He IN IMPULSIVE SOLAR FLARES BY ELECTRON FIREHOSE WAVES

A new mechanism for acceleration and enrichment of He during impulsive solar flares is presented. Low-frequency electromagnetic plasma waves excited by the Electron Firehose Instability (EFI) can account for the acceleration of ions up to 1 MeV amu energies as a single stage process. The EFI arises as a direct consequence of the free energy stored in a temperature anisotropy (T e ‖ > T e ⊥) of the bulk energized electron population during the acceleration process. In contrast to other mechanisms which require special plasma properties, the EFI is an intrinsic feature of the acceleration process of the bulk electrons. Being present as a side effect in the flaring plasma, these waves can account for the acceleration of He and He while selectively enhancing He due to the spectral energy density built up from linear growth. Linearized kinetic theory, analytic models and test-particle simulations have been applied to investigate the ability of the waves to accelerate and fractionate. As waves grow in both directions parallel to the magnetic field, they can trap resonant ions and efficiently accelerate them to the highest energies. Plausible models have been found that can explain the observed energies, spectra and abundances of He and He. Subject headings: Sun: flares – Sun: particle emission – Sun: abundances