“Fingers” of darkness reaching towards our source of light – the Sun – have puzzled astronomers for over 20 years. For the layman, far removed from astronomy, images of mysterious motions within a solar flare first discovered in 1999, might have stirred uneasy associations with some mythical demons – or even the hand of God.
While there have been some theories floating around about what these might actually be, a new explanation has now been offered.
In a study published in Nature Astronomy, astronomers at the Centre for Astrophysics | Harvard & Smithsonian (CfA) suggests that these motions form independently when fluids with different densities interact.
The flares are made up of bright energy that erupts from the Sun. But it was the downward flow of motion, as if material was falling back towards the Sun, that astronomers were at a loss to explain. They were discovered in 1999 after extreme ultraviolet and soft X-ray images of the Sun became available.
Described as “downward-moving dark voids,” resembling a hand or fingers of darkness curling down towards the Sun, astronomers at some point suggested a process called magnetic reconnection was driving them, which occurs when magnetic fields break, release energetic radiation, and then reform again.
“We wanted to know how these structures occur… What’s driving them and are they truly tied to magnetic reconnection?” said lead author and CfA astronomer Chengcai Shen.
There are a lot of magnetic fields pointing in different directions on the Sun, said study co-author and CfA astronomer Kathy Reeves. She explained that they are eventually pushed together, reconfiguring and releasing a lot of energy as solar flares.
"It’s like stretching out a rubber band and snipping it in the middle. It’s stressed and stretched thin, so it’s going to snap back," she added.
However, there was a flaw with the magnetic reconnection theory, as most downflows observed by scientists were “puzzlingly slow,” added co-author Bin Chen, an astronomer at the New Jersey Institute of Technology. She explained that typical reconnection models showed the downflows should be much quicker.
For their research, the team analysed downflow images captured by the Atmospheric Imaging Assembly (AIA) onboard NASA’s Solar Dynamics Observatory. It takes images of the Sun every twelve seconds in seven different wavelengths of light to measure variations in the Sun’s atmosphere.
After the scientists made 3D simulations of solar flares and compared them to the observations, the results showed most SADs were not generated by magnetic reconnection.
Instead, they appeared to form as a result of two fluids with different densities interacting.
"Those dark, finger-like voids are actually an absence of plasma. The density is much lower there than the surrounding plasma," said Reeves.
With future research into SADs planned using 3D simulations researchers believe this could help predict solar weather, what drives flares and eruptions. This, in turn, may ultimately help develop tools to mitigate its potentially devastating impacts.