CONICET | Buscador de Institutos y Recursos Humanos

Context. A large amount of magnetized plasma is frequently ejected from the Sun as coronal mass ejections (CMEs). Some of theseejections are detected in the solar wind as magnetic clouds (MCs) that have flux rope signatures.Aims. Magnetic clouds are structures that typically expand in the inner heliosphere. We derive the expansion properties of MCs in theouter heliosphere from one to five astronomical units to compare them with those in the inner heliosphere.Methods. We analyzeMCs observed by the Ulysses spacecraft using in situ magnetic field and plasma measurements. The MC boundariesare defined in the MC frame after defining the MC axis with a minimum variance method applied only to the flux rope structure.As in the inner heliosphere, a large fraction of the velocity profile within MCs is close to a linear function of time. This is indicativeof a self-similar expansion and a MC size that locally follows a power-law of the solar distance with an exponent called ζ. We derivethe value of ζ from the in situ velocity data.Results. We analyze separately the non-perturbedMCs (cases showing a linear velocity profile almost for the full event), and perturbedMCs (cases showing a strongly distorted velocity profile). We find that non-perturbed MCs expand with a similar non-dimensionalexpansion rate (ζ = 1.05 ± 0.34), i.e. slightly faster than at the solar distance and in the inner heliosphere (ζ = 0.91 ± 0.23). The subsetof perturbed MCs expands, as in the inner heliosphere, at a significantly lower rate and with a larger dispersion (ζ = 0.28 ± 0.52)as expected from the temporal evolution found in numerical simulations. This local measure of the expansion also agrees with thedistribution with distance of MC size, mean magnetic field, and plasma parameters. The MCs interacting with a strong field region,e.g. another MC, have the most variable expansion rate (ranging from compression to over-expansion).