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The aurora borealis, or northern lights, illuminates the sky over Stockholm on April 6, 2000. The red and yellow-green colors of this auroral display are due to oxygen gas hovering at two different altitudes. Oxygen in the atmosphere more than 150 miles above the surface of the Earth glows red when exposed to fast-moving particles from the magnetosphere, while oxygen 150 miles away or closer appears yellow-green or green, the brightest and most common auroral hue.
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The aurora borealis is seen here over Iceland's Vatnajokull Ice Cap, the largest ice cap in Europe, on February 15, 1999. The aurora appears in a band shape and gets its blue color from nitrogen 60 miles or less above the Earth's surface. There are many other auroral forms—coronas, rays, spirals, and arcs, to name a few. Observers of auroras sometimes think that the variety of shapes corresponds to fluctuating weather conditions. In reality, auroras take on roughly the same shape each time they appear, but they look different depending on the viewer's perspective. (They can sometimes also be blown by the wind.)
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A brilliant green section of the aurora borealis in its arc form shines over the skies of the Alaskan wilderness, a prime spot for aurora watching. This aurora occurred in the fall, when sky-watchers often report seeing auroras. Scientists don't yet know why, but geomagnetic storms that ignite auroras tend to happen more frequently during the months surrounding the fall and spring equinoxes. Green auroras like this one are the result of energized oxygen located 150 miles or less above Earth's surface.
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In April 1991 the space shuttle Discovery photographed this display of the aurora australis, or southern lights, over the Antarctic region. From above, you can see the magnetic field lines, a series of vortices in the Earth's magnetic field, along which electrons and protons move as they burst into the atmosphere. When space weather pours energy towards Earth and energizes its magnetic field, particles flow and collect at both ends of these field lines, culminating in 2,500-mile-wide rings encircling each pole.
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This color-coded image of our world at night during a display of the aurora borealis in 2001 shows on average the position of the northern lights. The aurora appears in light blue. The other colors represent city lights (yellow), oil production flares (red), burning vegetation (purple), and squid-fishing boats (green).
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NASA's UltraViolet Imager, an instrument on the Polar spacecraft, captured this image of the aurora borealis' distribution across the North Pole on April 6, 1996. Remarkably, it shows auroral activity in both the night and day hemispheres of Earth at the same time—the daytime side being the Western Hemisphere, which is visible in the lower part of the image. Though reflected sunlight would drown out the daytime aurora in a visible light image, ultraviolet light picks it up. The intensity of the aurora's concentration is color-coded from purple (weakest) through blue, green, yellow, and red (strongest). A sub-aurora, the large red area that appears in the upper part of the image, displays mostly in the night sky.
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On September 15, 1998, the NASA Dynamics Explorer 1 satellite recorded this image, the first of its kind showing the complete loop of the aurora borealis encircling the North Pole. The bright area in the upper left of the image is the sunlit portion of Earth.
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Half a billion miles away from Earth, this blue aurora glows in the atmosphere around Jupiter. Taken by the Hubble Space Telescope on December 14, 2000, the image shows the aurora centered over the planet's magnetic north pole. Auroral displays have been observed on Jupiter, Saturn, Uranus, and Neptune, as well as on the moons of these planets. The process for generating auroras is the same throughout the solar system; the only requirements for auroras are the presence of a magnetosphere and an atmosphere. Interestingly enough, Jupiter's moon Io does not have an atmosphere surrounding it, but its many active volcanoes on can create a temporary atmosphere around them when they erupt, allowing for eerie, ground-level auroras.
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The Hubble Space Telescope took this colored ultraviolet image of auroras at both of Saturn's poles in 2000.
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On September 16, 1770, while exploring the South Pacific on the HMS Endeavour, Captain James Cook saw what he described in his diary as "a phenomenon...in the heavens in many things resembling the Aurora Borealis." On the flip side of the Earth, in the Northern Hemisphere, residents of northern China observed an aurora on the same night. (The event was recorded in the Qingshi Gao, a Qing Dynasty almanac.)
Scientists have long suspected that auroras in the Northern and Southern hemispheres are conjugates, or mirror images, occurring at the same time. But hard evidence eluded them. For two decades, NASA studied the auroras and tried to capture an image of simultaneous auroral loops at both poles. Finally, their Polar spacecraft filmed this movie on October 22, 2001. It shows the auroras dancing around both poles at the same time. Analysis of the movie has shown that while the auroras appear to be mirror images, they have subtle details that do differentiate the two.
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