P. Demoulin, S. Dasso, M. Janvier
Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections (ICMEs) where a magnetic flux rope is detected. Is the difference between MCs and ICMEs without detected flux rope intrinsic or rather due to an observational bias? As the spacecraft has no relationship with the MC trajectory, the frequency distribution of MCs versus the spacecraft distance to the MCs axis is expected to be approximately flat. However, Lepping and Wu (2010) confirmed that it is a strongly decreasing function of the estimated impact parameter. Is a flux rope more frequently undetected for larger impact parameter? In order to answer the questions above, we explore the parameter space of flux rope models, especially the aspect ratio, boundary shape, and current distribution. The proposed models are analyzed as MCs by fitting a circular linear force-free field to the magnetic field computed along simulated crossings. We find that the distribution of the twist within the flux rope, the non-detection due to too low field rotation angle or magnitude are only weakly affecting the expected frequency distribution of MCs versus impact parameter. However, the estimated impact parameter is increasingly biased to lower values as the flux-rope cross section is more elongated orthogonally to the crossing trajectory. The observed distribution of MCs is a natural consequence of a flux-rope cross section flattened in average by a factor 2 to 3 depending on the magnetic twist profile. However, the faster MCs at 1 AU, with V>550 km/s, present an almost uniform distribution of MCs vs. impact parameter, which is consistent with round shaped flux ropes, in contrast with the slower ones. We conclude that either most of the non-MC ICMEs are encountered outside their flux rope or near the leg region, or they do not contain any.
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http://arxiv.org/abs/1211.5343
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