For all its fire, the visible surface of the sun is a relatively cool 6000 degrees Kelvin. The real heat rages in the sun’s outer atmosphere, or corona, where temperatures soar beyond a million degrees. The corona’s intense heat is one of the sun’s enduring mysteries, but in the 16 May PRL, solar physicists propose a first step to explaining it. Their theory, backed up by observations from two satellites, explains the dramatic and puzzling rise in temperature that occurs across a narrow layer just below the corona. According to the theory, cascades of shock waves are catapulted upward through the sun’s tenuous gas, launched by magnetic fields that bend and snap like slingshots.
In the late 1990s, researchers using the Solar and Heliospheric Observatory (SOHO) satellite identified a network of magnetic field lines lacing most of the sun’s visible surface, called the photosphere. The lines have both ends rooted below the surface, forming a dense “shag carpet” of magnetic loops. These loops constantly appear, merge explosively, and vanish. Solar physicists suspect the mergers somehow heat the base of the corona high above, but it hasn’t been clear how they propel so much energy through the solar transition region–the thin layer under the corona where temperatures jump by a factor of 100.
Now observations with SOHO and the Transition Region and Coronal Explorer (TRACE), another solar satellite, may have solved the puzzle. The satellites stared at the same spot on the sun for 2.2 hours. Instruments recorded magnetic fields at the surface and pulses of light from three different layers overhead. For instance, short bursts of ultraviolet light from oxygen atoms temporarily stripped of five electrons indicated points where the transition region was briefly heated to 300,000 degrees Kelvin. The combined data allowed Margarita Ryutova of the Lawrence Livermore National Laboratory in Livermore, California and Theodore Tarbell of the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, California, to determine how heat rifled upward toward the corona.
The patterns expose what Ryutova calls “an avalanche of shocks” within the transition region–shocks that start at the photosphere. When two magnetic field loops of opposite polarity approach and merge, they can form a pair of side-by-side arcs, like McDonald’s Golden Arches. But strong tension in the field lines quickly snaps out the kink in the center of the “M,” flinging plasma upward at supersonic speed. The shock waves from the catapulted plasma travel up to the transition region, spreading and distorting as they go.
With the relentless pace of magnetic mergers, the transition region is awash in shock waves, which continually crash into one another, Ryutova says. According to the team’s model, the transition region heating comes from shock wave collisions, which concentrate energy and can blast more hot plasma up to the base of the corona. Ryutova and Tarbell believe the collisions explain heat flashes and jets of plasma picked up by the satellites high in the transition region.
The scenario is plausible, says solar physicist Barry LaBonte of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “It’s an interesting perspective, and it’s a more complete and quantitative picture [of solar energy transfer] than we had before,” he says. More simultaneous studies of the sun at multiple wavelengths would test the model thoroughly, LaBonte notes.