In January 1999, scientists observed mysterious movements in a solar flare.
Unlike typical eruptions that showed light energy erupting outward from the Sun, this solar eruption also showed a downward flow of motion, as if matter were falling back toward the Sun. Described as “descending dark cavities”, astronomers wondered what exactly they saw.
Now, in a study published today (January 27, 2022) in Nature astronomy, astronomers at the Center for Astrophysics | Harvard & Smithsonian (CfA) offers a new explanation for the poorly understood downflows, now referred to as supra-arcade downflows (SADs) by the scientific community.
“We would like to know how these structures arise,” says lead author and CfA astronomer Chengcai Shen, who describes the structures as “dark finger-like features.” “What drives them, and are they really tied to magnetic reconnection?”
Researchers have assumed that SADs have been linked to magnetic reconnection since their discovery in the 1990s. The process occurs when magnetic fields break, release fast-moving and extremely energetic radiation, and then reform.
Atmospheric Imaging Assembly (AIA) on board NASA‘s Solar Dynamics Observatory captures a supra-arcade downflow in a solar eruption that occurred on November 29, 2020. Credit: NASA SDO / Sijie Yu
“On the Sun, it happens that you have a lot of magnetic fields 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,” he says. study co-author and CfA astronomer Kathy Reeves.
Reeves adds: “It’s like stretching a rubber band out and cutting it in the middle. It’s stressed and stretched thin so it’s going to slam back.”
Scientists assumed that the dark downpours were signs that the destroyed magnetic fields “snapped back” to the Sun after a solar flare.
But there was a catch.
Most of the downpours observed by scientists are “confusingly slow,” says co-author Bin Chen, an astronomer at New Jersey Institute of Technology.
Shen explains, “This is not predicted by classic reconnection models, which show that the downpour should be much faster. It is a conflict that requires a different explanation.”
To find out what happened, the team analyzed downflow images taken by the Atmospheric Imaging Assembly (AIA) aboard NASA’s Solar Dynamics Observatory. Designed and built in part at CfA and led by the Lockheed Martin Solar Astrophysics Laboratory, AIA takes images of the Sun every twelve 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 with the observations.
The results show that, after all, most SADs are not generated by magnetic reconnection. Instead, they are formed by themselves in the turbulent environment and are the result of two liquids with different densities interacting.
Reeves says scientists see essentially the same thing that happens when water and oil mix: the two different liquid densities are unstable and ultimately separate.
The dark, finger-like cavities are actually an absence of plasma. The density is much lower there than the surrounding plasma, ”says Reeves.
The team plans to continue studying SADs and other solar phenomena using 3D simulations to better understand magnetic reconnection. By understanding the processes that drive solar flares and eruptions from the Sun, they can ultimately help develop tools for predicting space weather and mitigating its effects.
Reference: “Origin of submerged plasma currents associated with magnetic reconnection in solar flares” 27 January 2022, Nature astronomy.
DOI: 10.1038 / s41550-021-01570-2
Additional co-authors on paper are Xiaoyan Xie from CfA; Sijie Yu of the New Jersey Institute of Technology; and Vanessa Polito of the Bay Area Environmental Research Institute.
This research was supported by grants from the National Science Foundation.