Proton and helium release times in SEP events observed with SOHO/ERNE
K. Huttunen-Heikinmaa, E. Valtonen, T. Laitinen
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 \ensuremath\pm 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.
A&A, 442, 673-685, 2005