Read the following article then click this link to find the questions you need to answer. Print out the question sheet and submit it with your answers handwritten directly below the questions.

Northern lights are many things to many people. Some native cultures in North America consider them spirits on a journey to another world. Western folk wisdom dictated aurorae were the reflection of sunlight off polar ice. Many urban dwellers often confuse them with light pollution. But to aficionados of the night, far away from the glare of city lights, the northern lights are an atmospheric light show taking wing in the form of glows, bands, and curtains. For an avid watcher, varied forms of aurorae are as much fun to observe and photograph as any deep-sky object or lunar vista.

Northern lights result when charged particles from the Sun plow into our planet’s upper atmosphere as they follow Earth’s magnetic field. A display’s brilliance and longevity depend on how much stuff the Sun spews toward Earth. Its shape depends on how these particles interact with Earth’s magnetic field. And its color provides clues about the nature of our atmosphere and the speed of culprit particles.

In Living Color

For the most part, northern lights look green. Electrons and protons from the Sun, accelerated by Earth’s magnetic field up to speeds of 37,000 miles a second, smash into oxygen atoms more than 60 miles up, causing the atoms to glow green (scientists call this a color type C aurora). Slower-moving particles don’t travel as deep into the atmosphere and hit atoms 120 miles high to create the red light of color type D aurorae. Green aurorae with top fringes of red (color type A) are caused by a mixture of slow- and fast-moving particles. If the particle stream is especially energetic, green aurorae sometimes sport red bases (color types B and E), caused by collision with nitrogen 42 miles up. And if there’s a Moon out or lingering twilight, try looking for still another color. Sunlight (moonlight is sunlight reflected from the Moon) causes upper atmospheric nitrogen ionized by particles to glow blue or purple (color type F).

Bands, Arcs and Blobs

But by far the most obvious feature of aurorae is form, which falls into two distinct categories: discrete and pulsating.

The most easily seen aurorae are discrete, characterized by their sharp boundaries. Common discrete forms include isolated bands and arcs; the latter have curved, symmetrical bottoms that span the horizon. Both are 100 or 200 miles tall but only 300 feet thick. Homogeneous bands and arcs do not display detail and look like a continuous glow of light. But draperies can form in both, giving them an undulating appearance.

Distinct segments, called rays, sometimes show up in discrete aurorae. These rays line up with Earth’s magnetic field lines. A rayed arc that’s low in the north is often called a picket fence aurora. If a section of discrete aurora folds upon itself, or spirals, it appears to momentarily brighten. In that area of the band or arc, the spiral is about 3,000 feet thick. Bright, discrete aurorae that form a spectacular crown overhead are called coronal. For the most part, discrete aurorae are active before midnight and often mark the peak of a display.

At peak display, bands and arcs undergo flickering, varying in brightness up to seven times a second. When the display is flaming, fast-moving waves of brightness move up the aurora’s length from its bottom. Lurking between brighter portions of the display might be bean-shaped pockets where there is no aurora light at all; these pockets are coined dark aurorae.

All these features signal an outburst’s peak. When things star to settle down, a second category of aurora dominates, pulsating aurorae. Common after midnight or toward the end of an active display, these have ill-defined boundaries. Their most obvious feature is a cyclical change in brightness, which varies anywhere from a tenth of a second to 20 seconds. To spot this aurora, lock your eyes on one section of the sky and try to spot blobs of light blinking in and out of view. Where a discrete display might last for several minutes, pulsating aurorae persist for hours.

There is a third, more elusive aurora subtly visible during peak discrete displays: the hydrogen arc. For the most part, particles spiraling along Earth’s magnetic field are responsible for the two most common forms of aurorae. Hydrogen arc aurorae, on the other hand, are caused by particles, mainly protons (hydrogen atoms missing their electron) that leach into the upper atmosphere from Earth’s radiation belts. To spot hydrogen arc aurorae, look for a uniform, non-pulsing, colorless band no brighter than the Milky Way to the south of a discrete display.

Look North for Aurorae

Seeing the northern lights depends on where you live and how active the Sun is. Particles that cause aurorae dive toward the magnetic north pole, located off the Perry Islands of Canada, at latitude 77^o north. They won’t reach the ground because Earth’s atmosphere gets in the way, so they create an 2,500-mile-wide auroral oval centered on the magnetic pole.

People who live under the fringes or inside the radius of this oval have a better chance of seeing aurorae than those who live farther south. Because the Sun always drizzles particles, the auroral oval is always present, and Alaskans and northern Canadians see displays almost every clear night. Those in the midlatitudes might see northern lights three to five nights a year, but far-southerners such as Floridians, southern Californians, and Texans see displays only once a year.

People that far south see aurorae only after the Sun has a massive outburst of particles generated by some solar flares. While Earth’s upper atmosphere comes under such an assault, the auroral oval creeps farther south. Where folks in the northern latitudes see a spectacular display, those in the south might spot an unanticipated red glow to the north. (During an active display in World War II, Californians thought they were under a Japanese air attack.)

Spectacular displays are predictable because particle streams from a solar flare take about three days to reach Earth. Besides crating aurorae, particle influxes also wreak havoc on Earth’s ionosphere and magnetic field, causing electrical and radio communication blackouts. Scientists scrupulously monitor solar activity and issue warnings when there’s an outburst. You can monitor these special notices by calling the Space Environmental Service Center’s solar hotline at (303) 497-3235, or listen to shortwave radio station WWV (5, 10, and 15 MHz) for updates at 18 minutes after the hour.

When forecasters predict a geomagnetic storm, be on the lookout for an upswing in auroral activity. If you do see a great display, note the date and watch the skies about a month later, when the Sun’s 27-day rotation brings the active region on its surface back into line with Earth.

Regardless of whether Earth withers under a geomagnetic storm or copes with its usual dose of Sunstuff, you may want to photograph aurorae. Load a camera with ISO 400 or 800 film and set it on a tripod. Use a 50mm (or wider) lens focused at infinity, set to its lowest f-stop number. With the camera’s shutter speed set on "B" or "T," use a cable release to take 30-second exposures. For bright, fast-moving displays, shoot short 5- to 15-second exposures. Leave the shutter open several minutes to record faint aurorae that often lie low in the north.

Northern lights remind us that it’s an unforgiving universe out there. Indeed, if not for a thin blanket of air between us and charged particle streams from the Sun, we wouldn’t be here to enjoy and photograph nature’s light show. So on chilly spring evenings, keep a sharp eye to the north, curl up in a blanket or sleeping bag under the starry sky, and enjoy.