On the continuum intensity distribution of the solar photosphere
For many years, there seemed to be significant differences between the
continuum intensity distributions derived from observations and
of the solar photosphere.
In order to settle the discussion on these apparent discrepancies, we
detailed comparison between simulations and seeing-free observations
that takes into account the crucial influence of instrumental image
We use a set of images of quiet Sun granulation taken in the blue,
green and red
continuum bands of the Broadband Filter Imager of the Solar Optical
The images are deconvolved with Point Spread Functions (PSF) that
account for non-ideal
contributions due to instrumental stray-light and imperfections.
In addition, synthetic intensity images are degraded with the
The results are compared with respect to spatial power spectra,
and the centre-to-limb variation of the intensity contrast.
The intensity distribution of SOT granulation images is broadest for
the blue continuum at
disc-centre and narrows towards the limb and for longer wavelengths.
The distributions are relatively symmetric close to the limb but
exhibit a growing
asymmetry towards disc-centre.
The intensity contrast, which is connected to the width of the
distribution, is found
to be (12.8 +/- 0.5) %, (8.3 +/- 0.4) %, and (6.2 +/- 0.2) %
at disc-centre for blue, green, and red continuum, respectively.
Removing the influence of the PSF unveils much broader intensity
with a secondary component that is otherwise only visible as
an asymmetry between the darker and brighter than average part of the
The contrast values increase to (26.7 +/- 1.3) %, (19.4 +/- 1.4) %,
and (16.6 +/- 0.7) %
for blue, green, and red continuum, respectively.
The power spectral density of the images exhibits a pronounced peak
at spatial scales characteristic for the granulation pattern and a
towards smaller scales.
The observational findings like the absolute values and centre-to-limb
of the intensity contrast, intensity histograms, and power spectral
are well matched with corresponding synthetic observables from
radiation (magneto-)hydrodynamic simulations.
We conclude that the intensity contrast of the solar continuum
intensity is higher
than usually derived from ground-based observations and is well
reproduced by modern
radiation (magneto-)hydrodynamic models.
Properly accounting for image degradation effects is of crucial
comparisons between observations and numerical models.
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