Differential Emission Measure Analysis of a Polar Coronal Hole During the Solar Minimum in 2007

Hahn, Michael; Landi, E.; Savin, D. W.

doi:10.1088/0004-637x/736/2/101

Cited by 29 publications

(38 citation statements)

References 26 publications

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“…This is typical for coronal holes and we found similar results in the DEM analysis for all three locations we studied in the CH-B observation (see also Hahn et al 2011). We suspect that the cooler emission comes from the coronal hole itself, while the warmer component is from thequiet Sun along the line of sight.…”

Section: Coronal Holessupporting

confidence: 84%

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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Section: Coronal Holessupporting

confidence: 84%

Inferring the Coronal Density Irregularity From Euv Spectra

Hahn

Savin

2016

ApJ

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“…Even a complex active region, when observed at the solar west limb, seemed to be composed by three near isothermal plasmas . Further studies carried out with Hinode/EIS observations, on the contrary, provided evidence that the quiescent solar corona is not strictly isothermal, but it is characterized by a rather narrow temperature structure with a tail that extends to higher temperatures Brooks et al 2009;Hahn et al 2011). The aim of the present paper is to assess how reliable such isothermal plasma claims are.…”

Section: Introductionmentioning

confidence: 91%

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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“…There is also a small contribution at high temperature, between T log 6.2 e = and 6.3, coming from quiet Sun material lying along the LOS. This signal complicates the interpretation of the results (Hahn et al 2011) and can distort the abundance measurements as the quiet Sun is subject to a small FIP effect (Feldman & Widing 2003 c for a statistical 26 degrees of freedom, chi-squared distribution is less than 5%. That implies a 95% chance that there is significant quiet Sun contamination in the spatial bin.…”

Section: The Fe and Si Demmentioning

confidence: 99%

Relative Abundance Measurements in Plumes and Interplumes

Guennou

Hahn

Savin

2015

ApJ

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We present measurements of relative elemental abundances in plumes and interplumes. Plumes are bright, narrow structures in coronal holes that extend along open magnetic field lines far out into the corona. Previous work has found that in some coronal structures the abundances of elements with a low first ionization potential (FIP) <10 eV are enhanced relative to their photospheric abundances. This coronal-to-photospheric abundance ratio, commonly called the FIP bias, is typically 1 for elements with a high-FIP (>10 eV). We have used Extreme Ultraviolet Imaging Spectrometer observations made on 2007 March 13 and 14 over a ≈24 hr period to characterize abundance variations in plumes and interplumes. To assess their elemental composition, we used a differential emission measure analysis, which accounts for the thermal structure of the observed plasma. We used lines from ions of iron, silicon, and sulfur. From these we estimated the ratio of the iron and silicon FIP bias relative to that for sulfur. From the results, we have created FIP-bias-ratio maps. We find that the FIP-bias ratio is sometimes higher in plumes than in interplumes and that this enhancement can be time dependent. These results may help to identify whether plumes or interplumes contribute to the fast solar wind observed in situ and may also provide constraints on the formation and heating mechanisms of plumes.

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Differential Emission Measure Analysis of a Polar Coronal Hole During the Solar Minimum in 2007

Cited by 29 publications

References 26 publications

Inferring the Coronal Density Irregularity From Euv Spectra

Inferring the Coronal Density Irregularity From Euv Spectra

Monte Carlo Markov chain DEM reconstruction of isothermal plasmas

Relative Abundance Measurements in Plumes and Interplumes

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