Energy dependence of ion charge states in CME related solar energetic particle events observed with ACE/SEPICA and SOHO/STOF

Abstract

We investigate the energy dependence of the charge states of heavy ions during a number of gradual solar energetic particle events related to coronal mass ejections. The data of two instruments with complementary energy ranges, the Solar Energetic Particle Ionic Charge Analyzer SEPICA on ACE and the time-of-flight mass and charge spectrometer STOF on SOHO are used to cover the energy range from ∼0.02 to 0.7 MeV/amu for O and Fe. Measurements with SAMPEX and SEPICA/ACE contain evidence that some gradual CME related SEP events show energy dependent charge distributions. In the present paper we are closing the gap in the lower energy interval (STOF data) thus making future comparison to solar wind values—representative for coronal conditions—more reliable. We find that energy dependent charge states of Fe are almost ubiquitous in gradual SEP events. Very large differences from event to event are observed however. In cases of local acceleration at the CME driven shock the variation in the energy interval up to 0.5 MeV/amu is ∼1–2 charge units. In alternative cases the mean ionic charge variation for Fe reaches ∼4 charge units over the same energy interval. The respective charge spectra could be interpreted as the result of ionization near the sun, in the dense coronal plasma, with or without reaching charge state equilibrium.