Alfvén waves may heat the Sun's corona

The closer an object is to its heat source, the hotter it gets—except in the case of the Sun’s corona. Somehow, energy makes the few-thousand-kilometer ascent from the Sun’s 10,000-K photosphere to the corona’s outskirts, raising the temperature to 3 million K. Alfvén waves seem like the ideal energy conveyor. They don’t bend backward as they propagate through regions of varying density and temperature. And the source of their restoring force, the solar magnetic field, extends from the photosphere up into the corona. But until now, no one had seen evidence of the waves in the first leg of their upward journey. David Jess of the Queen’s University Belfast in Northern Ireland and his collaborators observed the Sun with the Swedish Solar Telescope (SST) on La Palma, one of the Canary Islands. They looked at the center of the solar disk in a narrow waveband centered on the hydrogen-α absorption line, which originates from the 1000-km-thick region just above the photosphere. One area, measuring 430 000 km² and shown here, featured bright spots that usually indicate magnetic activity. There, Jess found the telltale signature of an Alfvén wave: a torsional twisting back and forth perpendicular to the propagation. The wave’s speed (20 000 km/s), amplitude (2.6 km/s), and period (400 s) are consistent with theoretical predictions. They also explain the past elusiveness of Alfvén waves: Other telescopes lack the SST’s combination of spatial and temporal resolution. If waves like this one do carry their energy into the corona and are numerous, Alfvén waves may indeed heat the corona. (D. B. Jess et al., Science 323, 1582, 2009.) — Charles Day