The outermost part of the Sun's atmosphere, the corona, doesn’t make sense. It is hundreds of times hotter than the Sun’s surface, and the mechanism that heats it up remains unclear. One proposed possibility is nanoflares, but they have not been observed yet.
A paper published in Nature Astronomy has shown the first possible detection of a nanoflare. A nanoflare needs to be produced by a magnetic reconnection event and could be heating up the corona.
By virtue of their size, only recently have we developed instruments that can spot them directly. The team who worked on this paper have now found some pretty compelling evidence of a phenomenon that appears to be the consequence of nanoflares.
Lead author Dr. Shah Bahauddin started this research during his PhD, where he investigated bright loops 100 kilometers (60 miles) across the layer below the corona.
“I thought maybe the loops made the surrounding atmosphere a bit hotter,” Bahauddin, who works at the University of Colorado, Boulder, said in a statement. “I never thought that it would make so much energy that it might actually propel hot plasma to the corona and heat it up.”
The loops were incredibly hot, in the range of millions of degrees. What was also surprising was how this energy is distributed. Heavier elements appeared to have more energy compared to lighter elements. The team performed a simulation showing that the atoms in the region were all bumping into one another during these events, but those with more mass (and so more momentum) were able to "steal" energy from the smaller atoms and blast off at much higher speeds.
The team was also able to link these events to the solar corona. 10 instances of bright loops were followed after a handful of seconds by heating in the corona, located tens of thousands of kilometers above it. The regions heated were right on top of the loops.
“And there it was, just a 20-second delay,” Bahauddin said. “We saw the brightening, and then we suddenly saw the corona got super-heated to multi-million degree temperatures,” Bahauddin said. “SDO gave us this important information: Yes, this is indeed increasing the temperature, transferring energy to the corona.”
The team is cautious in calling these particular events nanoflares, despite having the theoretical hallmarks of them. To confirm that they are indeed these fabled phenomena, the team is working on showing that these brightenings occur often enough all over the Sun. Although in progress, this is a very important piece of research.
“We have shown how a cool, low-lying structure can actually supply super-hot plasma to the corona,” Bahauddin said. “That, to me, was the most beautiful thing.”