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