Why Does Aurora Borealis Occur?

dancing waves of light in sky have awed humans for millennia

By Jonny Lupsha, Wondrium Staff Writer

The northern lights are one of the most beautiful natural phenomena on Earth. Some are lucky enough to have seen them in person. How do the northern lights happen?

Northern Light, Aurora borealis at Kirkjufell in Iceland. Kirkjufell mountains in winter.
The beauty of the northern lights, or the aurora borealis, is seen during winter at the Kirkjufell Mountain in Iceland. Photo by Guitar photographer / Shutterstock

For thousands of years, anyone lucky enough to be in the Arctic regions at night has marveled at the auroras—the world-renowned dancing waves of light in the sky. Whether in the far north or the far south, people have been wowed by the incredible sight of the auroras. Recently, a severe geomagnetic storm even brought the beauty of the aurora borealis as far south as North Carolina and Arizona.

Powerful storms notwithstanding, what causes these colorful streaks to regularly brighten the night sky for residents of places like Alaska and Iceland? In the video series National Geographic Polar Explorations, Dr. Edward M. Murphy, Associate Professor, General Faculty at the University of Virginia in Charlottesville, unravels the mystery of this beautiful natural phenomenon.

What Are the Auroras?

“The aurora appear as luminous rays, and arcs, bands, or curtains of green, red, blue, or even purple light in the sky,” Dr. Murphy said. “They occur when energetic charged particles strike the upper atmosphere of the Earth, exciting the atoms and molecules in our atmosphere to glow. In the north, they are called the northern lights, or the aurora borealis, and in the south, the southern lights or aurora australis.”

According to Dr. Murphy, the aurora occur along narrow, ring-shaped areas that are centered on the Earth’s magnetic poles in the north and south. The northern ring passes over Iceland, southern Greenland, northern Scandinavia, northern Russia, central Alaska, and northern Canada. On the other end of the planet, the southern oval passes over Antarctica, Tierra del Fuego, and several islands in the far South Pacific.

“In many of these areas under the main [northern] auroral oval, the aurora can be seen nearly every clear dark night,” he said. “There have even been a few recorded cases where it’s been seen as far south as the Caribbean.”

What Causes Aurora Borealis?

The solar wind is a continuous stream of charged particles constantly being emitted by the Sun. They include electrons, protons, and positively-charged ions. These particles carry some of the Sun’s magnetic field, also known as the interplanetary magnetic field, with them.

“When these particles and the interplanetary magnetic field encounter the Earth, they compress the Earth’s own strong magnetic field on the side facing the Sun and sweep the field on the side facing away from the Sun into a long magnetotail,” Dr. Murphy said. “This magnetotail contains lobes of opposite pointing magnetic fields, with a sheet of charged particles between the two lobes.”

Sometimes, these opposite-facing magnetic fields in the magnetotail reconnect with one another, and when they do, the field lines recoil, dragging the charged particles along with them and changing their magnetic energy into kinetic energy. They also accelerate electrons from Earth’s own magnetotail at high speeds.

“The electrons follow the lines of the magnetic fields back toward the Earth,” Dr. Murphy said. “When they strike the upper atmosphere, about 100 kilometers or about 60 miles above the surface, they excite the gas there to glow.”

In other words, in the auroras, electrons collide with gases in our atmosphere, causing them to glow.

National Geographic Polar Explorations is now available to stream on Wondrium.

Edited by Angela Shoemaker, Wondrium Daily