When the Heavens Dance
by Alan Dyer
How does nature stage a breathtaking light show called the aurora borealis?
The night starts out like any other clear evening for stargazing…until an unusual green “cloud” appears across the northern sky. The cloud brightens and begins to take shape, forming rippling curtains. They grow in size and climb the sky, and then patches of red appear, as if the heavens are catching fire. It's like nothing you've ever seen before.
Then the sky explodes. The curtains shoot up and cover the sky, meeting straight above you, like searchlights or lasers playing across the heavens. But this display is much more complex and unpredictable. It's as if you're looking down a tunnel of shimmering light. All around you the colorful curtains ripple and dance, like luminous veils being blown in a wind high above your head. Bright greens mix with deep reds and pinks. You might even catch sight of subtle blue and purple tints as you stand beneath the climax of nature's most spectacular light show. After a few minutes, the display dims and fades into random patches of light, switching off and on across the sky for the rest of the night.
What you have just witnessed is a grand display of northern lights, also known as the aurora borealis, the Latin words for “northern dawn.” Most people in North America might be lucky to see such an awe-inspiring spectacle once or twice a decade. Perhaps only once in a lifetime! People living in the northern United States (especially Alaska) and in Canada are more fortunate. They can see outstanding displays of northern lights every few months, with lesser displays shining in northern skies almost every week. By checking aurora-watch Web sites, you increase your chances of being outside and looking up when a great display begins to dance across your sky.
Though displays of auroras have been recorded by cultures around the world for thousands of years, only in the last few decades have we begun to understand what causes the light shows. While the lights do happen in our atmosphere, for their ultimate cause we need to look all the way back to the sun.
Massive explosions near the surface of the sun called flares release more energy in seconds than humans have generated in all of history. What triggers these flares is still unclear. The sun's bright surface is a tangled carpet of magnetic fields, twisting and coiling. The magnetic fields reach up into the atmosphere of the sun, called the corona, a region of thin but superhot gas glowing at temperatures of millions of degrees. (Technically, matter so hot isn't called a gas, but a plasma—a state of matter made of atoms ripped apart into bare nuclei and free-flying electrons.)
The sun's complex magnetic fields can twist together like rubber bands, which then trap the superheated coronal gas. The fields store up energy until, like a rubber band that can't be twisted and stretched anymore, the fields snap. They suddenly release torrents of intensely hot gas and searing radiation such as X-rays and ultraviolet light.
The expanding gas and radiation can blow a bubble in the sun's tenuous atmosphere. The bubble bursts, blowing a cloud of atomic particles away from the sun and into space. This event is called a coronal mass ejection. In time-lapse movies from satellites such as SOHO, the mass ejections make the sun look as if it is blowing off puffs of smoke.
One of the earliest discoveries in the space age was that the space between the planets always is filled with atomic particles blowing from the sun, in what we call the solar wind. Like the wind you feel outside on a pleasant day, the wind from the sun usually blows in a gentle “breeze.”
But those powerful solar flares can stir up storms in the solar wind. The bursting bubbles of the sun's atmosphere can shoot dense streams of particles at high speed across the solar system, creating the space version of a squall or windstorm on Earth. Sometimes, Earth lies directly in the path of one of these solar storms. About two or three days after it leaves the sun, the storm front reaches Earth and hits us with a gale-force blast of energized particles—electrons and protons—that were generated 149 million kilometers away on the sun.
Fortunately, Earth has a “deflector shield” that usually protects us from the full blast of the solar storm cloud. The shield is our planet's magnetic field. Earth's molten interior generates a magnetic field that surrounds our planet and extends far into space. When solar wind particles reach Earth, they usually flow around our magnetic “force field,” like water flowing around an island in a stream. But thousands of miles downstream, on the night side of Earth in our magnetic “tail,” some of those streaming particles tunnel their way into our magnetic shield, in a sneaky rear-guard action.
What happens then is still poorly understood, although orbiting satellites are beginning to provide a better picture. During an intense solar storm, our magnetic shield fills to the brim with energized particles that entered through the back door. These particles build up in number and intensity, like a battery being charged up. Then—zap! In a process perhaps similar to the twisting magnetic lines on the sun that created the storm in the first place, the magnetic field lines downstream from our planet twist and pinch off. They squeeze the energetic particles and squirt them toward Earth at the breakneck speed of 1,500 kilometers per second. The high-speed electrons and protons rain down onto our upper atmosphere from the region of space far above the night side of our planet.
All Charged Up
The particles act like a beam of electricity (in fact, that's what they are) shooting through a neon lamp. The lamp is almost a vacuum inside, with just a small amount of neon gas. When charged up by a jolt of electric current, the atoms first absorb the shot of energy, and then release it again in the form of light. The gas begins to glow.
In a similar process, the incoming beams of electrons and protons charge up atoms and molecules in our atmosphere such as oxygen and nitrogen. Zapped by the current of particles from space, these gases glow in shades of green, red, blue, and pink. Our atmosphere acts like a giant neon lamp, being hit with a billion kilowatts of energy in just a few minutes.
All that energy pulses far above us. The curtains of northern lights start about 500 kilometers up, so high that the space shuttle and space station sometimes fly through the tops of aurora displays. Very intense displays can penetrate down to an altitude of 80 kilometers. But usually the brightest auroral light we see comes from heights of 100–120 kilometers. That's still ten times higher than most jet airliners fly, and well within the layer of our atmosphere called the ionosphere, where the air is so thin that it remains a near-vacuum. You couldn't breathe up where the aurora dances, but electricity can work its magic to create a colorful light show.
So, if you are lucky enough to see a display of northern lights, think about where the action started, 149 million kilometers away at the sun. You are seeing the power of electricity, magnetism, chemistry, and atomic physics working together to make the heavens dance.
- What is a coronal mass ejection? What causes one to happen?
[anno: A coronal mass ejection is an event that occurs in the Sun's atmosphere. A solar flare erupts from the surface of the Sun, producing a lot of hot gas and radiation. This gas and radiation blow a bubble in the Sun's atmosphere. When the bubble bursts, it blows a cloud of atomic particles into space.]
- How is the energy released by a coronal mass ejection different from the kind of thermal energy normally released by the Sun?
[anno: Answers may vary but could include that sunlight normally travels to the Earth from the Sun in waves but that the energy released from a coronal mass ejection can become a dense stream of particles.]
- A coronal mass ejection causes an aurora borealis to appear over Alaska one evening. People head outside to watch the display. Does the energy that causes the aurora borealis to appear also increase the temperature of the air down on the ground? Why or why not?
[anno: The aurora borealis does not increase the temperature on the ground because the energy is captured in the magnetic field surrounding the Earth and contained in the upper atmosphere.]