The sun sends a constant stream of charged particles, know as the solar wind, into space. These energized particles interact with a protective magnetic shield that shrouds our planet.
Earth's magnetosphere, as the shield is called, is made up of invisible lines of a magnetic field that radiate out into space from the poles. The magnetosphere forces the solar wind to slide around it, protecting the planet from radiation.
The charged particles squeeze the magnetic field into a teardrop shape. In the magnetosphere, researchers say, the interaction creates electric fields and electromagnetic waves that transfer their energy to electrons, which then plunge into the atmosphere.
The charged particles excite oxygen and nitrogen in the atmosphere to create the aurora borealis, or northern lights. (The same condition creates the aurora australis, or southern lights.)
"The aurora varies in intensity from brightness equal to that of the Milky Way up to the equivalent of a full Moon," says climatologist Jan Curtis, who photographs auroras from his home in Alaska. "Colors range from mostly greens to reds, and take on the forms of discrete rays, homogeneous bands and arcs, or diffuse glowing clouds. Their movement can be stationary, or zip across the entire sky in seconds."
Thanks to satellites, researchers are getting their best look ever at auroras, and what's behind them. Using three satellites, including the Polar spacecraft, scientists now can make before-and-after measurements of the phenomenon when a gust of solar wind passes by.
James Spann, a space plasma physicist at NASA's Marshall Space Flight Center, explains a recent finding based on satellite data: When a pressure wave of solar radiation hits the leading edge of Earth's magnetosphere, known as the bow, it can cause the shield to contract, and the aurora brightens. As the pressure wave travels along the magnetosphere, the brighter area moves with it, said Spann and his colleagues in a study published in June of 1999.
As the sun approaches solar maximum, a peak of activity, expected in the coming months, flings more and more bursts of energy into space, forcing a corresponding increase in the auroras.