[♪ INTRO] Back in 1999, the turn of the century was shaping up to be an exciting time for astronomers.

If all went well, there would be exoplanet discoveries, new rovers on Mars, and groundbreaking research on the brand-new International Space Station. But one thing no one was expecting was to start off the century with a fresh mystery that would take over two decades to solve.

Then, in January of 1999, satellite photos of a solar flare revealed something odd: There seemed to be dark spikes coming out of the flare and flowing downward, toward the Sun… slowly. Like… too slowly. And it wasn’t until 2021 that researchers finally figured out why.

Now, astronomers have been observing solar flares since 1859, and we’ve become pretty familiar with them. Basically, they’re massive bursts of radiation that happen when magnetic field lines on the Sun get all twisted up and suddenly release a surge of magnetic energy. When that happens, bright light and plasma, or superheated gas particles, stream away from the Sun.

And unlike a lot of astronomical phenomena that have no direct effect on our lives, these outbursts can occasionally mess with communications and power grids if they erupt in our direction. So telescopes are constantly monitoring the Sun’s activity so that scientists can keep an eye on this kind of thing. And as telescopes have gotten better, they’ve given us a more and more precise look at solar flares themselves.

In the years leading up to 1999, scientists studying solar flares noticed that certain flares had a sort of dark fringe around them. But it wasn’t until that year that they got observations from the Japanese Yohkoh spacecraft that revealed how matter was moving in these fringe areas. And that’s when things got interesting.

The observations showed that as the solar flares erupted upward, the plasma in these dark regions was falling downward. At the time, scientists had an idea why this might be happening. They hypothesized that it had to do with the magnetic field lines.

See, after magnetic field lines that have been all twisted up release a bunch of energy, it’s kind of like if someone snipped a bunch of tightly tangled ropes. The tangle gets undone and the ropes, or magnetic field lines, resettle into a new configuration. The scientific term for this is magnetic reconnection.

And as magnetic field lines shift, that shifts the flow of plasma. So the team that made the 1999 discovery hypothesized that the downward-flowing regions were driven by magnetic reconnection. But there was one piece of the puzzle that didn’t quite fit.

These downflows, officially named supra-arcade downflows or SADs, were slow. Scientists compared real-life the observations of them with models that were based on the magnetic reconnection hypothesis… and they just didn’t line up. The real-life downflows were consistently slower than the ones in models.

Which left scientists scratching their heads and searching for a different explanation for these slow-moving plasma flows… for more than 20 years. To get to the bottom of this, a U. S.-based team of astronomers turned to data from NASA’s Solar Dynamics Observatory, which has been continuously imaging the Sun since 2010.

They gathered data on solar flares that had these mysterious downflows. Then they made some three-dimensional simulations of solar flares and compared these to the observations. Again, they concluded that magnetic reconnection just didn’t explain what they were seeing in the real world.

But after taking a close look at the data on actual solar flares, the researchers figured out that there was something different going on. And the explanation was both surprising… and kind of ordinary. Basically, the downflows that appear on solar flares happen at the boundary between areas of low- and high-density plasma.

Where the two meet, it’s like oil and water: Their different densities keep them from mixing, but they form little ripples and bubbles along the boundary. How exactly these ripples evolve into those dark spikes involves some messy physics, but it all comes down to fluid dynamics. Those dark spikes themselves are basically voids, where there is much less plasma than in the surrounding areas.

And they emerge because of the way fluids of different densities interact when they meet. So not exactly like a lava lamp, but not not like a lava lamp. In 2022, the team published their results in Nature Astronomy.

Over two decades since the 1999 discovery of these mysterious downflows, researchers finally had a pretty good idea what was causing them. The answer doesn’t just answer a nagging question that’s been hanging over astronomers since 1999. Any steps toward understanding solar activity help us keep better tabs on space weather and manage the risks of living in the orbit of a scorching ball of gas.

It also suggests a way to understand similar features in more exotic environments, like supernovas. The remnants of these exploded stars also have spike-shaped voids that emerge in turbulent environments, and they could come from a similar phenomenon. So it was worth the wait, because our Sun might help us understand worlds way beyond our own.

Thanks for watching this episode of SciShow Space. If you want to help us explain even more mind-blowing space stuff to the whole Internet, you can support us over at patreon.com/scishowspace. [♪ outro]