ENRICHMENT OF HE-3 AND HEAVY-IONS IN IMPULSIVE SOLAR-FLARES

The acceleration of He-3 and heavy ions by electromagnetic hydrogen cy clotron waves in a direct single-stage process in impulsive solar flar es is investigated analytically and with the help of test particle sim ulations. We illustrate in detail the mechanism by which a single mono chromatic wave can accelerate such ions to MeV and even GeV energies. While somewhat idealized, a monochromatic wave well illustrates the im portance of the background magnetic held gradient in the acceleration process. An interesting result of our analysis shows that the accelera tion rate is proportional to the magnitude of the magnetic held gradie nt and is independent of the wave amplitude, while the maximum energy gained increases with decreasing magnetic field gradient and increasin g wave amplitude. Heavy ions can also be accelerated by these electrom agnetic hydrogen cyclotron waves in a single-stage process by the seco nd or higher harmonic resonance. The acceleration of heavier ions by t he same mechanism raises the question of their low enrichment in compa rison to much higher enrichment of He-3. The solution is related to th e initial small acceleration of the thermal heavy ions at the higher h armonic resonances. The enrichment of the heavy ions increases with th e amplitude of the electromagnetic waves and decreases with the plasma density due to Coulomb collisions and absorption of wave energy. Comp arison between the rate of cooling of thermal heavy ions due to collis ions and heating by waves gives wave intensity and heavy ion ratios wh ich are consistent with observations. The relation between the acceler ated heavy ion abundances and their gyrofrequencies in the corona is u sed to estimate the temperature in the acceleration region. The existe nce of electromagnetic hydrogen cyclotron waves in flare plasmas is su pported by observations in auroral plasmas and by solution of the disp ersion relation, which shows that such waves can propagate over long d istances along coronal magnetic fields.