Velocity dispersion analysis of the onset of the SEP event at near-Earth spacecraft. The horizontal and vertical axes correspond to the reciprocal of the particle velocities (1/β = c/v) and onset times, respectively. The green and blue data points respectively identify the onsets of the 3DP electron and ERNE proton at the corresponding velocities (energies), with the respective errors indicated. The dashed line is the linear regression to fit all points. The legend gives the effective path length (L) and the estimated release time (t_inj) discussed in the main text.