An Unexpected Detection Reveals Gravity Waves in Our Atmosphere's 'Nightglow'

Quite by accident, a group of physicists based at Colorado State University has detected gravity waves rippling across Earth's highest atmospheric layers. The find, described in a paper published in the Proceedings of the National Academy of Sciences, is the first time that the phenomenon has been observed so far above the planet's surface on a global scale and with a high amount of spatial detail. The newfound detection ability may mean big things for weather and climate forecasting.

Above the Earth's surface is what can be imagined as the surface of a small pond—or several of them, in fact. At every point of transition between atmospheric layers is one of these surfaces, which are very much like the transitional regions between air and liquid that we experience regularly as the surfaces of bodies of water.

An odd wind current might disrupt one of these surfaces as a stone disrupts the metaphorical pond, and, as gravity works to restore the surface of the pond, so too does it work to restore the transitional boundary between layers of different densities following a disruption. The higher density air does not belong above air of lower density (it has more mass per unit of space), so the denser is pulled back to Earth with more force. Gravity is a function of mass.

The "global climatology" of gravity waves has been well-documented for the middle atmospheric layers of Earth, but the upper atmosphere has been more elusive. This enforces a limitation on complete models of global atmospheric circulation and, thus, on global climate models.

"These numerical models lack the observations necessary to constrain and improve upon gravity wave drag parameterizations, resulting in possible misrepresentation of important gravity wave processes," the CSU group writes. "Hence there is a clear and pressing need for high-resolution, global observations of the transient and episodic gravity wave spectrum reaching the upper atmosphere."

What the physicists found is that the Day/Night Band (DNB) radiometer on the Suomi National Polar-orbiting Partnership satellite happens to be quite capable of these observations. This arises from its ability to observe what's known as nightglow—a layer of atmosphere that becomes luminescent at night thanks to a variety of processes, such as the recombination of ions and electrons separated during the day thanks to the Sun's photon bombardment or energy released as cosmic rays collide with atmospheric particles of matter. It turns out that even without stars, there is no such thing as a truly dark night sky.

As the DNB is built to detect low levels of light, it's perfect for the job of uncovering structures within the nightglow. "On moonless nights, the observations provide all-weather viewing of waves as they modulate the nightglow layer located near the mesopause," the CSU paper explains. "These waves are launched by a variety of mechanisms ranging from orography to convection, intensifying fronts, and seismic and volcanic events."

Yes, volcanic events. One of the gravity wave-causing phenomena observed by the team was the eruption of Chile's Calbuco volcano in April 2015, which left a ring of concentric circles in the sky above. This opens the door to the possibility of using nightglow observations to detect other sorts of seismic events, such as "a large earthquake, which may produce an observable 'bright-sky' airglow response and/or gravity-wave train coupled to a tsunami front," Steven Miller, the study's lead author,tells Physics World.

Meanwhile, the study notes that, "Wave energy is recognized as the principal driver of upper atmospheric circulation, which in turn influences tropospheric weather patterns. For lack of global observations,information about upper atmospheric wave distribution and character is limited. Here, the DNB begins to fill a critical gap."

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