The heterosphere is the layer of an atmosphere where the gases are separated out by molecular diffusion with increasing altitude such that lighter species become more abundant relative to heavier species. The heavier molecules and atoms tend to be present in the lower layers of the heterosphere while the lighter ones are present higher up. The exact boundaries between the different molecules vary according to temperature and solar activity. The heterosphere extends from the

turbopause The turbopause, also known as the homopause, marks the altitude in an atmosphere below which turbulent mixing dominates. Mathematically, it is defined as the point where the coefficient of Eddy diffusion is equal to the coefficient of molecular d ...

to the edge of a planet's atmosphere and lies directly above the homosphere.

Earth's Heterosphere

The Earth's heterosphere begins at about 100 km altitude and extends to the outer reaches of its atmosphere. It incorporates most of the

thermosphere The thermosphere is the layer in the Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions; the ...

and all of the

exosphere The exosphere ( grc, ἔξω "outside, external, beyond", grc, σφαῖρα "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the densi ...

. The major constituents of Earth's heterosphere are nitrogen, oxygen, helium, and hydrogen. Nitrogen and oxygen compose the lower portion of the heterosphere. In the higher levels of the heterosphere, above about 1,000 km, helium and hydrogen are the dominant species present. The heterosphere also incorporates the

ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...

with ions present in the heterosphere's lower levels. These include O⁺, NO⁺, O₂⁺, and N₂⁺. Due to the diffused nature of the heterosphere's gases, its density at any given height is not entirely dependent on the temperature. Other factors contributing to density variations in the heterosphere include day and night cycles, solar activity, geomagnetic activity, and seasonal cycles. The heterosphere contains less than 0.001% of the Earth's atmosphere's total mass.

References

{{Reflist Atmosphere of Earth