Thermospheric density oscillations due to periodic solar wind highspeed streams

We report on periodic oscillations in thermosphere density, measured by the accelerometer on the CHAMP satellite during 2006, and relate these periodicities to oscillations observed in solar wind speed and K-p index. Common periodic oscillations at 4-5, 6-7, and 9-11 day periods are observed in the neutral density at 400 km in the 2006 data set, with the 7 day period being the predominant oscillation. Spectral analysis reveals that similar periodicities are present in both the solar wind and the planetary magnetic index K-p but not in the EUV solar flux proxy F-10.7. We suggest that the periodic oscillations observed in thermosphere density are a direct response to recurrent geomagnetic activity and associated high-speed streams in the solar wind. The lack of response in F-10.7 at the 7 day period enables storm effects on the thermosphere density to be isolated from solar flux effects. The K-p index for these events correspond to moderate levels of geomagnetic activity, and the resultant perturbations in thermosphere density are +/-20-30% of background levels. Although these levels of perturbation are small compared to major magnetic storms, their much higher occurrence frequency and characteristic long recovery time may lead to a cumulative effect on the state of the thermosphere and ionosphere.