Flux-rope coronal mass ejection geometry and its relation to observed morphology

A simple parameterization of a three-dimensional flux rope is used to determine a typical flux-rope geometry that corresponds to observed flux-rope coronal mass ejection (CME) morphologies (average apparent angular widths) at a leading-edge height of about 5.5 R-circle dot. The parameterized flux rope, the curved axis of which is assumed to trace out an ellipse, is described in terms of the eccentricity of the ellipse, the width (minor diameter d) of the flux rope at the apex, and the height of the apex above the solar surface 2R(1). Assuming self-similar expansion, there are only two geometrical parameters to be determined: the eccentricity is an element of and the axial aspect ratio Lambda(a) equivalent to 2R(1)/d. For each pair of geometrical parameters, an ensemble of 72 orientations is considered, with each being specified in terms of a latitude angle, a longitude angle, and a rotation about the direction of motion. The resulting ensemble of synthetic coronagraph images is used to produce statistical measures of the morphology for comparison to corresponding observational measures from St. Cyr et al. (2004). We find that a typical flux-rope CME has is an element of = 0.7 +/- 0.2 and Lambda(a) 1.1 +/- 0.3.