Evolution of magnetic clouds in the inner heliosphere

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

Saint Malo, France

Conferencia; Solar Wind 12; 2009

American Physical Society

Magnetic clouds (MCs) are objects in expansion during their travel throughthe heliosphere. {it In situ} observations indicate that its front travelsfaster than its back, showing a clear empirical signature of expansion.With the aim of quantifying the expansion rate of MCs, we present herean statistical study of all the MCs observed in the inner heliosphere(from 0.3 to 1 AU) by the spacecraft Helios 1 and 2, during their completeperiod of operations. From the analysis of the profile of the MC magneticfield components in its local frame, which is obtained from a rotationof the observed magnetic field vectors to a system of reference orientedas the main axis of the flux rope, we revise and redefine improved MCboundaries/orientation for each event.We then split the sample in two subsets according to the characteristics oftheir velocity profiles, (a) those MCs with a significantly perturbedvelocity profile due to the interaction with their surrounding solar wind(i.e. overtaken by streams) and (b) those that are not perturbed (no-overtaken).We analyze the expansion velocity ($V_{exp}$) respect to the cloud center,and its dependence with several MC properties, as its size, its helio-distance,its translation velocity, etc. From this heuristic approach we find and computea dimensionless local expansion rate ($zeta$) for MCs;this definition of $zeta$ is fully consistent with previous works,which obtained the same quantity from a different kind of analysis.We find also significantly different values of $zeta$ for overtakenand not-overtaken events. Not overtaken MCs expand at rates $zeta$ consistentwith the expected value from the global pressure decay in the surroundingsolar wind for increasing helio-distances, while overtaken ones may presentstrong departures from that general rule.We interpret this departures of $zeta$ for overtaken MCs as a consequenceof their interaction with streams in the local solar wind on their expansion.