Cirric
Hi. From the web. "The aurora occurs between 35 mi and 600 mi (56 km–970 km) above the earth." Any height above 100 Km is considered space.

Brant
What Cirric referred to is also known as the ionosphere.

lindajune
Auroras are produced by the collision of charged particles from the Earth's magnetosphere (mostly electrons but also protons and heavier particles) with atoms and molecules of the Earth's upper atmosphere (at altitudes above 80 km).

VGpugs
Auroras are produced by the collision of charged particles, mostly electrons but also protons and heavier particles, from the magnetosphere, with atoms and molecules of the Earth's upper atmosphere (at altitudes above 80 km). The particles have energies from 1-100 keV. Most originate from the sun and arrive at the vicinity of earth in the relatively low-energy solar wind. When the trapped magnetic field of the solar wind is favourably oriented (principally southwards) it reconnects with that of the earth and solar particles then enter the magnetosphere and are swept to the magnetotail. Further magnetic reconnection accelerates the particles towards earth.

Joe Jonas Luver
Really good site