Fitting of expanding magnetic clouds: A statistical study

Magnetic clouds (MCs) observed by Wind during the 1995–2003 years and listed in Lepping et al. [2006. A summary of WIND magnetic clouds for years 1995–2003: model-fitted parameters, associated errors and classifications. Ann. Geophys. 24(1), 215–245] are fitted using force-free cylindrical flux rope models. The cloud parameters yielded by a static model are compared with those obtained from a modified model that includes magnetic cloud expansion. The deviations of these fits from observations are quantified using 1 h averages of measured parameters by Wind. In the case of the static MC model, the deviations between fitted and observed magnetic fields increase with the magnetic strength and with the MC radius. The comparison of both the static and the dynamic models reveals that the modified model with expansion provides better fits in the range of low expansion speeds (<38km/s), whereas the results are not conclusive for larger expansion speeds. The dynamic model provides better fits than static one in 70%70% of investigated MCs. We conclude that the expansion of magnetic clouds plays an important role in the MC formation and propagation but a further progress requires determination of cloud parameters from a model with expansion self-consistently involved.