The Colors Of The Sky: What Is Airglow?

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Airglow is caused by sunlight exciting ions in the atmosphere of Earth. This causes faint light to be emitted is called airglow.

Humans have been exploring space in different forms for over 50 years now. Today, humanity is looking farther into the universe than ever before. However, the space closer to home has a few delights of its own to offer. At the junction of the end of Earth’s atmosphere and the beginning of outer space, there is a glowing bubble of green, blue and red light that seems to encapsulate the Earth. This glowing layer of atmosphere is called airglow.

ICON and Airglow - Nighttime View

Airglow as seen from the upper atmosphere. (Photo Credit : NASA /Wikimedia Commons)

In all probability, most people living in cities have never seen the tints of airglow painting the sky. The brightness of civilization makes the night look like a cold black vastness speckled with tiny shimmering stars. Airglow is almost uniformly present in the sky, but it’s so faint that other light sources, such as artificial human light, light from celestial stars, the sun and the moon outshine it.  If one ventures to a place unpolluted (relatively) by light, cameras—and perhaps even unaided human eyes—are able to capture the faint hues of airglow.

What causes Airglow?

Airglow is caused by the energy released by molecules in the upper atmosphere being excited by the harsh rays of the sun. The earth’s atmosphere is mainly composed of oxygen and nitrogen molecules, among others. These molecules, when excited by sunlight, as a result of all the excess energy, split up into atoms or ions.

These atmospheric ions and atoms are present in the upper atmosphere in a vague dispersed region called the ionosphere. The ionosphere spans from the mesosphere at about 60 km to the outermost reaches of the Earth’s atmosphere—the exosphere.

Now, these ions and atoms need to release this excess energy. There are multiple ways they can go about doing this. The best way is if they collide with another atom, forming a molecule and releasing energy in the process. This is possible in the lower atmosphere, where the air is dense with molecules with which to collide.

However, as one travels upwards, the outer reaches of the atmosphere are less dense and the probability of another atom being available to collide with decreases drastically. Not all of these interactions will lead to the production of light.

In the absence of any other matter to collide with, if the particle holds on to the energy long enough, it can then release this energy by transferring its electron to a lower energy state (or an orbital with a lower energy state). This jump from a higher energy state orbital to a lower one is accompanied by the release of energy in the form of a photon with a particular wavelength.

Different molecules emit different wavelengths, creating the different hues of airglow. This process is called chemiluminescence, and is similar to the process that happens in glowsticks and fireflies.

Airglow is present all the time, whether it’s visible or not. It is called dayglow during the day and is never seen because of the intense brightness of the sun, Twilightglow occurs around twilight and is one of the brightest varieties to behold, and the last is the one that occurs at night, called Nightglow (very creative naming, yes).

The colors of Airglow

As mentioned before, the ionosphere is not a distinct layer of the atmosphere. It stretches from about 50 km in the mesosphere all the way up to about 300 km. At different heights, a different hue of airglow is produced based on the different composition of ions, as well as other conditions, such as the intensity of solar radiation, temperature, and the effect of the Earth’s weather on that layer.

The concentration of ions at different heights are also different, so the color of airglow is different depending on how high (or low, from a satellite’s perspective) you look. The sodium layer at about 50 to 60 km above Earth’s surface radiates a yellow-orange light (approximately 590 nm wavelength).

OH and molecular oxygen can create hues of violet-blue wavelengths (380 to 490 nm) or red wavelengths (650 to 700 nm). Atomic oxygen colors the sky in three particular wavelengths—green (508 nm), orange-red (629 nm), and red (632 nm). Emissions in the infrared and UV regions of the electromagnetic spectrum also invisibly dominate the sky and are stronger than the emissions in the visible spectrum.

Why study Airglow?

Many of the spacecraft sent to space are not for exploring the universe, but rather for the practical purpose of GPS, TV, weather predictions, etc. These near-Earth satellites orbit relatively close to Earth and communicate with each other and stations on Earth through the use of lasers. The ionosphere and the ions present there tend to interfere with these signals. Changes in the ions, their density, and localization in the ionosphere will affect how satellites record data and communicate it.

But the ionosphere is just invisible air, after all, right? Correct, but airglow is like an indicator to see and observe changes in the ionosphere. Changes in the characteristic photometric spectrum emitted by the ionosphere is how changes in the ionosphere are detected.

The composition of the ionosphere is affected by the weather in space, as well as the weather conditions on Earth. The weather on Earth, the intensity of solar radiation, and gravity waves are some of the causes of changes in the ionosphere. Gravity waves are created when there is a disturbance between a fluid medium and gravity or buoyancy tries to restore the fluid to how it originally was or attain equilibrium. The ripples seen in a pond when a stone is dropped in is due to gravity waves. They are different from gravitational waves.

Orbitalaltitudes

The different altitudes of various near-Earth orbiting satellites (Photo Credit : Rrakanishu/Wikimedia Commons)

Since the time it was first discovered by Angstrom in 1868, studying airglow has been a challenging task. Dayglow is not easily visible or recordable from here on Earth, while the intensity of twilightglow and nightglow is not always strong enough to gather all the information desired. Here on Earth, spectroscopes are used to study which electromagnetic emissions make up airglow. Space travel now makes such observations easier and more accurate. ICON and GOLD are two of NASA’s near-Earth satellites that study the changes in airglow.

Auroras vs. Airglow

Airglow might sound the same as the auroras that dazzle us near the magnetic poles of the Earth, but they’re created through different processes. The auroras are caused when solar wind composed of plasma (different from solar radiation) interacts with matter in the atmosphere near the magnetic poles of the Earth—the magnetosphere.

This means that they only occur near the magnetic poles (North America and parts of northern Europe in the North, and New Zealand, Tasmania, and Antarctica in the South). We’ve covered the science of auroras in more detail here and here.

Airglow, on the other hand, is uniform and much less intense than auroras. It is created by the excitation of molecules in the ionosphere by solar radiation and their subsequent return to stability through the release of light. Traveling to see airglow would only require you to find somewhere with little to no light pollution.

It is fascinating to know that the dark skies we see above us are secretly filled with glowing light that is nothing short of magic. These lights help (or sometimes don’t) us transmit our GPS signals and TV broadcasts without ever making a fuss. Even as we continue to explore outer space, we cannot forget that the magic close to home can also dazzle and mystify us!

Reference:

  1. Nasa.gov
  2. Encyclopædia Britannica
  3. University Corporation For Atmospheric Research
  4. Atmospheric Optics
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About the Author:

Salama has a degree in Life Science and Biochemistry from St. Xavier’s College, Mumbai. She enjoys being in the water much more than being on land. She is passionate about science and wants to declutter science from its jargon to make people understand its beauty.

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