NASA Scientists Explain Why Sun's Corona is So Much Hotter Than Its Photosphere

A scientific conundrum, NASA has finally come up with a solution to why the sun's corona, or outer atmosphere, is so much hotter than its surface.

Scientists at the space agency point to nanoflares, which cannot be detected individually though they provide extra heat in small, rapid bursts. This theory was generated thanks to just six minutes of footage Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) mission.

"That's a bit of a puzzle," study lead author Jeff Brosius, a space scientist at Catholic University in Washington, D.C., and NASA's Goddard Space Flight Center in Greenbelt, Md., said in a NASA press release. "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."

Brosius' team published their work Aug. 1 in the Astrophysical Journal.

The space agency launched EUNIS, an inexpensive sounding rocket that collects data constantly in order to track the sun's complex properties, on April 23, 2013. The photosphere is the sun's surface we see from Earth and it is about 6,000 degrees Kelvin, but EUNIS suggests the corona is 300 times as hot.

The spacecraft's spectrograph picked up an "active region" and calculated 10 million degrees Kelvin coming from several different wavelengths. After analyzing the unique emission lines from the reading, the scientists theorized that nanoflares would definitely be able to produce a reading of 10 million Kelvin.

"The fact that we were able to resolve this emission line so clearly from its neighbors is what makes spectroscopists like me stay awake at night with excitement," Brosius said. "This weak line observed over such a large fraction of an active region really gives us the strongest evidence yet for the presence of nanoflares."

EUNIS does not capture images of the sun the way many are used to, but it has led to an important discovery regarding our sun. NASA said it will continue to make observations and examine the issue further.

"This is a real smoking gun for nanoflares," Adrian Daw, the current principal investigator for EUNIS at Goddard, said in the release. "And it shows that these smaller, less expensive sounding rockets can produce truly robust science."