The effect of periodic variations of thermospheric density on CHAMP and GRACE orbits

In this paper thermosphere densities observed by the CHAMP and GRACE satellites and their orbital parameters are used to investigate the effect of periodic oscillations in thermospheric densities (7-27 days) caused by solar rotation and periodic magnetic activity on satellite orbits during 2003-2005. Two new results are obtained in this study. First, the response of the mean radius of the satellite orbit per revolution (MRPR) to the oscillations in the mean atmospheric density per revolution (MDPR) increased linearly with oscillation periods. Therefore, MRPR had a strong oscillation near the 27 day period. However, it had no obvious 7, 9, and 13.5 day oscillations, although there were strong oscillations at the same periods in MDPR. Second, there was a phase difference of pi/2 between the oscillations of MRPR and MDPR. The phases of the oscillations in MRPR led the phases of the variations in MDPR. The correlation coefficient between the 27 day oscillations in MRPR and those in MDPR was 0.83 with a phase difference of -6.8 days for CHAMP; the correlation for GRACE was 0.67 with a phase difference of -6.4 days. The amplitudes of the oscillations in MRPR of CHAMP were larger than those of GRACE because GRACE had a higher orbit than CHAMP. These features are in good agreement with our theoretical analysis.