Physics
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsh41a..03v&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #SH41A-03
Physics
2114 Energetic Particles (7514), 2139 Interplanetary Shocks
Scientific paper
We have statistically studied Energetic Storm Particle (ESP) events associated with interplanetary fast forward shocks. The ESP events are based on SOHO/ERNE observations between May 1996 and April 2003. We deduced the magnetohydrodynamic modes of 606 shock candidates, compiled from various publicly available sources, by using solar wind data from WIND, ACE, and SOHO spacecraft, and ranked them according to the quality of the plasma signatures. A majority of the shock candidates was found to be fast forward shocks (54%). For 85% of the fast forward shocks ERNE data was available for the entire shock passage period. As the shock candidates, the observed ESP signals were also classified according to the quality of the signatures: intensity enhancements in several energy channels temporally close to the shock passage with no velocity dispersion. It was found that 29% of the fast forward shocks were "clearly" associated and 13% "probably" associated with high-energy (1.5-130 MeV/n proton and helium) ESP events. Very weak signals were found for 12% of the cases and 46% of the fast forward shocks had no associated high-energy ESPs. A more detailed analysis showed that roughly 40% of the interplanetary fast forward shocks accelerate ESPs up to energies greater than 1.5 MeV. A clear solar cycle variation was seen in the yearly occurrence frequency of fast forward shocks as well as in the high-energy ESP effectiveness of these shocks. We further studied the proton energy spectra at the time of fast forward shock passage (286 events) and their solar cycle dependence during the seven-year study period. The ESP spectra were compared with reference spectra calculated every day at 12:00 UT in the same manner as the ESP spectra. It was found that the standard power law well fits the spectra and that while ESP spectra are statistically significantly steeper than the reference spectra, the power law factors are larger for the ESP events.
Huttunen-Heikinmaa Kalle
Valtonen Eino
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