Implications of Using Spheroidal “Cone Model” CMEs in Solar‐Wind Models

Abstract Space‐weather forecasting requires advanced prediction of the arrival time and properties of coronal mass ejections (CMEs) in near‐Earth space. Kinematic properties of CMEs close to the Sun—such as speed, direction and angular width—are routinely estimated from coronagraph images by using three‐dimensional geometric models, such as the “cone model.” These are used to characterize a time‐dependent perturbation at the inner boundary of a numerical solar‐wind model, normally at 0.1 AU, enabling a forecast of the CME arrival time and speed at Earth. This perturbation is typically spheroidal in shape. In this study we show that spheroidal CMEs exhibit four features inconsistent with observations that may limit the accuracy of space‐weather forecasts: 1, Slow spheroidal CMEs intersect the model inner boundary for a long duration and hence resist acceleration by the ambient solar wind, producing longer transit times than observed; 2, The radial extent of a spheroidal CME is directly related to its angular width. Observations of CMEs at 1 AU do not display any relation between angular width and radial extent; 3, Fast‐and‐wide CMEs cannot be sufficiently decelerated by the ambient solar wind and arrive with higher speeds than observed; 4, Spheroidal CMEs show different magnitudes of interplanetary accelerations for different angular widths, contrary to observations. We show that fixing the CME duration at the inner boundary—which mimics observed CME expansion—alleviates these problems. The choice of fixed duration is a free parameter that needs to be calibrated against observations, but 8 hr works reasonably well.