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The Specific Acceleration Rate in Loop-structured Solar Flares - Implications for Electron Acceleration Models  

Jingnan Guo   Submitted: 2013-03-06 01:42

We analyze electron flux maps based on RHESSI hard X-ray imaging spectroscopy data for a number of extended coronal loop flare events. For each event, we determine the variation of the characteristic loop length L with electron energy E, and we fit this observed behavior with models that incorporate an extended acceleration region and an exterior ''propagation'' region, and which may include collisional modification of the accelerated electron spectrum inside the acceleration region. The models are characterized by two parameters: the plasma density n in, and the longitudinal extent L0 of, the acceleration region. Determination of the best-fit values of these parameters permits inference of the volume that encompasses the acceleration region and of the total number of particles within it. It is then straightforward to compute values for the emission filling factor and for the specific acceleration rate (electrons per second per ambient electron above a chosen reference energy). For the 24 events studied, the range of inferred filling factors is consistent with a value of unity. The inferred mean value of the specific acceleration rate above E_0=20 keV is sim10-2 s-1, with a 1-sigma spread of about a half-order-of-magnitude above and below this value. We compare these values with the predictions of several models, including acceleration by large-scale, weak (sub-Dreicer) fields, by strong (super-Dreicer) electric fields in a reconnecting current sheet, and by stochastic acceleration processes.

Authors: Jingnan Guo, A. Gordon Emslie, Michele Piana
Projects: RHESSI

Publication Status: ApJ, 2013, 766, 28
Last Modified: 2013-03-08 08:49
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Properties of the Acceleration Regions in Several Loop-structured Solar Flares  

Jingnan Guo   Submitted: 2012-06-18 06:40

Using {em RHESSI} hard X-ray imaging spectroscopy observations, we analyze electron flux maps for a number of extended coronal loop flares. For each event, we fit a collisional model with an extended acceleration region to the observed variation of loop length with electron energy E, resulting in estimates of the plasma density in, and longitudinal extent of, the acceleration region. These quantities in turn allow inference of the number of particles within the acceleration region and hence the filling factor f - the ratio of the emitting volume to the volume that encompasses the emitting region(s). We obtain values of f that lie mostly between 0.1 and 1.0; the (geometric) mean value is f = 0.20 imes div 3.9, somewhat less than, but nevertheless consistent with, unity. Further, coupling information on the number of particles in the acceleration region with information on the total rate of acceleration of particles above a certain reference energy (obtained from spatially-integrated hard X-ray data) also allows inference of the specific acceleration rate (electron s-1 per ambient electron above the chosen reference energy). We obtain a (geometric) mean value of the specific acceleration rate eta(20 keV) = (6.0 imes / div 3.4) imes 10-3 electrons s-1 per ambient electron; this value has implications both for the global electrodynamics associated with replenishment of the acceleration region and for the nature of the particle acceleration process.

Authors: Guo, Jingnan; Emslie, A. Gordon; Massone, Anna Maria; Piana, Michele
Projects: None

Publication Status: ApJ, in press
Last Modified: 2012-06-18 12:25
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Determination of the acceleration region size in a loop-structured solar flare  

Jingnan Guo   Submitted: 2012-06-03 11:26

In order to study the acceleration and propagation of bremsstrahlung-producing electrons in solar flares, we analyze the evolution of the flare loop size with respect to energy at a variety of times. A GOES M3.7 loop-structured flare starting around 23:55 on 2002 April 14 is studied in detail using Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations. We construct photon and mean-electron-flux maps in 2-keV energy bins by processing observationally-deduced photon and electron visibilities, respectively, through several image-processing methods: a visibility-based forward- fit (FWD) algorithm, a maximum entropy (MEM) procedure and the uv-smooth (UVS) approach. We estimate the sizes of elongated ares (i.e., the length and width of flaring loops) by calculating the second normalized moments of the intensity in any given map. Employing a collisional model with an extended acceleration region, we fit the loop lengths as a function of energy in both the photon and electron domains. The consistency of the estimates from these methods provides strong support that the model can reliably determine geometric parameters of the acceleration region. The acceleration region is estimated to be a substantial fraction (~ 1/2) of the loop extent, indicating that this dense flaring loop incorporates both acceleration and transport of electrons, with concurrent thick-target bremsstrahlung emission.

Authors: Jingnan Guo, A. Gordon Emslie, Eduard P. Kontar, Federico Benvenuto, Anna Maria Massone, Michele Piana
Projects: RHESSI

Publication Status: Accepted by A&A
Last Modified: 2012-06-08 07:28
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Relationship between Hard and Soft X-ray Emission Components of a Solar Flare  

Jingnan Guo   Submitted: 2010-12-17 01:38

X-ray observations of solar flares routinely reveal an impulsive high-energy and a gradual low-energy emission component, whose relationship is one of the key issues of solar flare study. The gradual and impulsive emission components are believed to be associated with, respectively, the thermal and nonthermal components identified in spectral fitting. In this paper, a prominent  50 second hard X-ray (HXR) pulse of a simple GOES class C7.5 flare on 20 February 2002 is used to study the association between high energy, non-thermal and impulsive evolution, and low energy, thermal and gradual evolution. We use regularized methods to obtain time derivatives of photon fluxes to quantify the time evolution as a function of photon energy, obtaining a break energy between impulsive and gradual behavior. These break energies are consistent with a constant value of  11 keV in agreement with those found spectroscopically between thermal and non-thermal components, but the relative errors of the former are greater than 15% and much greater than the a few percent errors found from the spectral fitting. These errors only weakly depend on assuming an underlying spectral model for the photons, pointing to the current data being inadequate to reduce the uncertainties rather than there being a problem associated with an assumed model. The time derivative method is used to test for the presence of a ?pivot energy? in this flare. Although these pivot energies are marginally consistent with a constant value of  9 keV, its values in the HXR rise phase appear to be lower than those in the decay phase. Assuming that electrons producing the high-energy component have a power law distribution and are accelerated from relatively hot regions of a background plasma responsible for the observed thermal component, a low limit is obtained for the low-energy cutoff. This limit is always lower than the break and pivot energies and locates in the tail of the Maxwellian distribution of the thermal component.

Authors: Jingnan Guo, Siming Liu, Lyndsay Fletcher, and Eduard P. Kontar
Projects: RHESSI

Publication Status: accepted by ApJ
Last Modified: 2010-12-21 01:02
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Abstracts by Author
The Specific Acceleration Rate in Loop-structured Solar Flares -- Implications for Electron Acceleration Models
Properties of the Acceleration Regions in Several Loop-structured Solar Flares
Determination of the acceleration region size in a loop-structured solar flare
Relationship between Hard and Soft X-ray Emission Components of a Solar Flare

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