aurora

aurora An emission of light from the Earth's high atmosphere, caused principally by oxygen atoms or nitrogen molecules that are excited by electrons accelerated within the magnetosphere. The visible aurora is dominated by the green (557.7 nm wavelength) and red (630 nm) emissions of oxygen, occurring respectively at altitudes of 100 km and above about 400 km, and red (661–686 nm) nitrogen emissions at about 95 km. Violet purple (391.4 nm) nitrogen emissions are sometimes seen in the sunlit uppermost parts of aurorae at altitudes of 1000 km.

Popularly known as the northern lights (or southern lights in the southern hemisphere), the aurora takes a number of characteristic forms. These may range from a glow, low over the northern horizon (from which the aurora borealis—‘northern dawn’—takes its name), through arcs and bands, which may be homogeneous, or may show vertical rays. Isolated rays and patches of auroral light may also be seen. Most spectacular of all is the corona, a perspective effect whereby rays appear to converge on a single region of the sky almost overhead during a particularly intense storm. During strong activity, the rays and other structures move, causing a ‘curtain’ effect, and there may often be rapid changes in brightness. The aurora is seen from the southern hemisphere as the aurora australis, a mirror-image of activity present at the same time over the opposite hemisphere of the Earth.

Auroral activity is present more or less continuously around the high-latitude auroral ovals. Observers at lower latitudes, such as in the British Isles, southern United States, or Australasia, see auroral activity only when the magnetosphere is disturbed by violent solar events. Mid-latitude aurorae are usually triggered by solar flares or coronal mass ejections, and are most likely around times of high sunspot activity. Aurorae also occur on Jupiter, Saturn, Uranus, and Neptune.