Periodic Variations of Drag Coefficient for the ANDE Spherical Satellites During its Lifetime

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A drag coefficient (<em>C</em>_D) inversion method is introduced to study the variations of the drag coefficient for orbital satellites with spherical geometry. Drag coefficients of the four micro satellites in the Atmospheric Neutral Density Experiment (ANDE) are compiled out with this new method. The Lomb-Scargle Periodgram (LSP) analysis of the four ANDE satellites' <em>C</em>_D series has shown that there are obvious 5, 7, 9, and 27 days' period in those data. Interesting results are found through comparing the LSP analysis with series of the daily solar radio flux at 10.7 cm (<em>F</em>_10.7 index), the <em>Ap</em> index, and the daily averaged solar wind speed at 1AU. All series in the same time interval have an obvious period of about 27 days, which has already been explained as the association with the 27 days' solar rotation. The oscillating periods less than 27 days are found in series of <em>C</em>_D, Ap and solar wind speed at 1AU, <em>e.g</em>., the 5, 7, 9 days period. However, these short periods disappeared in the time series of <em>F</em>_10.7 index. The same periodicities of 5, 7, 9 days in <em>Ap</em> and solar wind are presented at the same time interval during the declining phase of solar cycle 23. While in the ascending phase of solar cycle 24, these short oscillations are not so obvious as that in the declining phase of solar cycle 23. These results provide definite evidence that the <em>C</em>_D variations with period of 5, 7 and 9 days are produced by a combination of space weather effects caused by the solar wind and geomagnetic activity.