Solar flares are complex phenomena. They involve plasma, electromagnetic radiation across all wavelengths, activity in the Sun’s atmosphere layers, and particles travelling at near light speed. Spacecraft like NASA’s Solar and Heliophysics Observatory (SOHO) and the Parker Solar Probe shed new light on the Sun’s solar flares.

But it was a Japanese-led mission called Yohkoh that spotted an unusual solar flare in 1999. This flare displayed a downward flowing motion toward the Sun along with the normal outward flow. What caused it?

A team of researchers think they’ve figured it out.

When the 1999 Yuhkoh flare was spotted, astrophysicists wondered what caused the downward motion. They called these dark finger-like phenomena “downward-moving dark voids.” Astrophysicists have a more accurate term for them now: supra-arcade downflows (SADs.)

A group of researchers led by Chengcai Shen have an explanation for SADs. Shen is an astronomer at the CfA, the Harvard and Smithsonian Center for Astrophysics. The team’s paper is “The origin of underdense plasma downflows associated with magnetic reconnection in solar flares.” The journal Nature Astronomy published the work.

A solar arcade is an active area with multiple coronal loops. Coronal loops are magnetic structures that extend from the Sun’s photosphere out into the corona, looping back down to reconnect with the photosphere again. Coronal loops trap plasma magnetically, and that makes them visible.

This image is an example of solar coronal loops observed by the Transition Region And Coronal Explorer (TRACE). These loops have a temperature of approximately 106 K. These loops contrast greatly with the cool chromosphere below. Image Credit: By NASA Public Domain
This image is an example of solar coronal loops observed by the Transition Region And Coronal ExplorerTRACE). These loops have a temperature of approximately 106 K. These loops contrast significantly with the cool chromosphere below. Image Credit: By NASA Public Domain

A supra-arcade is a solar arcade with added features. Along with the loops, there are downflows above the arcade. Scientists thought supra-arcades are somehow connected with the magnetic reconnection behind solar flares, but the specifics were unknown.

“We wanted to know how these structures occur,” says lead author and CfA astronomer Chengcai Shen. “What’s driving them, and are they truly tied to magnetic reconnection?”

The Sun has complex magnetic fields that can become compressed and disfigured. They can break, releasing fast-moving and powerful radiation along magnetic lines, then reconnect to form loops.

“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.

“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,” Reeves added.

That knowledge is firmly established, so it’s reasonable to conclude that the same mechanism guided SADs. “A characteristic feature of magnetic reconnection is the production of fast reconnection outflow jets near the plasma Alfven speeds,” the authors write in their paper. “In eruptive solar flares, dark, finger-shaped plasma
downflows moving toward the flare arcade have been commonly regarded as the principal
observational evidence for such reconnection-driven outflows.”

But observations didn’t entirely back that explanation. It comes down to speed.

“However, they often show a speed much slower than that expected in reconnection theories, challenging the reconnection-driven energy release scenario in standard flare models,” they write.

In the elastic band analogy, the snap-back is rapid. But when scientists watched these SADs, most of them didn’t snap back. Instead, they reconnected more slowly with the Sun. If the same thing happened with an elastic band, we’d think somebody spiked our drink.

“This is not predicted by classic reconnection models, which show the downflows should be much quicker. It’s a conflict that requires some other
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