EVIDENCE FOR A COMMON ACCELERATION MECHANISM FOR ENRICHMENTS OF ³He AND HEAVY IONS IN IMPULSIVE SEP EVENTS

We have surveyed the period 1997–2015 for a rare type of ³He-rich solar energetic particle (SEP) event, with enormously enhanced values of the S/O ratio, that differs from the majority of ³He-rich events, which show enhancements of heavy ions increasing smoothly with mass. Sixteen events were found, most of them small but with solar source characteristics similar to other ³He-rich SEP events. A single event on 2014 May 16 had higher intensities than the others, and curved Si and S spectra that crossed the O spectrum above ∼200 keV nucleon⁻¹. Such crossings of heavy-ion spectra have never previously been reported. The dual enhancement of Si and S suggests that element Q / M ratio is critical to the enhancement since this pair of elements uniquely has very similar Q / M ratios over a wide range of temperatures. Besides ³He, Si, and S, in this same event the C, N, and Fe spectra also showed curved shape and enhanced abundances compared to O. The spectral similarities suggest that all have been produced from the same mechanism that enhances ³He. The enhancements are large only in the high-energy portion of the spectrum, and so affect only a small fraction of the ions. The observations suggest that the accelerated plasma was initially cool (∼0.4 MK) and was then heated to a few million kelvin to generate the preferred Q / M ratio in the range C–Fe. The temperature profile may be the distinct feature of these events that produces the unusual abundance signature.