Airglow - What is it?
Airglow from Maui’s Haleakala National Park summit looking towards the Big Island with the Milkyway.
On my first solo trip to try night photography I drove up to the summit of Haleakala National Park. At high altitude Haleakala’s summit provides stunning views of our cosmos. A Bortel 2 rating means the light pollution is low providing exceptional night skies. Tripod, Nikon camera and a 28mm lens I set up to capture the Milkyway. A great starting point for night photography is camera on manual, lens wide open, ISO 6400 and about 10 seconds shutter speed. Play around with these settings. It was a wonderful evening. On returning home I excitedly uploaded my shots to Adobe Bridge to start editing. I could not believe how green the sky was. What was going on? My Milkyway master class colleagues provided the answer: AIRGLOW!
WHAT IS AIRGLOW?
Airglow is a faint, natural light emitted by the Earth's atmosphere. Unlike the bright light from stars or artificial sources, airglow is produced by chemical reactions occurring high above our heads. It happens primarily in the upper layers of Earth's atmosphere, at altitudes ranging from about 80 kilometers (50 miles) to 100 kilometers (62 miles).
At night, we typically associate the sky with darkness, but airglow serves as a subtle reminder that the atmosphere is still "alive" with activity. While it isn't as striking as a lightning storm or the flash of shooting stars, airglow is present nearly every night, just waiting for you to notice.
How Does Airglow Form ?
The main players in airglow are the atoms and molecules in the Earth's atmosphere, particularly oxygen and nitrogen. Here's a breakdown of how airglow comes to life:
Solar Radiation: During the day, the Sun's ultraviolet (UV) radiation strikes Earth's atmosphere, exciting atoms and molecules. This energy is absorbed and stored in the form of "excited states" of particles. However, the Sun is no longer present at night to keep this energy flowing.
Re-emission of Light: As the Sun sets and the night begins, these excited particles don't stay in their high-energy states. Instead, they relax and return to their normal state, releasing the energy they absorbed. This release of energy is in the form of light. The wavelength and color of the light depend on the type of molecule involved.
Oxygen atoms at around 90 kilometers (56 miles) altitude are primarily responsible for the green glow in the sky. When these oxygen atoms return to their ground state, they emit light at a specific wavelength that appears green to the human eye.
Nitrogen molecules and ions can also contribute to airglow, though they emit different colors like red, blue, and purple. These wavelengths are less common and often fainter than the green glow from oxygen.
Why Does Airglow Appear Green?
The iconic green glow is the result of oxygen atoms emitting light at a wavelength of about 557.7 nanometers, a color that falls within the green part of the visible light spectrum. This happens when the oxygen atoms lose the energy they gained from solar radiation during the day and release it in the form of light. This particular green color is what gives airglow its signature hue, and it is the most commonly observed part of the phenomenon.
While oxygen is the primary contributor, other elements and molecules can emit different colors as well, but they are typically much dimmer or less noticeable. For example, red emissions are often caused by excited oxygen atoms at higher altitudes, while blue or purple hues are attributed to nitrogen.
Where Can You See Airglow?
One of the beautiful aspects of airglow is that it is visible from almost anywhere on Earth, though it is most easily seen in areas far from city lights, where light pollution doesn’t overpower the subtle glow.
Tropical and High-altitude Areas: Airglow is often more noticeable in areas closer to the equator or at high altitudes. The tropics offer clear, dark skies with little light interference, making airglow more visible. High-altitude regions, like mountain peaks or observatories, also provide excellent vantage points.
Remote Locations: Airglow tends to be most visible in areas that are far away from artificial lighting, such as national parks, rural settings, or out at sea. The darker the surroundings, the more vibrant the airglow becomes.
Clear Nights: The best time to observe airglow is on clear, dark nights, especially when there's little to no moonlight. A dark, moonless sky is ideal for picking out the faint glow in the atmosphere.
Airglow vs. Aurora: What’s the Difference?
While both airglow and auroras (like the Northern and Southern Lights) involve light emissions in the atmosphere, the processes that create them are quite different.
Auroras: These are caused by interactions between the solar wind (charged particles from the Sun) and the Earth's magnetic field. When these particles collide with atoms in the atmosphere, they excite the atoms, which then release light. Auroras typically occur near the poles, where the Earth's magnetic field is strongest. They can display a range of colors, including green, red, and purple, depending on the type of atom involved.
Airglow: Unlike auroras, airglow is not caused by solar wind but rather by the aftereffects of sunlight interacting with the atmosphere during the day. It occurs globally and is present throughout the night, even in the absence of solar activity.
The Beauty and Mystery of Airglow
Airglow is a reminder of the quiet yet powerful processes that shape our planet's atmosphere. Though it’s often too faint to be easily noticed, once you become aware of it, it adds a touch of magic to the night sky. Whether you're gazing up in the tropics, standing on a mountain peak, or simply out on a dark, clear night, the faint green glow of airglow can be a stunning sight to behold.
Next time you find yourself in a remote location with a clear sky, take a moment to look up. You might just witness this invisible yet beautiful phenomenon—and perhaps even appreciate the quiet, glowing beauty of our atmosphere. Amazing Maui Stars!