On the continuum intensity distribution of the solar photosphere

Wedemeyer, Sven; Voort, L. Rouppe van der

doi:10.1051/0004-6361/200911983

Cited by 81 publications

(48 citation statements)

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“…This has been demonstrated repeatedly in the literature, for example, in studies looking at the limb darkening function (Pierce & Slaughter 1977;Neckel & Labs 1994), the intensity contrast of granulation (Sánchez Cuberes et al 2000;Danilovic et al 2008;Wedemeyer-Böhm & Rouppe van der Voort 2009;Afram et al 2011), and the brightness of small-scale magnetic concentrations (Title & Berger 1996;Schnerr & Spruit 2011) and sunspots (Albregtsen & Maltby 1981;Mathew et al 2007).…”

Section: Introductionmentioning

confidence: 57%

“…A quantitative comparison of the rms intensity contrast of the quiet Sun presented here for HMI and other measurements reported in the literature would require taking into account instrumental differences such as the spatial resolution and bandpass, which is beyond the scope of this study. Due to HMI's limited spatial resolution, the rms contrast, even after image restoration with K 2 , remains below the values returned from spaceborne and balloon-borne (i.e., similarly seeing-free) observatories at finer spatial resolutions, namely Hinode (Danilovic et al 2008;Mathew et al 2009;Wedemeyer-Böhm & Rouppe van der Voort 2009) and SUNRISE (Hirzberger et al 2010). (Note though, that the divergence is also due in part to the different bandpass of the various instruments.…”

Section: Granulation Contrastmentioning

confidence: 97%

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Point spread function of SDO/HMI and the effects of stray light correction on the apparent properties of solar surface phenomena

Yeo

Feller

Solanki

et al. 2013

A&A

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Aims. We present a point spread function (PSF) for the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) and discuss the effects of its removal on the apparent properties of solar surface phenomena in HMI data. Methods. The PSF was retrieved from observations of Venus in transit by matching it to the convolution of a model of the Venusian disc and solar background with a guess PSF. We described the PSF as the sum of five Gaussian functions, the amplitudes of which vary sinusoidally with azimuth. This relatively complex functional form was required by the data. Observations recorded near in time to the transit of Venus were corrected for instrumental scattered light by the deconvolution with the PSF. We also examined the variation in the shape of the solar aureole in daily data, as an indication of PSF changes over time.Results. Granulation contrast in restored HMI data is greatly enhanced relative to the original data and exhibit reasonable agreement with numerical simulations. Image restoration enhanced the apparent intensity and pixel averaged magnetic field strength of photospheric magnetic features significantly. For small-scale magnetic features, restoration enhanced intensity contrast in the continuum and core of the Fe I 6173 Å line by a factor of 1.3, and the magnetogram signal by a factor of 1.7. For sunspots and pores, the enhancement varied strongly within and between features, being more acute for smaller features. Magnetic features are also rendered smaller, as signal smeared onto the surrounding quiet Sun is recovered. Image restoration increased the apparent amount of magnetic flux above the noise floor by a factor of about 1.2, most of the gain coming from the quiet Sun. Line-of-sight velocity due to granulation and supergranulation is enhanced by a factor of 1.4 to 2.1, depending on position on the solar disc. The shape of the solar aureole varied, with time and between the two CCDs. There are also indications that the PSF varies across the FOV. However, all these variations were found to be relatively small, such that a single PSF can be applied to HMI data from both CCDs, over the period examined without introducing significant error. Conclusions. Restoring HMI observations with the PSF presented here returns a reasonable estimate of the stray light-free intensity contrast. Image restoration affects the measured radiant, magnetic and dynamic properties of solar surface phenomena sufficiently to significantly impact interpretation.

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Section: Introductionmentioning

confidence: 57%

Section: Granulation Contrastmentioning

confidence: 97%

Point spread function of SDO/HMI and the effects of stray light correction on the apparent properties of solar surface phenomena

Yeo

Feller

Solanki

et al. 2013

A&A

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“…Magnetohydrodynamical simulations with a horizontal cell size of 20 km or larger are widely used in the literature (Afram et al 2010(Afram et al , 2011Hirzberger et al 2010;Orozco Suárez 2008;Röhrbein et al 2011;Wedemeyer-Böhm & Rouppe van der Voort 2009). We found a reduction of the effective BP diameter of 20% and a near doubling of the BP number density by doubling the horizontal grid resolution.…”

Section: Discussionmentioning

confidence: 99%

Comparison of solar photospheric bright points between Sunrise observations and MHD simulations

Riethmüller

Solanki

Berdyugina

et al. 2014

A&A

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Bright points (BPs) in the solar photosphere are thought to be the radiative signatures (small-scale brightness enhancements) of magnetic elements described by slender flux tubes or sheets located in the darker intergranular lanes in the solar photosphere. They contribute to the ultraviolet (UV) flux variations over the solar cycle and hence may play a role in influencing the Earth's climate. Here we aim to obtain a better insight into their properties by combining high-resolution UV and spectro-polarimetric observations of BPs by the Sunrise Observatory with 3D compressible radiation magnetohydrodynamical (MHD) simulations. To this end, full spectral line syntheses are performed with the MHD data and a careful degradation is applied to take into account all relevant instrumental effects of the observations. In a first step it is demonstrated that the selected MHD simulations reproduce the measured distributions of intensity at multiple wavelengths, line-of-sight velocity, spectral line width, and polarization degree rather well. The simulated line width also displays the correct mean, but a scatter that is too small. In the second step, the properties of observed BPs are compared with synthetic ones. Again, these are found to match relatively well, except that the observations display a tail of large BPs with strong polarization signals (most likely network elements) not found in the simulations, possibly due to the small size of the simulation box. The higher spatial resolution of the simulations has a significant effect, leading to smaller and more numerous BPs. The observation that most BPs are weakly polarized is explained mainly by the spatial degradation, the stray light contamination, and the temperature sensitivity of the Fe i line at 5250.2 Å. Finally, given that the MHD simulations are highly consistent with the observations, we used the simulations to explore the properties of BPs further. The Stokes V asymmetries increase with the distance to the center of the mean BP in both observations and simulations, consistent with the classical picture of a production of the asymmetry in the canopy. This is the first time that this has been found also in the internetwork. More or less vertical kilogauss magnetic fields are found for 98% of the synthetic BPs underlining that basically every BP is associated with kilogauss fields. At the continuum formation height, the simulated BPs are on average 190 K hotter than the mean quiet Sun, the mean BP field strength is found to be 1750 G, and the mean inclination is 17 • , supporting the physical flux-tube paradigm to describe BPs. On average, the synthetic BPs harbor downflows increasing with depth. The origin of these downflows is not yet understood very well and needs further investigation.

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“…Danilovic et al 2008;Wedemeyer-Böhm & Rouppe van der Voort 2009;Pereira et al 2009c;Hirzberger et al 2010;Afram et al 2011). We used a kernel with two components, i.e.…”

Section: Spatial Degradationunclassified

“…Asplund et al 2000;Wedemeyer-Böhm & Rouppe van der Voort 2009;Asplund et al 2009;Afram et al 2011), as well as in terms of the match between average synthetic and observed profiles (e.g. Asplund et al 2000).…”

Section: Introductionmentioning

confidence: 98%

Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations

Beck

Fabbian

Moreno‐Insertis

et al. 2013

A&A

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Context. Numerical three-dimensional (3D) radiative (magneto-)hydrodynamical [(M)HD] simulations of solar convection are nowadays used to understand the physical properties of the solar photosphere and convective envelope, and, in particular, to determine the Sun's photospheric chemical abundances. To validate this approach, it is important to check that no excessive thermodynamic fluctuations arise as a consequence of the partially incomplete treatment of radiative transfer causing radiative damping that is too modest. Aims. We investigate the realism of the thermodynamics in recent state-of-the-art 3D convection simulations of the solar atmosphere carried out with the Stagger code.Methods. We compared the characteristic properties of several Fe i lines (557.6 nm, 630 nm, 1565 nm) and one Si i line at 1082.7 nm in solar disc-centre observations of different spatial resolution with spectra synthesized from 3D convection simulations. The observations were taken with ground-based (Echelle spectrograph, Göttingen Fabry-Pérot Interferometer (GFPI), POlarimetric LIttrow Spectrograph, Tenerife Infrared Polarimeter, all at the Vacuum Tower Telescope on Tenerife) and space-based instruments (Hinode/Spectropolarimeter). We degraded the synthetic spectra to the spatial resolution of the observations, based on the distribution of the continuum intensity I c . We estimated the spectral degradation to be applied to the simulation results by comparing atlas spectra with averaged observed spectra. In addition to deriving a set of line parameters directly from the intensity profiles, we used the SIR (Stokes Inversion based on Response functions) code to invert the spectra. Results. The spatial degradation kernels yield a similar generic spatial stray-light contamination of about 30% for all instruments. The spectral stray light inside the different spectrometers is found to be between 2% and 20%. Most of the line parameters from the observational data are matched by the degraded HD simulation spectra. The inversions predict a macroturbulent velocity v mac below 10 m s −1 for the HD simulation spectra at full spatial resolution, whereas they yield v mac 1000 m s −1 at a spatial resolution of 0. 3. The temperature fluctuations in the inversion of the degraded HD simulation spectra do not exceed those from the observational data (of the order of 100−200 K rms for −2 log τ 500 nm −0.5). The comparison of line parameters in spatially averaged profiles with the averaged values of line parameters in spatially resolved profiles indicates a significant change in (average) line properties on a spatial scale between 0. 13 and 0. 3. Conclusions. Up to a spatial resolution of 0. 3 (GFPI spectra), we find no indications of excessive thermodynamic fluctuations in the 3D HD simulation. To definitely confirm that simulations without spatial degradation contain fully realistic thermodynamic fluctuations requires observations at even higher spatial resolution (i.e. <0. 13).

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On the continuum intensity distribution of the solar photosphere

Cited by 81 publications

References 50 publications

Point spread function of SDO/HMI and the effects of stray light correction on the apparent properties of solar surface phenomena

Point spread function of SDO/HMI and the effects of stray light correction on the apparent properties of solar surface phenomena

Comparison of solar photospheric bright points between Sunrise observations and MHD simulations

Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations

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