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The Astrophysical Journal


We report the first examples of ³He-rich solar energetic particle (SEP) events with heavy ion enrichments that are not ordered generally by mass. A second new feature is the large enrichment of ¹⁴N, the first time large enrichments have been seen for heavy ions lighter than O or Ne. These SEP events were too small and at too low an energy (<500 keV nucleon-1) to have been observed previously. The best observed event, 2001 July 16, had abundance ratios C : N : O : Ne : Mg : Si : S : Ca : Fe of ∼0.8 : 1.0 : ≡ 1 : 0.5 : 0.5 : 2.0 : 2.4 : 0.2 : 0.9, corresponding, respectively, to enhancements over the slow solar wind of ∼1.2 :15 : ≡ 1 : 5. : 3.4 : 12 :49 : 19 : 7. This pattern is incompatible with a gravitational settling enrichment mechanism. The simultaneous large enhancements of ¹⁴N and ²⁸Si, which have identical charge-to-mass ratios for certain ionization states, suggest that selective plasma wave heating plays a key role, as has long been suspected. Enhancement of ¹⁴N implies relatively low temperatures (<1.5 × 10 6 K) since fully stripped ¹⁴N cannot be preferentially heated. Also, the presence of enhanced species such as ¹⁴N and ²⁸Si, along with others with small or no enhancements ( 16)O, ²⁰Ne), may indicate that the plasma waves must operate over a relatively narrow range of frequencies and that the material must be heated while the waves are present.



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