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.
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