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.

"Lower in the atmosphere, where there are lots of molecules, this energy can be dissipated kinetically through collisions with other molecules. At very high altitudes where the atmosphere is very thin, the preferred mechanism is the release of a photon of light.  But if you get too high up, there just aren't enough light-emitting reactions taking place to produce an appreciable amount of light.  There is a kind of sweet spot in the atmosphere where there are enough reactions taking place to produce appreciable (detectable) light, but not so many that the dissipation is going to be predominately via molecules running into each other."

That sweet spot is about 85 to 95 kilometers, or roughly 52-59 miles, up. And that layer is, of course, where the Texas bullseye shows up.

The process that created it, however, began much lower in the atmosphere, with a thunderstorm that blew across the southwestern tip of Texas in the wee hours of the morning on April 4.

In a thunderstorm like this, there is a very strong convective current moving upward: the updraft. As water vapor condenses and the resulting droplets freeze into ice, the thunderstorm releases heat into the air.

"This dramatically warmed air strengthens the updraft further and as this buoyant air accelerates and eventually impacts against the 'lid' of the troposphere," Miller said.

Imagine tossing a stone into water: what we're seeing here is the remnant of the collision of a storm and the tropopause (the "lid"), around 12-15 km above the surface.

In that kind of impact, Miller said, "a whole spectrum (many different amplitudes, many different wavelengths) of upward-moving waves are formed, carrying energy to the upper atmosphere." Most of them break up before they reach the airglow sweetspot, but some of them don't. And as those "pass through [the] critical layer of the atmosphere where nightglow occurs, the layer is perturbed ... and the structure of the wave is revealed."

It's important to understand that these kinds of waves happen all the time, all over the world, "not just by thunderstorms but by many other processes, including air flowing over the mountains, strong changes in wind speed associated with the jet stream, etc."

And that's what makes this research more than a quirky exercise in planetary lightshows. It's not just that there are these previously hidden features of the atmosphere that we can now see, but that they are significant features of the way the earth's global circulatory system works. If we want to model weather and the climate, scientists need to understand the interactions between the lower levels of the atmosphere we're familiar with and the more mysterious upper reaches.

"Models that attempt to predict weather and climate must account for the full circulation of the atmosphere, since circulations happening in the upper atmosphere can feed back down to influence our near-surface weather at a variety of space and time scales," Miller said. "Right now, these models have little information on what is going on with the upper atmospheric waves—they must make simple assumptions that are likely not capturing the true processes. This limits the skill of prediction."

But, of course, until the Suomi satellite launched, it was difficult to study those interactions.

The bullseye, then, isn't only the remnant of a storm's encounter with the outer atmosphere, but a target for climate scientists, too. As we seek to explain the planetary changes that we've unintentionally set in motion, this is one areas of dynamism that bears further scrutiny.

This week, Miller watched for nightglow through the satellite's eyes as the Earth moved between the sun and the moon, causing a lunar eclipse. That allowed him, he said, "to catch a glimpse of some nightglow waves, which was kind of neat. These hidden signals in the darkness."

Via Jayson Prentice