Observational insights into Sr I 4607 Å scattering polarization with DKIST/ViSP

Abstract

Scattering polarization signals in the Sr I 4607 Å spectral line are among the strongest originating from the solar photosphere, offering a powerful diagnostic of tangled magnetic fields in the 3--300 G range via the Hanle effect. However, measuring them with sub-arcsec resolution remains a significant challenge. We analyze spatially resolved quiet-Sun observations of these signals performed with the Visible Spectropolarimeter (ViSP) at the Daniel K. Inouye Solar Telescope (DKIST) and identify its current observational limits. We present high-resolution, high-precision spectropolarimetric observations in a spectral window including the Sr I 4607 Å line at various limb distances. We apply consistent instrumental corrections across all spectral lines, enabling the adjacent lines to serve as reliable references. At a limb distance of $μ= 0.74$, the signal-to-noise ratio is low but sufficient in the total linear polarization map to directly reveal sub-arcsec structures in the Sr I line for the first time, which can be attributed to scattering polarization. Disk-center measurements are still dominated by noise related to the current limitations of the observational setup. By combining high spatio-temporal and spectral resolution with exceptional polarimetric precision, DKIST enables measurements of solar photospheric scattering polarization at fine scales. However, current signal-to-noise limitations still hinder direct detection of disk-center scattering polarization and must be addressed before further progress can be made.

Figures (11)

• Figure 1: Intensity spectra at disk center and close to the limb compared to the FTS atlas Neckel1999 and the Second Solar Spectrum atlas Gandorfer2002, respectively. Vertical dashed black lines indicate the spectral positions of the line cores (Sr i and Fe i) and the selected continuum position. Vertical red lines indicate the wavelength positions of the red wing where Stokes $V/I$ is plotted later in the paper. We normalized each spectrum to the continuum.
• Figure 2: Full Stokes spectrograms of the first scan and fifth slit position at disk center, clipped to the spectral region of Sr i (bottom spectral line) and Fe i (top spectral line).
• Figure 3: Linearly polarized spectra (obtained as explained in the text) at different limb positions $\mu$ (see legend). For reference, the data from the Second Solar Spectrum atlas Gandorfer2002 is also shown.
• Figure 4: Four upper rows: Full Stokes maps for Sr i (left column), Fe i (central column), and the continuum (right column), averaged over all three scans and spatially binned to 0$\overset{\prime\prime}{.}$1$\times$0$\overset{\prime\prime}{.}$1 sampling. The intensity and linear polarization is plotted for the spectral line core wavelength positions, while the circular polarization is taken at a small wavelength offset in the red wing (see Fig. \ref{['fig:center_spectrum']} for the offset). Each Stokes parameter has the same scale shown on the right. The intensity is scaled by the factor given in the upper left of the intensity panel. Limb distance contours are given. The limb ($\mu=0$) is given by the inflection point of the continuum intensity. We also show the approximate position of $\mu=0$ determined from the ViSP coordinates in the continuum intensity image. Bottom rows: Center-to-limb variation (CLV) for the intensity and linear polarization parallel to the limb. Note that the continuum polarization is scaled by a factor of 5. Reference low spatio-temporal resolution Sr i data Zeuner2022Malherbe2025 are shown in dark blue and red; see text for details. Note that the intensity is plotted on a logarithmic scale.
• Figure 5: Full Stokes maps in Sr i, Fe i, and continuum from ViSP scanning a 61$\overset{\prime\prime}{.}$8$\times$1$\overset{\prime\prime}{.}$1 region at $\mu=0.74$. The data is spatially binned to 0$\overset{\prime\prime}{.}$1$\times$0$\overset{\prime\prime}{.}$1 sampling. Each colorbar applies to all panels left from it, except Stokes $I$, whose range applies only to the continuum. The disk center direction is indicated by the red arrow and the scanning direction is upwards (positive direction of the ordinate). Note that the scale in the scanning direction is much smaller compared to the scale along the slit.