The source of ghostly shadows in the Sun's atmosphere could finally be identified

We might finally have an explanation for mysterious shadows of falling material in the atmosphere of the Sun, observed during solar flares.

First spotted in 1999, these mysterious streaks of shadow – referred to as "downward-traveling dark voids" – were thought to be related to the magnetic field interactions that trigger solar eruptions. Now, solar physicists have found that's not actually the case; rather, these "supra-arcade downflows" are the result of fluid interactions in the solar plasma.

The phenomenon is very similar to structures observed at the shock interfaces in supernova remnants, where instabilities also result in long, finger-like structures. The finding will help us better understand the wild behavior of our turbulent Sun.

The idea that the structures might have something to do with the solar magnetic field is not unreasonable, since the Sun's extremely complicated and messy magnetic fields are what generate flares to start with.

Our star is a roiling, turbulent ball of incredibly hot plasma, a fluid made up of charged particles that interacts strongly with electromagnetic forces. Because the Sun is a sphere, the equatorial surface rotates faster than the poles. This results in the solar magnetic field growing tangled, which in turn can produce strong localized magnetic fields all over the Sun, opening up the sunspots from which flares emerge.

In these localized magnetic fields, the field lines can grow chaotic. At the roots of solar flares, opposing lines connect, snap, and reconnect. Powerful sheets of electric current also stretch across the core solar flare region. This magnetic reconnection results in the release of energy and acceleration of electrons to relativistic speeds.

"On the Sun, what happens is you have a lot of magnetic fields that are pointing in all different directions. Eventually the magnetic fields are pushed together to the point where they reconfigure and release a lot of energy in the form of a solar flare," said astronomer Kathy Reeves of the Harvard & Smithsonian Center for Astrophysics.

"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."

Supra-arcade downflows, embedded in fan-like structures, closely resemble predicted reconnection outflows seen in simulations of magnetohydrodynamics – the movement of electrically conducting fluids. But with one huge problem: they're about 15 percent slower than the simulated outflows, which scientists found difficult to resolve.

A team of researchers led by astronomer Chengcai Shen of the Harvard & Smithsonian Center for Astrophysics wanted to get to the bottom of this strange discrepancy, so they took and carefully studied images of the downflows from NASA's space-based Solar Dynamics Observatory.

Then, they ran simulations of solar flares, and compared these to the observation data. They found that magnetic reconnection is not responsible for the majority of the shadows.

Rather, when magnetic reconnection downflows meet the flare's closed loops of magnetic field, they create a termination shock. Supra-arcade downflows spontaneously form in the turbulent interface region below the termination shock, and are the result of fluids of different densities interacting – like oil and water, the researchers noted.

That interface region is similar to the region sandwiched between two forward and reverse shocks in a supernova remnant, where finger-like structures can also be found.

"Those dark, finger-like voids are actually an absence of plasma. The density is much lower there than the surrounding plasma," Reeves said.

The results reveal that the interface region might be more complex than we thought, which could help us understand how magnetic energy is released during solar flares. The team plans to continue conducting 3D simulations of solar phenomena to investigate further.

The research has been published in Nature Astronomy.

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