Expansion Speed of Coronal Mass Ejections

Abstract

A large set of limb coronal mass ejections (CMEs) are used to determine the accurate relationship between radial (V_rad) and expansion (V_exp) speeds of CMEs. It is demonstrated that this relation is exceptionally well described by the function f(w)=1/2(1+cot w), representing a full cone model for the CME with a half-width, w. We also demonstrate that for extremely fast CMEs (V_exp>3000 km s⁻¹), it is better to use the approximation V_rad≈V_LE. This implies that such CMEs expand spherically above the solar surface.