The bedazzling northern lights have befuddled humans for
centuries. Today we know a bit more about their magic.
by Joan Jarvis Ellison
Very early one winter morning in 1989, my husband woke me. He'd seen an aurora through our bedroom window and knew that I love auroras. We bundled up in heavy winter jackets and pants, wool hats, scarves, mittens, and boots and ventured outside.
The northern horizon was awash in light. We tipped our heads up and up and up. The more we tipped our heads, the more we saw. White light streamed down from overhead. When our necks began to ache, we lay down on our backs in the snow and gazed, awestruck. The entire sky pulsed with light.
Rays of silver flowed down in all directions, like the spokes in a celestial wheel. It looked like the center of the universe was right over our heads. My husband and I had read about the charged particles in the upper atmosphere that caused auroras. We had seen faint displays of the northern lights many times from our farmhouse outside of Pelican Rapids, and still those moving silver rays appeared magical.
How much more supernatural might such a display have seemed to Paleo-Indians along the shores of Glacial Lake Agassiz 10,000 years ago. Did the Minnesota Woman, a young girl whose bones were found on those shores just north of our home, fear what she saw glowing in the sky? Or did she perhaps look upon the streaming lights as a dance of the gods?
Ancient storytellers told tales of the aurora borealis (named for the Roman goddess of the dawn, Aurora, and the Greek god of the north wind, Boreas), or northern dawn. Some scholars believe that references in ancient literature to red Chinese dragons writhing in the sky were actually reports of intense displays of the aurora.
During high-energy displays, the northern lights can be seen as far south as southern Europe. Seeing red light in the sky, the Roman emperor Tiberius sent his troops marching into the night to the aid of the port of Ostia in A.D. 37, fearing that the city was in flames, only to find the aurora. Norse mythology, compiled in the 13th century, speaks of the bridge Bifröst—a burning rainbow arc across the sky—over which the gods could travel from heaven to earth.
A belief among Anishinaabe in Minnesota is that the northern lights are a representation of ancestors' souls dancing with one another in the sky. Another Anishinaabe belief describes how the northern lights came into being. After the great flood, the story goes, Earth tipped on its axis, throwing the northern part of the world into perpetual darkness. The people prayed to the Gitchi Manitou, who captured the light of the hidden sun in the ice crystals around the pole and refracted the rays up into the sky as the northern lights, thus allowing people to make their way to a new land.
Some Inuit people of North America felt that the aurora would bring good weather, and they believed they could control its motions by clapping their hands. Other native peoples saw it as an omen of evil and wore their weapons when they went outside during auroral displays.
Even as late as the 20th century, auroral displays inspired superstition and probably accounted for many reports of UFOs.
Not all ancient observers classified the auroras as mystical events. Aristotle listed them with comets and meteors as astronomical phenomena. King's Mirror, an instruction book written for a Viking monarch in the 13th century, suggested that the auroras were caused by one of three things: a fire encircling Earth, light absorbed and re-emitted by ice and snow, or light from the sun. The French mathematician Descartes proposed that the aurora came from sunlight reflected by particles in cirrus clouds. In 1716 the English astronomer Edmund Halley theorized a connection between the presence of the lights and Earth's magnetic field.
By the 20th century, scientists recognized that sunspot activity was associated with the appearance of auroras. Scientists used balloons and rockets to monitor the sun visually, photographically, and by radio. They studied the solar wind—high-energy charged particles streaming toward Earth. Scientists also studied the magnetosphere—the complex, shifting magnetic field in the upper atmosphere generated by the planet's core. Eventually they began to piece together what actually happens. Just 60 years ago, scientists confirmed that auroras occur when Earth's magnetic fields funnel electrons into the upper atmosphere.
The green, red, or blue drifts of light we see dancing in the sky begin with the solar wind, ejected from the surface of the sun. This wavering, thin stream of hot plasma, mostly protons and electrons, carries its own magnetic field. When the solar wind is calm and steady, it flows around Earth's magnetosphere without disturbing it.
Normally, the magnetosphere protects us from incoming charged particles. Even a mild solar wind interacts with the magnetosphere to some extent and sends high-energy particles into our atmosphere, creating faint auroras. However, when large solar flares or eruptions occur on the sun, the speeds of the wind can increase to nearly 1 million miles per hour. This turbulence can produce intense magnetic fields within the solar wind. This violent space-weather generates spectacular auroras.
Researchers are still trying to figure out how this happens. NASA launched a group of five small spacecraft into orbit around Earth in 2007 to study geomagnetic storms. The spacecraft, along with 20 observatories on the ground, measured what happens during these magnetic storms. Scientists determined that solar wind pushes and stretches the magnetosphere, dragging it out behind the night side of Earth. Solar storms increase the intensity of solar wind. As a result, more charged particles move faster and reach Earth's atmosphere.
When the storm's high-energy electrons smash into oxygen ions in the atmosphere 200 miles above Earth, red light is emitted. When the electrons interact with oxygen molecules 60 miles above Earth, the molecules emit yellow and green light. Nitrogen ions produce red light when they collide with the charged particles, and nitrogen molecules glow red purple. As a result of these collisions, we see a magical, shimmering display of color.
A diffuse glow in the sky usually signals the onset of an auroral event. When the aurora becomes more active, an arc forms. Vertical rays shoot down from the arc. In more energetic auroras, the arc and rays create dancing curtains of light with brighter colors. Rarely, during extremely energetic displays, the aurora fills the sky from the northern horizon toward the south. This produces a corona, where the rays seem to stream down from directly overhead, as they did that March night when my husband and I watched.
Historically, the number of mass ejections from the sun varies on an 11-year cycle. The last big year for auroras visible in most of Minnesota was 1989. Those substorms produced aurora that could be seen during nighttime in the Twin Cities, in spite of all the light pollution, and during the day in Stockholm, Sweden. Some scientists predicted that we were approaching a solar minimum, a long period with little or no sunspot activity and thus few spectacular displays of the aurora borealis. But in the past year or two, the incidence of sunspots and aurora displays has increased.
Astronomers predict this sunspot cycle will be the smallest in 100 years and will peak in October 2013. Though it probably won't produce auroras as exciting as the ones in 1989, aurora displays should continue to increase up until that peak. Any opportunity to watch the glow of energetic, charged particles along the magnetic field lines of Earth's atmosphere is a chance to see magic in the sky.
Tricks for Catching Magic
To see the northern lights, find a place with clear, dark skies. Light pollution will completely overwhelm the northern lights unless they are incredibly bright. Improve your odds of seeing the spectacle with the following tricks:
—Find your magnetic latitude. To know if you have a chance of viewing from your place, you need to know this latitude and the current level of geomagnetic activity. The higher your latitude, the better your chances of seeing an aurora. Rochester has magnetic latitude of about 54, Minneapolis/St. Paul just over 55, Duluth and Fargo/Moorhead about 57, and International Falls about 58.
—Find the geomagnetic activity level (Kp number). A map at this site shows the Kp number needed for viewing at various latitudes. The higher the Kp number, the better your chances of viewing the aurora.
—Look at
www.spaceweather.com for a map of the auroral distribution and daily photographs of the aurora. (Be warned, some days the photo is completely black.) Sign up at this site for email updates on sunspots that may produce aurora. You can pay to get a phone call every time an aurora is visible from your location.
—View within an hour or two of midnight, when the best displays appear.
—Wait for a cloudless night.
—Be patient and let your eyes adapt to the dark to see subtle auroras.
—Learn about the night sky from University of Minnesota programs. Go to
www.astro.umn.edu and click on Public Events.
—Be flexible and enjoy the night sky even without the aurora.
