Solar Energetic Fe Charge State Measurements: Implications for Acceleration by Coronal Mass Ejection–driven Shocks

Abstract

A variety of studies have demonstrated that large (gradual) solar energetic particle (SEP) events are produced by shocks driven by fast coronal mass ejections (CMEs). As the CME-driven shocks propagate through the corona and interplanetary space, they accelerate energetic particles from ambient plasma. A key piece of evidence supporting the CME-driven shock acceleration scenario is the SEP Fe mean charge state measurements from ~1 MeV nucleon^-1 to 200-600 MeV nucleon^-1, which are consistent with each other and imply acceleration in either the high corona or interplanetary space. However, the SEP Fe mean ionic charge state measurements are generally inconsistent with typical quasi-stationary interstream or coronal hole solar wind Fe charge state measurements, despite the fact that gradual SEP time profiles indicate that shock acceleration subsequent to departure from the corona dominates the SEP observations. These Fe results indicate that neither overtaken solar wind nor charge state biasing of the solar wind as it is swept up and accelerated by the shock are the dominant component accelerated by the CME-driven shock. We suggest that the dominant component for both the plasma and SEP populations appears to be expelled "coronal" plasma stored in the so-called sheath region (between the CME and the shock front), and that swept-up solar wind generally makes only a minor contribution to the SEPs. The only concurrent observation of SEP and solar wind charge states made to date (by instruments on board ISEE 3 in 1978 September) supports this view. We suggest that coordinated SEP and solar wind charge state observations should be given high priority by state-of-the-art instruments currently deployed on board the Solar Anomalous Magnetospheric Particle Explorer, Wind, and the Solar and Heliospheric Observatory.