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 energies as a single stage process.
The EFI arises as a direct consequence of the free energy
stored in a temperature anisotropy (T_parallel>T_perp) 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 3He and 4He
while selectively enhancing 3He 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 3He and 4He.
Authors: G. Paesold, R. Kallenbach and A.O. Benz
Publication Status: Astrophysical Journal, in Press
Last Modified: 2002-09-08 10:55
Spatial analysis of solar type III events associated with narrow band spikes at metric wavelengths
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 Nancay Radioheliograph (NRH) at several frequencies. Five events were selected showing spikes at one or two and type III bursts at two or more Nancay 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.
Authors: G. Paesold, A.O. Benz, K.-L. Klein, N. Vilmer
Publication Status: A & A (in press)
Last Modified: 2001-03-16 08:57
Electron Firehose instability and acceleration of electrons in
solar flaressolar flaresAn electron distribution with a temperature anisotropy T_parallel/T_perp>1 can lead to the Electron Firehose instability (Here _parallel and _perp denote directions relative to the background magnetic field B_0). Since possible particle acceleration mechanisms in solar flares exhibit a preference of energizing particles in parallel direction, such an anisotropy is expected during the impulsive phase of a flare. The properties of the excited waves and the thresholds for instability are investigated by using linearized kinetic theory. These thresholds were connected to the pre-flare plasma parameters by assuming an acceleration model acting exclusively in parallel direction. For usually assumed pre-flare plasma conditions the electrons become unstable during the acceleration process and lefthand circularly polarized waves with frequenci- frequencies of about ~|Omega_p| are excited at parallel propagation. Indications have been found, that the largest growth rates occur at oblique propagation and the according frequencies lie well above the proton gyrofrequency.
Authors: Paesold, G. and Benz, A.O.
Publication Status: A&A (in press)
Last Modified: 1999-11-04 16:04