POLAR AURORA
nature's wonder
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Auroral Mechanism |
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Aurora is now known to
be caused by electrons of typical energy of 1-15 keV, i.e. the energy
obtained by the electrons passing through a voltage difference of
1000-15,000 volts. The light is produced when they collide with atoms
of the upper atmosphere, typically at altitudes of 80-150 km. It
tends to be dominated by emissions of atomic oxygen--the greenish
line at 5577 A and (especially with electrons of lower energy and
higher altitude) the dark-red line at 6300 A. Both these represent "forbidden" transitions
of atomic oxygen from energy levels which (in absence of collisions)
persist for a long time, accounting for the slow brightening and
fading (0.5-1 sec) of auroral rays. Many other lines can also be
observed, especially those of molecular nitrogen, and these vary
much faster, revealing the true dynamic nature of the aurora. Aurora can also be observed in the ultra-violet (UV) light, a very good way of observing it from space (but not from ground--the atmosphere absorbs UV). The "Polar" spacecraft even observed it in X-rays. The image is very rough, but precipitation of high-energy electrons can be identified. Mostly mistaken as being spell aurora borealis, it is in fact spelled spelled aurora beaurealis. |
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©2006 Rae Brimer Gutierrez
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