Recently we've featured some really amazing images of Earth taken from the International Space Station (ISS).
At 225 miles up, astronauts aboard the ISS enjoy an unprecedented and privileged view of our world, literally from the edge of space. In addition to having a vantage point from which to view the countries, continents and oceans below, they also get to observe amazing atmospheric phenomena from a unique viewpoint that few humans have ever experienced.
From lightning flashing strobe-like within storm clouds to the undulating ribbons of the aurorae, the ISS crew sees it all from above –- and shares with us in the photos they take.
One phenomenon that's not so visible from Earth's surface but shows up regularly in astronaut images is a glowing hazy band known as airglow.
A photochemical reaction that occurs high in the atmosphere, airglow is created by atoms, molecules and ions that have been excited by ultraviolet radiation from the sun. They release that energy as visible - as well as infrared and ultraviolet - light when they return to their normal state... not entirely unlike glow-in-the-dark toys or paint!
This light is most visible in photos when the ISS is orbiting above the night side of the planet, as seen above. Although present around the entire globe, airglow appears as a thin band around Earth's limb because viewing the atmosphere at a shallow angle –- rather than directly down through it - dramatically increases the layer's relative visibility.
Most visible airglow is created by oxygen atoms and molecules which glow green, also commonly seen in auroras. Other airglow elements include sodium atoms and nitrogen ions. While these elements are present in the atmosphere at many layers, the region that glows visibly is constrained to a region 85-95km (53-60 miles) up in a band about 6-10km (4-6 miles) wide. The reason for this is that below those heights the atoms and molecules are more concentrated and collide more readily, releasing their energy sooner, and above that altitude the density of the atoms is too low to do much colliding at all.
There are other factors involved with airglow as well, such as temperature and altitude, as well as different kinds of airglow depending on when in the day they occur.
Find out more about airglow in this article by the Institute of Astronomy and Astrophysics (Instituto de Astronomía y Física del Espacio) in Buenos Aires, Argentina.
Image credit: NASA.