Evolution of magnetic clouds in the inner heliosphere

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

Proc. of Solar Wind 12 Conference

AIP Conf. Proc.

Año: 2010; p. 391 - 394

Magnetic clouds (MCs) are objects in expansion during their travel through the heliosphere. In situ observationsindicate that their front travel faster than their back, showing a clear empirical signature of expansion. With the aim ofquantifying the expansion rate of MCs in the inner heliosphere (0.3 to 1 AU), we present here a statistical study of eventsobserved by the spacecraft Helios 1 and 2, during their complete period of operations. From the analysis of the profile of theMC magnetic field components in its local frame, which is obtained from a rotation of the observed magnetic field vectorsto a system of reference oriented as the main axis of the flux rope, we revise a list of events identified in previous worksand redefine improved MC boundaries/orientation for each event. We then split the sample into two subsets according to thecharacteristics of their velocity profiles, (a) those MCs with a significantly perturbed velocity profile due to the interaction withtheir surrounding solar wind (i.e. overtaken by streams) and (b) those that are not perturbed. We compute the dimensionlesslocal expansion rate (z ) for MCs defined by several works [e.g. Démoulin et al., Solar Phys, 250, 347-374 (2008)]. We find significantly different distribution of values for z when perturbed and non perturbed events are considered. Non perturbed MCs expand at rates z 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 z for perturbed MCs as a consequence of interaction with streams on their expansion.