Influence of the drag force on the leading edge of a coronal mass ejection

Abstract

Context. The drag based model (DBM) is frequently used to analyse the kinematics of coronal mass ejections (CMEs) in interplanetary space. The DBM incorporates a 2D solution with the leading edge of the CME. Aims. Certain aspects of the evolution of the CME leading edge in the DBM have not been fully and accurately described previously. The main goal of this paper is to clarify these issues. Methods. We analysed the behaviour of the leading edge according to the DBM equations by studying the dependence of the radial distance of each segment of the leading edge on the angular coordinate, ϕ, and observed the limits as time goes to infinity. We also analysed the behaviour of the velocity profile, v(ϕ). Results. We showed that for isotropic solar wind conditions, the distance between the apex and the flank is a monotonically increasing function of time that converges at infinity to a constant value. The leading edge never becomes fully circular. The analysis of the v(ϕ) profile shows that the speed of all CME leading-edge segments converges towards the solar wind speed, w, but the speed of the CME flank never exceeds that of the apex. Conclusions. The drag force alone cannot flatten the leading edge of a CME in isotropic solar wind conditions. This also holds for any model that uses the drag as a description of the CME kinematics if the typical initial assumption that the flank is slower and farther behind than the apex is used. On the other hand, non-isotropic solar wind conditions can change this conclusion and even introduce a deformation of the leading edge. A similar effect can be obtained with temporal variations in solar wind conditions along the path of the CME, or by introducing other forces.