Proton and helium release times in SEP events observed with SOHO/ERNE

Abstract

We determine the proton and helium nuclei release times at the Sun for 25 SEP events within the time period May 8, 1996-June 20, 2001, and compare them to each other. The event onset times at 1 AU are determined with a Poisson-CUSUM method, and the solar release times are obtained by the velocity dispersion analysis. Velocity dispersions were derived in the energy range 14-51 MeV/n. If the release times of protons and helium nuclei do not overlap within their error limits, and if protons are released earlier than helium, then the helium event is defined as "delayed". Seventeen helium events (68%) were found to be "delayed" and only eight (32%) "non-delayed". The average delay was (34 ± 18) minutes. The event-related flare location had an important role in the "non-delayed" events. A significant difference was found in the helium-to-proton ratios at the time of maximum intensity of the "delayed" and "non-delayed" events. No clear evidence was found that the "delayed" and "non-delayed" events would correspond the classical gradual-impulsive division of SEP events. We find that the delay is related to the poor magnetic connection between the flare site and the spacecraft. © ESO 2005.