Ultraviolet and X-ray Spectroscopy of the Solar Atmosphere

Phillips, K. J. H.; Feldman, U.; Landi, E.

doi:10.1017/cbo9780511585968

Cited by 134 publications

(131 citation statements)

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“…These spatially resolved observations indicate appearances of distinct coronal and footpoint X-ray sources. Hard X-ray (HXR) footpoint sources are usually described in terms of collisional thick-target models (Dubov 1963;Elliot 1969;Arnoldy et al 1968;Lingenfelter 1969;Brown 1971;Syrovatskii & Shmeleva 1972;Brown 1973a;Lin & Hudson 1976), where the electrons accelerated in the corona travel downwards inside magnetic loops marking footpoint sources in dense chromospheric plasma by emission in X-rays, EUV, and white-light emissions (Phillips et al 2008). The observations show that the HXR source height in the chromosphere decreases with increasing X-ray energy (Matsushita et al 1992;Aschwanden et al 2002;Kontar et al 2008;Saint-Hilaire et al 2010;Battaglia & Kontar 2011a) as anticipated from downward propagating electron beams.…”

Section: Introductionmentioning

confidence: 99%

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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Section: Introductionmentioning

confidence: 99%

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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“…Many different DEM diagnostic techniques have been developed in the past (see the reviews of Phillips et al 2008;and Harrison & Thompson 1992). Harrison & Thompson (1992) carried out a comparative analysis aimed at determining which, among six different DEM diagnostic techniques, was most successful at reproducing pre-defined DEM curves using a set of emission lines.…”

Section: Introductionmentioning

confidence: 99%

Monte Carlo Markov chain DEM reconstruction of isothermal plasmas

Landi

Reale

Testa

2012

A&A

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Context. Recent studies carried out with SOHO and Hinode high-resolution spectrometers have shown that the plasma in the off-disk solar corona is close to isothermal. If confirmed, these findings may have significant consequences for theoretical models of coronal heating. However, these studies have been carried out with diagnostic techniques whose ability to reconstruct the plasma distribution with temperature has not been thoroughly tested. Aims. In this paper, we carry out tests on the Monte Carlo Markov chain (MCMC) technique with the aim of determining: 1) its ability to retrieve isothermal plasmas from a set of spectral line intensities, with and without random noise; 2) to what extent can it discriminate between an isothermal solution and a narrow multithermal distribution; and 3) how well it can detect multiple isothermal components along the line of sight. We also test the effects of 4) atomic data uncertainties on the results, and 5) the number of ions whose lines are available for the DEM reconstruction. Methods. We first use the CHIANTI database to calculate synthetic spectra from different thermal distributions: single isothermal plasmas, multithermal plasmas made of multiple isothermal components, and multithermal plasmas with a Gaussian DEM distribution with variable width. We then apply the MCMC technique on each of these synthetic spectra, so that the ability of the MCMC technique at reconstructing the original thermal distribution can be evaluated. Next, we add a random noise to the synthetic spectra, and repeat the exercise, in order to determine the effects of random errors on the results. We also we repeat the exercise using a different set of atomic data from those used to calculate synthetic line intensities, to understand the robustness of the results against atomic physics uncertainties. The size of the temperature bin of the MCMC reconstruction is varied in all cases, in order to determine the optimal width. Results. We find that the MCMC technique is unable to retrieve isothermal plasmas to better than Δ log T 0.05. Also, the DEM curves obtained using lines calculated with an isothermal plasma and with a Gaussian distribution with FWHM of log T 0.05 are very similar. Two near-isothermal components can be resolved if their temperature separation is Δ log T = 0.2 or larger. Thus, DEM diagnostics has an intrinsic resolving power of log T = 0.05. Atomic data uncertainties may significantly affect both temperature and peak DEM values, but do not alter our conclusions. The availability of small sets of lines also does not worsen the performance of the MCMC technique, provided these lines are formed in a wide temperature range. Conclusions. Our analysis shows the present limitations in our ability to identify the presence of strictly isothermal plasmas in stellar and solar coronal spectra.

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“…These lines are usually chosen to be from the same ion so that the dependence on charge states and elemental abundances cancel. The physics behind density-sensitive line ratios are explained in detail in Phillips et al (2008). Table 1 lists the transitions we use here as density diagnostics.…”

Section: Density-diagnostic Linesmentioning

confidence: 99%

Inferring the Coronal Density Irregularity From Euv Spectra

Hahn

Savin

2016

ApJ

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Understanding the density structure of the solar corona is important for modeling both coronal heating and the solar wind. Direct measurements are difficult because of line-of-sight integration and possible unresolved structures. We present a new method for quantifying such structures using density-sensitive extreme ultraviolet line intensities to derive a density irregularity parameter, a relative measure of the amount of structure along the line of sight. We also present a simple model to relate the inferred irregularities to physical quantities, such as the filling factor and density contrast. For quiet-Sun regions and interplume regions of coronal holes, we find a density contrast of at least a factor of 3-10and corresponding filling factors of about 10%-20%. Our results are in rough agreement with other estimates of the density structures in these regions. The irregularity diagnostic provides a useful relative measure of unresolved structure in various regions of the corona.

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Ultraviolet and X-ray Spectroscopy of the Solar Atmosphere

Cited by 134 publications

References 0 publications

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

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

Monte Carlo Markov chain DEM reconstruction of isothermal plasmas

Inferring the Coronal Density Irregularity From Euv Spectra

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