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SYFY WIRE Bad Astronomy

Red and Green Ghosts Haunt the Stormy Night

By Phil Plait

Randy Halverson is a photographer and has quite a gift for time-lapse video (I’ve featured his work many times on the BA blog).

He recently sent me a video he took in central South Dakota that is quite astonishing. While photographing a storm at night, he caught two very rare events at the same time: sprites, and gravity waves rippling through airglow!

First, here’s the video, because it’s amazing.

Did you see the sprite? It happens just before the six-second mark in the video. Look to the right, just above the storm cloud. The red flash is obvious once you spot it.

Sprites are a phenomenon associated with lightning storms; they’re electrical discharges from the top of the storm cloud, not the bottom. They’re not really understood, but they occur simultaneously with lightning between clouds or from cloud to ground, and glow eerily red. They were only first discovered in 1989 (officially, that is; pilots have been reporting them for decades, but no one outside that cadre really took them seriously) so not a huge amount is known about how they operate.

On a larger scale, you can also see green ripples moving across the sky. The green is airglow, molecules in the upper atmosphere that are energized by the Sun during the day, and give off that energy as light at night. This occurs via chemoluminescence; a process where the excited nitrogen and oxygen atoms molecules bump each into other and form bonds, giving off that light.

The rippling is due to gravity waves. This is simply an up-and-down oscillation of something under the influence of gravity. For example, waves on the surface of the water in your bathtub are gravity waves; the water gets pushed up a little bit (maybe when you plop your rubber ducky into the water), and then gravity pulls that crest of water back down. But the water itself pushes back, and you get oscillatory motion.

This can happen in air, too (air is a fluid, after all). Currents of air in the upper atmosphere bob up and down pretty often, similar to the water in your tub. This motion can disturb the process that creates airglow, so you get those rippling waves moving across the sky. I only recently learned about this phenomenon, when I saw a time-lapse video taken in Chile, a world away from South Dakota.

Several years ago, Randy sent me an email asking about a weird rippling glow he saw in some footage he had taken. I wasn’t sure at that time what it was (though I was sure it wasn’t an aurora), and we agreed it could be airglow. It’s funny that he would send me this new video right after I finally learned what that rippling was! If Randy had asked me two weeks ago I wouldn’t have known. He found out on his own, and now we both understand. And I hope now you do too.

The sky above us is just incredible. There’s still so much to discover, so much to figure out, so much to explore. And it’s all right there, just over our heads.

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