Auroras occur when electrons and protons from the Sun strike gas molecules in Earth's upper atmosphere. As the solar particles encounter Earth's magnetosphere, they are drawn along the magnetic field lines and funneled toward the North and South poles. There, high above Earth's surface, they collide with atmospheric molecules, energizing them and causing them to glow. The colors that result depend on the gas molecules involved. The brightest and most common auroral color, a brilliant yellow-green, is produced by the glow of oxygen molecules roughly 60 miles above Earth. Ionized nitrogen molecules emit blue light when hit by solar particles; neutral nitrogen molecules emit a purplish-red light. All-red auroras are rare; they are caused by the glow of oxygen atoms 200 miles above Earth. The size and intensity of the aurora varies from night to night, and moment to moment, depending on the strength of the solar wind. On April 6, 2001, a large geomagnetic storm produced an aurora that was seen as far south as Alabama. The scientific understanding of auroras has advanced enormously in recent years with the launch of satellites designed expressly to study them. Instruments aboard NASA's Polar spacecraft monitor ultraviolet radiation and chemical changes in the upper atmosphere, effectively offering an up-to-the-minute report on the shape and intensity of the aurora. The Imager for Magnetopause-to-Aurora Exploration (IMAGE) spacecraft, launched in 2000, studies Earth's magnetosphere in astounding detail. It can watch auroras evolve over a period of hours, and can even see auroras flickering in the far-ultraviolet wavelength. Recently and for the first time, scientists observed a phenomenon known as "black auroras." A black aurora isn't really an aurora at all: it's the dark, empty space within a colorful aurora where one would otherwise expect auroral activity to be visible. Nonetheless, black auroras exhibit distinct patterns, including curls, rings and writhing black patches. Nowadays, scientists often can forecast a spectacular aurora hours or days in advance, so it's worth checking space weather websites (See Related Links) with some regularity.
In the 1970s, with the aid of the Hubble Space Telescope, it became apparent that Earth is not the only planet with auroras. On both Jupiter and Saturn, auroras appear pink due to the large amounts of hydrogen in those planets' atmospheres. Jupiter's aurora has proved to be particularly intriguing. On Earth, the aurora is powered by a barrage of charged particles from the Sun. On Jupiter, auroras are generated instead by volcanic particles from the Jovian moon Io. These particles become ionized, expand and then are trapped in Jupiter's tremendous magnetic field. Rotating once every ten hours, Jupiter generates auroras many times more powerful than those on Earth. However, Earth's auroras remain unique in one respect: they are (at times, anyway) green. Indeed, Earth is the only known planet with green auroras, because it is the only known planet with an oxygen-rich atmosphere. As scientists look deeper into the universe for signs of other, potentially habitable worlds, auroras are one clue they examine. If a distant, unknown planet has shimmering green auroras, that's a strong indication that its atmosphere is rich in oxygen, perhaps enough to support life. Whether that life is capable of appreciating the auroras—well, that's another issue.