Flux‐Rope Coronal Mass Ejection Geometry and Its Relation to Observed Morphology

Abstract

A simple parameterization of a three-dimensional flux rope is used todetermine a ``typical flux-rope geometry'' that corresponds to observedflux-rope coronal mass ejection (CME) morphologies (average apparentangular widths) at a leading-edge height of about 5.5 Rsolar. Theparameterized flux rope, the curved axis of which is assumed to traceout an ellipse, is described in terms of the eccentricity of theellipse, the width (minor diameter d) of the flux rope at the apex, andthe height of the apex above the solar surface 2R1. Assumingself-similar expansion, there are only two geometrical parameters to bedetermined: the eccentricity ɛ and the axial aspect ratioΛa≡2R1/d. For each pair ofgeometrical parameters, an ensemble of 72 orientations is considered,with each being specified in terms of a latitude angle, a longitudeangle, and a rotation about the direction of motion. The resultingensemble of synthetic coronagraph images is used to produce statisticalmeasures of the morphology for comparison to corresponding observationalmeasures from St. Cyr et al. (2004). We find that a typical flux-ropeCME has ɛ=0.7+/-0.2 and Λa=1.1+/-0.3.