The atmosphere of the sun is composed of several layers, mainly the photosphere, the chromosphere and the corona. It's in these outer layers that the sun's energy, which has bubbled up from the sun's interior layers, is detected as sunlight.

The lowest layer of the sun's atmosphere is the photosphere. It is about 300 miles (500 kilometers) thick. This layer is where the sun's energy is released as light. Because of the distance from the sun to Earth, light reaches our planet in about eight minutes.

Image of the solar corona, taken by the SECCHI outer coronagraph (COR2) on the STEREO Ahead observatory on June 8, 2010 at 01:09:35 UT. Click to enlarge.
Image of the solar corona, taken by the SECCHI outer coronagraph (COR2) on the STEREO Ahead observatory on June 8, 2010 at 01:09:35 UT. Click to enlarge.
Credit: NASA/STEREO

The photosphere is marked by bright, bubbling granules of plasma and darker, cooler sunspots, which emerge when the sun's magnetic field breaks through the surface. Sunspots appear to move across the sun's disk. Observing this motion led astronomers to realize that the sun rotates on its axis. Since the sun is a ball of gas with no solid form, different regions rotate at different rates. The sun's equatorial regions rotate in about 24 days, while the polar regions take more than 30 days to make a complete rotation.

The photosphere is also the source of solar flares: tongues of fire that extend hundreds of thousands of miles above the sun's surface. Solar flares produce bursts of X-rays, ultraviolet radiation, electromagnetic radiation and radio waves. [Space Weather: Sunspots, Solar Flares & Coronal Mass Ejections]

The next layer is the chromosphere. The chromosphere emits a reddish glow as super-heated hydrogen burns off. But the red rim can only be seen during a total solar eclipse. At other times, light from the chromosphere is usually too weak to be seen against the brighter photosphere.

The chromosphere may play a role in conducting heat from the interior of the sun to its outermost layer, the corona. "We see certain kinds of solar seismic waves channeling upwards into the lower atmosphere, called the chromosphere, and from there, into the corona," Junwei Zhao, a solar scientist at Stanford University in Stanford, California, and lead author on a recent study that tracked waves from sunspots, said in a statement. "This research gives us a new viewpoint to look at waves that can contribute to the energy of the atmosphere."

The third layer of the sun's atmosphere is the corona. It can only be seen during a total solar eclipse as well. It appears as white streamers or plumes of ionized gas that flow outward into space. Temperatures in the sun's corona can get as high as 3.5 million degrees Fahrenheit (2 million degrees Celsius). As the gases cool, they become the solar wind.

Why the corona is up to 300 times hotter than the photosphere, despite being farther from the solar core, has remained a long-term mystery. 

"That's a bit of a puzzle," Jeff Brosius, a space scientist at Catholic University in Washington, D.C., and NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement. "Things usually get cooler farther away from a hot source. When you're roasting a marshmallow you move it closer to the fire to cook it, not farther away."

Recent research suggests that tiny explosions known as nanoflares may help push the temperature up by providing sporadic bursts reaching up to 18 million F (10 million C).

"The explosions are called nanoflares because they have one-billionth the energy of a regular flare," Jim Klimchuk, a solar scientist at NASA's Goddard Space Flight Center in Maryland, said in a statement. "Despite being tiny by solar standards, each packs the wallop of a 10-megaton hydrogen bomb. Millions of them are going off every second across the sun, and collectively they heat the corona." [VIDEO: Nanoflares: Why the Sun's Corona Is 300x Hotter Than Its Surface]

Giant super-tornados may also play a role in heating the sun's outer layer. These solar twisters are a combination of hot flowing gas and tangled magnetic field lines, ultimately driven by nuclear reactions in the solar core.

"Based on the detected events, we estimate that at least 11,000 swirls are present on the sun at all times," Sven Wedemeyer-Böhm, a solar scientist of the University of Oslo in Norway and lead author of the team that identified tornados on the sun, told Space.com.

Additional reporting by Nola Taylor Redd, Space.com contributor

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