The atmosphere above about 180 km is heated by the absorption by molecular oxygen of the spectral region from 1200Å to 1800Å of sunlight. Calculation Indicates that the temperature of the atmosphere from 180 km to 300 km may increase 50° per hour In the daytime, which causes molecular oxygen, and possibly nitrogen, to dissociate into atoms. Due to the heating and dissociation there is a marked daily expansion, and nightly contraction, of these outlying leve