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. Described as “downward-moving dark voids,” astronomers wondered what exactly they were seeing.
Now, in a study published today (January 27, 2022) in Nature Astronomy, astronomers at the Center for Astrophysics | Harvard & Smithsonian (CfAThe Harvard-Smithsonian Center for Astrophysics (CfA) is a joint venture between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. Founded in 1973, the Harvard-Smithsonian Center for Astrophysics is comprised of six research divisions: Atomic and Molecular Physics; Optical and Infrared Astronomy; High Energy Astrophysics; Radio and Geoastronomy; Stellar, Solar, and Planetary Sciences; and Theoretical Astrophysics.”>CfA) offer a new explanation for the poorly understood downflows, now referred to as supra-arcade downflows (SADs) by the scientific community.
“We wanted to know how these structures occur,” says lead author and CfA astronomer Chengcai Shen, who describes the structures as “dark finger-like features.” “What’s driving them and are they truly tied to magnetic reconnection?”
Scientists have assumed that SADs 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.
The Atmospheric Imaging Assembly (AIA) onboard NASAEstablished in 1958, the National Aeronautics and Space Administration (NASA) is an independent agency of the United States Federal Government that succeeded the National Advisory Committee for Aeronautics (NACA). It is responsible for the civilian space program, as well as aeronautics and aerospace research. It’s vision is “To discover and expand knowledge for the benefit of humanity.””>NASA’s Solar Dynamics Observatory captures a supra-arcade downflow within a solar flare that occurred on November 29, 2020. Credit: NASA SDO/Sijie Yu
“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,” says study co-author and CfA astronomer Kathy Reeves.
Reeves adds, “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.”
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,” says co-author Bin Chen, an astronomer at the New Jersey Institute of TechnologyFounded in 1881, the New Jersey Institute of Technology (NJIT) is a public research university in Newark, New Jersey. It began as Newark Technical School and advanced into the Newark College of Engineering. Then, with the addition of a School of Architecture in 1973, it grew into a polytechnic university that now hosts five colleges and one school.”>New Jersey Institute of Technology.
Shen explains, “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.”
To find out what was happening, the team analyzed downflow images captured by the Atmospheric Imaging Assembly (AIA) onboard NASA’s Solar Dynamics Observatory. Designed and built partially at the CfA and led by the Lockheed Martin Solar Astrophysics Laboratory, the 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 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 says 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 plasmaPlasma is one of the four fundamental states of matter, along with solid, liquid, and gas. It is an ionized gas consisting of positive ions and free electrons. It was first described by chemist Irving Langmuir in the 1920s.”>plasma. The density is much lower there than the surrounding plasma,” Reeves says.
The team plans to continue studying SADs and other solar phenomenon 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.
Reference: “The Origin of Underdense Plasma Downflows Associated with Magnetic Reconnection in Solar Flares” 27 January 2022, Nature Astronomy.
Additional co-authors on the paper are Xiaoyan Xie of the 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.