Have you ever looked at the clouds during the day and seen what appeared to be ripples or waves in the clouds? If you have, then you have seen a gravity wave. I am examining these waves for my Masters project, which involves counting how many waves arrive during the polar night, determining if there is a direction that the waves prefer to travel in, and how particular atmospheric conditions – like Sudden Stratospheric Warmings – affect the waves. Knowing how these waves are acting will help us understand more about polar dynamics.
Gravity waves are not gravitational waves. Both are fascinating, but they are distinct phenomena. Gravity waves are caused by parcels of air being moved around by buoyancy; they are responsible for transferring energy and momentum between different parts of the atmosphere. Gravitational waves are caused by large masses orbiting each other, and are of great interest to astrophysicists. The waves we observe in the atmosphere (gravity waves) are also, less confusingly, referred to as buoyancy waves.
I view gravity waves by using a camera called the All Sky Imager. This instrument is a camera that can take pictures of the whole sky. It is part of the suite of instruments located at the Polar Environment Atmospheric Research Laboratory (PEARL) near Eureka, Nunavut.
The All Sky Imager takes pictures of the very faint light at specific wavelengths arriving from the airglow layer. Airglow is the emission of light from the atmosphere itself. The “airglow layer” we are watching lies between 85 km and 100 km. Below about 85 km, air molecules tend to release energy through collisions instead of emitting light. Airglow light is very faint; the only way to see it with the naked eye is to go out on a clear night away from any source of light pollution. If the stars aren’t bright then you might be able to see a faint green glow across the whole sky. If you happen to be an astronaut aboard the International Space Station, you will have a particularly impressive view of the Earth’s airglow.
Since the All Sky Imager takes pictures every minute during the polar night, it is possible to see other interesting atmospheric phenomena. Meteorites flying through the atmosphere are one striking example of what we observe with this instrument. They appear as bright lines across the image. One of my colleagues recently referred to them as “rockets flying past” in the images while making the appropriate noises to accompany their appearance! Frequently, the imager observes the aurora borealis, which looks like a band going across the image that appears to be “dancing”. An event known as bore waves can also been in some of the images. These sorts of waves are very similar to tidal bores, an example of which is the one that can occur in the Petitcodiac River due to tides from the Bay of Fundy.
One way to be able to see gravity waves occurring in the atmosphere is to make what is known as a ‘difference movie’. These movies are made from differences between consecutive images over a whole day.
I could watch them all day because I never know what interesting feature I will see in them. Unfortunately, after the fourth or fifth viewing, reality starts suggesting I should get back to my research. Below is an example of a difference movie on YouTube made with the All Sky Imager that illustrates some interesting events such as aurora borealis, ripple events, and meteorites flying by. How many waves can you count in the movie?
The All Sky Imager has been running every winter since 2007 – that’s a lot of images and movies to look at! This motivated us to program a computer to find waves in images. This challenge has been the main work behind my Master’s project. In a couple of months, I hope to find some really interesting results.
– Chris Vail
A University of New Brunswick M.Sc.student who’s not procrastinating from writing his thesis by watching movies of his data…