If you thought the exterior of the sun was hot, check out its corona. Although our star's visible surface is less than 6000°C, its atmosphere blazes at up to 4 million°C. Now, thanks to a telescope briefly boosted high above Earth by a small rocket, astronomers have found evidence of long-theorized magnetic processes that could help explain these blistering temperatures.
One reason the sun's corona is so much hotter than its surface is comes down to a phenomenon known as wave heating, in which sound waves originating at the surface travel up through the corona. There, friction among the particles boosts the temperature, says Jonathan Cirtain, a solar physicist at NASA's Marshall Space Flight Center in Huntsville, Alabama. But wave heating provides only enough energy to heat the corona to about 1.5 million°C, he notes. And although most solar physicists agree that interactions among the sun's magnetic field lines play a role in heating the corona, those processes haven't been observed directly.
Now, Cirtain and his colleagues have gathered strong evidence to support the notion of a different kind of heating: magnetic heating. The team's 3.2-meter-long, 210-kilogram camera-and-telescope combo rode a rocket into space above southern New Mexico on 11 July last year, observing the sun in a narrow band of extreme ultraviolet wavelengths that is normally blocked by Earth's atmosphere. The researchers targeted that band because one particular type of atom emits radiation in that range—an iron atom in which 11 of its normal complement of 26 electrons have been stripped away, also known as Fe XII. Although those highly charged ions exist most often at a temperature of about 1.5 million°C, data simultaneously gathered from other Earth-orbiting instruments provide information about material at even higher temperatures, Cirtain says.
The team's telescope could spot features in the solar atmosphere that were only 150 kilometers across—the same resolution as being able to discern a dime at a distance of 6 km, and a sixfold improvement over instruments now orbiting Earth. And although the telescope stayed above our planet's atmosphere for only 5 minutes and was focused on only 3% of the sun's surface, it observed two different episodes where magnetic field lines strongly interacted to produce Fe XII in large quantities. In those episodes, the bending and flexing of magnetic field lines released enough energy to boost temperatures nearby to as much as 7 million°C. Such events, which likely occur on a near-continual basis in magnetically active regions of the sun's surface, provide a substantial source of heating to the whole corona, the researchers report online today in Nature.
"The new findings are a tantalizing glimpse of what is possible with such an instrument," says Peter Cargill, a solar physicist at Imperial College London. Although the small-scale details of what's going on in the solar corona are still unclear, the new findings provide data that researchers can use to test old hypotheses and develop new ones, he notes.
The team's images provide "stunning detail, but only for a few minutes," adds Karel Schrijver, a solar physicist at Lockheed Martin Advanced Technology Center in Palo Alto, California. Now that researchers have proven that such observations are possible, he notes, a next step might be to develop a similar instrument to orbit Earth full-time and gather data to complement the low-resolution images of the entire sun being taken by other sensors.
Cargill emphatically agrees: "It is insane not to fly such an instrument on any future mission to look at the solar corona."