Atmospheric conditions in 1995 from the orbital decay of the ODERACS II spheres

The orbital decay of a low-Earth-orbit satellite having constant cross-sect ional area in a near-circular orbit can furnish valuable information regard ing the dynamical conditions of the atmosphere. The slope of the orbital de cay curve gives the atmospheric density, whereas the curvature of the decay curve can indicate the expansion, stability or contraction of the: atmosph ere. The state of the atmosphere in 1994 was determined from the orbital de cay of the ODERACS I spheres by employing this technique. The state of the atmosphere in 1995 was determined from the orbital decay of the ODERACS II spheres and reported in this study. From days 40-215, 1995, the atmosphere was in a prolonged compression mode. During this period, ODERACS II Sphere 3 encountered progressively smaller densities as the density scale height d iminished steadily. This was followed by a period of relatively stable atmo sphere. During a brief period from days 280-300, the atmosphere was in a st rong expansion mode. which was followed by another compression mode until d ay 360. The results show great parallels between the atmospheric conditions in 1994 and 1995, including clear indications of the semi-annual variation . The atmosphere in 1995 was under greater compression than in 1994, consis tent with the phase of the sunspot cycle. However, the rate of atmospheric contraction was greater in 1994 than in 1995 in contrast to predicted model s. The observed 10.7 cm solar fluxes at Penticton, British Columbia, strong ly support the findings of this study. (C) 1998 Elsevier Science Ltd. All r ights reserved.