On the Effect of Pre-event Background in Determining Solar Particle Event Onset
T. Laitinen, K. Huttunen-Heikinmaa, E. Valtonen
The onset of a Solar Energetic Particle (SEP) event has been traditionally determined by using the velocity dispersion analysis (VDA), where the event onset at the Sun is determined by fitting the solar onset time and the particles' path length to match the observed event onset at 1 AU for several SEP energy channels. This approach has been studied by using energetic particle simulations, in order to understand the effect of the interplanetary scattering on the arrival time of the first particles to 1 AU. In these studies, the SEP event onset at 1 AU has been defined as the time when the intensities reach certain percentage of the maximum of the event. This is often not feasible for practical work, as a real event may be complex in its structure, and the pre-event background may mask the SEP onset to differing degrees in different energies. In order to estimate the usability of the VDA method, we study the simulated SEP onset on a pre-existing particle background by varying the pre-event background level, the difference of the pre-event background and the simulated event maximum spectral indices, the interplanetary mean free path and the injection profile. As in the previous studies, we find that large path length variation can be explained as an artifact resulting from the use of the VDA method. More importantly, the error in the obtained solar onset time depends strongly on the properties of the pre-event background.
Twelfth International Solar Wind Conference, 1216, 249-252, 2010