The true dimensions of interplanetary coronal mass ejections Original Research Article

The majority of observations of Interplanetary Coronal Mass Ejections (ICMEs) are made with a single spacecraft and their properties inferred from a fit to a cylindrically symmetric model. This process produces a single dimension for a magnetic cloud, its radial thickness. If the radius of curvature of an ICME were equal to its half thickness, the shock front driven by the ICME would be at a distance of only about 0.15 times the ICME thickness according to the gas dynamic model, while the observed median thickness is closer to 0.4. The agreement with theory can be improved if the obstacle has two radii of curvature, one corresponding to the rope cross-section and one corresponding to the curvature of the rope axis. This picture is in accord with the popular paradigm of an ICME as a twisted flux rope rooted in the sun. Nevertheless this correction is insufficient to bring full agreement, suggesting ICMEs have yet a third scale size, a cross-flow diameter that may be of the order of four times their radial thickness. This inference is in accord with both indirect evidence in the form of the distribution of impact parameters derived from the cylindrical cross-section model and with direct evidence from the fit of multi-point observations with oval cross-section ropes.