Ionic charge state measurements in solar energetic particle events

Abstract

Direct ionic charge state measurements have been made for solar energetic particle (SEP) events by the SEPICA instrument on the Advanced Composition Explorer satellite. In large events, the charge state may even be measured as a function of time, revealing features that may be created by processes 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. Several examples of charge state measurements in SEP events will be presented. Two events consist of magnetic clouds, in which charge state observations provide information about the cloud geometry and source region. Other events are characterized by energy dependent charge states, in which higher energy ions have higher charge states. These may be caused by at least three mechanisms: shock acceleration in the corona together with collisions, rigidity dependent acceleration from an interplanetary shock, or finally interplanetary shock acceleration operating on a mixture of coronal material and preaccelerated flare ions. A multiple event study has compared iron charge states, ranging from «Qå=10+ to 20+, to those of C, O, Ne, Mg and Si. The mean charge states of these ions all increased as the iron charge state increased. In events with the highest iron charge states, there were abundance enhancements in Ne with respect to oxygen, even though the mass/charge of the O and Ne were similar. In events with the lowest iron charge states, all these ions except Mg showed mean charge states generally consistent with coronal material of an equilibrium temperature of 1.3–1.6 million degrees K. These observations may be the result of several processes, including a mixture of plasma with different source and acceleration histories, and modification of abundances and possibly charge states by collisional or other means in the corona.