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Tiny Explosions May Power the Sun's Blazing Corona
The sun's outer atmosphere, called the corona, is much hotter than its surface — and a new study investigates why.
Credit: S. Wiessinger/NASA's Goddard Space Flight Center/SDO

The upper reaches of the sun's atmosphere are thousands of times hotter than its surface, and a new study offers a possible reason for that intense heat: countless explosions from the sun, each too small for scientists to detect.

The "surface" of the sun — the photosphere, or visible part of the sun — is about 10,000 degrees Fahrenheit (5,500 degrees Celsius). Strangely, even though it is farther away from the sun's core, the uppermost part of the sun's atmosphere — known as the corona — is much hotter, regularly reaching about 1.8 million to 3.6 million degrees F (1 million to 2 million degrees C) and sometimes reaching up to 72 million degrees F (40 million degrees C), according to NASA.

This mystery of why the sun's corona is so much hotter than its photosphere "is known as the coronal heating problem," study lead author Shin-nosuke Ishikawa, a solar physicist at the Japan Aerospace Exploration Agency, told Space.com. [The Sun Revealed: Photos of the Million-Degree Solar Corona]

One possible explanation for this mystery involves tiny solar flares dubbed "nanoflares" that are too small for current scientific instruments to see given the blinding interference from the rest of the sun, the researchers said. 

"We think nanoflares range from a billion to a million times smaller than a regular solar flare," Ishikawa said. That still means a nanoflare typically explodes with about as much energy as 10^19 joules, Ishikawa said, which is equivalent to about 10 billion tons of TNT.

Although nanoflares are fainter and less energetic than regular flares, they would, in theory, happen far more often. This could potentially inject an extraordinary amount of heat into the solar atmosphere, explaining why the corona is so hot, the researchers said.

To investigate the mystery of the sun's corona, Ishikawa's team analyzed X-ray data of solar activity taken by the Focusing Optics X-ray Solar Imager (FOXSI-2) sounding rocket. "Our instruments for X-ray detection from the sun were about 100 times more sensitive than past instruments," Ishikawa said.

Many of us take the sun for granted, giving it little thought until it scorches our skin or gets in our eyes. But our star is a fascinating and complex object, a gigantic fusion reactor that gives us life. How much do you know about the sun?
This image, captured by NASA's Solar Dynamics Observatory (SDO) on March 10, 2012, shows an active region on the sun, seen as the bright spot to the right. Designated AR 1429, the spot has so far produced three X-class flares and numerous M-class flares.
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Solar Quiz: How Well Do You Know Our Sun?
Many of us take the sun for granted, giving it little thought until it scorches our skin or gets in our eyes. But our star is a fascinating and complex object, a gigantic fusion reactor that gives us life. How much do you know about the sun?
This image, captured by NASA's Solar Dynamics Observatory (SDO) on March 10, 2012, shows an active region on the sun, seen as the bright spot to the right. Designated AR 1429, the spot has so far produced three X-class flares and numerous M-class flares.
0 of questions complete

The researchers discovered very energetic X-rays, a signature of solar plasma heated to more than 18 million degrees F (10 million degrees C), over a region of the sun that had no visible flares. They suggested that this superheated plasma could be generated only by nanoflares.

Next summer, the researchers plan to launch a sounding rocket loaded with upgraded scientific instruments "to detect much more faint X-ray sources," Ishikawa said. They are also working on a satellite that could detect nanoflares, he added. The hope is that such research could help scientists begin to estimate "how many nanoflares there actually are, and how much energy they actually have," Ishikawa said.

The scientists detailed their findings online today (Oct. 9) in the journal Nature Astronomy.

Follow Charles Q. Choi on Twitter @cqchoi. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.