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We identified 17 magnetic clouds (MCs) with durationslonger than 30 h, surveying the solar wind data obtained by the WIND andACE spacecraft during 10 years from 1995 through 2004. Then, the magneticfield structures of these 17 MCs were analyzed by the technique of theleast-squares fitting to force-free flux rope models. The analysis was madewith both the cylinder and torus models when possible, and the results fromthe two models are compared. The torus model was used in order toapproximate the curved portion of the MCs near the flanks of the MC loops.As a result, we classified the 17 MCs into 4 groups. They are (1) 5 MCevents exhibiting magnetic field rotations through angles substantiallylarger than 180° which can be interpreted only by the torus model; (2) 3other MC events that can be interpreted only by the torus model as well,though the rotation angles of magnetic fields are less than 180°; (3) 3MC events for which similar geometries are obtained from both the torus andcylinder models; and (4) 6 MC events for which the resultant geometriesobtained from both models are substantially different from each other, eventhough the observed magnetic field variations can be interpreted by eitherof the torus model or the cylinder model. It is concluded that the MC eventsin the first and second groups correspond to those cases where thespacecraft traversed the MCs near the flanks of the MC loops, the differencebetween the two being attributed to the difference in distance between thetorus axis and the spacecraft trajectory. The MC events in the third groupare interpreted as the cases where the spacecraft traversed near the apexesof the MC loops. For the MC events in the fourth group, the real geometrycannot be determined from the model fitting technique alone. Though anattempt was made to determine which model is more plausible for each of theMCs in this group by comparing the characteristics of associatedbidirectional electron heat flows, the results were not very definitive. Itwas also found that the radii of the flux ropes obtained from the torusfitting tend to be generally smaller than those obtained from the cylinderfitting. This result raises a possible problem in estimating the magneticflux and helicity carried away from the Sun by the MCs

Long-duration magnetic clouds: a comparison of analyses using torus- and cylinder-shaped flux rope models