Comparison of ionic charge states of energetic particles with solar wind charge states in CME related events

Abstract

With SWICS and SEPICA onboard ACE, ionic charge measurements are now available for a wide energy range, covering the solar wind and suprathermal energetic particles. The high sensitivity of SEPICA has been utilized to examine the heavy ion composition and mean Fe ionic charge state as a function of time for several large, gradual, CME related solar energetic particle events. We selected those events for further analysis that clearly show small Fe/O ratios and low mean Fe ionic charge states (QFe∼10–12) throughout the event, excluding by this method a possible admixture from impulsive events. We determine for these events with SEPICA the mean ionic charge states of C, O, and Fe in the energy range ∼0.2–0.6 MeV/nuc and with SWICS the ionic charge distribution of C, O, and Fe ions of the solar wind. We find for this sample of ‘pure’ gradual events for suprathermal ions consistently low mean ionic charge states of ∼5.5 (C), ∼6.7 (O), and ∼11–12 (Fe). We compare the suprathermal charge states with solar wind measurements. For events where local acceleration at an interplanetary shock is observed we find that C mean charge states are compatible with solar wind charge states and that the mean charge of O and Fe are ∼0.6 and ∼1–2 charge units larger for suprathermal ions than observed in the solar wind. We suggest that small systematic differences of ∼0.6 (O) and ∼1–2 (Fe) charge units between the solar wind source and suprathermal particles could be accounted for by Q/M dependent acceleration and propagation effects.