There is a very different world from ours, very near to the world we know – less than an hour away at the speed of highway traffic; five minutes away at the speed of a rocket accelerating into orbit. This is the world of outer space – a mere fifty miles above our heads.
The air there is unbreathably thin – near vacuum. And the elements of that air take forms very different from those we are accustomed to down on the surface. Where we live, the air is composed of molecules – O2, N2, CO2, OH2. But at the edge of space above us those molecules have been separated into free atoms by the unshielded solar radiation.
There are rocks in the space above us. But those rocks are in motion – they are falling toward Earth from space – accelerating as they get closer. At fifty miles up, they are traveling at fifty thousand miles an hour. Tons of this meteoric material are deposited in the atmosphere every day, by the shooting stars we see every night.
When they enter the atmosphere, these space rocks burn. They do not combust, they heat through atmospheric friction – rising far above combustion temperatures. They heat up until they become incandescent. At those temperatures, bonds between molecules are broken. During the final few fiery seconds of their billions of years of existence, those space rocks are atomized in our atmosphere. As free atoms, their elements are much unlike the molecular forms that make up the rocks we know down on the surface. One difference is that in their free state, those atoms glow – each with its own particular luminescence.
Our atmosphere is transparent – mostly. But we can see its faint colors – if we look through enough of it. The ocean is colored blue, but you can’t see that in a glass of sea water. You have to look through ten feet of it to catch the color. The same goes for the sky.
One way to see through a lot of the atmosphere is by optimizing the slant angle. When we look straight up, we are gazing along a sight-line that passes straight through the atmosphere – along its shortest possible path. This shows us the transparency. But if we look off at a slant, lowering our gaze toward the horizon, we can see more.
Dozens of elements exist as atomic vapor in the ionosphere. Sodium is the most fluorescent of the lot. A layer three miles thick, containing free-flying sodium atoms, lies seventy miles up. Those atoms settle out below the layer at the same rate they are replenished from space rocks falling in from above it.
Atoms of sodium vapor in the layer absorb visible light and jump up to an excited state. Less than a microsecond later, they re-emit the light as they return to the ground state. The atoms resonate between the ground state and the excited state, absorbing and re-emitting photons at a wavelength of 589 nanometers – the golden “sodium D” line.
Looking straight up, we cannot see that fluorescence in the twilight, or in the darkness of night. Its layer is thin and far away. But we can detect the fluorescence of atmospheric sodium if we increase the viewing slant angle.
One perspective that provides such an angle looks straight out toward the horizon from a few hundred miles up. At that angle, tangent to the curvature of the atmosphere and to the Earth’s surface, the sight-line stays immersed in the sodium vapor layer – below its ceiling and above its floor – for hundreds of miles. There is enough fluorescent sodium along that long line of sight to plainly see a bright golden arc – curving along just above the Earth’s distant horizon.
Another intensely colored layer rides just above the sodium vapor. At that next level, oxygen atoms become visible. They are phosphorescent – they can be raised to an excited state by visible light, just as sodium vapor can be. Excited oxygen atoms then re-emit that light – at the wave-length of the green aurora.
The air pressure delimits the phosphorescent oxygen layer. Seventy five miles up, the vacuum is already quite hard – less than a millionth of the pressure down on the surface. Higher up, the air pressure continues to fall, and the green flow fades as the oxygen itself grows progressively more scarce.
The lower altitude limit of oxygen phosphorescence is also set by the air pressure. Oxygen’s phosphorescent state has a half life of one second. During that time, excited oxygen atoms de-excite by the release of a green, 558 nm photon.
But an alternative pathway for the de-excitation can progress more quickly. That pathway releases multiple, lower energy (invisible) photons, through collisions with other air molecules. Collisions happen more frequently at lower altitudes – where the air pressure is greater.
At one atmosphere pressure, air molecules collide with each other billions of times every second. Their flight paths are bent sideways every few nanometers. That air density precludes emission of a green photon, which requires collisions to be avoided for a whole second. The colorful oxygen emission can never be seen in the air we breathe.
In the rarefied air seventy miles up, an oxygen atom flies for a meter between collisions – which happen only five hundred times per second. Some of the excited atoms will have time to emit the green photon before they lose their excitation. More of them will do so, the higher the altitude.
The half life-time of another excited state of oxygen atoms controls the altitude of another emission. Red photons (630 nm) are emitted from a different excited oxygen state. Their phosphorescence has a half life of almost two minutes. This red glow is seen from yet another layer – one hundred fifty miles up, where atoms fly for half a mile between collisions, which happen only every five seconds. At the greater air pressures below that altitude, the red emission is also pre-empted by collisions. The light of the red phosphorescent layer is not seen below that height, even at the otherwise very low pressure one hundred miles up.
So the sky glows. The luminescence is brightest by day, when unshielded sunlight resonates with free atoms of sodium and oxygen. But it is also least visible then, also because of the sunlight, which blinds our view.
Resonance fluorescence fades when the Earth’s shadow eclipses the light seventy five miles overhead during twilight. But the airglow continues through the night, due to chemical luminescence.
During the day, oxygen atoms absorb the sun’s ultraviolet light and jump up to ionized excited states. Excited oxygen can persist in metastable forms, such as O3 – ozone, through the twilight and into the night. The excited forms finally relax, through, for example, the regeneration of O2 molecules. During those reactions, their excitation energy is released into the dark as photons of visible light.
Under conditions of very good seeing, we can observe thousands of stars, down through sixth magnitude. Before them, we may notice the faint band of zodiacal light along the ecliptic. Through that, we can see Uranus. All around, we can also see the airglow. Its light is, in aggregate, brighter than that of all the stars. But the airglow is diffuse. It is most easily seen at a slant angle, close to the horizon. From ground level, its light casts the trees at the edge of the meadow in silhouette.
Under those good seeing conditions, structure in the luminous ionospheric layers becomes visible. The airglow behaves like strata of clouds. Though it is very thin, the ionosphere still conducts high speed winds. They ripple the luminescence into bands like ranks of cirrus, or push it aside, creating patches of transparency.
The airglow is intensely colored in green, golden, red, and in shades of purple where the primary colors mix. Uranus is colored sea green. But those colors are only visible in long photographic exposures. By eye, they are all colorless monochrome.
The luminous intensity of the airglow at any given spot is less, on average, than that of a third magnitude star. Third magnitude colors are too dim for our vision to perceive. We notice third magnitude lights through the rod cells in our eyes – cells that are color-blind – they see only in black and white.
Our optimal perception of the night sky does not kick in until our rod cells have become dark adapted. That may not happen until forty minutes after sunset – during Nautical Twilight. One indication that our rods have come on-line is that we can then perceive third magnitude stars coming out through the deepening dusk.
Another set of colors we can perceive at large slant angels shows in the stars themselves. Looking through the thickest part of the atmosphere, the stars twinkle most vigorously. This is seen when they are first arising – especially, above a flat land- or sea-scape.
Their white light is diffracted into prismatic flashes of color when it enters the atmosphere. It is diffracted the most when passing along the line through the thickest air – which contains the maximal number of different layers of air density. Light is diffracted when it passes from one density layer of air into another. The greatest slant angle view through the atmosphere is hundreds of miles long – much longer than the thickness of the atmosphere directly above. The air is warmest early in the evening, increasing the density variations between its layers. The effect on starlight diminishes rapidly, as the stars rise away from the horizon.
On the fall cross-quarter day, Capella rises in the northeast during Civil Twilight. As it rises, the Belt of Venus is rising above it, casting its pink arc across the horizon, centered on the anti-solar point. Below that arc, the sky is darkened by the rising shadow of the Earth. Against that backdrop, a rising Capella shows the yellow and orange flashes, which predominate in its particular color spectrum.
Sirius rises during twilight on the winter cross quarter day. When it is just above the southeastern horizon, it thrown its brightest darts of color, from deep crimson red to diamond blue. Occasionally, all of its light is diffracted away from the line of sight, and the star winks off for a split second. As a southern star, Sirius rises at a flat angle in the northern hemisphere. This keeps its image immersed in the thick air along the horizon for longer. That extends our chance to appreciate the long view – maximizing the transformative effect of the atmosphere on the starlight coming through it.
The rocks burn and the air glows: notes. Less than a thousand kg of sodium atoms are spread around the globe in the mesospheric layer 70 miles up. At the top of the layer, solar ultraviolet radiation leaves the atoms in a non-luminous, ionized form. At the bottom of the layer, the sodium atoms combine with oxygen into a non-luminous, oxidized form. Sodium vapor within the layer glows with resonance fluorescence (Gardner et al, 1989). Its faintness is balanced by the decrease in the ambient background brightness after darkness falls; and by the increase in the eye’s sensitivity to dim light after sunset. Background light intensity falls a million fold from daylight to midnight, allowing us to see the trace fluorescence. The great dynamic range of our vision allows us to distinguish differences over that range of intensities. This allows us to appreciate the evolution of the shades of twilight from afternoon until the stars are all out.
Gardner, C. S. et al, 1989 Sodium resonance fluorescence lidar applications in atmospheric science and astronomy. Proceedings of the Institute of Electrical and Electronics Engineers 77, 408 – 418