Neutral composition waves excited by auroral heat sources are investigated. For horizontal wavelengths of the order of 1000 km it is concluded that diffusion processes (1) play a significant role such that deviations from diffusive equilibrium prevail for He throughout the thermosphere, (1) produce phase differences of about 220¿ (or -140¿) between He and N2, and (3) account for He amplitudes comparable to those of the N2. These results basically explain recent AE-C satellite measurements which have revealed an anticorrelation between the heavier and light constituents of the thermosphere. The calculations also indicate that temperature and N2 concentration are generally out of phase by about 100¿.