Which atmospheric layer is where earth's northern and southern lights occur?

Thermosphere is the atmospheric layer where Earth's northern and southern lights occur.

Explanation:

These are the beautiful lights of different colors appear in the evening sky (generally at higher altitudes).  

These occur in thermosphere.

The solar wind (a hot flux of particles) are moved away from sun by the plasma. These solar wind particles may get strike on the thermosphere of earth and thus get in contact with magnetic field of earth which appears like bright colored light.