On the magnetosheath thicknesses of interplanetary coronal mass ejections

Interplanetary coronal mass ejections (ICMEs) often move relative to the ambient solar wind plasma at a speed that exceeds the fast magnetosonic velocity, producing a standing shock wave in the frame of the ICME and a magnetosheath whose plasma flows around the obstacle much as the terrestrial magnetosheath flows around the Earthʹs magnetosphere. The half-thickness of an ICME is typically 0.1 AU at 1 AU. If this dimension represented the characteristic scale size of the obstacle, then the thickness of the magnetosheath should be about 0.025 AU but it is typically closer to 0.1 AU. In order to treat this problem we convert the original Spreiter et al. (Planet. Space Sci. 14 (1966) 223) formula for the terrestrial magnetosheath thickness to one that is appropriate for the ICME magnetosheath. This treatment allows us to conclude that the characteristic radius of curvature of an ICME at 1 AU is about 0.4 AU. This radius of curvature is provided by both the bend of the axis of the rope and by an elongation of the ICME in the direction perpendicular to both the rope axis and the solar wind flow. Thus near 1 AU ICMEs have a radial thickness that is smaller than their other two characteristic dimensions.