Role of Expansion Velocity of Magnetic Clouds in Flux Rope Model

In order to examine the influence of magnetic cloud＇s expansion on its cylindrical flux rope model. The parameter of MC of 15 typical magnetic clouds with Dstmin 〈 -50 nT during 1998-2003 are fitted by applying static and expanding flux rope models, the Multi-MC events and the shock propagation in MC events are not take into account. It is found that the RMS deviations of the fitting results by the expanding model are all less than or equal to those by the static model, it reduced the error up to 30%. That the peak of the magnetic filed in MC is at the leading end in expanding models, this more consistence with the observations than static model. The inferred expansion speeds of these magnetic clouds are consistent with previous statistical results that the expanding speed of MC is at the order of backgoud Alfvén speed. Thus the expanding model matches much more to observed magnetic clouds than the static model. Some differences of the fitted magneticclouds＇ parameters between the two models are analyzed.