  Magnetic clouds (MCs) are the interplanetary counterparts of coronal mass
ejections (CMEs), and usually modeled by a flux rope. By assuming the
quasi-steady evolution and self-similar expansion, we introduce three types of
global motion into a cylindrical force-free flux rope model, and developed a
new velocity-modified model for MCs. The three types of the global motion are
the linear propagating motion away from the Sun, the expanding and the poloidal
motion with respect to the axis of the MC. The model is applied to 72 MCs
observed by Wind spacecraft to investigate the properties of the plasma motion
of MCs. First, we find that some MCs had a significant propagation velocity
perpendicular to the radial direction, suggesting the direct evidence of the
CME's deflected propagation and/or rotation in interplanetary space. Second, we
confirm the previous results that the expansion speed is correlated with the
radial propagation speed and most MCs did not expand self-similarly at 1 AU. In
our statistics, about 62\%/17\% of MCs underwent a under/over-expansion at 1 AU
and the expansion rate is about 0.6 on average. Third, most interestingly, we
find that a significant poloidal motion did exist in some MCs. Three
speculations about the cause of the poloidal motion are therefore proposed.
These findings advance our understanding of the MC's properties at 1 AU as well
as the dynamic evolution of CMEs from the Sun to interplanetary space.
