Abstract. In the course of the energization of electrons to energies of some tens of keV during the impulsive phase of a solar flare, the velocity distribution function of the electrons is predicted to become anisotropic with T e > T e ⊥ (Here, and ⊥ denote directions with respect to the background magnetic field). Such a configuration can become unstable to the so-called Electron Firehose instability (EFI). Left hand circularly polarized electromagnetic waves propagating along the magnetic field are excited via a non-resonant mechanism: electrons non-resonantly excite the waves while the protons are in resonance and carry the wave. The non-resonant nature of the instability raises the question of the response of the electron population to the growing waves. Test particle simulations are carried out to investigate the pitch-angle development of electrons injected to single waves and wave spectra. To interpret the simulation results, a drift kinetic approach is developed. The findings in the case of single wave simulations show the scattering to larger pitch-angles in excellent agreement with the theory. The situation dramatically changes when assuming a spectrum of waves. Stochasticity is detected at small initial parallel velocities resulting in significant deviations from drift kinetic theory. It enhances the scattering rate of electrons with initial parallel velocity below to the mean thermal perpendicular velocity. Increased scattering is also noticed for electrons having initial parallel velocity within an order of magnitude of the resonance velocity. The resulting pitch-angle scattering is proposed to be an important ingredient in Fermi-type electron acceleration models, particularly transit-time acceleration by compressional MHD waves.
Abstract. The spatial association of narrow band metric radio spikes with type III bursts is analyzed. The analysis addresses the question of a possible causal relation between the spike emission and the acceleration of the energetic electrons causing the type III burst. The spikes are identified by the Phoenix-2 spectrometer (ETH Zurich) from survey solar observations in the frequency range from 220 MHz to 530 MHz. Simultaneous spatial information was provided by the Nançay Radioheliograph (NRH) at several frequencies. Five events were selected showing spikes at one or two and type III bursts at two or more Nançay frequencies. The 3-dimensional geometry of the single events has been reconstructed by applying different coronal density models. As a working hypothesis it is assumed that emission at the plasma frequency or its harmonic is the responsible radiation process for the spikes as well as for the type III bursts. It has been found that the spike source location is consistent with the backward extrapolation of the trajectory of the type III bursts, tracing a magnetic field line. In one of the analyzed events, type III bursts with two different trajectories originating from the same spike source could be identified. These findings support the hypothesis that narrow band metric spikes are closely related to the acceleration region.
A new mechanism for acceleration and enrichment of 3 He during impulsive solar flares is presented. Lowfrequency electromagnetic plasma waves excited by the electron firehose instability (EFI) can account for the acceleration of ions up to 1 MeV amu À1 energies as a single-stage process. The EFI arises as a direct consequence of the free energy stored in a temperature anisotropy (T e k > T e ? ) 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 3 He and 4 He while selectively enhancing 3 He 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 3 He and 4 He.
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