Velocity dispersion analysis of the onset of the SEP event at STEREO-A. The horizontal and vertical axes correspond to the reciprocal of the particle velocities and onset times, respectively. The red points and circles respectively identify the electron and proton onsets at the corresponding velocities (energies), with the respective errors indicated. The black line is the linear regression fit to all points. The legend gives the effective path length (L) and the estimated release time (onset time) discussed in the text.