Simulation of Charge-Equilibration and Acceleration of Solar Energetic Ions

Recent measurements of the mean ionic charge states of solar energetic iron and silicon by SAMPEX and ACE during the large solar events of 1992 November 1 and 1997 November 6 show a mean ionic charge that increases with energy. This feature has implications for the use of the observed charge state as a probe of the coronal electron temperature and density, as well as for models of ion acceleration and transport in the coronal plasma. In this paper, we show results of a nonequilibrium model for the mean ionic charge that includes shock-induced acceleration in addition to charge-changing processes. The model is able to reproduce the general features observed without, however, specifying uniquely the acceleration time and the plasma electron density. Based on our simulations for iron and silicon for the 1992 and 1997 events, and assuming a characteristic shock-acceleration time of ~10 sec, our model suggests an equilibration-acceleration site at heights ~1 solar radius above the solar surface, a density ~10^9 cm^(–3), and an electron temperature ~1–1.33 MK. For ions with kinetic energy ≳ 30 MeV/nucleon we estimate the amount of coronal material the ions traverse to be ~100 µg/cm^2.