The thermosphere is a layer of the Earth's atmosphere situated directly above the mesosphere and below the exosphere. The thermosphere performs several important functions, such as temperature regulation and filtering powerful X-rays and ultraviolet radiation emitted by the sun. The thermosphere has the unique ability to expand to absorb heat and contains a diverse range of temperatures throughout its different layers.
Some temperatures in the thermosphere are fixed and inherent while others vary with the strength and duration of sunlight filtering through. The lower portion of the thermosphere generally contains the highest temperatures and has the densest atmosphere. Further up the thermosphere, temperatures level off, then decline towards the uppermost layer. The thermosphere, as with the surface of the Earth, is typically warmer during the day, then cools off at night. The difference in daytime and nighttime temperatures within this layer may be extreme: the thermosphere may become 900 degrees Fahrenheit warmer when the sun is most active, and during the summer months. The thermosphere is separated from the exosphere above it by a boundary called the thermopause; the mesopause, in contrast, forms the boundary between the thermosphere and the mesosphere below. The thermosphere is the least dense of all layers of the atmosphere, and extends for a considerable distance into the outermost regions of the Earth's atmosphere.