"The Enigmatic Aurora"

Katy McNitt - May 16, 1995
Expert on Ozone and Climate Change

    I have read a lot of articles about auroras lately, and it seems the only thing they agree on is this: for the most part, auroras are still a big mystery. I don't even know the correct plural form of the word: aurora, aurorae, auroras? The dictionary offers no assistance, so I'll stick with "auroras".

Apparently, the story starts with our sun. The sun produces heat, light, and something called the "solar wind": hot gas which is ejected from violent reactions on the sun's surface. This wind is filled with electrically charged particles, and it travels away from the sun at 900 km per second!

As part of this wind "blows" by the earth, some of the particles are caught by the earth's magnetic field. They seem to follow the magnetic lines of force to the north or south geomagnetic poles, like sand rolling down the sides of two big funnels.

It's the "funnel" part that scientists don't understand: the particles should bounce back and forth between the two magnetic poles; instead, they get "dumped" into the atmosphere.

Once in the atmosphere, these particles-- fast-moving protons and electrons-- collide with molecules in the air and release tremendous amounts of energy, causing the molecules to glow. The usual yellow-green color is emitted by atomic oxygen, but oxygen glows red, too, at higher altitudes. If you see red along the lower border of an aurora, it's probably produced by molecular nitrogen.

From outer space, you can see that the auroras occur in oval- shaped "rings of fire", around each geomagnetic pole, but from the earth, they look like broad curtains of light, sometimes stretching thousands of kilometers from east to west. Their north/south "thickness" is often less than a few hundred meters, but they reach high into the atmosphere, between 100 km and maybe 1,000 km above the earth's surface. Auroras are usually high- latitude phenomena, but during the intense solar storms of 1958, auroras were seen across the continental U.S., farther south than Salt Lake City, Denver, Chicago, and New York!

Because auroras are caused by disturbances in the sun, auroral activity changes with the sun's natural cycles. We're nearing a "solar minimum" this year, so the sun is relatively "quiet", but I've seen auroras almost every time the sky is clear. There's one outside right now: it's not as bright as usual because of the full moon, but it looks like a swirled green waterspout, touching down on the horizon. This is the "aurora australis": at the north pole it's called the "aurora borealis".

Sometimes the aurora is so faint I assume it's a cloud, until one end of it curls over and twists the whole shape into a bright, shimmering fold. They're hard to describe beyond "magic". I've seen wide washes of pale white-green that cover most of the sky; and swirling rivers of green, white, and pink; and rows of spiky teal needles that bounce and ripple. My favorite type of aurora is the "corona": technically, it's a "fan-shaped rayed band", but to me, it's more like an umbrella. The sky fills with long, thin, pulsating rays, all converging at one point and showering around the station on all sides. It's as if this particular aurora was made just for us. Often, my friends and I will lie down in the snow and watch the display, giggling and saying "WOW" over and over. It's the best form of entertainment at the South Pole!

It's also a hot topic for science in Antarctica. A group from the University of Maryland is studying magnetic micropulsations; U.S. groups are working with Russian scientists to digitize years of magnetometer data; and the National Science Foundation has established several unmanned research sites around Antarctica. Here at the South Pole Station, scientists from Embry Riddle Aeronautical University are conducting spectroscopic and interferometric studies of the aurora. Utah State University and Lockheed Research Labs use all-sky cameras to capture auroral images for study, and there are hosts of riometers, radiometers, and optical photometers pointed toward the sky. With a fluxgate magnetometer, scientists from AT&T Bell Labs and the University of Maryland can measure the magnitude and direction of variations in the earth's magnetic field. Researchers from the University of New Hampshire monitor magnetic pulsation sensors.

All of these people want to know more about auroras, and what they represent. How is energy transferred from the solar wind to the earth's magnetosphere and ionosphere? What causes the different types of aurora, especially the "arc", which looks like huge waves rising up from the horizon? Space satellites have explored the magnetosphere, but what else can we learn by studying auroras from the earth's surface?

I wouldn't mind knowing the answers to some of these questions; but sometimes, when I'm lying there in the snow and giggling to myself, I'd rather believe it's all just magic.

more soon: Katy McNitt

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