Tiny explosions add up to heat corona

A relentless onslaught of tiny explosions buffet the solar atmosphere, researchers report. These eruptions, dubbed nanoflares, might help solve the long-standing riddle about why the sun’s corona is millions of degrees hotter than its surface.

“This is a real breakthrough to solving one of the most important problems in space science,” James Klimchuk, an astrophysicist at the NASA Goddard Space Flight Center in Greenbelt, Md., said at a news conference April 28.

The nanoflares rapidly heat the plasma in the corona, the sun’s outer atmosphere, to about 10 million degrees Celsius, says Klimchuk. The plasma then quickly cools to a relatively balmy 2 million degrees or so, still much warmer than the roughly 5,500-degree surface of the sun.

Each eruption belches out roughly the same amount of energy as a 10-megaton hydrogen bomb, Klimchuk says. While that amount of energy is enormous by Earth standards, it’s just a blip on the sun. These nanoflares have just one-billionth the energy of their much larger cousins, the massive solar flares that hurl bits of the sun into space at millions of kilometers per hour. There are, however, millions of nanoflares erupting every second.

Astronomers can’t see individual flares, said Adrian Daw, another Goddard astrophysicist. Instead they see the superhot plasma in the sun’s atmosphere, which is the “smoking gun for nanoflares,” he says.

To peer into the corona, Daw and colleagues launched a sounding rocket known as the Extreme Ultraviolet Normal Incidence Spectrograph, or EUNIS, from White Sands, N.M., in April 2013. From about 320 kilometers above Earth’s surface, EUNIS observed ultraviolet light streaming from highly ionized atoms in the sun’s atmosphere. The amount of energy in the ultraviolet light indicated that small pockets of plasma were being heated to about 10 million degrees, the researchers reported at the Triennial Earth-Sun Summit in Indianapolis.

The nature of the corona’s thermostat has stumped researchers for 76 years, and nanoflares — first proposed as a solution in 1988 — are just one of many possible solutions. One other leading idea is that waves rippling through the plasma transport energy from the solar interior to the corona.

These new results lend significant support to nanoflares, says Amir Caspi, an astrophysicist at the Southwest Research Institute in Boulder, Colo., who was not involved with this research. Caspi detected X-rays from superhot plasma several years ago with another sounding rocket known as X123. The presence of such plasma, he says, suggests something very energetic and impulsive is powering it. Waves, on the other hand, should produce a lot less of the 10-million-degree plasma, he says. There are, however, many open questions about the underlying physics, he adds.

NASA’s NuSTAR satellite, which normally looks for X-rays radiating from far-flung black holes and exploding stars, corroborates the EUNIS results. In addition to probing the plasma temperature, NuSTAR looked for high-speed electrons zipping off the sun’s surface. Such electrons would indicate that the nanoflares are scaled-down versions of normal flares. No electrons were detected, which could just mean that the signal is swamped by other solar activity, or it could point to a different mechanism driving the nanoflares.

Researchers suspect nanoflares are triggered by abrupt changes in magnetic fields, which “behave like rubber bands,” says Klimchuk. If you keep twisting a rubber band, he says, eventually it snaps and releases energy. Magnetic fields operate the same way. On the sun, turbulent motion beneath the sun’s surface, similar to a boiling pot of water, winds up the magnetic fields, building energy that might eventually erupt in a nanoflare.

More detailed ultraviolet spectra of the hot plasma could show how the nanoflares interact with the rest of the corona, says Klimchuk. Visible light observations from the ground, which are much easier to work with than ultraviolet data, can also fill in some of the missing gaps. “What’s hard is arranging the moon to block out the sun for you,” Daw says. Fortunately, a total solar eclipse will march across the United States in 2017 and provide a rare opportunity to view the corona from the ground and track rapid changes better than what’s possible from a spacecraft.