  The propagation of 15 interplanetary coronal mass ejections (ICMEs) from
Earth's orbit (1 AU) to Mars (~ 1.5 AU) has been studied with their propagation
speed estimated from both measurements and simulations. The enhancement of
magnetic fields related to ICMEs and their shock fronts cause the so-called
Forbush decrease, which can be de- tected as a reduction of galactic cosmic
rays measured on-ground. We have used galactic cosmic ray (GCR) data from
in-situ measurements at Earth, from both STEREO A and B as well as GCR
measurements by the Radiation Assessment Detector (RAD) instrument onboard Mars
Science Laboratory (MSL) on the surface of Mars. A set of ICME events has been
selected during the periods when Earth (or STEREO A or B) and Mars locations
were nearly aligned on the same side of the Sun in the ecliptic plane
(so-called opposition phase). Such lineups allow us to estimate the ICMEs'
transit times between 1 and 1.5 AU by estimating the delay time of the
corresponding Forbush decreases measured at each location. We investigate the
evolution of their propagation speeds before and after passing Earth's orbit
and find that the deceleration of ICMEs due to their interaction with the
ambient solar wind may continue beyond 1 AU. We also find a substantial
variance of the speed evolution among different events revealing the dynamic
and diverse nature of eruptive solar events. Furthermore, the results are
compared to simulation data obtained from two CME propagation models, namely
the Drag-Based Model and ENLIL plus cone model.
