Determination of Stochastic Acceleration Model Characteristics in Solar Flares |
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Qingrong Chen Submitted: 2013-07-09 15:23
Following our recent paper (Petrosian & Chen 2010), we have developed an inversion method to determine the basic characteristics for the model of stochastic acceleration
of particles by plasma waves or turbulence directly and non-parametrically from observations in the framework of
the leaky box version of the Fokker-Planck kinetic equation.
These characteristics are determined by wave-particle interactions as described by the momentum and pitch angle diffusion coefficients. Our method relates these coefficients directly to observations. In particular,
we show that by inverting the Fokker-Planck equation to its integral form, one can derive the energy diffusion coefficient and direct acceleration rate by turbulence
in terms of the accelerated and escaping particle spectra.
We apply the analytic formulas to the suprathermal electrons
in solar flares, which produce hard X-ray emission at the coronal loop top (LT) region and two thick target footpoint regions of the flare loop. We utilize the regularized electron flux spectral images recently developed for
the Reuven Ramaty High Energy Solar Spectroscopic Imager ({it RHESSI}). From the spatially resolved electron flux spectra, we determine the electron escape time, which is related to the pitch angle scattering rate, and the energy diffusion coefficient at the LT acceleration region.
Results obtained from two relatively intense {it RHESSI} events indicate that the escape time increases with energy and the energy diffusion (or direct acceleration) time
and scattering time have dramatically different energy dependences. Such behaviors may be difficult to explain by
existing wave-particle interaction models, and may indicate that a different acceleration mechanism is at work or imply a breakdown of the basic interpretation of the escape of electrons being a random walk process. The discrepant energy dependences can be alleviated somewhat by a turbulence spectrum that is much steeper than the Kolmogorov-type spectrum. On the other hand, a more likely explanation could be that the escape of electrons out of the LT acceleration region is governed by converging field lines in a magnetic mirror geometry, in which the escape time is proportional to scattering time. The results demonstrate the critical importance of combined modeling of
electron acceleration by plasma wave turbulence and the large scale magnetic field variations in a reconnection environment.
Authors: Qingrong Chen and Vahé Petrosian
Projects: RHESSI
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Publication Status: submitted to ApJ
Last Modified: 2013-07-10 13:40
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On the Relationship between the Continuum Enhancement and Hard X-ray Emission in a White-Light Flare |
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Qingrong Chen Submitted: 2004-12-08 22:20
We investigate the relationship between the continuum enhancement and the hard X-ray (HXR) emission of a white-light flare on 2002 September 29. By reconstructing the RHESSI HXR images in the impulsive phase, we find two bright conjugate footpoints (FPs) on the two sides of the magnetic neutral line. Using the thick-target model and assuming a low-energy cutoff of 20 keV, the energy fluxes of non-thermal electron beams bombarding FPs A and B are estimated to be 1.0 1010 and 0.8 1010 ergs/cm2/s, respectively. However, the continuum enhancement at the two FPs is not simply proportional to the electron beam flux. The continuum emission at FP B is relatively strong with a maximum enhancement of about 8% and correlates temporally well with the HXR profile; however, that at FP A is less significant with an enhancement of only about 4-5%, regardless of the relatively strong beam flux. By carefully inspecting the Hα line profiles, we ascribe such a contrast to different atmospheric conditions at the two FPs. The Hα line profile at FP B exhibits a relatively weak amplitude with a pronounced central reversal, while the profile at FP A is fairly strong without a visible central reversal. This indicates that in the early impulsive phase of the flare, the local atmosphere at FP A has been appreciably heated and the coronal pressure is high enough to prevent most high-energy electrons from penetrating into the deeper atmosphere; while at FP B, the atmosphere has not been fully heated, the electron beam can effectively heat the chromosphere and produce the observed continuum enhancement via the radiative backwarming effect.
Authors: Q. R. Chen and M. D. Ding
Projects: RHESSI
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Publication Status: accepted for publication in ApJ
Last Modified: 2004-12-08 22:20
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