J/A+A/705/A116 HeI 10830Å triplet 6enter-to-limb variations (Pietrow+, 2026)
Center-to-limb variations of the He I 10830 Å triplet.
Pietrow A.G.M., Kuckein C., Verma M., Denker C., Trelles Arjona J.C.,
Kamlah R., Poppenhaeger K.
<Astron. Astrophys. 705, A116 (2026)>
=2026A&A...705A.116P 2026A&A...705A.116P (SIMBAD/NED BibCode)
ADC_Keywords: Atomic physics ; Sun ; Spectroscopy
Keywords: atomic data - radiative transfer - techniques: spectroscopic -
Sun: abundances - Sun: chromosphere
Abstract:
We present high-resolution spectroscopic observations of the quiet-Sun
center-to-limb variations (CLV) of the HeI triplet at 10830Å
and the nearby SiI 10827Å line, observed with GREGOR Infrared
Spectrograph (GRIS) and the improved High-resolution Fast Imager
(HiFI+). The observations cover the interval µ=[0.1, 1.0], where
µ is the cosine of the heliocentric angle. At each µ-position,
the spectra are spatially averaged over 0.02µ, and the resulting
CLVs are given both as these averaged data points and as smooth
polynomial curves fitted across each wavelength point. The HeI spectra
were inverted using the HAnle and ZEeman Light (HAZEL) code, showing
an increase in optical depth towards the limb and a reversed
convective blueshift for the red component, while the blue component
was entirely absent. In addition, we find a strong increase in the
steepness of the HeI CLV compared to that of the nearby continuum. The
SiI showed a behavior more typical of photospheric lines, namely
shallower CLV, a reduction in width and depth, and a more typical
convective blueshift.
Description:
These data provide quiet-Sun center-to-limb variation (CLV) spectra of
the HeI 10830Å triplet and the nearby SiI 10827Å line, observed
with the GREGOR Infrared Spectrograph (GRIS) at the 1.5m GREGOR solar
telescope. The spectra cover the wavelength range
10818.50-10833.31Å, with a spectral sampling of 14.678mÅ per
pixel, and an effective resolution of about R∼120000 after the final
smoothing described in the paper.
The observations span µ=[0.1, 1.0], where µ is the cosine of
the heliocentric angle. For each µ, spectra are spatially averaged
in bins of width Δµ=0.02, following the procedure described
in Sect. 2 of the paper. The continuum intensities are calibrated by
matching the continuum near 10824Å to the limb-darkening atlas of
Neckel (1994, Sol. Phys., 153, 91). The resulting CLV is available both
as binned spectra and as smoothed, polynomial-fitted spectra.
Two FITS files are provided:
1) clv_data.fits
CLV spectra as a function of µ and wavelength, and the polynomial
fits to these spectra, with the associated µ and wavelength axes.
FITS structure:
Primary HDU : mean_data (2-D image)
Extension 'FITTED' : fit_data (2-D image)
Extension 'MU' : mu (1-D image)
Extension 'WAVELENGTH': wav (1-D image)
HDU 0 (Primary): mean_data
--------------------------
Contents: Mean observed CLV spectra as a function of µ and wavelength.
Array type: 2-D image, with shape [Nmu, Nlam]
Axis convention:
Axis 0 (row index): µ index (0 .. Nmu-1)
Axis 1 (column index): wavelength index (0 .. Nlam-1)
Meaning: Each row contains the spatially averaged Stokes I spectrum for
one µ bin of width Δµ = 0.02, as described in Sect. 2.1
(CLV curve generation).
Units: Intensity values are on the same scale as in the paper, i.e.
scaled so that the continuum at 10823-10824 Å matches the Neckel
(1994, Sol. Phys., 153, 91) continuum limb-darkening curve at each µ.
See Sect. 2.1 of the paper for details.
HDU 1 (ImageHDU, EXTNAME = 'FITTED'): fit_data
----------------------------------------------
Contents: Polynomial-fitted CLV spectra corresponding to mean_data.
Array type: 2-D image, shape [Nmu, Nlam]
Axis convention: Same as HDU 0.
Meaning: For each wavelength point, a fourth-order polynomial in µ is
fitted to the mean_data CLV following the Neckel (1994, Sol. Phys.,
153, 91) approach. This HDU stores the resulting smooth CLV curves that
reduce small-scale intensity fluctuations caused by activity or
instrumental effects.
Units: Same intensity scale as HDU 0.
HDU 2 (ImageHDU, EXTNAME = 'MU'): mu
------------------------------------
Contents: µ-values (cosine of the heliocentric angle) for each CLV bin.
Array type: 1-D image, length Nmu
Meaning: µ[i] is the representative µ of row i in mean_data and
fit_data. The µ range is approximately from 0.1 to 1.0, with a bin
width of Δµ = 0.02, as described in Sect. 2.1.
Units: Dimensionless.
HDU 3 (ImageHDU, EXTNAME = 'WAVELENGTH'): wav
---------------------------------------------
Contents: Wavelength array corresponding to the spectral dimension.
Array type: 1-D image, length Nlam
Meaning: wav[j] gives the wavelength of column j in mean_data and fit_data.
Units: Angstroem (Å), covering the GRIS spectral window from
10818.50Å to 10833.31Å, see Sect. 2
(Observations and data processing).
2) clv_r.fits
Radially binned, mean spectra as a function of projected radius on
the solar disc, together with the corresponding radial coordinate.
FITS structure:
Primary HDU : meanspectraby_r (2-D image)
Extension 'r' : r_centers (1-D image)
HDU 0 (Primary): meanspectraby_r
------------------------------------------
Contents: Mean spectra binned in projected radius r on the solar disc.
Array type: 2-D image, with shape [Nr, Nlam]
Axis convention:
Axis 0 (row index): radial bin index (0 .. Nr-1)
Axis 1 (column index): wavelength index (0 .. Nlam-1)
Meaning: Each row contains a spatially averaged spectrum for one radial
bin. The radial coordinate is related to µ as described in the
paper, with the CLV also displayed as a function of radius
(see Fig. 1 and Fig. 2).
Units: Intensity scale is the same as for clv_data.fits, i.e. calibrated
to match the Neckel (1994, Sol. Phys., 153, 91) continuum limb-darkening
curve at the continuum reference near 10823-10824 Å.
HDU 1 (ImageHDU, EXTNAME = 'r'): r_centers
--------------------------------------------------
Contents: Radial coordinate of each bin corresponding to the rows of HDU 0.
Array type: 1-D image, length Nr
Meaning: r_centers[i] is the projected radial coordinate of the i-th bin
used to compute the mean spectrum in row i of the primary HDU.
The exact definition and units of r follow the description in the paper
(radius on the solar disc, as used in the CLV plots).
Units: Same units as used for the radial CLV in the paper (see Sect. 2.1
and Figs. 1-2). Users should refer to the paper for the precise
geometric mapping between r and µ.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 156 2 List of fits files, center-to-limb variation
(CLV) data
fits/* . 2 Individual fits files
--------------------------------------------------------------------------------
Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1 I1 --- HDU Number of HDU in the file (1)
3- 5 I3 Kibyte Size Size of FITS file
7- 19 A13 --- FileName Name of FITS file, in subdirectory fits
21-156 A136 --- Title Title of the FITS file
--------------------------------------------------------------------------------
Note (1): For clv_data.fits:
Primary HDU : mean_data (2-D image)
Extension 'FITTED': fit_data (2-D image)
Extension 'MU' : mu (1-D image)
Extension 'WAVELENGTH': wav (1-D image)
For clv_r.fits:
Primary HDU : meanspectraby_r (2-D image)
Extension 'r' : r_centers (1-D image)
--------------------------------------------------------------------------------
Acknowledgements:
From Alexander Pietrow, apietrow(at)aip.de
Usage and citation
Users of these data products are requested to cite the paper:
Pietrow, A.G.M., Kuckein, C., Verma, M., Denker, C., Trelles Arjona,
J.C., Kamlah, R., Poppenhager, K.,
"On the center-to-limb variations of the He I 10830 Å triplet",
Astronomy & Astrophysics, (2025, 705, A116).
and to include the standard acknowledgement to CDS/VizieR, where
appropriate, when using these data.
(End) Patricia Vannier [CDS] 20-Nov-2025