Abstract. We identified 17 magnetic clouds (MCs) with durations longer than 30 h, surveying the solar wind data obtained by the WIND and ACE spacecraft during 10 years from 1995 through 2004. Then, the magnetic field structures of these 17 MCs were analyzed by the technique of the least-squares fitting to force-free flux rope models. The analysis was made with both the cylinder and torus models when possible, and the results from the two models are compared. The torus model was used in order to approximate 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 MC events exhibiting magnetic field rotations through angles substantially larger than 180 • which can be interpreted only by the torus model; (2) 3 other 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) 3 MC events for which similar geometries are obtained from both the torus and cylinder models; and (4) 6 MC events for which the resultant geometries obtained from both models are substantially different from each other, even though the observed magnetic field variations can be interpreted by either of the torus model or the cylinder model. It is concluded that the MC events in the first and second groups correspond to those cases where the spacecraft traversed the MCs near the flanks of the MC loops, the difference between the two being attributed to the difference in distance between the torus axis and the spacecraft trajectory. The MC events in the third group are interpreted as the cases where the spacecraft traversed near the apexes of the MC loops. For the MC events in the fourth group, the real geometry cannot be determined from the model fitting technique alone. Though an attempt was made to determine which model is more plausible for each of the MCs in this group by comparing the characteristics of associated bidirectional electron heat flows, the results Correspondence to: K. Marubashi (k.marubashi@eos.ocn.ne.jp)were not very definitive. It was also found that the radii of the flux ropes obtained from the torus fitting tend to be generally smaller than those obtained from the cylinder fitting. This result raises a possible problem in estimating the magnetic flux and helicity carried away from the Sun by the MCs.

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Magnetic field structures are analyzed for both the ionospheric hole region and the magnetosheath‐ionosphere interaction region of the nightside of Venus, in search of possible coupling between these two regimes. A magnetic coordinate system based on the directions of the solar wind and the interplanetary magnetic field (IMF) is found to order the data reasonably well, allowing consistent superposition of observational data from individual passes of the Pioneer Venus orbiter. The main findings are (1) ionospheric holes form in a zone of ±45° magnetic latitude covering possibly the entire width of the Venus wake in the longitudinal direction; (2) the magnetic field within the holes is strong and ordered, and its direction is determined by the IMF direction, being consistent with the field line draping concept; (3) the magnetic field direction in the holes, particularly at low altitudes near the terminator, shows evidence for the existence of strong plasma flow toward the antisolar meridian in the direction parallel to the magnetic equatorial plane; and (4) the average magnetic field distribution in the nightside magnetosheath also evidences strong plasma flow toward the antisolar meridian. Together these results strongly indicate that the magnetosheath plasma flow in the wake region plays an important role in forming the ionospheric holes, through deformation of the nightside ionopause. These results are combined in a model of the three‐dimensional magnetic field structure around the ionosphere of Venus.

This study aims to provide a reference to different magnetic field models and reconstruction methods for interplanetary coronal mass ejections (ICMEs). In order to understand the differences in the outputs of those models and codes, we analyze 59 events from the Coordinated Data Analysis Workshop (CDAW) list, using four different magnetic field models and reconstruction techniques; force-free fitting (Goldstein, 1983; Burlaga, 1988;Lepping, Burlaga, and Jones, 1990), magnetostatic reconstruction using a numerical solution to the Grad-Shafranov equation (Hu and Sonnerup, 2001), fitting to a self-similarly expanding cylindrical configuration (Marubashi and Lepping, 2007) and elliptical, non-force free fitting (Hidalgo, 2003). The resulting parameters of the reconstructions for the 59 events are compared statistically, as well as in selected case studies. The ability of a method to fit or reconstruct an event is found to vary greatly: the Grad-Shafranov reconstruction is successful for most magnetic clouds (MCs) but for less than 10% of the non-MC ICMEs; the other three Al-Haddad et al methods provide a successful fit for more than 65% of all events. The differences between the reconstruction and fitting methods are discussed, and suggestions are proposed as to how to reduce them. We find that the magnitude of the axial field is relatively consistent across models but not the orientation of the axis of the ejecta. We also find that there are a few cases for which different signs of the magnetic helicity are found for the same event when we do not fix the boundaries, illustrating that this simplest of parameters is not necessarily always well constrained by fitting and reconstruction models. Finally, we look at three unique cases in depth to provide a comprehensive idea of the different aspects of how the fitting and reconstruction codes work.

[1] We investigate 17 coronal mass ejection (CME) events identified by the ACE spacecraft during solar cycle 23, focusing upon the fine structures of the sheath region between the CME and its associated shock to find their dependence on the shock parameters. We observe the planar magnetic structure (PMS) downstream of a quasiperpendicular shock when the Alfvén Mach number >2.0. Here, the PMS is characterized by the magnetic fields changing directions abruptly and intermittently within a plane parallel to the shock plane. The downstream PMS does not form when a magnetic cloud with b value <0.05 exists just upstream of a shock with Alfvén Mach number <2.0. The sheath magnetic fields become highly turbulent when the shock angle is <60 and/or the upstream b value is >0.5 and the upstream is dominated by Alfvenic fluctuations. Citation: Kataoka, R., S. Watari, N. Shimada, H. Shimazu, and K. Marubashi (2005), Downstream structures of interplanetary fast shocks associated with coronal mass ejections, Geophys. Res. Lett., 32, L12103,