27 Jan 2022

New findings published Jan. 27 in the journal Nature Astronomy have called into question decades of theoretical understanding used by astrophysicists to interpret an observational phenomenon central to understanding energy released during powerful eruptions from the Sun, known as solar flares.

Solar flares, which are triggered when magnetic field lines break and reconnect above the Sun’s surface — referred to as magnetic reconnection — can release a large amount of energy, energetic charged particles, and intense electromagnetic radiation, possessing the potential to disrupt communications technologies on Earth. 

For nearly two decades, scientists studying solar flares have observed dark, finger-like plasma outflows known as supra-arcade downflows (SADs) above their bright, arcade-like magnetic loops in extreme ultraviolet and X-ray wavelengths. These outflows have been considered key evidence of major solar flares being powered by magnetic reconnection which often occurs in enormous electric current sheets, offering a critical data point for estimating the rate of a solar flare’s energy release. 

However, researchers have been puzzled to observe these plasma flows travel substantially slower than speeds predicted in established theories, casting doubt over past interpretations of solar flares and similar explosive events that involve magnetic reconnection.

“We wanted to know how these structures occur,” said lead author and astronomer Chengcai Shen of the Harvard-Smithsonian Center for Astrophysics (CfA) in a statement. “What’s driving them and are they truly tied to magnetic reconnection? … This is not predicated by classic reconnection models, which show the downflows should be much quicker. It’s a conflict that requires some other explanation.”

“The speed of the reconnection-driven plasma outflows is an important observational input to determine how efficient solar flares release their energy,” said Bin Chen, associate professor of physics at NJIT and second-author of the study. “The latter is fundamental to our understanding of the underlying physics of solar flares and similar, but often more violent, explosive events across the universe. Therefore, if our interpretation of these key observed features is incomplete, then our current knowledge on solar flare energy release may be challenged.”

[Image]

Still image of several supra-arcade downflows, also described as “dark, finger-like features,” occurring in a solar flare on June 18, 2015. The downflows appear directly above the bright flare arcade.