DKIST resolves sub-arcsec photospheric scattering polarization

TL;DR

The paper tackles mapping scattering polarization in the solar photosphere, a key diagnostic of hidden magnetic fields influenced by the Hanle effect. Using DKIST's ViSP, the authors obtain sub-arcsecond two-dimensional spectropolarimetric maps of the Sr i 4607 Å line near disk center, achieving about $0.2''$ spatial resolution with high polarimetric sensitivity. They find strong, spatially structured total linear polarization in Sr i (up to ~0.4%), while a nearby Fe i line shows weaker linear polarization, indicating that Sr i signals arise primarily from scattering rather than Zeeman polarization; the patterns are coherent across scan directions, supporting a solar origin. This work demonstrates the diagnostic power of high-resolution spectropolarimetry to study small-scale solar magnetism and provides a benchmark for future modeling of scattering polarization in the photosphere, paving the way for more detailed analyses of granulation-related polarization and the solar local dynamo.

Abstract

Scattering polarization signals offer a unique diagnostics of the physical conditions in the solar atmosphere, in particular magnetic fields via the Hanle effect. However, their spatial structure remains poorly constrained due to the difficulty of achieving high spatial resolution and polarimetric sensitivity simultaneously. We present the first direct observation of sub-arcsecond structuring in the linear scattering polarization of the photospheric Sr i 4607 Å\, line near the solar disk center ($μ$ = 0.74), obtained with the Visible Spectro- Polarimeter (ViSP) at the Daniel K. Inouye Solar Telescope (DKIST). The data achieve about 0".2 resolution with 30 s integration and sufficient sensitivity to detect fine-scale patterns in the total linear polarization, which are evident in Sr i but absent in a nearby Fe i line that is simultaneously observed. Since this Fe i line is more Zeeman-sensitive than the Sr i 4607 Å\, this disparity confirms that the signals in the Sr i 4607 Å\, line arise from scattering. These data provide the first spatially resolved two-dimensional maps of photospheric scattering polarization at sub-arcsecond scales, enabled by the capabilities of a 4-meter solar telescope.

Figures (3)

• Figure 1: Intensity spectra from ViSP at $\mu=0.74$ compared to the FTS atlas Neckel1999, resampled to match the ViSP spectral sampling. Dashed vertical black lines mark the line centers of Sr i and Fe i. Dashed red lines indicate the red-wing wavelength positions used for plotting Stokes $V/I$. All spectra have been normalized to the continuum for display purposes.
• Figure 2: RMS intensity contrast variation with slit position (upper panel) and power spectrum of the intensity at the center of the Sr i line (lower panel). For the latter, the black dots show the power spectrum windowed by a Hann filter before computing the FFT, summed over all scan positions. The solid blue line shows a median filter applied to the Hann-windowed power spectrum. The dashed red vertical line indicates the frequency that equates to an effective spatial resolution of 02.
• Figure 3: Intensity, circular polarization and total linear polarization maps (from left to right) in Sr i (top panels) and Fe i (bottom panels) of a 1$\times$ 1 quiet Sun region scanned by ViSP at $\mu=0.74$. Data is spatially binned to 01 $\times$ 01 sampling. Color bars for the polarization maps are applied consistently for both lines. Note that the total linear polarization scale is saturated for Sr i. The approximate solar disk center direction is indicated by the red arrow in the intensity panels. The scanning direction is upwards. Contours in black are at the level of 0.3%. The intensity in Sr i is scaled by a factor of two to get the same intensity scale as for Fe i.