Indication of Charge State Variation During the November 1997 Solar Particle Events

Shown above are the ionic charge state distributions of oxygen in the energy range 0.35 - 1 MeV/nucleon for the time periods November 6, 1200 - 2400 UT and November 7, 0000 - 2400 UT, 1997, as obtained with the high resolution sensor on SEPICA. The dominant presence of O⁶⁺ and O⁷⁺ is evident with a small contribution from O⁸⁺. This demonstrates the capability of SEPICA to distinguish the contribution of individual charge states. Note that O⁶⁺ dominates during the first time period, which contains the starting phase of the largest event. During the second period, which includes the passage of an interplanetary shock associated with a coronal mass ejection, O⁷⁺ becomes more prominent. The variation of charge states O⁶⁺ and O⁷⁺ is very sensitive to temperature around 2 x 10⁶ K (Arnaud and Rothenflug, ApJ Suppl 60, 425, 1985). A rough estimate yields an estimated temperature of 2 x 10⁶ K for the first period and 2.3 x 10⁶ K for the second period. Although a complete quantitative interpretation has to await detailed modeling of the results with the sensor response, this appears to be the first measurement of a variation in the charge state composition during a solar particle event. The charge state distributions of other elements, such as He, C, N, Si and Fe, are currently under study.