Charge states of solar energetic iron: Nonequilibrium calculation with shock-induced acceleration

Observed charge states of solar energetic ions by Solar, Anomalous, and Magnetospheric Particle Explorer and Advanced Composition Explorer from recent large solar events in 1992 and 1997 show an energy-dependent feature that apparently has not been seen in earlier events, e.g., in 1978-1979. The mean charge state of iron, and to a lesser extent other heavy ions, was observed to increase with energy from similar to 0.5 to similar to 50 MeV nucleon(-1). We present a possible explanation for this new feature and discuss potential implications for models of particle acceleration. Our calculation suggests that this energy-dependent feature is a result of the charge-changing and energy-changing processes taking place concurrently and can be absent when the processes act over different timescales.