News in Science

Sun's hot mystery solved

Magnetic magic One of the Sun's greatest mysteries has been solved: why it gets hotter, rather than cooler, further away from its surface.

The Sun's visible surface, the photosphere, has a temperature of about 6000°C, which increases dramatically to millions of degrees in the Sun's upper atmosphere or corona.

Braided magnetic streams of superheated plasma are the most likely source for the Sun's 6-million degree coronal temperatures, say a team scientists led by Dr Jonathan Cirtain from NASA's Marshall Space Flight Centre.

The discovery means there are at least two different mechanisms heating the Sun's atmosphere, they report in the journal Nature.

"Alfven waves, which involve the low frequency oscillation of ionised gas in a magnetic field, combined with plasma jets called spicules, bring temperatures up to about 1.5 million degrees," says Cirtain.

"The new observations show braided magnetic strands are likely to provide the additional heating needed to reach temperatures of over six million degrees."

The Sun's activity, including solar flares, coronal mass ejections and plasma eruptions, are powered by magnetic fields generated deep below the solar surface. These magnetic fields form loops extending beyond the surface into the Sun's atmosphere. Plasma flows along these fields, outlining them with glowing threads, just like iron filings sprinkled around a bar magnet shows its magnetic field lines looping from one end of the bar to the other.

The scientists observed this process, known as 'magnetic reconnection', using the new High-resolution Coronal Imager (Hi-C) telescope.

Five spectacular minutes

Launched into space on a five-minute flight, the Hi-C photographed the Sun in ultraviolet wavelengths, which are blocked by Earth's atmosphere.

The Hi-C provided a degree of image resolution five times better than any previous observations, says Cirtain.

It showed that the Sun is dynamic, with magnetic fields constantly warping, twisting, and colliding in bursts of energy which boost the corona's temperature to millions of degrees, he adds.

"The suborbital flight only lasted five minutes, but gave us images showing interweaved magnetic fields braided like hair," says Cirtain.

"As these braids relaxed and straightened, they released enough energy to heat the plasma in the corona to over 6 million°C."

"We also saw an area where magnetic field lines crossed each other then straightened out as the fields reconnected, minutes later, that spot erupted with a mini solar flare."

In the future, Cirtain hopes to put Hi-C aboard a satellite that could observe the Sun continuously at the same high level resolution.

"We only had five minutes of observing time on the sub-orbital flight," says Cirtain.

"Imagine what we could learn by watching the Sun 24/7 with this telescope."