The ionic charge of solar energetic particles: a key for the localisation of the acceleration region

The mean ionic charge of solar energetic particles provides critical information for the determination of the source and the acceleration region of the ions. Gradual events show at energies < 0.5 MeV/amu a mean ionic charge of Q_m ≈ 10 for iron ions, suggesting a solar wind origin and acceleration by coronal or interplanetary shock waves. Impulsive events show a large increase of Q_m(Fe) at 0.1–0.5 MeV/amu from 11 to 20. This can only be explained by acceleration and ionisation in a plasma of sufficiently high density in the low corona, at altitudes of < 0.3 R_S (R_S: solar radius).