Iron charge state distributions for four large SEP events in the energy range 0.23-0.33 MeV/nuc: implications for source populations

Abstract

Charge state measurements of solar energetic particles (SEP) provide diagnostic information about the source populations and acceleration mechanisms and their locations in SEP events. The SEPICA instrument on board the ACE spacecraft can directly measure the ionic charge state for SEP iron ions with energies below 0.6 MeV/nuc. The direct measurement technique yields not only the mean charge of iron ions for an event, but the charge state dis- tribution as well. We determined event-averaged iron charge state distributions in the energy range 0.23 to 0.33 MeV/nuc for several large (gradual) SEP events. We find a large variability of iron charge distributions, ranging from distributions compatible with a coronal source at ~1MK to distributions with a pronounced tail at high charge states, suggesting the presence of multiple sources or additional effects, such as stripping in the low corona. We present some typical examples and discuss the implications for propagation and acceleration effects.