INVESTIGATION OF SEASONAL AND INTERANNUAL VARIATIONS OF INTERNAL GRAVITY-WAVE INTENSITY IN THE THERMOSPHERE OVER SAINT-SANTIN

Seasonal and interannual variations in the intensity of internal gravi ty waves (IGW) at altitudes of 100-300 km are investigated using measu rements of electron density, ion drift velocity parallel to the geomag netic field, and ion temperature with the Saint Santin incoherent scat ter radar between 1967 and 1987. IGW with periods of less than 6 hours produce ion temperature amplitudes of 5-80 K and ion velocity amplitu des along the geomagnetic field of 3-17 m/sec. Vertical profiles of IG W intensity have maxima in the altitude regions 140-180 and 250-280 km and vary on annual and interannual timescales. Usually, the IGW inten sity is at its maximum in winter and minimum in summer. Strong interan nual variations are also observed, whose long-term trends sometimes ap pear to follow the solar cycle, either positively or negatively, but a re often irregular. Model calculations show that maxima of IGW intensi ty during solar minimum can be explained by the diminution of atmosphe ric density and ion drag, which leads to increased amplitudes of IGW p ropagating to the thermosphere from lower layers of the atmosphere.