Ionic Charge State Measurements in Solar Energetic Particle Events

With the launch of the Advanced Composition Explorer, it has become possible with the SEPICA instrument to make ionic charge state measurements for individual Solar Energetic Particle events. In large events, the charge state may even be measured as a function of time, revealing changes that may be created by phenomena such as injections from different acceleration mechanisms, or confinement by magnetic field structures. The charge state can be a sensitive indicator of separate SEP populations. A wide range of iron charge states have been measured for many SEP events, ranging from <Q>=10+ to 20+. The mean charge states of C, O, Ne, Mg and Si all increased as the iron charge state increased. In the events with the lowest iron charge state, all these ions except Mg had mean charge states consistent with the solar wind and with an equilibrium temperature of 1.3-1.6 million degrees K. On the other hand, in events with the highest iron charge states, O and Mg charge states were not consistent with the temperature derived from the iron alone. Moreover, there were abundance enhancements in Ne with respect to oxygen in those cases, even though the mass/charge of the O and Ne were similar. Additionally, the November, 1997 event displayed a trend in which the mean charge state for several ions increased with energy. These measurements may be the result of several processes, including a mixture of plasma with different source and acceleration histories, and abundance formation and possibly additional charge state modification by collisional or other means in the corona.