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   Aurora, Northern Lights
  Aurora Borealis, Ørsta, Norway December 27, 2005 imaged by Geir Øye (site, aurora gallery). Tall curtains of colour silently shift and alter across the night sky. Their green base is somet 90-100 km (56-62 mile) high, the red extends much higher. Image ©Geir Øye, shown with permission.

  Aurorae occur in two ovals encircling the magnetic poles. Canada, northern USA, Scandinavia and northern Russia are well positioned for aurorae. The ovals expand during high auroral activity and mid latitude Europe and the US then see displays.



 
The aurora or 'Northern lights' is one of Nature’s greatest spectacles. A display might start as a few upward shafts of light almost imperceptible against a darkening twilight sky. The shafts then take form, they brighten into greens topped with reds, they join to make wide curtains, move and flicker, they disappear then quickly return again. The silence as they change is somehow more eerie than the lights themselves. Sometimes the display is confined to the north, in others reds and orange cover the whole sky.

Aurorae are best seen for 2-3 hours around midnight although they can be visible from dusk to dawn. A moonless night well away from light pollution is ideal. They are most frequent and at their finest at high latitudes. Aurorae are concentrated in two giant ovals around* Earth’s magnetic poles. The northern pole is currently** in the high Canadian Actic and Canada, Northern USA and Northern Europe are well placed for bright displays.
  
The auroral ovals enlarge during high activity and displays are then visible further south in England, Germany and mid latitude USA. Very exceptionally, aurorae are even seen in the tropics.

Aurorae are produced by solar storms and are most frequent during the maxima of the 11 year solar activity cycle. The last maximum was 2000/1 and although we are now (2007) near minimum activity there are still plenty of aurorae to see.

Solar disturbances take 2-3 days to reach Earth and aurorae are therefore to some extent predictable. For daily oval plots and aurora predictions see links.

   
  
The ovals are only approximately centered on each magnetic pole. They are furthest from it on Earth's midnight side.
**  Locating the exact position of the northern magnetic pole and characterising the Earth's magnetic field was one of the subsidiary quests during the long search for a North West Passage. The magnetic pole was then near Boothia Peninsula. It is presently (2007) northwest of Sverdrup Island at ~83°N 115°W . After a period of stability is is migrating quickly, ~ 40 km per year, towards Siberia. Bad news for 2060 North American aurora observers.