Why We Can See In The Dark

In search of a pitch black night?  Don't expect to find it on Earth. Thanks to starlight, zodiacal light, and especially airglow, true darkness doesn't exist.

Pitch black nights don't exist. Not on Earth, the Moon, Mercury, Mars, or anywhere else in the solar system where you can gaze up into the night sky. Find the darkest place on Earth, hold your splayed hand up against the sky and you'll see it in silhouette. Chances are, once your eyes have become properly dark adapted, you'll be able to carefully pick your way across the landscape without a light.

What makes your hand visible, anyway? Ignoring human-made light pollution and focusing only on natural sources, there are several contributors to nighttime illumination. The stars, of course, including the unresolved ones plus starlight reflecting off interstellar dust in the plane of the Milky Way. This amounts to at most one-third of nature's night light, making it more feeble than one might suppose.

Another major player is the zodiacal light, sunlight reflecting off comet and asteroid dust concentrated in the solar system plane. Zodiacal light emissions vary over time depending on your latitude, seasonal variation of the ecliptic's angle to the horizon, and solar activity.

But the most widespread contributor to night sky brightness comes from airglow. Look at any nighttime photo taken from the International Space Station and you'll see the arc of Earth encapsulated in a thin green shell of glowing air. Unlike the aurora, which concentrates in ovals centered on Earth's geomagnetic poles, airglow pervades mid-latitudes, equator regions and polar skies alike.

If the green color reminds you of the aurora borealis, it's because similar processes are at work. Both involve the excitation of atoms and molecules — in particular oxygen — at altitudes of around 60-65 miles (100 km). But different mechanisms get them jazzed.

In auroras, electrons and protons from the Sun physically crash into oxygen and nitrogen atoms and molecules at high speed, energizing electrons within the atoms to higher energy levels. When the atoms return to their rest states, they emit photons of green and red light. With gazillions of atoms and molecules at play, the amount of light released can create staggering auroral displays.

Airglow, which is present both day and night, arises from the Sun's ultraviolet light. UV light is powerful stuff as anyone who's experienced a nasty sunburn can attest. Solar UV galvanizes several different processes in the upper atmosphere that lead to airglow emission. These include excitation, where an energized atom returns to its ground state either by itself or by smacking into a nearby atom, and photo-ionizaton, where UV radiation knocks the electron right out of an atom. When it recaptures another, the satisfied atom releases a photon of light.

In still another reaction, UV cleaves oxygen molecules apart into separate atoms that are then free to combine with nitrogen to form NO (nitrous oxide), a process that also emits photons.The brightest emission, the one that typically shows in ISS and ground-based photos, originates from excited oxygen atoms beaming light at 557.7 nanometers, or yellow-green.

With today's digital cameras working at high ISOs, airglow frequently shows up in time-exposure photos taken from dark sky sites. Years ago, I'd notice streaks of pale light across the darkest of skies when no clouds were about. Back then I couldn't figure it out. Now, thanks to my camera, it's clear I was seeing airglow. When in doubt, I'll make a 30-second exposure at ISO to 3200 with the lens wide open then check the LCD screen for telltale green streaks. I use the camera both to confirm what I see and to hunt for patches and plumes I may have overlooked.

Airglow is visible across the seasons and best visible about 10–20° high along a line of sight through the thicker atmosphere. If you look lower, its feeble light is absorbed by denser air and dust. Looking higher, the light spreads out over a greater area and appears dimmer. That said, on certain nights, I've seen the green sheen up to 50°. Too dim to register color, it takes the form of streaks, featureless smears, and plumes.

Auroras put in appearances from my latitude of 47° North but they assume different forms, move around, and are generally much brighter. Airglow is visible whether or not aurora is present and appears all over the sky — north, south, east and west. Airglow varies with solar activity and season, becoming more pronounced during solar maximum. Once identified, you'll see it nearly every moonless night from dark skies. Allowing your eyes to fully dark adapt is key to seeing the phenomenon.

Based on my own viewing experience, airglow is more obvious and widespread in the spring and summer and less so in the winter. It also varies in shape, extent, and brightness during the night. Patches can disappear or multiply, and faint streaks may brighten up and then slowly fade. What will you find?

Two nights ago, I saw and confirmed many streaks, including a remarkable series of nearly parallel "bars" perpendicular to the band of the Milky Way, likely the work of gravity waves. Unlike the more familiar gravitational waves, gravity waves are created by jet stream shear, wind flowing over mountain ranges, and even thunderstorms in the lower atmosphere. Wave disturbances propagate upward into the ionosphere to spawn and shape multiple layers or streamers of airglow.

Airglow comes in multiple colors depending on whose doing the emitting or recombining:

* Green — The most common emission occurs when UV light breaks molecular oxygen or O₂ into individual atoms about 60 miles (95 km) overhead. Rife with excess energy, they radiate green photons to return to their rest states.

* Red — I’ve never seen it, but long-exposure photos often reveal red/pink mingled with the more common green caused by excited oxygen atoms at 90-185 miles (150-300 km) emitting light as they return to the rest state. Excited OH- (hydroxyl) radicals can also radiate deep red light in a process called chemoluminescence when they react with oxygen and nitrogen. Another chemoluminescent reaction takes place when oxygen and nitrogen molecules are split apart by ultraviolet light high in the atmosphere and recombine to form nitric oxide.

* Yellow — Due to sodium atoms around 57 miles (92 km) high. Sodium, a component of meteorites, "salts" the upper atmosphere when meteoroids vaporize as meteors.

* Blue — Weak emission occurring at approximately 59 miles (95 km) altitude when two separate oxygen atoms reunite to form an oxygen molecule (O₂).

Airglow is brightest in the daytime, but the glare of daylight masks its presence. The nighttime variety is a thousand times fainter in comparison. Good thing or we'd never know a dark sky!