Seeing the astronomical phenomenon known as the northern lights, a.k.a. aurora borealis (literally "northern dawn"), is a bucket-list item for many people. These dramatic curtains of colored light, which appear high in the night sky in the northern hemisphere, are most visible in the middle of the night and the dead of winter, and in remote, dark areas. Humans have seen and made stories about the lights since prehistoric times and, more recently, conducted scientific studies on them. Here are 10 facts you ought to know about the aurora.
1. EXCITED ELECTRONS AND MAGNETIC FIELDS CAUSE THE SPECTACLE.
The ethereal glow comes from collisions [PDF] between fast moving electrons from the magnetosphere (the region of space controlled by Earth’s magnetic field) and oxygen and nitrogen molecules in our upper atmosphere. Electrons transfer some of their energy to these molecules when they collide; this transfer of energy is said to “excite” them. An excited molecule eventually returns to its non-excited state by releasing photons, or light particles. Large numbers of these collisions create enough light for us to see.
2. SPECIFIC ATOMS CREATE SPECIFIC COLORS.
The colors of the polar lights depend on whether electrons collide with oxygen or nitrogen, and how energetically. The change in energy between “excited” and original states has a specific value and the resulting photon has a specific color, or wavelength, Don Hampton, a research assistant professor at the Poker Flat Research Range in Alaska, tells mental_floss.
Oxygen emits greenish-yellow or red light, while nitrogen generally gives off blue light; the blending of these produces purple, pink, and white. Oxygen and nitrogen also emit ultraviolet light, which can be detected by special cameras on satellites but not by the human eye. Researchers can use the different colors to figure out such things as the energy level of the electrons bombarding our atmosphere and creating the aurora.
3. THEY'RE ALWAYS THERE.
Auroras occur mostly in high latitudes, near the poles, because electrons travel along magnetic field lines and the Earth's magnetic field lines come out and go into the Earth near its poles. But auroras have been seen as far south as Mexico. In some areas, such as Alaska or Greenland, they may be visible most nights of the year. The lights also occur during the day, but we can only see them with the naked eye after dark. In fact, according to the University of Alaska Fairbanks Geophysical Institute, there's always an aurora somewhere on Earth.
4. THE AURORAS HAVE BEEN PROBED.
In January 2015, NASA-funded scientists launched a rocket, the Auroral Spatial Structures Probe, into the northern lights from the Poker Flat Research Range about 30 miles north of Fairbanks. The probe carried seven instruments to study the electromagnetic energy that can heat the thermosphere—the second highest layer of the atmosphere—during auroral events. On a related note, astronauts aboard the International Space Station often see, and photograph, the aurora.
5. THEY'VE BEEN OBSERVED FOR A LONG TIME.
Galileo Galilei and Pierre Gassendi witnessed a lights display in 1621, and a reference to the northern lights was made by Gregory of Tours way back in the 500s. The earliest datable account is on a Babylonian clay tablet that recorded observations made by the official astronomers of King Nebuchadnezzar II on the night of 12/13 March 567 BCE of an unusual “red glow.” But the earliest of all might be 30,000-year-old cave paintings that some believe to represent the aurora. The lights were observed and commented on by Benjamin Franklin in 1778 and Edmond Halley, of comet fame, in 1716.
6. SUNSPOT ACTIVITY AFFECTS THE LIGHTS.
The lights of the aurora are only the end product of a complicated interaction between the sun and Earth (an interaction that likely occurs around other stars, too). Space weather, or activity in the sun's upper atmosphere (which extends throughout the solar system), includes radiation and plasma released by the sun. That in turn can affect Earth's geomagnetic fields and, thus, the aurora.
Auroras tend to be more frequent and spectacular during high solar sunspot activity, which occurs on an 11-year cycle. Particularly spectacular displays include auroral storms noted during August and September 1859; February 11, 1958, when lights 1250 miles wide circled the Arctic from Oregon to New Hampshire; and March 13, 1989, when the sky turned a vivid red seen in Europe and in North America as far south as Cuba.
7. THEY'RE THE STUFF OF LEGENDS.
Some North American Inuit call the auroras aqsarniit, or football players, believing they result from spirits of the dead playing football with the head of a walrus. Other legends include a belief among ancient North Americans that the aurora was a narrow, torch-lit pathway to guide departed souls to heaven, and the Iglulik, who called the lights arsharneq or arshät and thought they were a powerful spirit who assisted shamans. In Roman times and in 16th century Europe, people viewed the displays, particularly red ones, as frightening omens.
8. THE LIGHTS ARE TOURIST ATTRACTIONS.
The best viewing of the northern lights occurs in high northern latitudes during the winter, in places like Fairbanks, Alaska; Dawson City, Yukon; Yellowknife, Northwest Territories, Canada; Gillam, Manitoba, Canada; the southern tip of Greenland; Reykjavik, Iceland; Tromsø, Norway; and the northern coast of Siberia (bundle up!). You need clear, dark skies, so take into account the weather, times of sunrise and sunset, and the moon phase. Sightings are most likely during the three or four hours around midnight. Look in the direction of the closest pole. The lights can reach heights of 620 miles, but usually are about 60 miles high.
9. SOME PEOPLE HEAR THEM.
According to scientists, the upper atmosphere where the lights occur is too thin to carry sound waves, and the aurora is so far away that it would take a sound wave five minutes to reach ears on the ground. Still, many people claim to hear whooshing and crackling noises when there is an aurora in the sky. There's at least one case where a microphone detected audible sound associated with an aurora, and recently, research groups in Denmark and Finland claim to have recorded sounds from the lights. However, some early explorers found that covering someone’s eyes made the sound disappear, so the Geophysical Institute suggests that the perception of sound could be caused by "signal leakage" from overstimulated visual centers in the brain or by energetic phenomena associated with the aurora. There are other theories about the perception of auroral noises, but no consensus as to whether they actually exist. Solving the mystery may reveal more about how the brain and sensory perception work than the phenomena of the lights.
10. THERE ARE SOUTHERN LIGHTS.
An aurora occurs around the southern magnetic pole, known as aurora australis. This aurora has to be fairly active before it can be seen from places other than Antarctica, with Hobart, Tasmania and the southern tip of New Zealand offering the best viewings. The large spirals sometimes seen in the aurora will be mirror images of each other at both poles. Of course, when it is winter and dark in the northern hemisphere, there is light around the clock at the South Pole, so this can only be observed when it is fall and winter in Antarctica, which corresponds to northern hemisphere summer.