Inferred Ionic Charge States for Solar Energetic Particle Events from 2012-2015 with ACE and STEREO

Abstract

Mean ionic charge states for SEP events can reflect source temperatures, stripping during acceleration and transport, and the composition of source material. Multi-spacecraft measurements of mean ionic charge states for single SEP events can also demonstrate longitudinal dependence depending on seed particle composition or acceleration conditions. in previous studies, we calculated inferred high-energy ionic charge states for SEP events. The analysis method fits the energy dependence of decay times for each element in SEP events, combined with charge-to-mass ratios relative to a calibration element, and derives mean charge state estimates for elements from O to Fe. Previously, we applied the method using ACE and STEREO data to SEP events through the beginning of 2012, in order to elucidate evidence on seed populations or longitudinal variations with charge state for single SEP events, with varying results. In this paper, we continue applying the method to new SEP events from 2012 to 2015 in the ACE and STEREO data. With the three spacecraft widely spread apart during this time period, there are fewer single SEP events with multi-spacecraft data, but the wide spacing allows more SEP candidate events to be considered, separately, than would be available with just a single spacecraft. Our new results for two SEP events continue to be consistent with observed correlation between Q(Fe) and Fe/O in previous events.