In which layer of the atmosphere are the ionized particles that cause the aurora borealis?
The upper layers of the atmosphere are the magnetosphere and the ionosphere.
The Solar Wind, protons and electrons emitted from the sun cause the Aurora Borealis. When these charge particles move past the earth they are attracted to the magnetic force field of the earth created by the magnetosphere. The force field is strongest at the poles particularly the north pole where the magnetic field is pulled into the earth.
These charged particles pulled into the earth's magnetic field by the magnetosphere then collide with gas particles in the ionosphere. In the ionosphere the charged particles collide with different types of gases, oxygen, ozone, molecular nitrogen gas, and free atoms of nitrogen. Each gas creates a different color when it collides with the energy of the charged particles of the Solar Wind.