CONICET | Buscador de Institutos y Recursos Humanos

Magnetic clouds (MCs) are objects in expansion during their travel through the heliosphere. In situobservations indicate that the MC front travels typically faster than its back, showing a clear empiricalsignature of expansion. With the aim of quantifying the expansion rate of MCs, we present here astatistical study of all the MCs observed in the inner heliosphere (from 0.3 to 1 AU) by the spacecraftHelios 1 and 2, during their complete period of operations. We first rotate the observed magnetic field into the MC frame (defined by the MC axis and thespacecraft trajectory). This allows a better localization of the MC boundaries for each event. We thensplit the sample in two subsets according to the characteristics of their velocity profiles, (a) those MCs with a significantly perturbed velocity profile due to the interaction with theirsurrounding solar wind (i.e. perturbed by streams), and (b) those that are not perturbed by them (not perturbed). We analyze the expansion velocity (defined with respect to the cloud center), Vexp and itsdependence with several MC properties, as its size, its helio-distance, its translation velocity, etc. Theremoval of the correlations found lead to the definition of a dimensionless local expansion rate (ζ) forMCs. This definition of ζ is fully consistent with previous works, which defined the same quantity fromtheoretical considerations. We find significantly different values of ζ for perturbed and not perturbed events. Not perturbedMCs expand at rates consistent with the expected value from the global pressure decay in thesurrounding solar wind (SW) for increasing helio-distances, while perturbed ones may present strongdepartures from that general rule. We interpret this departures for perturbed MCs as a consequence oftheir interaction with local SW streams on their expansion.