tokens: and 180° to the phase space densities at 90° are greater than three. It is easy to see that strong depletions occur within the first bar, and the depletion CSE interval was removed when we estimated the percentages of CSEs. One can also see that the bars in the rear half have two short intervals, which are caused by scattering of strahl electrons. We therefore identify the two short intervals as CSE strahls, and the percentage of CSE strahls is about 35%. Fig. 1. Suprathermal electron pitch angle distributions of 272 eV, magnetic field and plasma data measured by ACE during the October 21C22, 1999 ICME passage. The two vertical dashed lines denote the boundaries of the ICME. Furthermore, this study examined all the suprathermal electron PADs of the 272 ICMEs and estimated the percentages of CSEs. The results are listed in the sixth column of Table A.1 . Only 10 (9.9%) of the 101 MCs have no CSE, and the ratio is consistent with the results of Shodhan et al. (2000) , who examined the CSE signatures of 52 MCs and concluded that 6 MCs have no CSEs. As for the other 91 MCs, the percentage of CSEs varies from 6 to 100%, with a mean value of 57%. Meanwhile, 75 of the 171 nonMC ICMEs (43.9%) do not contain CSEs. The fraction of ICMEs without CSEs is thus clearly larger in nonMC ICMEs than in MCs. The extensive difference may imply that the two groups have different magnetic structures. For the other 96 nonMC events, their mean CSE percentage is 45%, which is smaller than for the MCs (57%). Among the 96 nonMC ICMEs, 21 have greater than 70% CSEs. Examination of the magnetic field component curves of these 21 ICMEs reveals that fields of most events have neither apparent smooth rotations nor disorder, but their magnetic field component curves have only slight rotations, and they are relatively stable or even close to the line. It is well known that most MCs can be described with a constant force-free field configuration, for example, the Lundquist solution ( Lepping et al. 1990 ; Feng et al. 2006 ). According to the Lundquist solution, the shapes of measured field component curves depend on the distance d 0 between the spacecraft trajectory and the rope axis (see Figs. 2–4 of Feng et al. 2006 ). The spacecraft will measure center-enhanced and bipolar curves if the spacecraft trajectory is close to the rope axis; otherwise, the bipolar curves will disappear. Figure 2 shows the variations of the magnetic field components for the case of d 0 = 0.9 R 0 , where R 0 is the radius of the flux rope, z is the axial direction, x is the radial direction pointing to the satellite contact position, and the direction y is obtained from the cross product of z and x . In Fig. 2 , all the three magnetic field components are relatively stable and exhibit only slight rotations. These trends indicate that the spacecraft that passes through the flank of a magnetic flux rope will measure relatively stable magnetic field variations. Therefore, the 21 highest percentage (more
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