Nanoflares could be the driving force behind the mysterious heating of the sun's corona. While the surface of the star has a temperature of more than 10,300 degrees Fahrenheit, the atmosphere of the sun, known as the corona, is measured around a few million degrees Fahrenheit.
Heat does not normally build up when measured further from the source of heat. Astronomers have long wondered how the corona could be 300 times hotter than the surface of the star. Nanoflares are newly discovered small explosions which erupt from the surface of the sun. Despite their size, these features can get as hot as 18 million degrees Fahrenheit, which could be driving the strange heating seen in the corona.
"The explosions are called nanoflares because they have one-billionth the energy of a regular flare. 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," Jim Klimchuk, a solar astronomer from the Goddard Space Flight Center, said.
The Extreme Ultraviolet Normal Incidence Spectrograph (Eunis) mission, flown aboard a sounding rocket, discovered nanoflares for the first time in December 2013. The spacecraft recorded a spectrum of light coming from the sun, examining the short-wavelength light that is produced by the extreme temperatures. This revealed the presence of millions of nanoflares erupting across the surface of our parent star. In areas where these tiny, super-hot explosions were taking place, the corona was seen growing hotter, even in the absence of larger, normal flares. This provides direct physical evidence the newly recognized phenomenon is responsible for heating the corona.
Astronomers had two main theories about the strange heating of the solar corona. The first was that heating was gradual, while the second idea held that dramatic events, such as flares, were responsible for the phenomenon.
Announcement of the finding was made on April 28 at the first Triennial Earth-Sun Summit (TESS), a meeting of heliophysicists held in Indianapolis, Ind. Another presentation delivered there presented evidence backing up the finding of nanoflares. Heliophysics studies the sun-Earth connection. The TESS meeting will occur every three years.
The Nuclear Spectroscopic Telescope Array (NuSTAR) recorded the sun in X-rays, also finding evidence of the miniature flares. This satellite usually studies X-rays from distant stars and black holes, but can also study the bright light of the sun, which isn't a capability for most observatories.
"X-rays are a direct probe into the high-energy processes of the sun," Iain Hannah from the University of Glasgow said.
A third presentation at the conference detailed mathematical models explaining why these features have been so hard to locate prior to the recent discovery.