So about how high are the aurora displays?
"About 90% of the time, the light from the aurora comes from an altitude of 100-250 km"(berklyey.edu). Although, Occasionally aurora with particles as high as 600 km above the earth atmosphere can give visible displays.
Magnetosphere
The magnetosphere
is the area of space surrounding the earth as
dictated by the earths magnetic field. Other
planets have magnetospheres but, earth's is one of
the strongest. This space is constantly
changing as a result of solar wind fluctuations.
It is the protection of the magnetosphere that
keeps most solar wind particles from hitting the
earth (science.NASA.gov). This
is where the atoms that make up the northern
lights are found. When displays of the northern
lights occur, particles from the sun found in the
earths magnetosphere are accelerated towards the
earths poles.
http://www.casde.unl.edu/glossary/s.php