J/A+A/712/A151 CLV measurements of solar active region features (Pietrow+, 2026)
Center-to-limb variations of solar active regions.
Observations of spots, faculae, and network in the 6173 Å continuum.
Pietrow A.G.M., Sumra S., Petit dit de la Roche D.J.M., Lossnitz E.J.,
Denker C., Abmus M.
<Astron. Astrophys. 712, A151 (2026)>
=2026A&A...712A.151P 2026A&A...712A.151P (SIMBAD/NED BibCode)
ADC_Keywords: Sun ; Magnetic fields ; Spectrophotometry ; Optical
Keywords: techniques: photometric - Sun: activity - Sun: photosphere -
sunspots - stars: activity
Abstract:
Accurate modeling of stellar active regions (ARs) remains a major
bottleneck for radial-velocity and transmission-spectroscopy studies
aimed at finding Earth-like planets. While much effort has been
devoted to AR modeling, their center-to-limb variations (CLV) have
been largely overlooked. We take a step toward remedying this by
measuring the CLV of the 6173Å continuum intensity for sunspots
(the whole spot, and separate umbrae and penumbrae), faculae, network,
and the quiet Sun using the Helioseismic and Magnetic Imager (HMI)
onboard the Solar Dynamics Observatory (SDO). This study is based on
four simple round α-sunspots and their surroundings, as well as
one strongly evolving region. After correcting for stray light, we
find that relative to the quiet Sun, all components except for the
umbra display reduced darkening towards the limb. Additionally,
strongly evolving active regions do not appear to display
significantly altered CLV profiles compared to stable active regions.
Faculae and network show contrast enhancements that peak near
µ~=0.3 before declining toward the limb, reaching maxima of
approximately 4% and 2% respectively in contrast excess relative to
the quiet Sun, while the spot-to-quiet-Sun contrast rises to
approximately 15% near the limb. For both types of AR, this change in
CLV behavior near the limb is likely related to the three-dimensional
structure of the active regions and the rapidly changing viewing
geometry. This behavior is not captured by synthetic CLVs based on
PHOENIX and ATLAS model atmospheres with solar values and a different
effective temperature, underscoring the need for more realistic
treatments of stellar activity.
Description:
Center-to-limb variation (CLV) measurements for five solar active
regions observed with the Solar Dynamics Observatory (SDO). Intensity
measurements are made in HMI continuum at 6173 Angstrom for five NOAA
active regions: 12519, 12526, 12670, 12673, and 12738. Six solar
feature types are distinguished: Umbra, Penumbra, and the full sunspot
(classified from HMI), and Quiet Sun, Plage, and Network (classified
from AIA, measured in HMI continuum). The dataset provides (1) raw
pixel-level scatter measurements of intensity vs. heliocentric angle
cosine mu=cos(theta) per active region, (2) polynomial fit curves to
each feature's CLV profile, and (3) mean CLV curves averaged over the
four alpha-type active regions. Active region NOAA 12673 (beta-type)
is excluded from the average curves.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
sca12519.dat 34 7175915 Scatter data of the active region NOAA 12519
alpha-type
sca12526.dat 34 7123597 Scatter data of the active region NOAA 12526
alpha-type
sca12670.dat 34 7288310 Scatter data of the active region NOAA 12670
alpha-type
sca12673.dat 34 7777802 Scatter data of the active region NOAA 12738
alpha-type
sca12738.dat 34 7234675 Scatter data of the active region NOAA 12673
beta-type
polyfits.dat 44 5430 Polynomial fits on a mu grid from 0.1 to 1.0
for all 5 ARs
averaged.dat 47 1086 Intensities averaged over only alpha-type AR
which corresponds to figure 6 Mean CLV intensity
variations averaged over the four α-spots
of section Results and discussion
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Seealso:
J/A+A/705/A116 : HeI 10830Å triplet 6enter-to-limb variations
(Pietrow+, 2026)
J/A+A/692/A83 : Transmission spectrum of WASP-69b
(Petit dit de la Roche+ 2024)
J/A+A/671/A130 : CLV curves of 10 spectral lines taken with SST
(Pietrow+, 2023)
J/MNRAS/527/2940 : Solar activity components intensity profiles
(Cretignier+ 2024)
J/ApJS/255/16 : Quiet-Sun MgII h & k line profiles from IRIS (Gunar+, 2021)
Byte-by-byte Description of file: sca*.dat
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Bytes Format Units Label Explanations
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1- 8 A8 --- Feature Solar feature center-to-limb-variations CLV of
active regions AR polynomial coefficients type
as Full, Network, Penumbra, Plage, Quiet and
Umbra (FEATURE) (G1)
10- 21 E12.7 --- mu Heliocentric angle cosine mu = cos(theta) from
0.1 to 1.0 (MU)
23- 34 E12.7 --- I HMI continuum intensity as raw counts and not
normalised, see POLYFITS/AVGCURVE for
normalised values (INTENSITY)
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Byte-by-byte Description of file: polyfits.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 A9 --- AR Active region name as
NOAA12519,12526,12670,12738,12673 (AR_NAME)
11- 18 A8 --- Feature Solar feature type 905 occurence times for
each Full, Network, Penumbra, Plage, Quiet and
Umbra (FEATURE) (G1)
20- 31 E12.7 --- mu Heliocentric angle cosine mu = cos(theta) from
0.1 to 1.0 (MU)
33- 44 E12.7 --- I/I1 Polynomial CLV fit, normalised to disk centre
(I/Imu=1) corresponds to figure fitted CLV
intensity variations for five ARs of the
section Results and discussion (INTENSITY_FIT)
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Byte-by-byte Description of file: averaged.dat
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Bytes Format Units Label Explanations
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1- 8 A8 --- Feature Feature type 181 occurence times for each
Full, Network, Penumbra, Plage, Quiet and
Umbra (FEATURE) (G1)
10- 21 E12.7 --- mu Heliocentric angle cosine mu = cos(theta)
from 0.1 to 1.0 (MU)
23- 34 E12.7 --- Imean/I1 Mean CLV intensity normalised (I/I1)
(MEAN_INTENSITY)
36- 47 E12.7 --- e_Imean/I1 Standard deviation CLV intensity normalised
(STD_INTENSITY)
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Global notes:
Note (G1): Feature classifications are as follows:
Umbra = Sunspot umbra (segmented from HMI continuum)
Penumbra = Sunspot penumbra (segmented from HMI continuum)
Full = Full sunspot, umbra+penumbra combined (HMI)
Quiet = Quiet Sun (segmented from AIA; intensity measured in HMI)
Plage = Plage region (segmented from AIA; intensity measured in HMI)
Network = Network region (segmented from AIA; intensity measured in HMI)
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Acknowledgements:
A.G.M. Pietrow, apietrow(at)aip.de
License: CC-BY-4.0
(End) Luc Trabelsi [CDS] 01-Jul-2026