  We investigate energetic particle transport in Corotating Interaction Regions
(CIRs) through a case study. The CIR event we study occurred on $2008$ February
$08$ and was observed by both the Advanced Composition Explorer (ACE) and the
twin Solar TErrestrial RElations Observatory (STEREO)-B spacecraft. An in-situ
reverse shock was observed by STEREO-B ($1.0$ AU) but not ACE ($0.98$ AU).
Using STEREO-B observations and assuming the CIR structure does not vary
significantly in the corotating frame, we estimate the shock location at later
times for both the STEREO-B and ACE observations. Further assuming the
accelerated particle spectral shape at the shock does not vary with shock
location, we calculate the particle differential intensities as observed by ACE
and STEREO-B at two different times by solving the focused transport equation
using a Monte-Carlo simulation. We assume that particles move along Parker's
field and experience no cross-field diffusion. We find that the modulation of
sub-MeV/nucleon particles is significant. To obtain reasonable comparisons
between the simulations and the observations by both ACE and STEREO-B, one has
to assume that the CIR shock can accelerate more particles at a larger
heliocentric distance than at a smaller heliocentric distance.
