Wave-like variations and sudden density decreases in the lower thermosphere as measured by the San Marco V satellite

Neutral density measurements were carried out by the microaccelerometer on board the Italian San Marco V satellite in 1988. During the final week of i ts existence the satellite's perigee decreased to as low as 130 km. Measure d density values were compared to the corresponding CIRA '86 (MSIS '86) or to our dMSIS model values. The residuals reveal a wavy structure of differe nt time scales. Characterising the wave amplitude by the average deviation of the residuals, its dependence on different parameters was studied. These investigations demonstrated that the wave-amplitude varies with height, lo cal solar time and geomagnetic disturbance level. There is a particularly d eveloped wave pattern in the average deviations below 200 km. Case studies indicated that there are sudden density decreases of 20-30 sec duration tha t might be in connection with plasma bubble crossings by the satellite. Alt ogether 261 such cases were identified and their distribution as a function of height, LST and longitude have been investigated. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.