Dr. Chengcai Shen from the Harvard & Smithsonian Center for Astrophysics and colleagues have analyzed images of the so-called supra-arcade downflows captured by the Atmospheric Imaging Assembly (AIA) onboard NASA’s Solar Dynamics Observatory.
This image shows several supra-arcade downflows occurring in a solar flare; the downflows appear directly above the bright flare arcade; this solar flare occurred on June 18, 2015. Image credit: SDO / NASA.
In January 1999, solar astronomers 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.
Described as downward-moving dark voids, scientists wondered what exactly they were seeing.
“We wanted to know how these dark finger-like structures occur,” Dr. Shen said.
“What’s driving them and are they truly tied to magnetic reconnection?”
Scientists have assumed that supra-arcade downflows 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,” said Dr. Kathy Reeves, an astronomer with the Harvard & Smithsonian Center for Astrophysics.
“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.”
“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.”
The scientists assumed the dark downflows were signs of the broken magnetic fields ‘snapping back’ to the Sun after a solar flare eruption.
“But there was a catch. Most of the downflows observed by scientists are puzzlingly slow,” said Dr. Bin Chen, an astronomer at the New Jersey Institute of Technology.
“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,” Dr. Shen said.
Observations and 3D modeling of the energy-release region of a solar flare. Image credit: Shen et al., doi: 10.1038/s41550-021-01570-2.
To find out what was happening, the team analyzed downflow images captured by the Atmospheric Imaging Assembly (AIA) onboard NASA’s Solar Dynamics Observatory.
They then made 3D simulations of solar flares and compared them to the observations.
The results show that most supra-arcade downflows 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.
“Those dark, finger-like voids are actually an absence of plasma. The density is much lower there than the surrounding plasma,” Dr. Reeves said.
A paper on the findings was published in the journal Nature Astronomy.
_____
C. Shen et al. The origin of underdense plasma downflows associated with magnetic reconnection in solar flares. Nat Astron, published online January 27, 2022; doi: 10.1038/s41550-021-01570-2
