lockheed’s Alan M. Title.

The TRACE findings may also help astronomers better predict the behavior of coronas surrounding other stars, says Aschwanden.

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