  On scales of days, the galactic cosmic ray (GCR) flux is affected by coronal
mass ejections and corotating interaction regions (CIRs), causing so-called
Forbush decreases and recurrent Forbush decreases (RFDs), respectively. We
explain the properties and behavior of RFDs recorded at about 1 au that are
caused by CIRs generated by solar wind high-speed streams (HSSs) that emanate
from coronal holes. We employed a convection-diffusion GCR propagation model
based on the Fokker-Planck equation and applied it to solar wind and
interplanetary magnetic field properties at 1 au. Our analysis shows that the
only two effects that are relevant for a plausible overall explanation of the
observations are the enhanced convection effect caused by the increased
velocity of the HSS and the reduced diffusion effect caused by the enhanced
magnetic field and its fluctuations within the CIR and HSS structure. These two
effects that we considered in the model are sufficient to explain not only the
main signatures of RFDs, but also the sometimes observed "over-recovery" and
secondary dips in RFD profiles. The explanation in terms of the
convection-diffusion GCR propagation hypothesis is tested by applying our model
to the observations of a long-lived CIR that recurred over 27 rotations in
2007-2008. Our analysis demonstrates a very good match of the model results and
observations.
