Enrichment of3He and Heavy Ions in Impulsive Solar Flares

Abstract

The acceleration of 3He and heavy ions by electromagnetic hydrogen cyclotron waves in a direct single-stage process in impulsive solar flares is investigated analytically and with the help of test particle simulations. We illustrate in detail the mechanism by which a single monochromatic wave can accelerate such ions to MeV and even GeV energies. While somewhat idealized, a monochromatic wave well illustrates the importance of the background magnetic field gradient in the acceleration process. An interesting result of our analysis shows that the acceleration rate is proportional to the magnitude of the magnetic field gradient and is independent of the wave amplitude, while the maximum energy gained increases with decreasing magnetic field gradient and increasing wave amplitude. Heavy ions can also be accelerated by these electromagnetic hydrogen cyclotron waves in a single-stage process by the second or higher harmonic resonance. The acceleration of heavier ions by the same mechanism raises the question of their low enrichment in comparison to much higher enrichment of 3He. The solution is related to the initial small acceleration of the thermal heavy ions at the higher harmonic resonances. The enrichment of the heavy ions increases with the amplitude of the electromagnetic waves and decreases with the plasma density due to Coulomb collisions and absorption of wave energy. Comparison between the rate of cooling of thermal heavy ions due to collisions and heating by waves gives wave intensity and heavy ion ratios which are consistent with observations. The relation between the accelerated heavy ion abundances and their gyrofrequencies in the corona is used to estimate the temperature in the acceleration region. The existence of electromagnetic hydrogen cyclotron waves in flare plasmas is supported by observations in auroral plasmas and by solution of the dispersion relation, which shows that such waves can propagate over long distances along coronal magnetic fields.