The solar cycle variability of diurnal and semidiurnal thermospheric temperatures

Diurnal and semidiurnal components of exospheric temperature derived from midlatitude incoherent scatter measurements corresponding to 100 ≤F 10.7 ≤200 between 1969 and 1972 are interpreted in the light of recent theoretical results. Diurnal temperature amplitudes are in excellent agreement with theoretical calculations previously calibrated with equatorial temperature variations inferred from satellite drag data. Present evaluation of the semidiurnal temperature component indicates that semidiurnal temperatures in the upper thermosphere are primarily due to thermal excitation above 100 km, except during very quiet solar conditions, when contributions due to upward propragating tides are comparable. Current theoretical estimates of the (2, 2) tidal mode propagating into the thermosphere are inferred to be approximately a factor of 2 too large; thus excitation and damping mechanisms affecting the (2, 2) tidal mode below 150 km require continued examination.