HOW COME? Solar flares, Earth's magnetic field create auroras

What are auroras, and what causes their colors? asks Tanvir Khan, of Sunnyside, Queens.

The ancient Norse people of Scandinavia said that auroras were sunlight flashing from the shields of the Valkyries, mythic warrior women who carried dead heroes off to heaven. It's not hard to see why. On a night sprinkled with stars, sheets of light suddenly spring up into the sky. A flat sheet of white light pleats into a rippling pink or green curtain, with glowing patches of red or purple. Then the curtain of light twists into long, swaying columns, rising up and up into the night.

Today we know that when auroras appear on Earth, they are set off by events 93 million miles away, on the Sun. Solar flares shooting out of the Sun's turbulent surface send bits and pieces of atoms zipping into space. Torn from their atoms, protons and electrons stream through the solar system, in a "solar wind."

As these charged particles near Earth, they begin to feel the pull of our planet's magnetic field. The magnetic force lines, curving out into space, attract passing protons and electrons. The particles are drawn in, traveling in arcs toward the top and bottom of the planet. Caught like fish in a net, the particles are dumped into the Earth's atmosphere near the North and South poles.

High in the air, the incoming particles collide with nitrogen and oxygen atoms. Some of the atoms temporarily lose an electron.

Others get "excited," gaining energy. When these atoms return to normal after their run-ins with the charged particles, they give off photons of light. Depending on the energy and wavelength of the photons, and our eye's sensitivity to different wavelengths, we see an array of colors.

At high altitudes, oxygen atoms emit photons of red light. Further down, where collisions are fast and furious, oxygen and nitrogen emit higher-energy green light. Blue and violet light come mainly from nitrogen atoms at even lower altitudes. Pinks and yellows appear when red, green, or blue light combine. Result: The glowing air of an aurora.

The glow near the North Pole is called the "aurora borealis." The "aurora australis" lights up skies around the South Pole. When sunspot activity spawns violent storms, and the solar wind intensifies, the northern aurora can sometimes be seen as far south as Mexico.

And Earth is not the only planet with glowing skies. Jupiter has enormous auroras arcing out from its north pole, and Saturn has long-lasting auroras that pulse over both poles.

Watch amazing time-lapse videos of auroras at www.space.com/21243-aurora-nights-in-finland-almost-60-in-time-lapse-video.html

and at http://vimeo.com/85134959.