  Forbush decreases (FDs), which are short-term drops in the flux of galactic
cosmic rays, are caused by the shielding from strong and/or turbulent magnetic
structures in the solar wind, especially interplanetary coronal mass ejections
(ICMEs) and their associated shocks, as well as corotating interaction regions.
Such events can be observed at Earth, for example, using neutron monitors, and
also at many other locations in the solar system, such as on the surface of
Mars with the Radiation Assessment Detector instrument onboard Mars Science
Laboratory. They are often used as a proxy for detecting the arrival of ICMEs
or corotating interaction regions, especially when sufficient in situ solar
wind measurements are not available. We compare the properties of FDs observed
at Earth and Mars, focusing on events produced by ICMEs. We find that FDs at
both locations show a correlation between their total amplitude and the maximum
hourly decrease, but with different proportionality factors. We explain this
difference using theoretical modeling approaches and suggest that it is related
to the size increase of ICMEs, and in particular their sheath regions, en route
from Earth to Mars. From the FD data, we can derive the sheath broadening
factor to be between about 1.5 and 1.9, agreeing with our theoretical
considerations. This factor is also in line with previous measurements of the
sheath evolution closer to the Sun.
