Which process causes the temperature to decrease as one goes higher in the mesosphere?

Above the stratopause and extending to an altitude of about 80 Km is the atmospheric layer called the mesosphere.In this layer,the temperature decreases as the altitude increases.In fact,the mesosphere is the coldest layer of the atmosphere,dropping to a temperature of -90C.The upper boundary of the mesosphere,called mesopause ,is market by a return to increase temperatures.