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Question:
What is an aurora borealis made of and why does it only appear sometimes? Why do aurorae appear near the magnetic poles? What is the ionosphere?



Replies:
Aurora Borealis is made of charged particles moving in Earth's magnetic field. These particles emit light as they interact with the ionosphere. These particles are emitted by Sun in small quantities (as compared to Sun's mass) and whenever these cross Earth, we see the display. Solar activity has an 11 year period so the Auroral activity also varies over this period.

The ionosphere is the uppermost layer of atmosphere. It is completely ionized (hence the name ionosphere) by the radiation from the Sun and cosmic rays. Near magnetic poles, the lines of magnetic field (of Earth) enter the atmosphere. Charged particles emitted by the Sun move along the lines of magnetic field. Therefore they enter the atmosphere where the lines enter the atmosphere -- at the magnetic poles. Here they interact with a atmo- sphere and produce Aurora.

Jasjeet S Bagla



The Sun is continually sending out particles (mostly protons and electrons) into space; some are blasted out by solar flares and sunspots, others "boil" off from the very hot corona (the Sun's upper atmosphere) into the solar system. This is called the solar wind. The Earth's magnetic field shields the Earth from most of these particles; some get trapped in doughnut-shaped regions surrounding the Earth called the Van Allen belts. Particles continually "leak" from these regions, traveling down along the Earth's magnetic field lines (that is the reason they are usually seen only near the north and south poles), until they crash into the Earth's upper atmosphere. The collisions transfer energy to atoms of gas, which then give off light, just like a neon sign. Pictures from space show that there is a permanent "ring" of auroral activity around the poles; but during times when the Sun is very active, northern auroras may be seen as far south as Mexico.

RC Winther


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