  Large magnetic structures are launched away from the Sun during solar
eruptions. They are observed as (interplanetary) coronal mass ejections (ICMEs
or CMEs) with coronal and heliospheric imagers. A fraction of them are observed
insitu as magnetic clouds (MCs). Fitting these structures properly with a model
requires a better understanding of their evolution. In situ measurements are
done locally when the spacecraft trajectory crosses the magnetic configuration.
These observations are taken for different elements of plasma and at different
times, and are therefore biased by the expansion of the magnetic configuration.
This aging effect leads to stronger magnetic fields measured at the front than
at the rear of MCs, an asymmetry often present in MC data. However, can the
observed asymmetry be explained quantitatively only from the expansion? Based
on self-similar expansion, we derive a method to estimate the expansion rate
from observed plasma velocity. We next correct for the aging effect both the
observed magnetic field and the spatial coordinate along the spacecraft
trajectory. This provides corrected data as if the MC internal structure was
observed at the same time. We apply the method to 90 best observed MCs near
Earth (1995-2012). The aging effect is the main source of the observed magnetic
asymmetry only for 28\% of MCs. After correcting the aging effect, the
asymmetry is almost symmetrically distributed between MCs with a stronger
magnetic field at the front and those at the rear of MCs. The proposed method
can efficiently remove the aging bias within insitu data of MCs, and more
generally of ICMEs. This allows one to analyse the data with a spatial
coordinate, such as in models or remote sensing observations.
