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The solar minimum of 2008 was exceptionally quiet, with sunspot numbers attheir lowest in 75 years. During this unique solar minimum epoch, however,solar wind high - speed streams emanating from near-equatorial coronal holesoccurred frequently and were the primary contributor to the recurrentgeomagnetic activity at Earth. These conditions enabled the isolation offorcing by geomagnetic activity on the preconditioned solar minimum state ofthe upper atmosphere caused by Corotating Interaction Regions (CIRs).Thermosphere density observations around 400 km from the CHAMP satellite areused to study the thermosphere density response to solar wind high - speedstreams/CIRs. Superposed epoch results show that thermosphere density respondsto high - speed streams globally, and the density at 400 km changes by 75% onaverage. The relative changes of neutral density are comparable at differentlatitudes, although its variability is largest at high latitudes. In addition,the response of thermosphere density to high - speed streams is larger at nightthan in daytime, indicating the preconditioning effect of the thermosphereresponse to storms. Finally, the thermosphere density variations at the periodsof 9 and 13.5 days associated with CIRs are linked to the spatial distributionof low - middle latitude coronal holes on the basis of the EUVI observationsfrom the STEREO.

Impact of CIR Storms on Thermosphere Density Variability during the Solar Minimum of 2008