Officially known in the Northern hemisphere as the aurora borealis, the Northern Lights are natural phenomena featuring beautifully colored light displays over the Earth. In 1621, a French scientist, Pierre Gassendi, saw the lights in the north and named after the Roman goddess of dawn, Aurora. He added the word "borealis" for the Roman god of the north wind, Boreas. In the southern hemisphere, they are called aurora australis, meaning "southern." The lights are usually seen after dusk near both poles. Although they look elegant and calm, aurorae are produced from millions of explosions of magnetic energy.
What Causes Aurora?
These haunting lights are a form of intense space weather, a result of the atmosphere shielding the Earth against fierce solar particles that would otherwise make our planet uninhabitable. Millions and millions of electrically charged particles in the solar wind wash over Earth and smash into upper atmospheric gases. The energy from each collision is released as photons -- particles of light. This causes the particles to glow. Aurorae are typically seen at the poles because Earth's magnetic field syphons them around the planet. Think of water moving around a rock protruding the surface of a river.
When Is The Best Time To See The Lights?
The lights are the most frequent between September and October (autumn) and then occur again between March and April (spring) because of Earth's tilt in relation to the sun. They are also visible sometimes in the winter. When darkness overtakes the sky, the lights stand out even brighter and can be seen longer.
How are the Aurora Measured?
Tasked with measuring aurora, NASA's THEMIS team has 20 ground-based observatories (GBOs) across Alaska and Canada. Each station includes a digital camera with a fish-eye lens to capture images of the aurora every three seconds and a magnetometer to measure changes in Earth's magnetic field due to electric currents surging through the upper atmosphere. This visualization shows the ground station locations' radial coverage. Each blue circle represents a circular distance of 540 kilometers (335 miles).
The shape of the aurorae can vary widely. They can appear in a dull glow, arcs, swirls or streaks across the sky called "curtains" that always run east-west, moving and changing constantly. Their "shimmering" effect is actually producing by fading particle explosions just as new ones occur. Although harmless to life on Earth, the aurora can cause power disruptions in satellite communications and in radio and television broadcasts.
The higher in the sky these collisions occur, the more intense the color of the lights. Most aurorae occur about 60 to 620 miles above the earth's surface. They're most commonly green. Only displays extremely high in the upper atmosphere will turn red or purple. Atmospheric gases -- hydrogen, nitrogen, oxygen -- interacting with the solar particles also play a role in the manner in which the colorful display appears.
A Mean Green
During peaks in the solar cycle, when solar flares produce the most intense spurts of wind from the sun, more particles collide with Earth's atmosphere causing more brilliant aurorae. These vibrant green aurorae, shown here above the Alaskan wilderness, are the most dominant aurora color. They occur from about 100 kilometers (about 62 miles) to 250 kilometers (about 155 miles) above the Earth's surface and are caused by the reaction of solar particles with oxygen in the atmosphere.
Blue aurorae are found at the lowest parts of the atmosphere, around 60 miles above the Earth's surface. They are produced from collisions with molecular nitrogen.
Fire in the Sky
During magnetic solar storms, aurorae may shift from the polar regions toward the equator because eruptions from the sun interfere with Earth's magnetic field. When this happens, residents as far as the Dakotas can see intense Northern Lights such as the ones pictured here.
NASA scientists are on an endless search to find Earth-like features off our planet, and Saturn's aurorae fit the bill. The aurorae on Saturn are generated from charged particles released by the sun interacting with Saturn's upper atmosphere, causing them to glow. Using invisible light, Cassini cameras were able to capture aurorae at Saturn's poles for the first time in 2008. The aurorae appear at the poles because the ringed planet's magnetic field forces them pole-ward, exactly what happens on Earth.
Jupiter is no different. The gas giant also showcases aurora at its poles for the same reasons they appear on Saturn and Earth. However, unlike Earth, Jupiter's aurora include several bright streaks and dots, instead of a more uniform "curtain" pattern. Magnetic fields connecting Jupiter to its largest moons -- specifically Io, Ganymede and Europa -- are to blame.