We discuss possible mechanisms of acceleration of particles in solar flares and
show that turbulence plays an
important role in all the mechanism. It is also argued that stochastic particle
acceleration by turbulent plasma waves is the most likely mechanism for
production of the high energy electrons and ions responsible for observed
radiative signatures of solar flares and for solar energetic particle or SEPs,
and that the predictions of this model agrees well with many past and recent
high spectral and temporal observations of solar flares. It is shown that, in
addition, the model explains many features of SEPs that accompany flares. In
particular we show that it can successfully explain the observed extreme
enhancement, relative to photospheric values, of 3He ions and the relative
spectra of 3He and 4He. It has also the potential of explaining the
relative abundances of most ions including the increasing enhancements of heavy
ions with ion mass or mass-to-charge ratio.
Authors: Vahe' Petrosian
Publication Status: to appear in the proceedings of ISSI workshop on He rich SEPs
Last Modified: 2008-09-23 20:51
On the Spatial Distributioin of Hard X-rays from Solar Flares
The aim of this paper is to investigate the spatial structure of the impulsive phase hard X-ray emission from solar flares. This work is motivated by the YOHKOH and the forthcoming HESSI observations. Summarizing past results, it is shown that the transport effects can account for the observations in inhomogene- inhomogeneous loop models where there is a strong field convergence and/or density enhancement at the top of the flaring loop. Scattering by plasma turbulence at the acceleration site or pancake type pitch angle distribution of the accelerated electrons can also give rise to enhanced emission at the loop tops. These could be a natural consequence of acceleration by plasma waves. This paper considers a general case of stochastic scattering and acceleration that leads to an isotropic pitch angle distribution and an enhanced emission from the loop tops or the acceleration site. Following the formalism developed in earlier papers the strength and the spectrum of the radiation expected from the acceleration site and the foot points are evaluated and their dependence on the parameters describing the acceleration process and the flare plasma are determined. The theoretical ratio of these two intensities and relative values of their spectral indices are compared with the YOHKOH observations, demonstrat- demonstrating that the above mentioned parameters can be constrained with such observations. It is shown that future high spatial and spectral resolution observations, for example those expected from HESSI, can begin to distinguish between different models and constrain their parameters.
Authors: Petrosian, Vahe', & Donaghy, T. Q.
Publication Status: ApJ, in press vol 527
Last Modified: 1999-11-19 23:02
Plasma Turbulence and Stochastic Acceleration in Solar Flares
Observational aspects of solar flares relevant to the acceleration process of electrons and protons are reviewed and it is shown that most of these observations can be explained by the interaction with flare plasma of a power law energy distribution of electrons (and protons) that are injected at the top of a flaring loop, in the so-called thick target model. Some new observations that do not agree with this model are described and it is shown that these can be explained most naturally if most of the energy released by the reconnection process goes first into the generation of plasma turbulence, which accelerates, scatters and traps the ambient electrons near the top of the loop stochastically stochastically. The resultant bremsstrahlung photon spectral and spatial distri- distributions agree with the new observations. This model is also justified by some theoretical arguments. Results from numerical evaluation of the spectra of the accelerated electrons and their bremsstrahlung emission are compared with observations and shown how one can constrain the model parameters describing the flare plasma and the spectrum and the energy density of the turbulence.
Authors: Petrosian, Vahe'
Publication Status: Proceedings of Workshop on Plasma Turbulence, Sept. 1999, Cracow, Poland
Last Modified: 1999-11-19 23:02