In January 1999, scientists observed mysterious motions within a solar flare. Unlike typical flares that showed bright energy erupting outwards from the Sun, this solar flare also displayed a downward flow of motion, as if material was falling back towards the Sun. Astronomers wondered what exactly they were seeing.
Now, in a study published Jan. 27 in the journal Nature Astronomy, astronomers at the Center for Astrophysics | Harvard & Smithsonian (CfA) offer a new explanation for the poorly understood downflows.
"We wanted to know how these dark finger-like structures occur. What's driving them and are they truly tied to magnetic reconnection?" said lead author astronomer Chengcai Shen. Scientists have assumed that structures are tied to magnetic reconnection since their discovery in the 90s. The process occurs when magnetic fields break, releasing fast moving and extremely energetic radiation, and then reform.
"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 study co-author astronomer Kathy Reeves.
Reeves added: "It's like stretching out a rubber band and snipping it in the middle, so it's going to snap back."
Scientists assumed the dark downflows were signs of the broken magnetic fields "snapping back" to the Sun after a solar flare eruption. Most of the downflows observed by scientists are "puzzlingly slow." This is not predicted by classic reconnection models, which show the downflows should be much quicker. It's a conflict that requires some other explanation, said co-author Bin Chen, an astronomer at the New Jersey Institute of Technology.
To find out what was happening, the team analyzed downflow images captured by the Atmospheric Imaging Assembly (AIA) onboard NASA's Solar Dynamics Observatory. The AIA takes images of the Sun every 12 seconds in seven different wavelengths of light to measure variations in the Sun's atmosphere. They then made 3D simulations of solar flares and compared them to the observations.
The results show that most SADs are not generated by magnetic reconnection after all. Instead, they form on their own in the turbulent environment and are the result of two fluids with different densities interacting. Reeves said scientists are essentially seeing the same thing that happens when water and oil are mixed together: the two different fluid densities are unstable and ultimately separate. "Those dark, finger-like voids are actually an absence of plasma. The density is much lower there than the surrounding plasma," Reeves explained.
The team plans to continue their studies using 3D simulations to better understand magnetic reconnection. By understanding the processes that drive solar flares and eruptions from the Sun, they may ultimately help develop tools to forecast space weather and mitigate its impacts.
