thermopause

The thermopause is the outer boundary of Earth's thermosphere, the layer of the atmosphere that lies above the mesopause and extends outward toward space. It marks the transition to the exosphere and is not a fixed physical surface. The altitude of the thermopause varies with solar activity and latitude, but is typically cited as roughly 500 to 1,000 kilometers above the Earth's surface, and can shift by several hundred kilometers under different conditions.

In practice, the thermopause is a notional boundary used in atmospheric science. It represents the height at which the atmosphere becomes extremely rarefied and collisions between particles become infrequent, so the thermosphere effectively gives way to the exosphere. Because the thermosphere contains a wide range of densities and temperatures, the exact location of the thermopause is model-dependent rather than a single sharp limit.

Conditions within the thermosphere respond strongly to solar radiation. Temperatures rise with altitude in this region due to heating by extreme ultraviolet and X-ray photons. However, the concept of temperature in the thermosphere is nuanced: the gas is highly rarified and often not in local thermodynamic equilibrium, so a single temperature value does not describe all particle populations uniformly.

Variability in solar activity, geomagnetic conditions, and solar cycle influences the position and properties of the thermopause. Shifts in altitude affect satellite drag, orbital decay, and the interpretation of upper-atmosphere measurements. The thermopause is studied using satellite data, radar, and empirical models to improve space weather forecasting and the understanding of atmospheric escape processes.

See also: homopause, exosphere, exobase, thermosphere.