Auroras: The Northern and Southern Lights

Auroras are the vibrant displays of red, green, and purplish-blue lights that appear around Earth's Poles, sometimes seeming to shimmer or pulse in the night sky. Auroras seen in the Northern Hemisphere are called the aurora borealis, or northern lights, while displays around the Southern Hemisphere are called the aurora australis, or southern lights.

Auroras aren't visible while the sun is shining, but our stormy star is the source of these nighttime shows. The sun generates a constant stream of charged particles, or plasma, that's ejected in all directions into space. When this so-called solar wind slams into the invisible magnetic field surrounding Earth, it produces currents of charged particles, mostly electrons, which flow toward the Poles. In the upper atmosphere, solar particles collide with gas atoms and "excite" them with extra energy, which then gets released as light.

An aurora's brilliant colors are determined by the compositions and densities of atmospheric gases—mostly oxygen and nitrogen—found at different altitudes. Reds are the highest of the auroral colors, appearing above 150 miles (240 kilometers). It takes almost two minutes for an excited oxygen atom to emit a red photon, and if the atom collides with another air particle before releasing its light, the color may never emerge. That's why red appears only in the thinner air found at very high altitudes. Bright greens are most common 150 to 60 miles (240 to 100 kilometers) above Earth. Green photons are discharged in less than a second, so they're able to occur in the moderately dense atmosphere at middle elevations. In the very thick lower atmosphere, less than 60 miles (100 kilometers) above the planet's surface, we see a purplish mixture of red and blue lights—the signature colors of molecular nitrogen.

Under the right conditions, auroras might be seen at any time of the year from many places around the globe. But the light shows are far more common closer to the Poles, in chilly hot spots such as Alaska, Canada, Scandinavia, Siberia, and Antarctica. When solar activity is stronger, lower-latitude regions such as North Dakota, Michigan, Quebec, or Tasmania may experience auroral shows. Spring and fall are by far the best seasons for aurora spotting in all locations.

The northern lights have been documented as far back as 568 B.C., when astronomers of Babylon's King Nebuchadnezzar II recorded them on a clay tablet. Today sky-watchers can count on space weather reports to predict when auroras are more likely to emerge. Periods of high sunspot activity tend to produce more frequent and more spectacular auroral storms, such as a February 1958 storm that produced an aurora borealis seen as far south as Mexico City.

But the same charged particles that produce spectacular auroras can also wreak havoc when they occur in high numbers, posing problems for electronics, satellite communications, and the power grid.

Earth isn't the only planet to boast auroras. All of the planets in our solar system are subject to the solar wind, so all planets with a dense enough atmosphere-and even some moons-can produce auroras. But not all auroras are alike. Venus, for example, has no global magnetic field, so its auroras are very irregular. Uranus and Neptune's magnetic fields aren't aligned with their rotational axes, so their auroras may appear near their equators. The gas giants Jupiter and Saturn, meanwhile, have magnetic fields that act more like our own, so their supersized auroras tend to form around their poles and take similar shapes.

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