The surface of the Sun, called the photosphere, has a temperature of around 5,505 °C (9,941 °F). But the Sun’s external atmosphere, or corona, regularly reaches temperatures of several million degrees Celsius. So why is there such a disparity?
"That's a bit of a puzzle,” said lead researcher Jeff Brosius, a space scientist at NASA's Goddard Space Flight Center, in a 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.”
One theory, proposed by astrophysicst Thomas Gold in 1964, was that thousands of tiny ‘nanoflares’ are constantly peppering the corona with millions of degrees of heat every second to keep it much warmer than the photosphere below. They do this by releasing energy, which heats the plasma nearby to temperatures of around 10 million degrees Celsius. But because nanoflares are believed to cool down so quickly - leaving little evidence for their super-heated activities - and because they're impossible to observe individually, this theory has been very difficult to prove. Until now.
Thanks to a little research rocket that NASA launched towards the Sun in April 2013, Brosius and his team have been able to identify the heating activities of nanoflares in the corona in unprecedented detail.
To do so, they looked at six minutes of data from the rocket, which is designed to record a snapshot of the solar atmosphere once every 1.3 seconds. Part of NASA's EUNIS mission (short for Extreme Ultraviolet Normal Incidence Spectrograph), the rocket is equipped with a super-sensitive spectrograph, which gathers information about what material is present at any given temperature based on the light it emits. The rocket scanned one of the ‘active regions’ of the Sun - which are special, highly magnetic regions where sunspots, solar flares and coronal mass ejections are common occurrences - for light emissions.
As light from this active region streamed into the EUNIS spectrograph, the instrument separated it into various wavelengths. The wavelengths were plotted out on what's known as an emission line, which allowed the researchers to identify the atoms present in the region. From this, they identified the presence of material with a temperature of 10 million degrees Celsius. This, they report, is the strongest evidence we have that tiny nanoflares are heating the plasma up 10 million degrees to keep the corona hotter than the Sun's surface.
"The fact that we were able to resolve this emission line so clearly from its neighbours is what makes spectroscopists like me stay awake at night with excitement," said Brosius in the press release. "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."