Bibcode
DOI
Sánchez Cuberes, M.; Bonet, J. A.; Vázquez, M.; Wittmann, A. D.
Bibliographical reference
The Astrophysical Journal, Volume 538, Issue 2, pp. 940-959.
Advertised on:
8
2000
Journal
Citations
32
Refereed citations
29
Description
We have measured the center-to-limb variation (CLV) of parameters
describing geometric and photometric statistical properties of the solar
granulation at 6708 Å. This work is based on an excellent series
of white-light images obtained with the Swedish Vacuum Solar Telescope
at Roque de los Muchachos Observatory, La Palma, during the partial
solar eclipse of 1994 May 10. The lunar limb profile, which is visible
in each frame, was used as a calibration tool for estimating the
point-spread function of the combined optical system formed by the
atmosphere and the telescope. Before restoration, noise was removed from
the images by a novel application of the so-called optimum filter for
two-dimensional objects. The latter was optimized in terms of rms error
and was constructed from very precise smoothed models of the specific
power spectrum of the granulation at each position on the solar disk.
The determination of the positions on the solar disk was achieved with
high accuracy by matching the position of the Moon's limb in our images
to a numerical simulation of the eclipse geometry. The CLV curve of the
ΔIrms granular contrast shows one of the steepest
gradients among those reported in the literature and quite a high value
(9.6%) at the disk center considering that our working wavelength is in
the far-red range of the solar spectrum. The elliptical shape of the
restored power spectra with ellipticities equal to those expected just
from foreshortening proves that radiative transfer effects do not alter
the isotropy of the horizontal intensity pattern of the solar
granulation, at least up to μ=0.4. The mean wavenumber, k, derived
from the two-dimensional power spectra azimuthally integrated along the
ellipses amounts to a value of 6.15 Mm-1 at the center of the
solar disk and then shows a decrease toward the limb. Apart from the
power spectra analysis, a direct statistical study of the granulation
size and brightness, based on the image segmentation for defining
granular contours, has also been performed. A general increase in both
granular and intergranular areas is found as we move toward the solar
limb. The mean granular cell area varies from 1.36 Mm2 at
μ=1 up to 2.06 Mm2 at μ=0.6, and in parallel, the
granular filling factor (the percentage of area of the image covered by
granules) decreases from 44.2% to 42.8%. In the small area range, the
granular brightness increases linearly with the granular cell size and
is preserved constant, on average, for granular cells larger than ~2.0".
No slope variation is found for the intergranular intensities versus
granular cell areas. Observations close to the solar limb detect
granular structures as small as 0.53" or even smaller up to a distance
of at least ~0.5" from the limb, showing that the ΔT associated
with the granulation persist at least until z~200 km. However, this
penetration could be different for small and large granules because we
find several hints indicating the progressive disappearance of small
structures toward the limb.