The atmosphere can be divided into different layers based on temperature variations. These are the troposphere (0–18 kilometres above the earth), stratosphere (18–50 kilometres), mesosphere (50–90 kilometres), and thermosphere (90–600 kilometres). The upper part of the mesosphere, and most of the thermosphere, is also known as the ionosphere (80–400 kilometres). Auroras occur here, generally at between 100 and 400 kilometres above the earth. Beyond the thermosphere is the exosphere, which is really a transitional zone between the atmosphere and outer space.
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