Comparison of Toroidal Interplanetary Flux-rope Model Fitting with Different Boundary Pitch-angle Treatments.

We developed a new fitting method of a toroidal model for the analysis of interplanetary flux ropes (IFRs). The pitch angle of the magnetic field at the IFR boundary [ α p ] is fixed to 90 ∘ in the toroidal model of Romashets and Vandas (Geophys. Res. Lett.30, 2065, 2003) (conventional method). To relax this condition for our method, the pitch angle was made one of the free parameters (generalized method). In this paper, we examine the difference between the results of these two toroidal IFR model fitting methods by applying them to magnetic-obstacle (MO) events observed by the Wind and Solar Terrestrial Relations Observatory (STEREO) spacecraft. We found that the generalized method gives an α p near 90 ∘ (within the range of 90 ∘ ± 30 ∘ ) for approximately 60% of events. Thus, the assumption in the conventional method that α p is 90 ∘ is valid for many cases. This also implies that many flanks of IFR have a magnetic field line nearly perpendicular to the IFR axis. We also found that the generalized method gives a similar normal direction of the torus plane and poloidal magnetic flux as the conventional method. However, the generalized method gives a toroidal magnetic flux that is significantly different from that given by the conventional method for a non-negligible number of events. Thus, we concluded that it is better to use the generalized method than the conventional method in order to get more accurate estimation of the toroidal magnetic flux of IFRs. [ABSTRACT FROM AUTHOR]