郑国霖 朋友聂远:Navis.gr - The 'Aurora Borealis' Show

AuroraBorealisOne of thegreatest natural light shows on Earth is the Aurora Borealis (Northern Lights). The best place to see it is near the ArcticCircle in Scandinavia, Canada or Alaska at Springequinox. Click on the Aurora Image to Watch the 'Aurora Show'
See also the 'Dancing Northern Lights' animation [105 KB]
An aurora is a display of coloured light in the night skythat occurs primarily in high latitudesof both hemispheres. Auroras in the Northern Hemisphereare called the northern lights, or aurora borealis. Inthe Southern Hemisphere auroras are called the southernlights, or aurora australis.

The aurora is usually white with a greenish tinge but maytake on a yellowish or reddish cast. Vertical rays, likesearchlight beams, are common. In the beautiful coronaform of aurora, rays seem to meet overhead in a starlikeshape. In the spectacular flame type, tonguelike raysripple upward. Vertical rays rising from curving bandsare called draperies.

Auroras appear when highly charged particles fromsunspots and solar flares excite the thin gases of theupper atmosphere and make them glow. Displays are mostfrequent in spring and fall because the Earth is then most nearly in linewith zones of the sun where sunspots are large andfrequent. However, auroras may be most frequent duringwinter in certain areas.

The particles from the sun are deflected by the Earth'smagnetic field toward the geomagnetic poles. (These polesmark the axis of the Earth's magnetic field and are notthe same as the magnetic poles, which mark points atwhich lines of magnetic force are vertical.) Theparticles then collide with oxygen and nitrogen atoms,knocking away electrons to leave ions in excited states.These ions emit radiation at various wavelengths,creating the colours of the aurora.

In the spring of 1989, astronomers across the NorthernHemisphere as far south as southern California (Perspective View[43 KB]), Florida,and Arizona observed a series of spectacular auroras.Especially prominent were the auroras observed in themonths of March (auroras which appeared unusually red)and May (auroras which consisted of arcs, rays, andsheets of greenish light).