Authors: Klecker, B., E. Möbius, M.A. Popecki, L.M. Kistler, H. Kucharek, M. Hilchenbach
Reference: Solar Wind 11 - SOHO 16, Contributed Paper, June 2005, Whistler, Canada
The experiments SEPICA onboard ACE and STOF onboard SOHO provide information on suprathermal heavy ion ionic charge states in solar energetic particle (SEP) events over the extended energy range of ~ 0.02 to 0.70 MeV/n. For events correlated with interplanetary shocks the ionic charge states in this energy range are mostly compatible with solar wind (SW) charge states or differ from SW charge states, e.g. for Fe, by only 1--2 charge units. In SEP events of short duration, low intensity and enrichment in heavy ions and 3He (usually called 'impulsive events'), we find significantly higher ionic charge states of ~10-12 (Mg), ~11-14 (Si) und ~14-20 (Fe), with a remarkable increase of the mean ionic charge in this energy range by several charge states (~ 2-5 for Fe). This large increase of the mean ionic charge with energy at energies < 1 MeV/n can be best explained by additional ionization of the ions by charge stripping in a sufficiently dense environment, during or after the acceleration. We compare the measured mean ionic charge of Mg, Si, and Fe with model calculations for the mean ionic charge as a function of particle energy, including the effect of energy dependent impact ionization. We show how these ionic charge measurements, in combination with model calculations and information on acceleration time scales, can be used to infer the altitude of the acceleration region, low in the corona.