Applicability of Milne-Eddington inversions to high spatial resolution observations of the quiet Sun

Suárez, D. Orozco; Rubio, L. R. Bellot; Vögler, A.; Iniesta, J. C. del Toro

doi:10.1051/0004-6361/200913421

Cited by 27 publications

(11 citation statements)

References 51 publications

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“…Since we obtained similar distributions and trends with the two methods, in the following we only present results obtained for average magnetic-field strengths. Note that Orozco Suárez et al (2010) showed that Milne Eddington inversions statistically provide reliable estimates of magnetic-field properties at τ 500 = 0.1, therefore the magnetic-field strength values computed over B0 maps are good representations of results obtained from inversions.…”

Section: Mhd Simulations and Data Analysismentioning

confidence: 91%

The statistical distribution of the magnetic-field strength in G-band bright points

Criscuoli

Uitenbroek

2014

A&A

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Context. G-band bright points are small-sized features characterized by high photometric contrast. Theoretical investigations indicate that these features have associated magnetic-field strengths of 1 to 2 kG. Results from observations, however, lead to contradictory results, indicating magnetic fields of only kG strength in some and including hG strengths in others. Aims. To understand the differences between measurements reported in the literature, and to reconcile them with results from theory, we analyzed the distribution of the magnetic-field strength of G-band bright features identified in synthetic images of the solar photosphere and its sensitivity to observational and methodological effects. Methods. We investigated the dependence of magnetic-field strength distributions of G-band bright points identified in 3D magnetohydrodynamic simulations on feature selection method, data sampling, alignment, and spatial resolution. Results. The distribution of the magnetic-field strength of G-band bright features shows two peaks, one at about 1.5 kG and one below 1 hG. The former corresponds to magnetic features, the second mostly to bright granules. Peaks at several hG are obtained only on spatially degraded or misaligned data. Conclusions. Simulations show that magnetic G-band bright points have typically associated field strengths of a few kG. Field strengths in the hG range can result from observational effects, which explains the discrepancies presented in the literature. Our results also indicate that results from spectro-polarimetric inversions with an imposed unit filling-factor should be employed with great caution.

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Section: Mhd Simulations and Data Analysismentioning

confidence: 91%

The statistical distribution of the magnetic-field strength in G-band bright points

Criscuoli

Uitenbroek

2014

A&A

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“…1 show a map of the vertical field component at continuum optical depth τ 630 = 0.1 directly from the simulation (left panel) in comparison with the map of B LOS from the inversion of the synthetic Stokes profiles (right panel). Orozco Suárez et al (2010) showed that Milne-Eddington inversions fairly well represent the conditions in the atmospheric layers around the τ 630 = 0.1 level. The maps indicate that the morphology of the magnetic structures is well preserved in the maps of B LOS , albeit somewhat smeared owing to instrumental image degradation.…”

Section: Synthetic Observationsmentioning

confidence: 98%

On the relation between continuum brightness and magnetic field in solar active regions

Danilović

Röhrbein

Cameron

et al. 2013

A&A

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Context. Variations of solar irradiance are mainly determined by the changing coverage of the visible solar disk with magnetic flux concentrations. The relationship between brightness and field strength is an important ingredient for models and reconstructions of irradiance variations. Aims. We assess the effect of limited observational resolution on the relationship between brightness and magnetic field by comparing comprehensive MHD simulations with observational results. Methods. Simulations of magnetoconvection representing the near-surface layers of a plage region were used to determine maps of the continuum brightness and Stokes profiles for the Fe  line at 630.22 nm. After convolving with instrumental profiles, synthetic observations of the magnetic field were generated by applying a Stokes inversion code. We compare the resulting relation between brightness and apparent vertical magnetic field to the corresponding outcome derived from real observations of a plage region with the Hinode satellite. Results. Consideration of the image smearing effects due to the limited resolution of the observations transform the largely monotonic relation between brightness and field strength at the original resolution of the simulations into a profile with a maximum at intermediate field strength, which is in good agreement with the observations. Conclusions. Considering the effect of limited observational resolution renders the relation between brightness and magnetic field from comprehensive MHD simulations consistent with observational results. This is a necessary prerequisite for the utilization of simulations for models and reconstruction of solar irradiance variations.

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“…On the other hand, the simplifying assumptions of the ME approximation (no gradients of the parameters along the LOS) often raise questions about the validity of the results. Although this is a topic that is constantly under examination, it is generally accepted that the Milne-Eddington approximation gives fair estimates over the range of formation of the line used (Westendorp Plaza et al 1998) and this is not only the case at active regions, but also at finer magnetic concentrations, as has been demonstrated by Orozco Suárez et al (2010). In this study, a crosscomparison with MHD simulations has shown that the magnetic field flux for the Fe I lines closely matches the simulated flux at log τ = −1.…”

Section: Observationsmentioning

confidence: 95%

Hinode SOT/SP and SoHO/MDI quiet Sun magnetic field. Implications of their differences on the extrapolated chromospheric field and the height of the magnetic canopy

Kontogiannis

Tsiropoula

Tziotziou

2011

A&A

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Aims. We explore the differences in the measurement of the magnetic field of the quiet solar photosphere provided by the Michelson Doppler Imager (MDI) onboard SoHO and the SpectroPolarimeter (SOT/SP) onboard Hinode and the ensuing implications for the extrapolated chromospheric magnetic field and the determination of the location of the magnetic canopy. Methods. We employ potential field extrapolation to reconstruct the chromospheric magnetic field using the magnetic field of the photosphere provided by the two instruments. We also calculate the plasma-β parameter using the VAL C model atmosphere of the quiet Sun to determine the height of the magnetic canopy. Results. MDI underestimates the magnetic field of the quiet Sun sometimes by a factor of five, which leads to an overestimation of the height of the magnetic canopy by up to ∼550 km. Although the overall magnetic field configuration does not differ significantly when calculated with either MDI or SOT/SP, the data of the latter lead to lower and more extended canopies. The difference in the resolution of the two instruments does not seem to affect the chromospheric magnetic field higher than 1000 km. Conclusions. The height of the magnetic canopy is an important parameter to consider when investigating wave propagation and the oscillatory properties of the quiet Sun regions in the network and internetwork. The canopy height's derivation depends very much on the sensitivity of the instruments used to measure the photospheric magnetic field. Consequently precise measurements of the photospheric magnetic field are crucial to accurately reconstruct the chromospheric magnetic field and to distinguish between the various wave modes.

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Applicability of Milne-Eddington inversions to high spatial resolution observations of the quiet Sun

Cited by 27 publications

References 51 publications

The statistical distribution of the magnetic-field strength in G-band bright points

The statistical distribution of the magnetic-field strength in G-band bright points

On the relation between continuum brightness and magnetic field in solar active regions

Hinode SOT/SP and SoHO/MDI quiet Sun magnetic field. Implications of their differences on the extrapolated chromospheric field and the height of the magnetic canopy

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