1) Atmospheric boundary 
The stratopause (formerly Mesopeak) is the level of the atmosphere which is the boundary between two layers, stratosphere and the mesosphere. In the stratosphere the temperature increases with altitude, and the stratopause is the region where a maximum in the temperature occurs. This occurs not only on Earth, but on other planets with an atmospher...
The boundary layer between the stratosphere and the mesosphere at about 50 to 55 km.
(from the article `atmosphere`) The stratopause caps the top of the stratosphere, separating it from the mesosphere near 45–50 km (28–31 miles) in altitude and a pressure of 1 ... ...the photochemical production of ozone is not significant below about 20 km (12 miles). This absorption of solar energy is very important in ... ...
the upper boundary of the stratosphere
The stratopause is a relatively thin atmospheric transition layer found between the stratosphere and the mesosphere. The height of this layer is about 50 kilometers above the Earth's surface.
stratopause The boundary layer between the stratosphere and the mesosphere, at around 30 miles/50 km above the Earth's surface
the boundary layer between the stratosphere and the mesosphere at about 50 to 55 kilometers (31 to 34 miles).
the boundary or transition layer between the stratosphere and mesosphere.