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A magical northern lights display, most common near the icy north and south poles, begins 93 million miles away on that giant fireball we call the sun.
When concentrated pockets of the sun’s energy burst away from its surface, masses of energized particles shoot out into space – sometimes right toward Earth. If we do happen to be in the way, lucky sky watchers in places like Alaska, Canada and Norway might see some glowing aurora borealis dancing around the night sky.
Unlike solid Earth-like planets, our sun – one of countless trillions of stars in the universe – is a mass of super-hot electrically charged particles (mainly protons and electrons) called plasma, composed of about 71% hydrogen, 27% helium, and small amounts of oxygen, carbon, and other heavier elements. Gravity holds the sun’s swirling plasma together, creating immense pressure and high temperatures – at least 10,000°F on its fiery surface, and many times hotter down in its center and up in the sun’s atmosphere, called the corona. Plasma – which is not a gas, liquid or solid – constantly flows from the corona as a stream called the solar wind, which blows away from the sun in all directions throughout the solar system.
The sun’s intensely magnetic surface loops and undulates, often bunching into dark regions we know as sunspots. At times, that bunched-up magnetic energy explodes in the form of a solar flare, shooting high-energy particles and radiation off into space, says Kelly Korreck a solar physicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “It’s like a knot in your muscle, that concentrated energy that you want to release,” Dr. Korreck explains. Solar flare particles reach Earth in less than10 minutes and can trigger northern lights displays.
Powerful explosions known as coronal mass ejections (CMEs) can occur alongside solar flares. CMEs blast billions of tons of plasma away from the sun and send it shooting through space at thousands of miles per second. Even at that speed, Earth-directed CMEs take a few days to reach us, and upon arrival can wreak a little havoc or stir a little magic (or both). But first they have to reckon with two sets of Earth armor: the magnetosphere and the atmosphere.
The magnetosphere (mag-NEE-to-sphere) – an enormous oblong bubble much larger than our atmosphere – protects us from space radiation, like the deflector shield in science fiction that allows most of it to slip by, says Dr. Korreck. The motion of molten iron deep inside Earth’s core generates this huge magnetic field, which can extend up to 40,000 miles from Earth’s sun-facing side, and even farther on the night side. (Our regular atmosphere, by comparison, measures about 300 miles.) The magnetosphere connects to Earth through invisible magnetic field lines at the north and south poles.
When a CME collides with the magnetosphere – a disturbance referred to as a geomagnetic storm – some electrons and protons penetrate the shield and slide down those magnetic field lines toward the poles, explains Mark Hammergren an astronomer at the Adler Planetarium in Chicago. Once they reach Earth’s upper atmosphere, electrons energize or “excite” oxygen and nitrogen atoms, which release photons of light in green, red, and sometimes blue – the typical colors of a northern lights display.
“Between about 60 to 300 miles up, most oxygen atoms will give off a pretty intense greenish glow,” Dr. Hammergren explains, “and at lower altitudes where there is more of the gas bouncing around, oxygen can give off a reddish glow.” Nitrogen atoms, which he says make up the majority of earth’s atmosphere, give off a bluish glow at around 60 miles up, and if particles slide low enough into the atmosphere, nitrogen gives off a lilac or purplish color. “It’s rare,” Dr. Hammergren says, “because it requires an intense blast from the sun.”
Ancient scientists dubbed the phenomenon of the northern lights aurora borealis after Aurora, the Roman goddess of the dawn, and the Greek word “boreas” meaning “north wind.” In the southern hemisphere, they are called the aurora australis. Auroras shift and morph constantly, appearing as glowing veils or patches, rippling curtains, vertical shafts of light that seem to meet at a center point, and streams that pulse across the sky.
From space, the northern lights appear to shimmer and twirl within two irregularly-shaped rings encircling the poles called the auroral ovals. Roughly 2,000 miles in diameter, the auroral ovals occasionally stretch and bend southward when the solar wind is particularly strong, pushing northern lights to those of us at lower latitudes.
The sun goes through an 11-year cycle of magnetic activity, says Dr. Hammergren, with periods of high magnetic activity and a lot of sunspots (called the “solar maximum”) to periods of low or “minimum” magnetic activity, each alternating about every five and a half years. During the maximum, solar flares and CMEs happen more frequently and intensely, increasing the chances of auroras.
The sun’s gift of northern lights has a dark side. Solar flares and CMEs put airline crews and astronauts at risk of radiation exposure, especially on flights passing near the poles where the protective magnetosphere thins and weakens. Geomagnetic storms can disrupt radio and military communications, cause temporary drag on satellites, and reduce the precision of global positioning systems (GPS). A severe hit can even shut down power grids. “If a power company knows it is coming, they can adjust the loads so things don’t get destroyed,” Dr. Korreck explains, “but if there is failure, it could take six months to repair.” So far, she says, we’ve been lucky.
The last solar maximum occurred around 2013-2014, Dr. Hammergren says, and we are heading toward a solar minimum in about 2020, when solar activity such as flares and CMEs will quiet down. But he says that auroras can also be triggered by solar wind streams emitted by coronal holes – regions of the corona where magnetic field lines open up and allow high speed solar wind particles to stream away from the sun.
An Earth-facing coronal hole just opened up, according to Spaceweather.com, and the resulting solar wind may spark polar auroras on Jan. 27. A similar event might explain an unexpected aurora display Dr. Hammergren saw in July, 2009 while visiting Montana.
“Not only was it near solar minimum,” he says, “but also it was the middle of summer and we were up in the north, so the night was short. You never know when the northern lights will strike.”