A Bullseye in the Sky Over Texas

This is a thin glowing layer of Earth's atmosphere rippling in the wake of a huge thunderstorm.

When we see patterns in the atmosphere from space, they tend to be in the clouds of powerful storms. These all have roughly the same form: they look like a spiral galaxy with arms spinning out from the core. 

But meteorologists have detected other organizational principles at work. Like, take the fascinating image above. It shows .... well, I wasn't sure exactly what it showed. A meteorologist's blog post described them as "convectively-generated mesospheric airglow waves," but that did not quite explain how they worked or what they were.

So I got in touch with Steven Miller, senior research scientist and deputy director of the Cooperative Institute for Research in the Atmosphere (CIRA) at Colorado State University. Miller and his colleagues discovered these concentric rings while working with the newish satellite Suomi satellite's next-generation low-light sensor. (They published a paper on the discovery in PNAS.)

Miller told me I was looking at glowing ripples in the atmosphere itself!

"These are literally 'ripples of glowing atmosphere' whose structure is the result of a train of gravity waves that is passing through a thin layer of the atmosphere that produces a very faint veil of light called 'nightglow,'" he said. "These are not clouds (although they were forced by the thunderstorms below), and they do not occur in the troposphere, where our 'weather' is. They are much higher up—at the interface between the mesosphere and the thermosphere—about 90 km [55 miles] above the surface! The glow is revealing important dynamics of our atmosphere that would otherwise be completely invisible to us."

Satellites carry imaging equipment that can see light far outside the human visual range. So I figured that these ripples would not be visible to a person. It turns out, though: they are! Nightglow shines, in part, in the spectrum our eyes work in. While the light is strongest in the infrared we can't see, it is present in the range that we can: violet (380 nanometer) to deep red (740 nanometer). 

"And we do have some sensitivity to it—in fact, the nightglow is a source of background light in the nighttime sky that explains why on the darkest of nights and far away from surface lights you can still see the silhouette of your hand held up against the blackness of space," Miller said.

But, sadly, "for the most part, this light is simply too faint for us to notice," Miller said. "However, there is indeed enough overlap with human eye's response to permit detection of the strongest nightglow features with dark-adjusted human vision on a moonless night.  There have been cases when the waves can be discerned by the naked eye, but this is rare."

Imagine being out somewhere in Texas, way out beyond where the cities' light pollution reaches, on a moonless night. A thunderstorm arrives. You dismount and take cover. When it passes, you look up, and it's like there's a faintly glowing bullseye in the sky. No wonder people used to believe in magic.

OK, ending cowboy fantasy. Back to nightglow waves.

The next step in understanding what's going on here is to understand why a layer of our atmosphere glows. Reactions in the atmosphere can release excess energy. For example, UV light can break down oxygen molecules, which then recombine to form new molecules. Sometimes, the energy release of the reaction releases a photon of light. The general term for the phenomenon is "airglow" and the process that creates it is chemiluminescence.

Sit back for a minute and appreciate this: our planet glows. 

Annotated image from the International Space Station (Miller et al in PNAS).

"The exact processes vary between the day (dayglow), terminator (twilight glow), and the night (nightglow—which we are seeing here), but the basic idea is that chemical species react with each other to form other species, and sometimes the result is an amount of 'leftover energy' that must be dissipated," Miller explained.

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