Mesospheric Bore

Nov. 29, 2016: This month, a lot is happening in the mesosphere. The mesosphere is a layer of Earth’s atmosphere above the stratosphere; it is the realm of sprites, noctilucent clouds (NLCs), and airglow. Starting on Nov. 17th, NASA’s AIM spacecraft spotted bright noctilucent clouds forming in the mesosphere above Antarctica. Then, in an apparently unrelated development on Nov. 24th, the normal dome of airglow over China split in two. Xiao Shuai photographed the event from Mount Balang in Sichuan:

This is called a “mesospheric bore”–and not because it’s dull.  A bore is a type of atmospheric wave with deep ripples at its leading edge.  Indeed, you can see the ripples in Shuai’s photo separating the zone of airglow from clear sky.

Bores fall into the category of “gravity waves”—so called because gravity acts as the restoring force essential to wave motion. Analogy: Boats in water. When a boat goes tearing across a lake, water in front of the boat is pushed upward. Gravity pulls the water back down again and this sets up a wave.

In this case, instead of water, rarefied air is the medium through which the wave propagates.  The sudden boundary in the airglow layer is probably akin to a hydraulic jump.  But what created the disturbance in the first place?  (What is the ‘boat’?) No one knows.

“There may be updates in the coming days as scientists from NASA and the Chinese Academy of Science check data from satellites to learn more about this event,” says Jeff Dai, who has been helping Xiao Shuai process and communicate his extraordinary images. “Also, we encourage other photographers from Thailand, Myanmar, Bangladesh and India to submit their images of the wave.”

Realtime Space Weather Photo Gallery

Amazing Airglow above Easter Island

by Dr. Tony Phillips (Spaceweather.com)

Oct. 4, 2016: Not every colorful light in the night sky is an aurora.  Especially not in the South Pacific. Yuri Beletsky was on a beach in Easter Island, Chile, two nights ago when the starry canopy turned red:

“There was no fire,” says Beletsky. “This is an amazing display of airglow.”

Airglow is aurora-like phenomenon caused by chemical reactions in the upper atmosphere. Human eyes seldom notice the faint glow, because it is usually very faint, but it can be photographed on almost any clear dark night, anywhere in the world.

Beletsky is a veteran photographer of airglow, having captured it dozens of times from sites in Chile and the South Pacific. “The intensity of airglow varies, and sometimes it can be more prominent, as it was on Oct. 2nd,” he says.

The curious thing about Beletsky’s photo is not the intensity of the airglow, but rather its color–red. Airglow is usually green, the color of light from oxygen atoms some 90 km to 100 km above Earth’s surface. Where does the red come from? Instead of oxygen, OH can produce the ruddy hue. These neutral molecules (not to be confused with the OH- ion found in aqueous solutions) exist in a thin layer 85 km high where gravity waves often impress the red glow with a dramatic rippling structure.

Realtime Airglow Photo Gallery