Selective enrichment of energetic ions in impulsive solar flares

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We have shown previously that heavy ions as well as 3He can be accelerated to MeV energies by obliquely propagating electromagnetic proton cyclotron waves in impulsive solar flares. Impulsive flares are characterized by an increase of up to four orders of magnitude in the abundance of energetic 3He ions and up to one order of magnitude in energetic heavy ions compared to the abundances in long duration events and in the solar corona. 3He is accelerated when the first harmonic resonance is satisfied, while the heavier ions can be accelerated by the same wave at the second harmonic resonance when the wave frequency is twice the ion gyrofrequency. We postulate that obliquely propagating proton cyclotron waves exist in impulsive solar flares in analogy with the Earth’s aurora, where such waves are often the most intense waves present. The heating of 3He and of heavy ions can be substantially more efficient in a nonuniform plasma than in a uniform plasma. The heating efficiencies are nearly equal at higher energies, though at lower energies first harmonic heating is more efficient. The relatively small enrichments of the heavier ions compared to 3He can be qualitatively understood as a result of the damping of the waves by the heavier ions and by the less efficient initial acceleration at lower energies at the second harmonic resonance.