After 20 years, scientists discover the cause of a mysterious solar flare with ‘dark finger-like features’

CAMBRIDGE, Mass. — A strange solar flare has baffled astronomers for over 20 years, but a new study has finally discovered the cause of its mysterious downward movement.

A team from the Center for Astrophysics | Harvard & Smithsonian (CfA) says their findings could help forecast space weather and limit any damage it could cause. Astronomers have been observing solar flares, eruptions of radiation in the Sun’s atmosphere, for many years. These sudden bursts of energy projecting outwards often look like giant tentacles stretching out from the Sun.

However, in January 1999, scientists witnessed an unusual solar flare moving downwards, as if solar material was falling back towards the burning star. The strange motion of the flare, dubbed “downward-moving dark voids,” has kept the scientific community guessing until now.

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

One old theory could not explain the downward flare

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

Magnetic reconnection, when fast moving and extremely energetic radiation is released after a magnetic field breaks, has been one possible explanation for this event since the 90s.

“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,” co-author Dr. Kathy Reeves explains.

“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.”

Unfortunately, that theory did not hold water, as the solar flares’ so-called down flows were “puzzlingly slow.”

“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,” Reeves continues.

Solar flare ‘fluids’ mixing like oil and water

In this study, researchers analyzed images of the Sun captured by the Atmospheric Imaging Assembly (AIA) onboard NASA’s Solar Dynamics Observatory. AIA takes a picture of the Sun every 12 seconds in seven different wavelengths of light to measure variations in the star’s atmosphere.

3D simulations of solar flares were then created and compared with their observations. Results show these strange downward moving flares were not the product of breaking magnetic fields after all. Instead, they were formed in the turbulent environment on their own by the mixing of two fluids with different densities.

Like with oil and water, when the two fluids are mixed together, they become unstable and separate.

“Those dark, finger-like voids are actually an absence of plasma. The density is much lower there than the surrounding plasma,” Dr. Reeves details.

The researchers will continue studying these events and other solar phenomena using 3D simulations. Shedding light on these processes could help develop tools which forecast space weather and mitigate its impact on the Earth.

The findings are published in the journal Nature Astronomy.

South West News Service writer Tom Campbell contributed to this report.

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