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Northern Lights

The aurora borealis, or northern lights, have to be seen to be disbelieved.

Sometimes they appear as a bold, yellow-green arc of light bursting out of the horizon and stretching clear across the night sky, and at other times they look like a gigantic, pleated, multicolored curtain of undulating light wiggling furiously away in the darkness-like a celestial swath of phosphorescent silk a hundred miles high. Sometimes enormous snaky bands of white or green or reddish light writhe overhead, only to disperse in a sudden series of lavender and pale lime green, pencil-thin rays that race toward the zenith to form a huge fan across the heavens. And sometimes an arc, or a band, or vaguely shaped cloud just hangs there, rhythmically pulsating in the darkness, like something with a heart.

In ancient Greenland, the elders got together and decided that the aurora represented, obviously, the spirits of the dead playing catch with the head of a walrus. To the ancient Norse, the northern lights symbolized a procession of Valkyrie galloping across the night sky, transporting on their backs the souls of dead warriors to Valhalla. Some groups of Eskimos called the aurora "Keoeeit," a term suggesting a series of torches held aloft by spirits charged with leading the recently departed into heaven, and in the folklore of the northern Hebrides and Scotland, the northern lights stood for a battle. A scarlet aurora that appeared over Ireland in 1854 seemed, to its rather freakedout observers, the dancing blood of those killed at Balaclava, where in October of that year the Light Brigade made their ill-advised, ill-fated charge, "Into the jaws of Death . . . the mouth of Hell."

As the name implies, the northern lights appear most often in the far north, but they can, and do, show up in the subtropical south-if less often and less dramatically. Visible some three hundred nights of the year from Churchill, Manitoba, and Point Barrow, Alaska, the aurora appears feebly overhead in northern Florida on perhaps four nights a year. Over Duluth, Minnesota, and Quebec City, auroral displays occur on perhaps forty nights a year. One can't really say the farther north the better, as the aurora occurs most frequently along a narrow band that circles the geomagnetic North Pole, passing along the southern coast of Greenland and running through northern Quebec, central Hudson Bay, and along the northern coast of Alaska. Farther north than this the displays occur less frequently.

As a general rule of thumb, however, the farther north you go, the more auroral displays you will see. The reverse holds true in the Southern Hemisphere, where the aurora australis, or southern lights, occur. Satellite photos of the dark side of the earth often depict simultaneous occurrences of the northern and southern lights, which appear on the shadowed globe like flat, fiery berets or burning half-halos at opposite ends of the sphere.

Although all the facts have yet to come in, since the advent of spacecraft and satellites we have learned a great deal about the causes of the aurora. Numerous celestial and terrestrial factors come into play, and the whole process probably turns on grander cycles we but dimly perceive-even though we have managed, by exploding nuclear devices in space, to create artificial auroras that closely resemble the real thing.

The real thing occurs when charged, high-energy particles-mostly electrons-collide with, and in the process excite, atoms and molecules in the upper atmosphere. When a molecule or atom gets excited, it shifts into a higher level of energy, remains there for a hundred-millionth of a second or so, and then returns to its former energy level, emitting light in the process. Indeed, much of what we know about the ingredients of the upper atmosphere we discovered by analyzing the auroral spectrum of light.

The various colors in this spectrum can be traced to specific molecules or atoms. Oxygen molecules, for example, emit red or yellow light after an electron smashes into them, whereas an individual oxygen atom, after such a collision, glows green. When in the upper atmosphere an electron collides with a proton, the excited couple glows red for a ten-thousandth of a second or so and then wanders off as a relatively content hydrogen atom. Hydrogen atoms also form when protons bounce like billiard balls off other molecules in the upper atmosphere and then, at an altitude of about 70 miles up, abscond with an electron from a nitrogen molecule, which over the loss glows purple.

These charged, high-energy electrons, protons, and other particles that collide with molecules in the upper atmosphere come from the sun by way of the solar wind, or solar plasma, a high-speed stream of ionized (electrically charged) gas-95 percent of it hydrogen nuclei-that races toward the earth at slightly more than a million miles an hour. (The gas inside a neon sign is another form of plasma.) Fortunately for us, the earth's magnetic field deflects the solar wind with all its positively and negatively charged ions. But winds will be winds. The force of the solar wind flattens the earth's magnetic field so that it assumes a shape rather like that of a comet, whose elongated tail, due to the force of the solar plasma, always points away from the sun. So does the earth's magnetotail.

Aligned along and strongest near the North and South Poles, the earth's magnetic field affects the atmosphere in direct relation to altitude, its influence increasing with height. Of little significance at altitudes below 40 miles up, the magnetic field becomes an important factor at heights of 40 to 400 miles, where a high percentage of charged particles makes up the ionosphere. Most auroral displays take place within the ionosphere, at altitudes ranging from approximately 65 to 150 miles up. Above the ionosphere, the earth's magnetic field becomes the dominant force in determining the motions of atmospheric particles, which at such altitudes are almost invariably charged. We call this upper layer of the atmosphere, where charged particles swirl in loops around the earth's magnetic lines of force, the magnetosphere. These charged particles are known as the Van Alien radiation belts.

If it weren't for the solar wind, the magnetosphere would be exactly that-a sphere. The force of the solar plasma, however, flattens the part of the magnetosphere facing the sun, and blows the part on the dark side of the earth hundreds of thousands of miles into space. On the side facing the sun, the earth's magnetosphere extends perhaps 40,000 miles into space, whereas the comet-like magnetotail extends, according to data sent back by the International SunEarth Explorer 3 satellite, at least 850,000 miles out. As the world spins, so does the magnetosphere, which acts as something of a protective envelope that surrounds the earth and deflects ionized (or charged) particles-gamma rays and X rays, for example.

So here comes the solar wind, a gaseous plasma of electrically charged particles moving at a thousand miles per second, crushing the magnetosphere facing it to the scant depth of 40,000 miles and blowing the part in darkness a million miles downwind. As fastmoving solar particles encounter the magnetosphere it repels them, rather like a speeding automobile repels falling snowflakes, and they rush around it, some of them spilling into the atmosphere along the whirlpool-like force lines of the magnetic poles and others eddying into space along the elongated lines of the magnetotail, which draws them into its wake just like the vacuum behind the speeding auto pulls along snowflakes.

All this wind and all the turbulence it encounters generate complex patterns of circulation, so that while some charged particles spill into the atmosphere along the force lines of the magnetic poles, others trail along hundreds of thousands of miles beyond it, while still others get trapped by the Van Alien radiation belts.

During an intense auroral display, charged particles from all these circulation patterns bombard the ionosphere, the strongest of them penetrating to within 40 miles of the earth's surface. We have known for years that the force lines of the earth's magnetic poles pulled ions from the passing solar wind and the circulating Van Alien radiation belts into the ionosphere, but we only recently discovered how ions blown out along the peripheries of the magnetotail contribute to the aurora. It seems that rather like the snowflakes drawn into the wake of the speeding automobile, charged particles from the solar wind swirl and eddy at the far end of the magnetotail, where they spin as though in a plasma generator, a dizzying whirligig of electrified gas and magnetic force lines, which increases their energy until it reaches something of a critical mass, a level too intense to exist. The resulting explosion fires a stream of electrons and protons upwind at the dark side of the earth, whose attenuated magnetotail actually serves as a protective sheath, or magnetized conduit, for these high-speed ions, which hurry toward the magnetic poles.

Themselves the product of motion and electricity, the magnetic poles change constantly. (As Heraclitus' student Cratylus observed, "You can't step in the same river even once.") The easiest way to visualize the magnetic axis of the earth is to imagine a huge magnetized rod passing right through the planet and emerging at the poles-covered by ice and water, and therefore, impossible to see-but such is not the case. The inner core of the earth has no stable magnetic charge because nothing remains permanently magnetic at temperatures above about 932 degrees Fahrenheit, a value known as the Curie Point. Even easily magnetized minerals like iron cannot sustain magnetic charges at such temperatures, and at depths of more than 12 to 18 miles below the crust of the earth, such temperatures perenially prevail. The heat from radioactivity deep inside the planet prevents the formation of a permanent magnetic field.

The magnetic poles shift approximately a tenth of a degree a year, which seems pretty slight, just under 7 miles or so, but represents a rather startling rate of change as far as geological events go. Observers in London, for example, noted an 18-degree variation in magnetic north over the last four centuries: in 1580, compass needles pointed 11 degrees east of true north, and in 1980, 7 degrees west of it. Stonehenge, the while, didn't move at all.

This radical shift of magnetic north owes itself to the outer layer of the earth's fluid-iron core, which the heat from residual radioactivity further within the planet stirs into convective motion. These convection currents of fluid iron interact with stray, minuscule magnetic fields and generate electric currents along their lines of force, creating not only the magnetic poles but also a self-generating dynamo that produces as much electrical current as all the power plants humans currently operate. This million amperes of electric current produces, finally, a magnetic field as strong as the sun's, as powerful, if one can imagine such a thing, as a toy magnet.

The surface of the sun, however, experiences intense magnetic storms, which appear to us as sunspots and represent relatively cool, and consequently dark, areas on the solar disc. Tornado-like whorls within the gaseous surface of the sun, sunspots generate magnetic fields 3000 times more intense than normal. Solar flares, enormous atomic eruptions that discharge far higher than normal levels of charged particles, accompany sunspots, the net result being an increase in the intensity of the solar wind. The number of sunspots and solar flares fluctuates according to a well-defined eleven-year cycle; at the minimum point usually fewer than 10 sunspots exist. During maximum periods of sunspot activity, anywhere from 43 to 193 sunspots wreak magnetic havoc on the surface of the sun. At such times, auroral displays occur more frequently, and more dramatically, south of the geomagnetic North Pole.

Three years or so before the last period of maximum sunspot activity, a flock of honking Canada geese woke me in the middle of the night, and I threw on a heavy terry cloth robe and stepped outside to maybe catch a glimpse of them. It was late September, at the northern tip of the Gaspe Peninsula, 38 degrees, a half hour before midnight. When I stepped outside and the cool air hit me, it must have anesthetized me as well, for it took a full minute before I realized the sky was ablaze with a heavens-high curtain of rosy yellow light, rippled like a flag in strong wind, moving to the left, sounding, sure enough, like Canada geese.




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