You may have seen the famous blue marble or pale blue dot images showing Earth from 18,000 and 3.7 billion miles away, respectively. But closer to home — some 300 miles above Earth’s surface — you might encounter an unfamiliar sight: vibrant swaths of red and green or purple and yellow light emanating from the upper atmosphere.
This light is airglow.
Airglow is created when atoms and molecules in the upper atmosphere, excited by sunlight, emit light to shed excess energy. Or, it can happen when atoms and molecules that have been ionized by sunlight collide with and capture a free electron. In both cases, these atmospheric particles emit light in order to relax again. The process is similar to how auroras are created, but while auroras are driven by high-energy solar wind, airglow is energized by day-to-day solar radiation.
Since sunlight is constant, airglow constantly shines throughout Earth’s atmosphere, and the result is a tenuous bubble of light that closely encases our planet. Its light is too dim to see easily except in orbit or on the ground with clear, dark skies and a sensitive camera — it’s one-tenth as bright as the light given off by all the stars in the night sky.
Airglow highlights a key part of our atmosphere: the ionosphere. Stretching from roughly 50 to 400 miles above Earth’s surface, the ionosphere is an electrified layer of the upper atmosphere generated by extreme ultraviolet radiation from the Sun. It reacts to both terrestrial weather below and solar energy streaming in from above, forming a complex space weather system. Turbulence in this ever-changing sea of charged particles can manifest as disruptions that interfere with Earth-orbiting satellites or communication and navigation signals.
Understanding the ionosphere’s extreme variability is tricky because it requires untangling interactions between the different factors at play — interactions of which we don’t have a clear picture. That’s where airglow comes in. Each atmospheric gas has its own favored airglow color, hangs out at a different height and creates airglow by a different process, so we can use airglow to study different layers of the atmosphere.
Airglow carries information on the upper atmosphere’s temperature, density, and composition, but it also helps us trace how particles move through the region itself. Vast, high-altitude winds sweep through the ionosphere, pushing its contents around the globe — and airglow’s subtle dance follows their lead, highlighting global patterns.
Two NASA missions take advantage of precisely this effect to study the upper atmosphere: ICON — short for Ionospheric Connection Explorer — and GOLD — Global-scale Observations of the Limb and Disk.
ICON focuses on how charged and neutral gases in the upper atmosphere behave and interact, while GOLD observes what drives change — the Sun, Earth’s magnetic field or the lower atmosphere — in the region.
By imaging airglow, the two missions will enable scientists to tease out how space and Earth’s weather intersect, dictating the region’s complex behavior.
Keep up with the latest in NASA’s airglow and upper atmosphere research on Twitter and Facebook or at nasa.gov/sunearth.
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