Coherence of Coronal Mass Ejections in Near-Earth Space

Interplanetary coronal mass ejections (ICMEs) primarily move radially as they propagate away from the Sun, maintaining approximately constant angular width with respect to the Sun. As ICMEs have typical angular widths of around 60., plasma elements on opposite flanks of an ICME separate in the non-radial direction at a speed, v(G), roughly equal to the ICME radial speed. This rapid expansion is a limiting factor on the propagation of information across an ICME at the local Alfven speed, v(A). In this study, the 1-AU properties of ICMEs are used to compute two measures of ICME coherence. The first is the angular separation for which v(G) exceeds the local v(A). The second measure is the angular extent over which a wavefront can propagate as an ICME travels from a given heliocentric distance to 1 AU. For both measures, ICMEs containing magnetic clouds show greater coherence than non-cloud ICMEs. However, even for magnetic clouds, information is unable to propagate across the full span of the structure. Thus interactions of ICMEs with other solar wind structures in the heliosphere are likely to lead to localised distortion, rather than solid-body like deflection. For magnetic clouds, the coherence length scale is significantly greater near the centre of the spacecraft encounter than at the leading or trailing edges. This suggests that magnetic clouds may be more coherent, and thus less prone to distortion, along the direction of the magnetic flux-rope axis than in directions perpendicular to the axis.