Last week, the moon cast daylight briefly away as it slid precisely between the sun and Earth. If you watched the resulting total solar eclipse – either in person (lucky you!) or on one of the many live broadcasts – you may have noticed the enormous fountain of gas that appeared to be erupting from the sun’s edge.
Called a prominence, the glowing spray was easily visible when the sun was still a crescent, long before the moon blotted out its light. Then, as the sun briefly disappeared, the prominence blazed into view along with the fiery halo of the sun’s corona.
Though I’ve seen a few eclipses before, I was completely unprepared for how striking the sight was. Space-based telescopes that stare at the sun see such spurts all the time, but seeing one from an Alaska Airlines flight, a beach in Micronesia, or the living room couch doesn’t happen often.
So, I caught up with Alex Young, a solar astrophysicist at NASA’s Goddard Space Flight Center, and asked him to help me put the spectacle in context. Do prominences often appear during eclipses? How big is this one?
Turns out, such prominences are often visible during eclipses. And last week’s flaming streamer is only as tall as several Earths, which means it’s “not that big,” Young says. The sun itself is about 108 Earths across, and these types of streamers – which are called filaments when they’re visible on the sun’s disk rather than its edge – can often span the majority of our star, stretching for hundreds of thousands of kilometers.
But what exactly are these things? Though prominences might appear to be erupting from the sun’s surface, they aren’t exactly eruptions. That can happen later. Putting it simply, they’re gassy loops that are still attached to the sun, tethered to its magnetic fields.
Occasionally, however, prominences can produce what are called coronal mass ejections, or CMEs. These eruptions occur when the sun’s magnetic field lines twist and twist and twist until they snap, kind of like a rubber band, and launch a pile of gas and charged particles into space; if that blob of stuff happens to collide with Earth, it can cause magnificent auroras, swaddle high-altitude airplanes in radiation, and — in the worst-case scenario – take down power grids and communication satellites.
Here’s where it gets interesting. At optical wavelengths, or in visible light, last week’s prominence didn’t look exceptionally special. But when Young studied it in different wavelengths, he noticed the prominence had a bit of a cavity. That means, he says, that it could end up blasting charged particles into space.
When scientists look at coronal mass ejections, “we often see a bright expanding loop, which is followed by a darker cavity and a bright core associated with the prominence,” he says. “This prominence may very well produce a CME but unfortunately we probably won’t know until it happens. But we could see it start to get higher and higher, which would be an indication that it could be undergoing a runaway type expansion and about to fly off into space.”
A few years ago, a different prominence had a cavity that took on a rather spooky shape: It was a nice, neat circle, which prompted internet speculation about some mysterious, interloping spheroid was draining energy from the sun (in reality, no such thing was happening).
In any case, if this prominence does produce a coronal mass ejection, don’t worry. It happens all the time, and Earth almost never notices.