The night sky is full of wonder, from the spiraling arms of the Milky Way to meteor showers. The most fabled phenomenon, though, is probably the aurora borealis, the northern lights. Before you head north to maximize your chances of catching the show, however, read up on these facts you likely never knew about the colorful display.
They're Centered Around the Magnetic Poles
The northern lights occur as a result of the Earth's magnetic field interacting with particles from the sun, so their location is dependent on the magnetic poles of the planet, not the geographic poles. The particles come from the sun's solar wind, which is driven away from the sun — and towards the earth — by the hot plasma on the edge of the sun's atmosphere.
Some of these particles reach earth's magnetic field, at which point they travel down magnetic lines into the atmosphere, much like radio waves reaching an antenna. This occurs at both the north and south magnetic poles, giving us — in addition to the northern lights — the southern lights, or aurora australis.
They're Always There
While we may think of catching the northern lights as a lucky, once-in-a-lifetime occurrence, the lights themselves are, in fact, always present as a ring around the (magnetic) poles. What changes is the extent of their visibility. This is due to factors both on earth, such as light pollution and atmospheric conditions, as well as activity on the sun, approximately 93 million miles away.
When solar activity is high — a period associated with a lot of sunspots — the particles from the sun can end up traveling further from the magnetic poles in earth's atmosphere, allowing us to see the auroras at lower latitudes. When solar activity is low, however, the auroras hug the poles, meaning only those areas much closer to the poles will be treated to the display. This is a bit of bad news for anyone hoping to see the auroras anytime soon: the sun's activity waxes and wanes on an 11-year cycle, and we're currently in the midst of its lowest point.
They've Been Seen in Some Not-So-Northern Places
A solar outburst in 2011 caused the lights to be seen in cities such as Memphis and Atlanta — south of the aurora's normal reaches, which can extend to northern U.S. cities such as Seattle or Minneapolis.
In 1859, what became known as the Carrington Event saw the aurora extend as far south as Jamaica and Cuba, with records possibly indicating it was even seen in Colombia, just eight degrees north of the equator. The colorful lights of that event were visible for three nights, and the strength of the electromagnetic activity meant mayhem for telegraph operators, whose machines were either not working or, more shockingly, transmitting without being plugged in.
Different Colors are a Result of Different Elements
Green is the color most commonly seen in the northern lights, and it results from the solar particles' collision with oxygen at about 60 to 150 miles above the earth. At the bottom of the display is where you'll most likely see blue and purple, colors that occur when the particles interact with nitrogen, the most common element in the atmosphere.
Altitude can also affect the color of the lights. At altitudes higher than 150 miles, the reaction between the solar particles and oxygen produces a red color, which tends to occur only during very high solar activity. These events can also display a yellow color, which will be visible at intermediate altitudes as the result of a combination between red and green.
Other Planets Also Have Them
Because they are simply a consequence of solar particles interacting with a magnetic field, auroras are not unique to Earth. Jupiter, Saturn, Uranus, Neptune, Mars, and Venus all have their own versions of the phenomenon, and it's quite likely that planets in other star systems experience their own auroras as well.
On Jupiter, for example, its many moons affect the magnetic field to produce whirling patterns in the light. The strength of the planet's magnetic field also means that its auroras aren't dependent on solar activity, so they are a constant feature in the atmosphere as a result of particles pulled from nearby space. Saturn, meanwhile, has an aurora that consists solely of ultraviolet light, making it invisible to the naked eye.
The auroras on Mars and Venus are due to slightly different processes, as neither planet has an encompassing magnetic field. On Venus, auroras can been as a result of the sci-fi-sounding magnetotails, which are essentially a result of the sun's own magnetic field wrapping around the planet. Mars, on the other hand, has residual magnetism in various spots on its crust, causing localized auroras only over those areas.
Photographs Will Look Better to You Than the Real Thing
Because of how the human eye works, it's difficult to pick up the faint colors of the auroras in the night sky. Cones are our eyes' "high-res" vision cells, which we use to see colors in the bright light of day. At night, however, our eyes' rod cells are at work, which help us see in minimal light but only image the world in black, white, and gray. Consequently, when we're watching the northern lights, there generally isn't enough light to engage our cone cells, and we see the colors as shades of gray or, at best, faint hues.
Cameras, of course, don't have this limitation, and they can be set to longer exposures to capture even more light. The stunning photographs you might see of the auroras, then, while they represent the light and colors that were indeed present in the sky — no filter needed! — will be far more vivid than anyone present saw with their own eyes.