Temperature of the Source Plasma in Gradual Solar Energetic Particle Events

Scattering during interplanetary transport of particles during large, "gradual" solar energetic-particle (SEP) events can cause element abundance enhancements or suppressions that depend upon the mass-to-charge ratio [] of the ions as an increasing function early in events and a decreasing function of the residual scattered ions later. Since the -values for the ions depend upon the source plasma temperature [], best fits of the power-law dependence of enhancements vs. can determine . These fits provide a fundamentally new method to determine the most probable value of for these events in the energy region . Complicated variations in the grouping of element enhancements or suppressions match similar variations in at the best-fit temperature. We find that fits to the times of increasing and decreasing powers give similar values of , in the range of 0.8 -aEuro parts per thousand 1.6 MK for 69 % of events, consistent with the acceleration of ambient coronal plasma by shock waves driven out from the Sun by coronal mass ejections (CMEs). However, 24 % of the SEP events studied showed plasma of 2.5 -aEuro parts per thousand 3.2 MK, typical of that previously determined for the smaller impulsive SEP events; these particles may be reaccelerated preferentially by quasi-perpendicular shock waves that require a high injection threshold that the impulsive-event ions exceed or simply by high intensities of impulsive suprathermal ions at the shock. The source-temperature distribution of ten higher-energy ground-level events (GLEs) in the sample is similar to that of the other gradual events, at least for SEPs in the energy-range of . Some events show evidence that a portion of the ions may have been further stripped of electrons before the shock acceleration; such events are smaller and tend to cluster late in the solar cycle.