Solar Mystery Solved

Japanese spacecraft finds conclusive evidence of waves that make sun's corona much hotter than its surface

The sun's corona—a hazy plasma that makes up the outermost portion of the sun's atmosphere and is filled with exotic ions such as iron(XIV)—should, by rights, be cooler than the fiery surface. Yet the corona's temperatures reach more than 1 million oC, whereas the sun's surface is only about 6,000 oC.

Researchers analyzing data from the Japanese Aerospace Exploration Agency's spacecraft Hinode (Sunrise), which has been orbiting the sun since last year, have found conclusive evidence for one explanation for this temperature mystery: "Alfvén waves," which are energy-transporting oscillations that can heat the corona to fantastic temperatures as they ripple along the lines of magnetic fields spreading out from the sun.

These subtle waves were predicted more than 50 years ago by Swedish Nobelist Hannes Alfvén and have been difficult to detect, although some evidence for their existence has been reported (Science 2007, 317, 1192).

In 10 articles on the Hinode mission results, international research teams report the observation of these oscillations in thin streams of matter that jet out from the sun (Science 2007, 318, 1572, 1574, 1577, 1580, 1582, 1585, 1588, 1591, 1594, and 1597). The researchers also report that these waves are strong enough to accelerate the blast of high-energy plasma, known as the solar wind, that bathes the solar system.

University of Sheffield solar physicists Robertus Erdélyi and Viktor Fedun note in an accompanying commentary that "these pioneering results have serious consequences for solar and stellar coronal heating theories."