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Using RHESSI imaging spectroscopy techniques it is possible to estimate the rate of accelerated electrons flowing through the top of the flaring loop and also into the chromospheric footpoints to account for the observed photon spectra. The X-ray spectra of the coronal and footpoint sources are obtained and fitted with thin- and thick-target models, respectively. The thick-target model directly provides the electron rate ''NFP'' necessary to explain the observed footpoint emission. To obtain the electron rate at the looptop source ''NLT'' from the thin-target model, we need to infer the coronal target, given by the length of the non-thermal looptop source ''L'', which can be measured from RHESSI maps given certain care, and the thermal plasma density ''n'', which can be inferred from the thermal component of the HXR spectrum.
Using RHESSI imaging spectroscopy techniques it is possible to estimate the rate of accelerated electrons flowing through the top of the flaring loop and also into the chromospheric footpoints to account for the observed photon spectra. The X-ray spectra of the coronal and footpoint sources are obtained and fitted with thin- and thick-target models, respectively. The thick-target model directly provides the electron rate ''NFP'' necessary to explain the observed footpoint emission. To obtain the electron rate at the looptop source ''NLT'' from the thin-target model, we need to infer the coronal target, given by the length of the non-thermal looptop source ''L'', which can be measured from RHESSI maps given certain care, and the thermal plasma density ''n'', which can be inferred from the thermal component of the HXR spectrum.
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[[File:201f1.gif|center|thumb|800px|Figure 1:  
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[[File:201f1.png|center|thumb|800px|Figure 1:  
Imaging spectroscopy method to analyse the photon spectra produced at the coronal looptop source and at chromospheric footpoint sources.
Imaging spectroscopy method to analyse the photon spectra produced at the coronal looptop source and at chromospheric footpoint sources.
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[[File:201f2.gif|center|thumb|800px|Figure 2:  
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[[File:201f2.png|center|thumb|800px|Figure 2:  
RHESSI maps for the four flares analysed, showing the coronal looptop thermal source at 10 keV (red), the chromospheric footpoints at 60-70 keV (blue contours) and non-thermal coronal source (green contours) at the photon energy they appear in each flare.
RHESSI maps for the four flares analysed, showing the coronal looptop thermal source at 10 keV (red), the chromospheric footpoints at 60-70 keV (blue contours) and non-thermal coronal source (green contours) at the photon energy they appear in each flare.
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Revision as of 13:06, 13 May 2013


Nugget
Number: 300
1st Author: Paulo Simões
2nd Author: Eduard Kontar
Published: May 13, 2013
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Introduction

The X-ray images of solar flares with spectral capabilities taken by the Hard X-ray Telescope (HXT) on board of Yohkoh and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have revealed a wealth of information on spatially resolved distributions of energetic electrons in solar flares. These observations indicate distinct coronal and footpoint X-ray sources. Hard X-ray (HXR) footpoint sources are usually described in terms of collisional thick-target models where the electrons accelerated in the corona travel downwards inside magnetic loops marking footpoint sources in dense chromospheric plasma. Coronal X-ray sources are prominently visible at lower energies below ~10 keV are usually due to thermal bremsstrahlung and the presence of accelerated electron populations. The looptop sources are believed to be connected to the process of energy release in flares, and some models place the magnetic reconnection at the looptop, where the X-ray emission is a consequence of the plasma heating and particle acceleration.


Evidence for accelerated electrons at the looptop

We analysed four well-observed events in which the common loop structure could be identified in HXR: two footpoint sources at higher photon energies and a looptop source at lower energies. For these flare, a non-thermal source at the looptop was also observed at energies above 20 keV. Using RHESSI imaging spectroscopy techniques it is possible to estimate the rate of accelerated electrons flowing through the top of the flaring loop and also into the chromospheric footpoints to account for the observed photon spectra. The X-ray spectra of the coronal and footpoint sources are obtained and fitted with thin- and thick-target models, respectively. The thick-target model directly provides the electron rate NFP necessary to explain the observed footpoint emission. To obtain the electron rate at the looptop source NLT from the thin-target model, we need to infer the coronal target, given by the length of the non-thermal looptop source L, which can be measured from RHESSI maps given certain care, and the thermal plasma density n, which can be inferred from the thermal component of the HXR spectrum.

Figure 1: Imaging spectroscopy method to analyse the photon spectra produced at the coronal looptop source and at chromospheric footpoint sources.

In the classical thick-target model with near instantaneous acceleration, where accelerated electrons stream freely from the acceleration region at the top of the loop down to the chromospheric footpoints, one would expect that the electron rate on both regions would be the same, i.e. their ratio would be close to 1. 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 2 − 8) higher than at the footpoints. The results suggest that the number of electrons accelerated in the looptop is larger than needed to explain the precipitation into the footpoints and imply that electrons accumulate in the looptop.

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 2 − 8) higher than at the footpoints. The results suggest that the number of electrons accelerated in the looptop is larger than needed to explain the precipitation into the footpoints and imply that electrons accumulate in the looptop.


Figure 2: RHESSI maps for the four flares analysed, showing the coronal looptop thermal source at 10 keV (red), the chromospheric footpoints at 60-70 keV (blue contours) and non-thermal coronal source (green contours) at the photon energy they appear in each flare.


Summary

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. Our results suggest that the magnetic trapping should not be too strong and, at the same time, favour scattering with mean free path about 2-8 times smaller than the size of coronal source. 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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