Solar flares produced by the sun can have an impact on the Earth, with the most powerful flares causing global power outages and communications disruptions. However, these solar flares are relatively mild compared to the "superflares" seen by NASA's Kepler and TESS missions. These "superflares" come from stars and are 100 to 10,000 times brighter than solar flares.
The physics of solar flares and superflares are thought to be the same: a sudden release of magnetic energy. Superflare stars have stronger magnetic fields and therefore brighter flares, but some show an unusual behavior - an initial brightness boost that lasts only a short time, followed by a longer but less intense secondary flare. A research team led by Yang Kai, a postdoctoral researcher at the University of Hawaii's Institute of Astronomy, and Sun Xudong, an associate professor, built a model to explain this phenomenon, which was published in The Astrophysical Journal.
"By applying what we've learned about our Sun to other, cooler stars, we're able to identify the physics that drive these flares, even though we'll never be able to see them directly," Young said. "The changes in the brightness of these stars over time actually help us 'see' these flares, which are simply too small to observe directly."
The visible light in these flares is thought to come only from the lower layers of the star's atmosphere. Energetic particles produced by magnetic reconnection rain down from the hot, fragile corona (the star's outer layers), heating these layers. Recent research has hypothesized that superflare stars can also detect radiation from coronal loops -- hot plasma trapped by the Sun's magnetic field -- but that the density of these loops would have to be very high. Unfortunately, astronomers have no way to test this because there's no way to see these rings on stars other than our own sun.
Other astronomers, using data from the Kepler and TESS telescopes, have discovered that stars have a peculiar light curve - similar to a celestial body's "spike," a jump in brightness. It turns out that this light curve is similar to a solar phenomenon in which an initial burst is followed by a second, more gradual peak. These light curves remind us of a phenomenon we see on the Sun called late solar flares.
The researchers asked: "Could the same process - energizing large stellar rings - produce similar late brightness enhancements in visible light?"
To solve this problem, Yang adapted a fluid simulation often used to simulate solar flare rings and scaled up the ring's length and magnetic energy. He found that the flare's huge energy input pumped massive amounts of mass into the loop, producing a dense, bright emission of visible light, just as predicted.
These studies show that we only see this "impact" flash when the ultra-hot gas cools down in the ring's highest reaches. Under the influence of gravity, these luminous materials will fall, forming what we call "coronal rain", which is a phenomenon we often see on the sun. This convinced the team that the model must be real.
Compiled source: ScitechDaily