Analysis of Magnetic Clouds Evolution from 0.3 to 5 Astronomical Units

DASSO S.; GULISANO A.M.; DEMOULIN, P.; MARSCH, E.

Conferencia; 38th COSPAR Scientific Assembly; 2010

Magnetic clouds (MCs) are transient structures in the solar wind, formed by large scale magnetic flux ropes as deduced from in situ observations of magnetic field. Moreover, in situ observations of the plasma velocity frequently show a clear evidence of an expansion in the radial direction from the Sun, unlike in the solar wind. This expansion has important consequences on the MC evolution, such as on the rates of decrease of mass density or magnetic field intensity. The aim of this work is to show properties of the evolution of MCs in the inner and outer heliosphere from 0.3 to 5 AUs. We present here an analysis of MCs observed by the Helios and Ulysses spacecraft, using in situ magnetic field and bulk plasma measurements. We analyze the dependence of several properties of MCs with the distance to the Sun, and in particular we analyze the dimensionless local expansion rate (ζ) [e.g. Démoulin et al., 2008]. We explore the dependence of the expansion speed on the MC size, the translation velocity, and the heliocentric distance, finding that there is a subset of MCs with almost the same non-dimensional expansion rate (ζ). The time velocity profile observed in this subset of MCs is almost linear, then we call them non perturbed MCs. These MCs expand at rates ζ consistent with the expected value from the global pressure decay in the surrounding solar wind for increasing helio-distances, while perturbed ones may present strong departures from that global rule. We interpret these departures of ζ for perturbed MCs as a consequence of the interaction of MCs with fast streams on their expansion.