The atmospheric region extending from the temperature minimum at the tropopause, at about 18 km, to the mesopause (and sometimes higher, depending on context and author) is commonly referred to as the middle atmosphere. It is inaccessible to the large majority of meteorological rockets and satellites. Interactions there are complex and hard to model realistically. The region is so difficult to study that it has affectionately and sometimes despairingly become known as the “ignorosphere.”

The mesopause is the temperature minimum occurring in Earth's atmosphere near 90 km and represents the boundary between the mesosphere and thermosphere. The mesosphere lies at 50–90 km altitude and has a negative temperature gradient; the thermosphere lies above 90 km and has a positive temperature gradient (see Figure 1). The thermal and dynamical structure of the region around the mesopause, extending for present purposes from about 80 km to 150 km, is characterized by a diversity of physical processes that dynamically and energetically couple the middle and upper atmosphere.