Distribution of the optical thickness profiles obtained from the filament apex. Panel (a) shows the distribution of the fast component, while panel (b) the distribution of the main component. The fast and main components refer to the measurements obtained from the leading and the central part of the erupting feature, respectively, detected in the optical thickness spectra (see the text). The enhanced regions correspond to the multiple peaks of the profiles that are Doppler-shifted to large velocities during the filament eruption. The blue profiles are the line of sight velocity υ_los of the fast and main components calculated by tracking the enhanced regions. For comparison, it is also plotted the plane of sky velocity profile υ_pos (black) calculated from the time-distance diagrams presented in Fig. 3.