Peak proton intensities and composition variations of heavy ions during large solar energetic particle events: Uleis observations Original Research Article

We have examined low energy proton and heavy ion intensities in large solar energetic particle (SEP) events from 1997 to the end of 2001 using the ULEIS instrument on ACE spacecraft. We selected 33 events that are isolated and show clear onsets at 1 AU. The optical and radio signatures for all of our events are identified and where data are available we also located the associated Coronal Mass Ejection (CME) data from instruments on SOHO. We found at low energy (0.23 − 0.45 MeV), the peak proton intensities and the associated CME speeds are not correlated. In addition, we also found the measured ambient pre-event intensities are not at all related to the event peak intensities. These results are contrary to what was found by for ions greater than 10 MeV in 71 large SEP events. At low energy, the ambient particle intensity that we measured before the event onset may not be a good measure of what is later being accelerated in these large SEP events. The suprathermal particles at a few times the solar wind energy (not measurable with our instrument) may play a bigger role in the low energy ion intensities in large solar energetic particle events in our study. We also examined the event averaged abundance ratio of FeO as a function of solar longitude and CME speed. No obvious correlation is evident. However, the mean FeO ratio for magnetically well-connected (W61° − W70°) events has a statistically larger value (0.64) than those from other longitudes.