  The transport of energetic particles such as Cosmic Rays is governed by the
properties of the plasma being traversed. While these properties are rather
poorly known for galactic and interstellar plasmas due to the lack of in situ
measurements, the heliospheric plasma environment has been probed by spacecraft
for decades and provides a unique opportunity for testing transport theories.
Of particular interest for the 3D heliospheric transport of energetic particles
are structures such as corotating interaction regions (CIRs), which, due to
strongly enhanced magnetic field strengths, turbulence, and associated shocks,
can act as diffusion barriers on the one hand, but also as accelerators of low
energy CRs on the other hand as well. In a two-fold series of papers we
investigate these effects by modeling inner-heliospheric solar wind conditions
with a numerical magnetohydrodynamic (MHD) setup (this paper), which will serve
as an input to a transport code employing a stochastic differential equation
(SDE) approach (second paper). In this first paper we present results from 3D
MHD simulations with our code CRONOS: for validation purposes we use analytic
boundary conditions and compare with similar work by Pizzo. For a more
realistic modeling of solar wind conditions, boundary conditions derived from
synoptic magnetograms via the Wang-Sheeley-Arge (WSA) model are utilized, where
the potential field modeling is performed with a finite-difference approach
(FDIPS) in contrast to the traditional spherical harmonics expansion often
utilized in the WSA model. Our results are validated by comparing with
multi-spacecraft data for ecliptical (STEREO-A/B) and out-of-ecliptic (Ulysses)
regions.
