Ask A Scientist© Astronomy Archive

Author:      amanda smith, jeff, burce s maccallum,  and  jan s belzer
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?

Response #:  1 of 2
Author:      jasjeet s bagla
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.



Response #:  2 of 2
Author:      rcwinther
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.