Observation of energy-dependent ionic charge states in impulsive solar energetic particle events

We measured the mean ionic charge state of Fe in several 3He-rich and heavy-ion rich energetic particle events during the time period 1998-2000. We combined the ionic charge measurements of CELIAS/STOF onboard SOHO (10-100 keV/nuc) with the measurements of SEPICA onboard ACE (180-550 keV/nuc) to investigate the ionic charge distribution of Fe over a wide energy range. Whereas the mean ionic charge of Fe, Qm(Fe), increases strongly with energy from ∼14-16 at 180-250 keV/nuc to ∼16-20 at 350-550 keV/nuc, the mean ionic charge at lower energies of 10-100 keV/nuc was found to be significantly lower (12.5 ± 0.9). A comparison of the results with steady-state models that include impact ionization by protons and electrons in the low corona shows that the measured increase of Qm(Fe) with energy is often steeper than predicted by the model, and other effects, as for example adiabatic deceleration during the propagation, may play an important role.