Global thermospheric density variations caused by high-speed solar wind streams during the declining phase of solar cycle 23

Thermosphere densities at 400 km altitude from accelerometer measurements on the CHAMP satellite are used to investigate oscillations at periods of less than 13 days during the declining phase of solar cycle 23 (2002–2007). The periodic oscillations around 7 and 9 days in neutral density tend to occur during the latter part of the declining solar cycle when periodically recurrent fast streams in the solar wind modulate the level of geomagnetic activity in the geospace environment. It is interesting that the periodic oscillations in neutral density are felt globally and are proportional to the periodic Kp perturbations at the same frequency. Moreover, the periods of 7 and 9 days apparently reflect subharmonics of the 27‐day rotation and may be related to the longitudinal distribution of coronal holes; however the comparison of the temporal evolution of the periodicities between the coronal holes area and solar wind in 2005 indicates that their relationships are rather complex.