Deducing Electron Properties from Hard X-ray Observations

Many problems in solar physics require analysis of imaging data obtained in multiple wavelength domains with differing spatial resolution in a framework supplied by advanced three-dimensional (3D) physical models. To facilitate this goal, we have undertaken a major enhancement of our IDL-based simulation tools developed earlier for modeling microwave and X-ray emission. The enhanced software architecture allows the user to (1) import photospheric magnetic field maps and perform magnetic field extrapolations to generate 3D magnetic field models; (2) investigate the magnetic topology by interactively creating field lines and associated flux tubes; (3) populate the flux tubes with user-defined nonuniform thermal plasma and anisotropic, nonuniform, nonthermal electron distributions; (4) investigate the spatial and spectral properties of radio and X-ray emission calculated from the model; and (5) compare the model-derived images and spectra with observational data. The package integrates shared-object libraries containing fast gyrosynchrotron emission codes, IDL-based soft and hard X-ray codes, and potential and linear force-free field extrapolation routines. The package accepts user-defined radiation and magnetic field extrapolation plug-ins. We use this tool to analyze a relatively simple single-loop flare and use the model to constrain the magnetic 3D structure and spatial distribution of the fast electrons inside this loop. We iteratively compute multi-frequency microwave and multi-energy X-ray images from realistic magnetic flux tubes obtained from pre-flare extrapolations, and compare them with imaging data obtained by SDO, NoRH, and RHESSI. We use this event to illustrate the tool's use for the general interpretation of solar flares to address disparate problems in solar physics.

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“…Similarly, the thin-target hard X-rays are calculated as described in (e.g., Holman et al 2011;Kontar et al 2011, as the recent RHESSI reviews)…”

Section: X-ray Emissionmentioning

confidence: 99%

Three-Dimensional Radio and X-Ray Modeling and Data Analysis Software: Revealing Flare Complexity

Nita

Fleishman

Kuznetsov

et al. 2015

ApJ

109

104

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“…2002 July 23 flare, demonstrated that the actual spectrum does contain deviations from a simple power-law (Piana et al 2003;Kontar et al 2011a), we assume a single power-law in the electron spectrum for the purposes of our analysis. For each flare, the looptop source was fitted with a thin-target model and the footpoint sources were fitted using a thick-target model (see Kontar et al 2011a, for details). Assuming looptop source is a thin-target emission, the photon flux detected at the Earth is the convolution of the electron distribution and the bremsstrahlung cross-section.…”

Section: Spectral Fittingmentioning

confidence: 99%

“…Dennis et al 2011;Holman et al 2011;Kontar et al 2011a, as recent reviews). These spatially resolved observations indicate appearances of distinct coronal and footpoint X-ray sources.…”

Section: Introductionmentioning

confidence: 98%

Implications for electron acceleration and transport from non-thermal electron rates at looptop and footpoint sources in solar flares

Simões¹,

Kontar²

2013

A&A

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The interrelation of hard X-ray (HXR) emitting sources and the underlying physics of electron acceleration and transport presents one of the major questions in high-energy solar flare physics. Spatially resolved observations of solar flares often demonstrate the presence of well-separated sources of bremsstrahlung emission, so-called coronal and footpoint sources. Using spatially resolved X-ray observations by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and recently improved imaging techniques, we investigate in detail the spatially resolved electron distributions in a few well-observed solar flares. The selected flares can be interpreted as having a standard geometry with chromospheric HXR footpoint sources related to thick-target X-ray emission and the coronal sources characterised by a combination of thermal and thin-target bremsstrahlung. Using imaging spectroscopy techniques, we deduce the characteristic electron rates and spectral indices required to explain the coronal and footpoint X-ray sources. We found that, during the impulsive phase, the electron rate at the looptop is several times (a factor of 1.7−8) higher than at the footpoints. The results suggest that a sufficient number of electrons accelerated in the looptop explain the precipitation into the footpoints and imply that electrons accumulate in the looptop. We discuss these results in terms of magnetic trapping, pitch-angle scattering, and injection properties. Our conclusion is that the accelerated electrons must be subject to magnetic trapping and/or pitch-angle scattering, keeping a fraction of the population trapped inside the coronal loops. These findings put strong constraints on the particle transport in the coronal source and provide quantitative limits on deka-keV electron trapping/scattering in the coronal source.

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“…The "cold thick-target" model CTTM (Brown 1971;Syrovatskii & Shmeleva 1972) has proved popular because it provides a straightforward relationship between the bright X-ray emission from the chromospheric footpoints and the source coronal electron distribution. However, many RHESSI observations are not consistent with the CTTM or produce challenging results (Holman et al 2011;Kontar et al 2011), which demonstrates that essential physics is missing from the standard model.…”

Section: Introductionmentioning

confidence: 99%

Effect of turbulent density-fluctuations on wave-particle interactions and solar flare X-ray spectra

Hannah

Kontar

Reid

2013

A&A

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Aims. The aim of this paper is to demonstrate the effect of turbulent background density-fluctuations on flare-accelerated electron transport in the solar corona. Methods. Using the quasi-linear approximation, we numerically simulated the propagation of a beam of accelerated electrons from the solar corona to the chromosphere, including the self-consistent response of the inhomogeneous background plasma in the form of Langmuir waves. We calculated the X-ray spectrum from these simulations using the bremsstrahlung cross-section and fitted the footpoint spectrum using the collisional "thick-target" model, a standard approach adopted in observational studies. Results. We find that the interaction of the Langmuir waves with the background electron density gradient shifts the waves to a higher phase velocity where they then resonate with higher velocity electrons. The consequence is that some of the electrons are shifted to higher energies, producing more high-energy X-rays than expected if the density inhomogeneity is not considered. We find that the level of energy gain is strongly dependent on the initial electron beam density at higher energy and the magnitude of the density gradient in the background plasma. The most significant gains are for steep (soft) spectra that initially had few electrons at higher energies. If the X-ray spectrum of the simulated footpoint emission are fitted with the standard "thick-target" model (as is routinely done with RHESSI observations) some simulation scenarios produce more than an order-of-magnitude overestimate of the number of electrons >50 keV in the source coronal distribution.

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Deducing Electron Properties from Hard X-ray Observations

Cited by 173 publications

References 155 publications

Three-Dimensional Radio and X-Ray Modeling and Data Analysis Software: Revealing Flare Complexity

Three-Dimensional Radio and X-Ray Modeling and Data Analysis Software: Revealing Flare Complexity

Implications for electron acceleration and transport from non-thermal electron rates at looptop and footpoint sources in solar flares

Effect of turbulent density-fluctuations on wave-particle interactions and solar flare X-ray spectra

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