Northern And Southern Lights

Dance of the Aurora tte northern and southern lights are one of the most magnificent and earliest-known manifestations of the myriad links between the Sun and the Earth, ttey illuminate the Arctic and Antarctic skies, where curtains of multi-colored light dance and shimmer across the night sky far above the highest clouds, like giant dragons or snakes (Figs. 8.7, 8.8).

Perhaps because they can be colored red like the rising morning Sun, they are often called the aurora after the Roman goddess of the rosy-fingered dawn; this designation has been traced back to the time of the Italian scientist Galileo Galilei (1564-1642). tte auroras seen near the north and south poles have been given the Latin names aurora borealis, for "northern lights", and aurora australis, for "southern lights."

Aurora activity is not rare; it is almost always present! Residents in far northern locations regularly see the aurora borealis, every clear and dark winter night. Even today, the winter aurora brings solace to circumpolar inhabitants, reminding them of the eventual return of life-sustaining sunlight. But most people never see the awesome lights, for auroras are normally confined to high latitudes in the north or south polar regions with relatively few inhabitants. In addition, only an exceptionally intense aurora can be noticed against bright city lights.

Some of the earliest written accounts of the northern lights are found in Mediterranean countries where schools and libraries flourished long ago. Greek records of the aurora date back to at least Aristotle (384-322 BC), who described an aurora, probably the one occurring in 349 BC, as being blood red in color and with chasm-like or trench-like shapes. However, spectacular auroras rarely extend as far south as Greece, perhaps every 50 to 100 years.

Since civilization began, the multi-colored lights have always been most frequently observed at far northern latitudes, so some of the most vivid and comprehensive accounts come from the Scandinavian countries, tte oldest written records by Norwegians concerning the northern lights go back to the Viking period (500 to 1300). In one Norse chronicle, called Kongespeilet or King's Mirror and written about 1250, it is described by:

"ttese northern lights have this peculiar nature, that the darker the night is, the brighter they seem, and they always appear at night but never by day, most frequently in the densest darkness and rarely by moonlight. In appearance they resemble a vast flame of fire viewed from a great distance. It also looks as if sharp points were shot from this flame up into the sky, they are of uneven height and in constant motion, now one, now another darting highest; and the light appears to blaze like a living flame ...It seems to me not unlikely that the frost and the glaciers have become so powerful there that they are able to radiate forth these flames.33

FIG. 8.7 Aurora borealis Swirling walls and rays of shimmering green and red light are found in this portrayal of the fluorescent Northern Lights, or Aurora Borealis, painted in 1865 by the American artist Frederic Church (1826-1900). (Courtesy of the National Museum of American Art, Smithsonian Institution, gift of Eleanor Blodgett.)

FIG. 8.7 Aurora borealis Swirling walls and rays of shimmering green and red light are found in this portrayal of the fluorescent Northern Lights, or Aurora Borealis, painted in 1865 by the American artist Frederic Church (1826-1900). (Courtesy of the National Museum of American Art, Smithsonian Institution, gift of Eleanor Blodgett.)

FIG. 8.8 Northern lights Spectacular green curtains of light illuminate the northern sky, like a cosmic neon sign. Forest Baldwin took this photograph of the fluorescent Northern Lights, or Aurora Borealis, in Alaska. (Courtesy of Kathi and Forest Baldwin, Palmer, Alaska.)

In his book Fram Expedition, published in 1897, the Norwegian explorer Fridtjof Nansen (1861-1930) provided this account, written while he was trapped through the long Arctic winter in the frozen pack ice:

"tte glowing fire-masses had divided into glistening, many colored bands, which were writhing and twisting across the sky both in the south and north, "tte rays sparkled with the purest, most crystalline rainbow colors, chiefly violet-red or carmine and the clearest green. Most frequently the rays of the arch were red at the ends, and changed higher up into sparkling green It was an endless phantasmagoria of sparkling color, surpassing anything that one can dream. Sometimes the spectacle reached such a climax that one's breath was taken away; one felt that now something extraordinary must happen - at the very least the sky must fall.34

Nansen, who won the Nobel Peace Prize in 1922, wrote a few more sentences and could not continue; being thinly dressed and without gloves, he had no feeling left in body or limbs.

Auroras occur with the same frequency and simultaneously in both the southern and northern polar regions of the Earth. Indeed, the two auroras are almost mirror images of each other. But the aurora australis have never achieved a renown comparable to the northern lights, probably because the southern ones are not usually located over inhabited land and are instead seen from oceans that are infrequently traveled. In fact, the first recorded sighting of the southern lights did not occur until 1770, by the British Captain James Cook (1728-1779) during the voyage of the HMS Endeavor.

Nowadays we can use spacecraft to view both the northern and southern lights from above (Fig. 8.9). tte Space Shuttle has even flown right through the northern lights, which extend a few hundred kilometers above the ground. While inside the display, astronauts could close their eyes and see flashes of light caused by the charged aurora particles, which ripped through the satellite walls and passed through their eyeballs, making them glow inside.

tte view from space is just as magnificent as that from the ground. As the Czechoslovakian astronaut, Vladimir Remek (1948- ), expressed it:

Suddenly, before my eyes, something magical occurred. A greenish radiance poured from the Earth directly up to the [Space] Station, a radiance resembling gigantic phosphorescent organ pipes, whose ends were glowing crimson, and overlapped by waves of swirling green mist.35

tte play of northern lights in the sky has also been described in the folk lore of Arctic cultures, where they are often interpreted as the spirits of the dead either fighting or playing in the air. tte Vikings thought the auroras represented an eternal battle between the spirits of fallen warriors. In Norse mythology, the aurora is a bridge of fire that permits ttor, the God of War, to travel between Heaven and Earth. Eskimos have described the flickering lights as a dance of the dead, amusing themselves in the absence of light from the Sun, or as signals from the deceased trying to contact their living relatives. An Eskimo word for aurora, aksarnirq, translates into "ball player." For

FIG. 8.9 Aurora from the shuttle The eerie, beautiful glow of auroras can be detected from space, as shown in this image of the Aurora Australis or Southern Lights taken from the Space Shuttle Discovery. The green emission of atomic oxygen extends upward from 90 to 150 kilometers above the Earth's surface, where it is created by beams ofhigh-speed electrons moving down into the atmosphere, exciting the oxygen atoms and making them fluoresce. (Courtesy ofNASA.)

Alaskan Eskimos the spirits are playing ball with the heads of children who dared venture outside during the northern lights.

tte flickering, colored lights have inspired many poets, such as the English poet Robert Browning (1812-1889), whose vision of Judgment Day was inspired by an aurora, "the final belch of fire like blood, over broke all heaven in one flood", as from a dragon's nostril, and the American poet Wallace Stevens (1879-1955) who wrote of "its polar green, the color of ice and fire and solitude."36

In Norway or Sweden it was a common belief that the northern lights were reflections from silvery shoals of herring that flashed light against the clouds when swimming close to the water's surface. According to King's Mirror, the Arctic snow and ice absorbed large amounts of light from the long summer midnight Sun and re-radiated it as the northern lights in wintertime. Almost 400 years later, the French philosopher Rene Descartes (1596-1650) attributed the aurora to sunlight scattered from ice particles found high in the atmosphere at cold northern locations. But all these ideas were eventually shown to be wrong.

Since auroras become more frequent as one travels north from tropical latitudes, it was thought that the northern lights would occur most often at the highest polar latitudes. Early Arctic explorers were therefore surprised to find that their frequency of occurrence did not increase all the way to the poles. In 1860 Elias Loomis (1811-1889),

Professor of Natural Philosophy at Yale University, mapped out their geographic distribution, showing that the northern lights form a luminous ring encircling the North Pole, tte Swiss engineer and physicist Herman Fritz (1830-1883) extended Loomis' work, publishing a similar conclusion in 1881 in his then-well-known book Das Polarlicht. Loomis and Fritz showed that the intensity and frequency of auroras were greatest in an oval-shaped aurora zone centered on the North Pole with a width of about 500 kilometers and a radius of about 2,000 kilometers. Auroras can occur every night of the year within this zone.

About a century later, in 1957-58, the aurora distribution was mapped out in greater detail using all-sky cameras during the International Geophysical Year. An analysis of hundreds of thousands of photographs, each portraying the sky from horizon to horizon, confirmed that the aurora zone is an oval-shaped band that is centered on the Earth's magnetic pole.

Today spacecraft look down on the aurora oval from high above the north polar region, showing the northern lights in their entirety (Fig. 8.10). ttey indicate that the luminous aurora oval is constantly in motion, expanding toward the equator or contracting toward the pole, and always changing in brightness. Such ever-changing aurora ovals are created simultaneously in both hemispheres and can be viewed at the same time from the Moon.

Visual auroras normally occur at 100 to 250 kilometers above the ground, ttis height is much smaller than either the average radius of the aurora oval, at 2,250 kilometers, or the radius of the Earth, about 6,380 kilometers. An observer on the ground therefore sees only a small, changing piece of the aurora oval, which can resemble a bright, thin, windblown curtain hanging vertically down from the Arctic sky.

The Sun's Brightening and Dimming Switch

Loomis and Fritz established a general correlation between the occurrence of sunspots and northern lights. When the number of sunspots is large, bright auroras occur more frequently, and when there are few spots on the Sun the intense auroras are seen less often. So, the frequency of occurrence of the bright auroras tends to follow the 11-year sunspot cycle of solar activity, suggesting that the Sun somehow controls the brilliance of the northern lights. Loomis even suggested a physical connection, noting that an exceptionally intense aurora occurred on 2 September 1859; the day after the first solar flare was discovered.

tte Sun's influence was more fully explained in 1896 when the Norwegian physicist Kristian Birkeland (1867-1917) showed that electrons from the Sun might be directed and guided along the Earth's magnetic field lines to the polar regions. Birkeland demonstrated his theory by sending electrons toward his own magnetized sphere, or ter-rella, using phosphorescent paint to show where electrons struck it. An electromagnet was placed in the sphere, creating a dipolar magnetic field, and the entire apparatus was placed in a low-density vacuum that represented outer space, tte resulting light indicated that the electrons are curved down toward and around the magnetic poles, and the glowing shapes reproduced many of the observed features of the auroras.

Particle detectors onboard rockets launched into auroras in the early 1960s showed that Birkeland was right, at least in part! tte aurora is electrified, principally excited

FIG. 8.10 The aurora oval Instruments aboard the POLAR spacecraft look down on the aurora from high above the Earth's north polar region on 22 October 1999, showing the northern lights in their

- entirety. The glowing oval, imaged in ultraviolet light, is 4,500 kilometers across. The most stantly in motion, expanding toward the equator or contracting toward the pole, and always changing in brightness. Such ever-changing aurora ovals are created simultaneously in both hemispheres. (Courtesy of the Visible Imaging System, University of Iowa and NASA.)

intense aurora activity appears in bright red or yellow, toward the night side of the Earth; it is typically produced by magnetic reconnection events in the Earth's magnetotail. The luminous aurora oval is con-

by energetic electrons bombarding the upper atmosphere with energies of about 6 keV, or 6,000 electron volts, and speeds of about 50 kilometers per second. As the electrons cascade down the polar magnetic field lines into the atmosphere, they are slowed down by collisions with the increasingly dense air, exciting the gaseous atoms and causing them to glow like a cosmic neon sign, tte luminous aurora shimmers as electrons are injected down from different locations, exciting atoms that shine in long, thin vertical sheets like the folds of a curtain.

Currents can be produced along the aurora ovals that are as strong as a million amperes, ttese currents flow down from the magnetosphere, through the ionosphere in the upper atmosphere, around the aurora oval, and back out and up to the magnetosphere.

When the electrons slam into the upper atmosphere, they collide with the oxygen and nitrogen atoms there and excite them to energy states unattainable in the denser air below, tte most abundant constituents of our atmosphere are oxygen and nitrogen, respectively comprising 21 and 78 percent of the air, and their energized aurora transitions are "forbidden" at high densities, tte pumped-up atoms quickly give up the energy they acquired from the electrons, emitting a burst of color in a process called fluorescence. It is similar to electricity making the gas in a neon light shine or a fluorescent lamp glow, tte process also resembles the beam of electrons that strikes the screen of your color television set, making it glow in different colors depending on the type of chemicals, or phosphors, that coat the screen.

tte colors of the aurora depend on which atoms or molecules are struck by the precipitating electrons, and the atmospheric height at which they are struck. Excited oxygen atoms radiate both green (557.7 nanometers) and red (630.0 and 636.4 nanometers) light; the green oxygen emission appears at an altitude of about 100 kilometers and the red oxygen light at 200 to 400 kilometers, tte bottom edge of the most brilliant green curtains are sometimes fringed with the pink glow of neutral, or un-ionized, molecular nitrogen, and rare blue or violet colors are emitted by ionized nitrogen molecules.

So, energetic electrons colliding with oxygen and nitrogen atoms in the air cause the multi-colored aurora light show, but where do the electrons come from and how are they energized? Since the most intense auroras occur at times of maximum solar activity, it was once thought that the aurora electrons came from explosions on the Sun, hurled directly down into the upper atmosphere through the Earth's narrow, funnel-shaped polar magnetic fields. After all, the frequency of the solar explosions also peak at the maximum of the solar magnetic activity cycle.

In another popular theory, solar wind electrons were supposed to be held within the Van Allen radiation belts before being squirted out into the aurora zones, like a squeezed tube of toothpaste, as the result of excessive solar activity. However, solar particles coming in from the polar route apparently do not have enough energy to make all of the auroras, and there are not enough particles in the Van Allen belts.

Even though changing conditions on the Sun may trigger the intense northern and southern lights, we now know that the electrons that cause the auroras arrive indirectly at the polar regions, from the Earth's magnetic tail, and that these electrons can be energized and accelerated locally within the magnetosphere. Changing solar-wind conditions can temporarily pinch off the Earth's magnetotail, opening a valve that lets the solar-wind energy cross into the magnetosphere and additionally shooting electrons stored in the magnetic tail back toward the aurora zones near the poles.

During this magnetic reconnection process, the magnetic fields heading in opposite direction - having opposite north and south polarities - break and reconnect downwind of Earth on its night side. Electrons are pushed up and down the tail, and can be accelerated within the magnetosphere as they travel back toward the Earth and into its polar regions, tte electrons that are thrown Earthward follow the path of magnetic field lines, which link the magnetotail to the polar regions and map into the aurora oval.

tte entry from the magnetotail also explains why the aurora oval is not completely illuminated when seen from space. It shines mostly on the night side of the Earth, which is magnetically connected to the reconnection site in the magnetotail. Magnetic fields on the sunlit dayside are directly connected to the poles rather than to the magnetotail.

tte rare, bright, auroras seen at low latitudes in more clement climates become visible during very intense geomagnetic storms that enlarge the magnetotail. tte aurora ovals then intensify and spread down as far as the tropics in both hemispheres. Since these great magnetic storms are produced by solar explosions, known as coronal mass ejections, it is really the Sun that controls the intensity of the brightest, most extensive auroras, like the brightening and dimming switch of a cosmic light, even though auroras are most often fueled by particles accelerated in the magnetosphere.

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