The thermopause is the atmospheric boundary of Earth's energy system, located at the top of the thermosphere.
Below this, the atmosphere is defined to be active[clarification needed] on the insolation received, due to the increased presence of heavier gases such as monatomic oxygen.
Beyond (above) this, the exosphere describes the thinnest remainder of atmospheric particles with large mean free path, mostly hydrogen and helium.
[1] The exact altitude varies by the energy inputs of location, time of day, solar flux, season, etc.
Although these are all named layers of the atmosphere, the pressure is so negligible that the chiefly-used definitions of outer space are actually below this altitude.