A beautiful photo taken from the International Space Station (ISS) has captured two spectacular atmospheric phenomena around Earth in one picture.
Snapped just before dawn on March 16 during Expedition 62 as the ISS flew over the southern tip of the Alaskan peninsula, the image reveals a glowing aurora and airglow around Earth.
Both phenomena are caused by particles in Earth’s upper atmosphere interacting and releasing light. The green curve on the left is an aurora, which is created when charged particles from the solar wind penetrate through Earth’s magnetic shield and collide with atoms and molecules like oxygen and hydrogen in the atmosphere, producing bursts of light.
Oxygen atoms in the upper atmosphere release this excess energy as a red glow, while oxygen and nitrogen at lower altitudes produce the more familiar green glow.
The orange band following the curvature of Earth is airglow, which, unlike an aurora, is usually too dim to see from the ground and is best observed from Earth’s orbit. Also unlike aurorae, which are fleeting, airglow shines throughout Earth’s atmosphere constantly, day or night, encasing our planet in a bubble of light that can appear many different colors, depending on the combination of gases and how high up they are in the atmosphere.
Airglow is the natural “glow” of Earth’s atmosphere as sunlight interacts with the molecules in the atmosphere. These colorful lights reflect changes in the ionosphere, the ionized part of Earth's upper atmosphere about 80-650 kilometers (50-400 miles) above Earth's surface, not quite still Earth but not quite fully space.
Airglow is caused by sunlight depositing energy into the atmosphere during the day, which is transferred to oxygen molecules. This extra energy causes the molecules to rip apart, forming individual oxygen atoms. When they eventually recombine, it releases energy in the form of light.
Nightglow, the airglow that shines brightest in green, occurs 90-100 kilometers (55-62 miles) up, while the weaker red-orange seen here occurs due to the oxygen atoms in a lower energy excited state higher up, between 150-300 kilometers (93-185 miles), where the collisions are so infrequent the atoms have longer to radiate away their energy.
“Each atmospheric gas has its own favored airglow color depending on the gas, altitude region, and excitation process, so you can use airglow to study different layers of the atmosphere,” explains Doug Rowland, an astrophysicist at NASA’s Goddard Space Flight Center.
Each type of airglow contains information about the composition, density, and temperature of the upper atmosphere, so scientists use it as a proxy to understand how particles move through the ionosphere and what kind of particles exist there, which is key for helping us understand how Earth and space weather interconnect. Airglow holds clues to how our atmosphere affects weather in space and how space weather affects us.
Oh, and they're beautiful to look at.