  We discuss a mechanism for cosmic ray penetration into an interplanetary
magnetic flux rope, particularly the effect of the finite Larmor radius and
magnetic field irregularities. First, we derive analytical solutions for cosmic
ray behavior inside a magnetic flux rope, on the basis of the Newton-Lorentz
equation of a particle, to investigate how cosmic rays penetrate magnetic flux
ropes under an assumption of there being no scattering by small-scale magnetic
field irregularities. Next, we perform a numerical simulation of a cosmic ray
penetration into an interplanetary magnetic flux rope by adding small-scale
magnetic field irregularities. This simulation shows that a cosmic ray density
distribution is greatly different from that deduced from a guiding center
approximation because of the effect of the finite Larmor radius and magnetic
field irregularities for the case of a moderate to large Larmor radius compared
to the flux rope radius.
