‘Real breakthrough’: Discovery of nanoflares solves sun’s coronal heat mystery

NASA's EUNIS sounding rocket examined light from the sun in the area shown by the white line (imposed over an image of the sun from NASA's Solar Dynamics Observatory) then separated the light into various wavelengths (as shown in the lined images – spectra – on the right and left) to identify the temperature of material observed on the sun. (Credits: NASA / EUNIS / SDO)
What has long remained a major question for astronomers – the significant jump in the temperature of the sun’s corona in comparison to its visible surface – may have been solved by NASA scientists. A new theory suggests the heat source may be nanoflares.

The solar corona is a thin aura of plasma with the temperature exceeding that at the surface by about 300 times, extending millions of kilometers into space. Astronomers have been dazzled by this mystery for a long time.

NASA astronomers used a sounding rocket, a black-hole-hunting telescope and computer modeling to study the phenomenon. They have found that the corona is heated by tiny explosions called nanoflares.

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“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, explained 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.”

The existence of nanoflares was suggested a long time ago, yet researchers weren’t able to prove it. Nanoflares are too small and ephemeral to catch, and the corona itself can be observed with the naked eye only during a solar eclipse, when the moon’s silhouette covers the sun. Despite being far from the sun’s core, its corona can reach the temperature of one to three million Kelvin, while the photosphere, or the surface, has an average temperature of 5,800 Kelvin.

The evidence was collected by NASA's EUNIS sounding rocket, sent on a 15-minute flight into space with a spectrograph on board in December 2013. The mission proved the existence of nanoflares.

NASA’s NuSTAR telescope was also used in the research. With its help, NASA scientists managed to examine the regions of the sun where X-ray energy is emitted despite no normal-sized flares being detected.

Iain Hannah, an astrophysicist at the University of Glasgow in Scotland, said that “X-rays are a direct probe into the high-energy processes of the sun.”

According to Stephen Bradshaw, a solar astrophysicist at Rice University in Houston, Texas, it is difficult to spot nanoflares, yet this phenomenon could be explained by a “sophisticated computational model”.

Heliophysics researchers suppose that nanoflares could contribute to the space weather, thus influencing the Earth’s environment and technology.