Interplanetary fast forward shocks and energetic storm particle events above 1.5 MeV

Abstract. Interplanetary (IP) shock passages are usually identified by abrupt changes in the plasma parameters, but sometimes they are also associated with energetic storm particles (ESPs). The maximum observed energies of ESPs usually reach a few MeVs per nucleon and occasionally even a few hundred MeVs per nucleon. We have carried out a statistical study of ESP events observed by SOHO/ERNE above 1.5 MeV during the seven-year period between May 1996 and April 2003. In the first stage, we gathered a comprehensive database of IP shock candidates using several ready-made shock lists. We defined a qualitative classification for the ESP signals and studied their association with fast forward shocks. We present a survey of the overall statistics of ESP associations with fast forward shocks and the yearly amount of the shocks and associated ESP events during the 7-year study period. Our most important findings are that only 40% of the observed interplanetary fast forward shocks accelerate ESPs to energies greater than 1.5 MeV and that the high-energy ESP-effectiveness of the fast forward shocks has a solar cycle dependence. The yearly ESP-effectiveness varied from 11%, in May 1996–April 1997 (~activity minimum), to 53% in May 2000–April 2001 (~activity maximum). We also performed a quantitative analysis of the proton power law spectra at the time of the shock passage. We found that the average spectral index of ESPs was −3.6 with the standard deviation of the distribution of 1.3. The ESP events had significantly larger power law factors than the reference spectra, calculated every day at a certain time for comparison.