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

Abstract

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, Q m(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 Q m(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.