On the determination of the harmonic components of the diurnal variation of the thermosphere from satellite drag data

The accuracy of the diurnal and semidiurnal harmonics of density variation determined from satellite drag data is discussed and compared with data from the Jacchia empirical atmospheric model. The phase discrepancies between densities and temperatures, which are more pronounced in summer, and the extrema of the density data are described. The ratio of the semidiurnal to diurnal component of kinetic temperature, determined from Salah and Evans incoherent scatter radar observations, also indicates a seasonal variation. The radar data are compared with the Jacchia model, and a third harmonic is discussed.