Solar Energetic Particle event onset times have been traditionally studied by using the velocity dispersion analysis. The recently introduced simulation approach in the onset research takes into account the effect of the interplanetary scattering on the arrival time of the first particles. These studies have shown that scattering can easily explain the large apparent path lengths. The main focus of the simulation studies has been in the particle transport, and in the profile of the injection. In this study, we concentrated to study how the different pre-event background spectra can modulate the observed velocity dispersions of simulated event profiles. We found out that the pre-event background conditions have a major impact to the velocity dispersion curves of a fixed event profile.
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