Why Does the Sun’s Corona Get So Hot? NASA Launches Telescope to Find Out

As plasma is ejected from the sun’s surface, its temperature skyrockets--and so far physicists have not been able to explain why
NASA's Interface Region Imaging Spectrograph


Above the surface of the sun, plasma roiling in the star’s atmosphere does something that so far defies explanation, and seems to defy physics: It gets hotter as it moves farther out.

In the corona, the expansive outer layer of the solar atmosphere that extends millions of kilometers from the sun’s surface, temperatures reach millions of kelvins. The surface, by contrast, is a tepid 6,000 K (around 5,700 degrees Celsius). Although astronomers have developed a few possible explanations in recent years, no one can say precisely how or why the corona gets so hot. A new satellite will scrutinize the underlying regions of the sun’s atmosphere, giving physicists a chance to dig down like botanists studying a plant’s roots and uncover information that may help them solve the mystery.

The satellite—NASA’s Interface Region Imaging Spectrograph (IRIS), a new ultraviolet space telescope—will examine the chromosphere, a long-ignored layer of plasma beneath the corona, in unprecedented detail. “I wonder if maybe we were staring too hard at the corona to understand the corona,” says IRIS scientist Charles Kankelborg, a physicist at Montana State University. “It may be that by backing out we can get some vital clues to what’s happening.”

A carrier aircraft will carry IRIS and a Pegasus rocket booster aloft from Vandenberg Air Force Base in California on June 26, and then launch it from there into a polar orbit. From that vantage point the telescope will observe a small section of the chromosphere, a violently variable region between the corona and the surface. IRIS will not only photograph the sun but will also return spectra—detailed breakdowns of the star’s light that can reveal subtle physical processes at work. Other telescopes, such as the Sunrise 2 balloon that recently completed a five-day flight around the Arctic circle, have looked at the chromosphere but haven’t returned such detailed information. “You won’t just see beautiful images with fine-scale structure, but you’ll also be able to measure what the temperature is and what the density is,” says Eric Priest, a solar physicist at the University of Saint Andrews in Scotland who is not part of the IRIS team. “It’s revolutionary.”

Unlike the corona’s wispy prominences or the spotted, fiery surface, the chromosphere is tricky to behold. It absorbs and reemits some light from the surface, but it also emits its own UV light, making it difficult to identify where the photons originated, says Bart de Pontieu, the science lead for IRIS at the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, Calif. Only in the last 10 years have physicists developed computer models sophisticated enough to track the photons coming from this region and to sufficiently simulate chromospheric activity. That is a key reason why IRIS’s time is now. “With these models we have now a fighting chance of understanding the light that we see coming from the chromosphere,” de Pontieu says.

Physicists are also eager to observe solar outbursts with IRIS. And they should see plenty: IRIS will launch near the peak of the sun’s 11-year activity cycle. The IRIS team will use information from other satellites that observe the whole sun, such as Japan’s Hinode and NASA’s Solar Dynamics Observatory, to identify active areas of the sun and point IRIS toward flares as they grow, when it will obtain spectra every two seconds. De Pontieu compares the mission with studying the air just above the ocean, watching as water evaporates and condenses: “You’re seeing the process that feeds the clouds, and the process that depletes the clouds. By figuring what’s going in and out, you can figure out what’s going on up there.”

IRIS’s detailed readings will also help physicists track small-scale solar phenomena, such as the fleeting, thin fountains of hot plasma first discovered by de Pontieu and his colleagues in 2007. These jets, called “type 2 spicules,” climb as high as Earth is wide but last only 100 seconds or so. These jets may inject energy that keeps the corona at a rolling boil. Even so, astronomers are not sure how the sun’s magnetic fields drive these massive energy transfers. Magnetic fields do not naturally heat particles, Kankelborg notes: “If you take a magnet and suspend it on your refrigerator, the sausages don’t spoil.”

IRIS will test the two leading hypotheses that seek to explain the corona’s extreme heat. One idea holds that magnetic field lines twist and braid as they skitter around the sun, building up tension and unleashing massive amounts of energy when they finally break like rubber bands. Or magnetic waves rolling from deep within the sun could transfer energy and heat into the chromosphere and corona. “We don’t know whether any of these are right or whether we need new concepts,” Priest says. “It’s the observations from IRIS that are going to be able to determine that.”

In addition to the coronal heating mystery, IRIS will shed light on the processes that drive the solar wind, solar storms, ultraviolet radiation and other phenomena that can hinder electronic communications and negatively affect human health on Earth. Ultimately, Priest hopes IRIS observations will bolster studies of more distant stars as well. “If we understand these structures properly on the sun,” he says, “we will predict how they scale in other structures in the universe.”

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