The aurora borealis or northern lights is a natural light display in the Earth‘s sky.
It occurs most frequently in a belt of radius 2500 km (1550 mi) centered on the magnetic north pole. This zone extends over northern Scandinavia, Iceland, the southern tip of Greenland and continuing over northern Canada, Alaska and along the northern coast of Siberia.
The activity that creates auroras begins on the sun. The sun is a ball of superhot gases made up of electrically charged particles called ions. The ions, which continuously stream from the sun’s surface, are called the solar wind.
As solar wind approaches the Earth, it meets the Earth’s magnetic field. Without this magnetic field protecting the planet, the solar wind would blow away Earth’s fragile atmosphere, preventing all life.
The aurora occur in a region of the atmosphere 100 km (60 mi) above the earth, while rays can extend from this level to 500 km (about 300 mi).
It is most often seen in a striking green color, but it also occasionally shows off its many colors ranging from red to pink, blue to purple, white to yellow to orange.
The reason that the aurora is seen in so many colors is that our atmosphere is made up of many different compounds like oxygen and nitrogen. When the charged particles that come from the sun hit the atoms and molecules of the Earth’s atmosphere, they excite those atoms, giving off light. Different atoms give off different colors of the spectrum when they are excited.
Auroras can appear as long, narrow arcs of light, often extending east to west from horizon to horizon. At other times they stretch across the night sky in bands that kink, fold, and swirl, or even ruffe like curtains. They can spread out in multi-colored rays, like vertical shafts of light that stretch far up into space. And sometimes they engulf the sky in a thin cloud or veil – “Like snowfakes, no two are ever quite the same.”
We associate the aurora borealis with wintertime, although in reality it is present the year round – it’s just that we can’t see it when the nights are light as the background sky has to be fairly dark.
The word “aurora” is derived from the name of the Roman goddess of the dawn, Aurora, who travelled from east to west announcing the coming of the sun. The word “borealis” is derived from Boreas, the Greek god of the cold north wind and the bringer of winter.
Ancient folklore from China and Europe describes auroras as great dragons or serpents in the skies. In Scandinavia, Iceland, and Greenland, an aurora was often seen as the great bridge Bifrost, the burning archway by which the gods traveled from heaven to Earth. Some Native American tribes pictured spirits carrying lanterns as they sought the soulsof dead hunters, while Eskimos saw souls at play, using a walrus head as a ball.
Some of the brightest minds in history have puzzled over the aurora.
In the 4th century B.C. Aristotle made one of the frst truly scientifc accounts of the aurora borealis, describing “glowing clouds” and a lightthat resembled fames of burning gas.
The real advances in auroral science began when scientists started connecting auroras to magnetism. In the late 16th century, William Gilbert conducted experiments that led him to propose that the Earth itself was a giant magnet, with a North and South Pole as if a great bar magnet had been buried inside.
In the 17th century, Anders Celsius proposed that the lights were caused by moonlight refected by ice and water in the air. Rene Descartes (France) and other scientists asserted that the refraction of moonlight and the refection of colored rays by ice crystals in the atmosphere somehow caused the aurora. Some of these misconceptions survive even today.
It wasn’t until the late 1800s and early 1900s that spectroscopic measurements of auroral light identifed oxygen and nitrogen as the color sources for the aurora.