Aurora Borealis and the Visible Wavelength Spectrum
by Meg Smith
Late on a Sunday night in November 2004 as I was driving home from a wedding, I noticed an
unusual glow of palest green in the sky above Northwestern’s Fisk Hall. Campus lights? It was awfully
bright for that. Were they filming a movie? Normally my curiosity might have ended there but this
greenish light seemed out of place. In a moment it occurred to me there might be a more cosmic
explanation, however far-fetched, and I had to find out what it was. As I swung into NU lake front parking
lot and made a beeline to the far end, my hopes grew. Finally I yanked the parking brake into place,
jumped out of the car, and my wildest suspicions were confirmed: over the lake hung a colossal expanse
of eerie green light, like a ghostly glacier suspended in mid-air far over the water, and I realized I was
seeing for the first time in my life … the northern nights!
Directly above me, spikes of green light shot sharply skyward almost to a point, as though I was
looking up into the center of a luminescent pyramid. To the north, the entire horizon glowed as if strung
from end to end with a pleated green curtain. The northern lights rarely reach this far south as more than
faint glowing patches that are hardly noticeable, but this unearthly display filled the entire sky, morphing
slowly and silently around the cloudless night. Later, the mass and streaks and curtain that had hung so
still launched into a lively dance of pale glowing ribbons rippling across the sky. Thick, wavy streams of
light pulsed from one horizon to the other. I half expected cracks of thunder to accompany the lighteninglike flashes, but the turbulent light storm was soundless. Throughout the evening as I took it all in, I
wondered what is all this?
Galileo dubbed the mysterious phenomenon of the northern lights aurora borealis, after Aurora
http://www.thaliatook.com/OGOD/aurora.html, the Roman goddess of the dawn, and the Greek word
“boreas” meaning “north wind.” Apparently from his latitude on the planet (somewhere around Pisa,
Italy), Galileo saw mostly red auroras -- like the red light of dawn -- and he concluded that auroras were
caused by sunlight reflecting off the ice crystals in the atmosphere. Later spectral analyses of auroral
light determined that it is not the same as sunlight, although auroras are caused by the sun. For our
viewing enjoyment, auroras occur within the visible spectrum -- the range of the electromagnetic spectrum
visible to our appreciative human eyes.
According to the Geophysical Institute at the University of Alaska http://odin.gi.alaska.edu/FAQ/
auroras start with a “coronal mass ejection” (CME) http://www.youtube.com/watch?v=nI44kE1j6nI, a
disruption on our fiery sun that blasts up to a billion tons of matter away from its surface. This matter -solar plasma consisting mostly of ions, protons and electrons -- hurls through space on the “solar wind” at
thousands of miles per second. If the CME is aimed in our direction, it meets Earth's magnetic
atmosphere within two days, bursts through the outer atmosphere at the two magnetic poles (north and
south), and collides with particles of oxygen and nitrogen sparking a “geomagnetic storm” in the upper
atmosphere, at altitudes anywhere from 60 to 200 miles above Earth.
During this storm, oxygen and nitrogen electrons are slammed by the solar wind, become
“excited,” and jump up to higher quantized orbits around their atomic nuclei. Some electrons are bumped
out of their orbits completely, resulting in ionized atoms of oxygen and nitrogen. Then the particles return
to their normal energy levels -- the excited electrons release photons (or ionized atoms regain missing
electrons), the electrons hop back down to a lower quantized orbit, and photons are emitted into the
atmosphere as colored auroral light that we see in visible spectrum.
DEMONSTRATION VIDEO: http://www2.gi.alaska.edu/asahi/movies/neon.html electron
absorbing energy, then emitting it as a photon of light, and continuing demonstration of these photons
creating auroras.
The various auroral colors depend on which atoms photons are released from, and at what
altitude. The chart below shows which atoms produce certain colors, starting from the lowest altitude up
to the highest:
Red:
atoms of oxygen higher then 150 miles from Earth
low frequency long wavelengths of light
Green:
atoms of oxygen up to 150 miles from Earth
mid-frequency mid-wavelengths
Purple/violet:
nitrogen above 60 miles from Earth
high-frequency short wavelengths of light
Blue:
ionized nitrogen up to 60 miles from Earth
high frequency short wavelengths
Green is the most common auroral color. The atmosphere at high altitudes contains a greater percentage
of atomic oxygen and is very thin, giving the atoms ample opportunity to emit red light. At more moderate
altitudes, the combination of red, blue, and green lights from oxygen and nitrogen combine to produce
greenish-white light. At the lower edge of the curtain, the density of molecules doesn't permit oxygen to
emit light; the pinkish color comes from a combination of red and blue from nitrogen.
Most auroras are visible closer to the north pole and the south pole, where they are called the
aurora australis. Particularly powerful geomagnetic storms occasionally push the auroras to skywatchers
at lower latitudes than normal. Spaceweather.com reported that the display I serendipitously caught
lasted from November 7-10, 2004 and was seen in every U.S. state except Hawaii. I’m still amazed no
one I know, except my brother in Madison, was even aware it happened.
I wrapped myself in a woolly blanket and lay in the sand at Evanston’s Lighthouse Beach for a
better view. For the hour I was able to stay awake and reasonably warm I scanned the sky intently, eager
to catch every last flicker. In that time, only three other spectators appeared: a couple with their dog, and
a man who saw the lights from the road and pulled in. “Like a camp fire in the sky!” he exclaimed. Within
a few minutes he drove off. Finally, the chill and weariness in my bones called more strongly than the
fascination overhead. I hated to leave while there was even a hint of light left in the sky, but I pulled
myself away from the beach and drove home. A few nights later I caught the farewell display of that multinight performance: a pink wash in the northern sky and a faint but unmistakable green glow over Lake
Michigan. I watched for maybe half an hour, sad to see them fading away but thoroughly uplifted and
transformed by having seen them at all. The thrill of that first spectacular show will live with me forever,
and I hold out hope that I’ll see the northern lights once again over my hometown. And next time I’m
waking everyone up to tell them about it!
• Spaceweather.com aurora gallery from November 2007: http://www.spaceweather.com/aurora/
gallery_01nov04.htm
• Time lapse auroras in Finland: http://www.youtube.com/watch?v=Lc3FxNXjBs0
• Aurora Australis from the ISS: http://www.youtube.com/watch?v=ndQEq48IZA4
• What causes the colors of the aurora: http://www.webexhibits.org/causesofcolor/4D.html
• Auroral colors and spectra: https://www.windows2universe.org/earth/Magnetosphere/tour/
tour_earth_magnetosphere_09.html
• How stuff works-Aurora borealis: http://science.howstuffworks.com/nature/climate-weather/atmospheric/
question471.htm
• Stars and northern lights as seen from the ISS: https://vimeo.com/38409143