Abstract. The center-to-limb variation (CLV) in the contrast of photospheric faculae is of importance both to the physics of magnetic flux tubes and to the understanding of variations in the total solar irradiance. Measurements of the CLV of faculae are difficult and have resulted in widely disparate results. We tried an accurate measurement of the photospheric facular contrast by using both PSPT and SOHO/MDI observations, with the aim of understanding the reasons of the different results of facular photospheric contrast measurements already presented in literature

I Ermolli

M Centrone

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Mem. S.A.It. Vol. 74, 671
c© SAIt 2003 Memorie della
The center to limb variation
of photospheric facular contrast
M. Centrone1 and I. Ermolli1
INAF Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio
Catone, Italia
e-mail: name@mporzio.astro.it
Abstract. The center-to-limb variation (CLV) in the contrast of photospheric fac-
ulae is of importance both to the physics of magnetic flux tubes and to the un-
derstanding of variations in the total solar irradiance. Measurements of the CLV
of faculae are difficult and have resulted in widely disparate results. We tried an
accurate measurement of the photospheric facular contrast by using both PSPT
and SOHO/MDI observations, with the aim of understanding the reasons of the
different results of facular photospheric contrast measurements already presented
in literature.
Key words. Solar Atmosphere – Solar variability
1. Introduction
Solar irradiance variations on scales of days
up to the solar activity cycle length are
closely related to the evolution of the solar
surface magnetic field, because the emer-
gence and the evolution of active regions
on the solar surface is reflected in the irra-
diance records. Sunspots and active regions
faculae are considered to be the dominant
contributors to solar irradiance changes on
time scales of days to weeks while it is ex-
pected that small scale magnetic elements
that compose the network contribute to the
observed irradiance increase during activ-
ity maximum. Uncertainties in the contrast
of faculae and the network are yet one of
Send offprint requests to: I. Ermolli
Correspondence to: INAF OAR, Via Frascati
33, 00040 Monte Porzio Catone
the major sources of errors in the modeling
of solar irradiance variations. Incidentally,
because of these errors it is still debated
whether other mechanisms of non-magnetic
origin like temporal changes in latitude de-
pendent surface temperature of the Sun,
may contribute significantly to the mea-
sured irradiance variations.
Measurements of the CLV of faculae are dif-
ficult and have resulted in widely disparate
results. Some measurements have been re-
stricted to near the limb, while others have
been near disk center. In some cases, at-
tempts have been made to study facular
contrast across the solar disk at moderate
resolution or at fairly low spatial resolution.
The result of these investigations has not
given a clear picture of the CLV of the fac-
ulae. Part of the problem is the random-
ness in the appearance of facular patches
672 M. Centrone and I. Ermolli: Center to limb variation of facular contrast
which causes changes in the filling factor,
i.e. the density of flux tubes within a fac-
ular patch. The different spatial resolution
between the different investigations is also
of importance. Another problem is the very
low contrast of faculae as one approaches
the center of the solar disk making identi-
fication very difficult. Facular contrast has
been reported with negative, zero or posi-
tive values near disk center depending on
the magnetic flux of the flux tubes. The
change in aspect as one views faculae at
the different positions leads to difficulties
in interpretation of contrast observations.
As a result of these difficulties, there are
a variety of physical models of facular flux
tubes and collections of flux tubes. For a re-
view of the experimental results see Ahern
and Chapman (2000), while for a review
of physical models of facular flux tubes see
Steiner (1994).
Since uncertainties in the CLV of faculae
at different wavelengths makes modeling of
irradiance variations due to faculae a diffi-
cult task (Solanki and Unruh 1998, Harvey
and White 1999, Unruh Solanki and Fligge
1999), we tried an accurate measurement of
the facular contrast with the aim of under-
standing the reasons of the different results
already presented in literature.
2. Observations and data analysis
In order to determine the CLV of both the
facular and network contrast and to study
the dependence of the CLVs on the solar
activity, feature geometry and feature iden-
tification criteria, we analyzed near simul-
taneous full-disk images obtained at three
pass-band and the line-of-sight magnetic
field measurements provided respectively
by the PSPT network and MDI on board
of SOHO. In particular, we analyzed 591
1kx1k images extracted from the archive of
the daily observations carried out with the
PSPT a the Rome Observatory (Ermolli et
al. 1997, Centrone et al. 2001) during the
latest four Summer seasons (from 1999 up
to 2002) on 197 observing days. The im-
ages analyzed correspond to the observa-
tions acquired each day in sequence in less
than two minutes, at the three band-pass
centered at CaIIK (393.3±0.25nm), Blue
continuum (409.2±0.25nm) and Red con-
tinuum (607.1±0.5nm). The sample of im-
ages analyzed shows active regions spread
over almost all µ values. They are also sep-
arated in time so duplications are avoided.
All the images analyzed have been cali-
brated for the instrumental effects (i.e. dark
and flat-field corrections) and corrected for
the center-to-limb variation of the quiet
Sun. The latter correction was done com-
puting the average radial profile of the in-
tensity over the solar disk, excluding active
regions from this computation through an
intensity threshold criterion. Together to
the PSPT images, we analyzed also mag-
netograms obtained by MDI on board of
SOHO during Summer 2002, acquired near
simultaneously to the PSPT images. All the
images were first resized in order to get the
same solar disk size, then have been rotated
to co-align them and to compare images
pixel by pixel. Care has been taken to use
images as close in time each other as pos-
sible. In all cases but one, the images an-
alyzed were recorded within 10 minutes of
each other. The facular regions were iden-
tified on the sample of images analyzed by
applying three different automated meth-
ods, based on intensity threshold of con-
tiguous bright regions to pairs of images,
like described in detail in the following.
3. The identification of facular
regions
We use both the structural and the photo-
metric properties of facular regions to per-
form their identification on the solar disk.
The first identification method applied is
very often presented in literature, i.e. use
CaII K images to identify facular regions
on corresponding continuum images with
results largely dependent on the intensity
threshold used. In fact, first we identify
isolated long-lived bright regions on the
disk (step 1), finding pixel whose contrast
is greater a given value on a spatially fil-
M. Centrone and I. Ermolli: Center to limb variation of facular contrast 673
tered binary image, that is obtained select-
ing all the bright pixel in the CaII K image.
We then applied a straightforward intensity
threshold on the pre-identified connected
bright regions, to point out non redundant
chromospheric regions to be used to pick
up the corresponding photospheric facu-
lar regions. Note that photospheric faculae
are obtained excluding from the contrast
computation all the pixels corresponding to
photospheric spots and pores, like selected
in the continuum images analyzed, by ex-
cluding all connected pixels with negative
contrast. The second method applied, not
so often presented in literature with respect
to the previous, uses pairs of correspond-
ing magnetic and intensity images to select
facular region by threshold of magnetic flux
values on the solar disk. In particular, we
applied a threshold criterion of magnetic
flux values on the long-lived bright regions
on the disk isolated at step 1. At last, we
tried the identification of photospheric fac-
ular regions through straight photometric
properties of the photospheric facular re-
gions. In particular, we computed a color
image by the difference of the two contin-
uum images (Blue minus Red), in order to
isolate the magnetic regions by differential
photometry, like proposed by Foukal and
Duvall (1985). The difference signal of con-
tinuum images is sensitive to changes in the
temperature gradient near τ5000 = 1 be-
tween quiet photosphere and faculae. We
identified the photospheric facular regions
by thresholding the color image on the pre-
selected bright long lived regions isolated
at step 1.
Using the three identification methods we
make three masks of facular regions for
each day, that are used to compute the fac-
ular contrast dependence over the disk.
The contrast of the facular regions identi-
fied is defined, for each heliocentric angle,
from the ratio between the intensity of the
facular pixels belonging to the angle, for
heliocentric annuli of equal area, and the
value of the quiet sun intensity.
Fig. 1. Center to limb variation of the fac-
ular contrast: first identification method
(CaIIK intensity).
Fig. 2. Center to limb variation of the fac-
ular contrast: second identification method
(magnetic flux).
Fig. 3. Center to limb variation of the fac-
ular contrast: third identification method
(differential photometry).
4. Results
We have analyzed the facular contrast de-
pendence on the disk position, on the ac-
tivity level and on the automated method
used to identify the facular regions over the
solar disk. The simultaneous use of three
different methods to identify facular re-
gions allows us to explain some of the dif-
ferences among the results of the measure-
ments of the CLV of the facular contrast
already presented in literature. Figures 1,
2, 3 show the facular contrast dependence
on the disk position by the three identifi-
cation methods. The contrast of the facu-
lar regions identified at the disk center in
the images acquired with the two PSPT
continuum pass-bands are positives, in ac-
cordance with the results previously ob-
674 M. Centrone and I. Ermolli: Center to limb variation of facular contrast
tained by authors that do not used identi-
fications methods based on magnetic field
flux thresholds or dynamic intensity thresh-
olds. A negative value for the contrast of
facular regions at the disk center is found
without excluding from the analyzed re-
gions small spots or pores.
The CLV of the facular regions at the disk
edge does not change with the magnetic
activity. In particular, It is still increasing
up to µ = 0.2. The contrast value close to
the disk edge depends sensitively from the
identification method used to select facu-
lar regions. In particular, the use of a dy-
namical threshold criterion, changing with
the position on the solar disk (i.e. a con-
stant threshold on the magnetic flux value
or an intensity threshold changing with µ)
produces an increase of the mean contrast
computed, up to a factor two.
5. Discussion
Comparison with other contrast observa-
tions is not easy because of the differences
in selected wavelength, spatial resolution,
range of studied heliocentric angles,
magnetic filling factor and size of the
analyzed features. All these factors con-
tribute to the scatter between the exist-
ing contrast measurements. Anyway, by the
present work, we can assert the contrast
dependence over the disk depends sensibly
from the identification method used to de-
velop one’s own study.
References
Ahern, S., & Chapman, G.A. 2000, Sol.
Phys. 191, 71
Centrone, M., Ermolli, I., Fofi, M., Torelli,
M. 2001, Mem. SAIt. 72, 673
Ermolli, I., Fofi, M. et al. 1998, Sol. Phys.
177, 1
Foukal, P., & Duvall, T. Jr. 1985, ApJ 296,
739
Harvey, K.L., & White, O.R. 1999, ApJ
515, 812
Solanki, S.K., & Unruh, Y.C. 1998, A&A
329, 747
Steiner, O. 1994, IAU Symp. 154, 407
Unruh, Y.C. Solanki, S.K., & Fligge, M.
1999, A&A 345, 635


The center to limb variation of photospheric facular contrast

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The center to limb variation of photospheric facular contrast

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