Authors: Paesold, G.; Kallenbach, R.; Benz, A. O.
Reference: Astrophys. Jou., 582, 495-505, 2003
Abstract:
A new mechanism for acceleration and enrichment of 3He 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-1 energies as a single-stage process. The EFI
arises as a direct consequence of the free energy stored in a temperature anisotropy (Te∥>Te?) of
the bulk energized electron population during the acceleration process. In contrast to other mechanisms
that 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 3He and 4He while selectively enhancing 3He as a result of 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 3He and 4He.