stratosphere vs substratosphere

stratosphere

noun
  • The region of the uppermost atmosphere where temperature increases along with the altitude due to the absorption of solar ultraviolet radiation by ozone. The stratosphere extends from the tropopause (10–15 kilometers) to approximately 50 kilometers, where it is succeeded by the mesosphere. 

substratosphere

noun
  • The area of the atmosphere directly below the stratosphere. 

How often have the words stratosphere and substratosphere occurred in a corpus of books? (source: Google Ngram Viewer )