Christmas Colors in the Aurora
"Nice column you did about the new aurora videotape," said an acquaintance. "Only thing is, you left out the science." Doggone! Knew I'd forgotten something...
The column I wrote a while back (which may or may not have appeared in your newspaper) ended up being on "The Aurora Explained" tape prepared by scientists at the Geophysical Institute, but I'd intended to write about something mentioned on the tape. Research Associate Gina Price tells what causes the colors appearing in the northern lights. (See, Christmas was coming, and I thought something about colored lights would be seasonally appropriate...Never mind. I meant well.)
With more detail taken from The Aurora Watcher's Handbook here's the basic explanation. First, envision a neon sign. Plug it into a source of electric current, and the sign lights up. That happens because, as the electrons of the flowing current hit the molecules of the neon gas, the neon emits light. The light has a specific color, characteristic for neon. Thus, the tubular letters glow a brilliant red---"Eat at Joe's" or some such message will beckon through the darkness.
With auroras the message isn't so obvious, but the mechanism is very similar. A stream of charged particles---usually negatively charged electrons, just as in the neon sign---comes whipping into the upper atmosphere. Its source is the complex interaction of the sun's activity with the earth's electromagnetic envelope rather than the operation of a utility's generator, but a molecule of gas doesn't care about the source of the electron that hits it. It just reacts to the energy.
A physicist would tell you that an atom or molecule of a gas struck by an energetic particle emits a photon of light, a quantum of energy having a specific wavelength that indicates not only what color the eye detects but also exactly how much energy the photon carries away from the gas molecule. That's precisely the case, of course, but I'm no physicist; I prefer to think of the photon as a kind of visible Ouch! given off by a gas particle when it's been hit by a speeding electron.
Earth's upper atmosphere contains mostly oxygen and nitrogen. Oxygen is responsible for the green in auroras; what auroral scientists call the "oxygen green line" falls at a point on the visible spectrum most people would describe as pale green with a slightly yellow cast.
Under other circumstances, oxygen emits a clear red color. Unfortunately, the human eye is only a fifth as sensitive to light in that part of the spectrum as it is to light near the oxygen green line, so we can't fully appreciate most red auroral light. The rare and memorable all-red auroras come from light emitted by oxygen atoms very high in the atmosphere.
Like oxygen, nitrogen emits more than one color, depending on the energy level and electrical status of the stricken molecule. (To hold with my earlier analogy, the different colors or energy content of the photons are like the different complaints given off by playing children who get hit---a soft 'ouch' from a tired child, an assertive OUCH! from a lively one.). Some nitrogen light emissions fall on the violet end of the spectrum, where the human eye is really insensitive, and others are closer to true red. The red lower borders decorating some of our prettiest auroras come from nitrogen molecules. They react quickly when hit by an electron, kicking out their photons perhaps seven-tenths of a second faster than oxygen ejects its green photon, which is why the red lower borders sometimes seem to be moving more quickly than the green curtains above them.
And the message in these colored lights? I like to think it's "Welcome to the north!"
