Diagnostic capabilities of spectropolarimetric observations for understanding solar phenomena

Abstract

Future ground-based telescopes will expand our capabilities for simultaneous multi-line polarimetric observations in a wide range of wavelengths, from the near-ultraviolet to the near-infrared. This creates a strong demand to compare candidate spectral lines to establish a guideline of the lines that are most appropriate for each observation target. We focused in this first work on Zeeman-sensitive photospheric lines in the visible and infrared. We first examined their polarisation signals and response functions using a 1D semi-empirical atmosphere. Then we studied the spatial distribution of the line core intensity and linear and circular polarisation signals using a realistic 3D numerical simulation. We ran inversions of synthetic profiles, and we compared the heights at which we obtain a high correlation between the input and the inferred atmosphere. We also used this opportunity to revisit the atomic information we have on these lines and computed the broadening cross-sections due to collisions with neutral hydrogen atoms for all the studied spectral lines. The results reveal that four spectral lines stand out from the rest for quiet-Sun and network conditions: Fe I 5250.2, 6302, 8468, and 15 648 Å. The first three form higher in the atmosphere, and the last line is mainly sensitive to the atmospheric parameters at the bottom of the photosphere. However, as they reach different heights, we strongly recommend using at least one of the first three candidates together with the Fe I 15 648 Å line to optimise our capabilities for inferring the thermal and magnetic properties of the lower atmosphere.

DKIST Unveils the Serpentine Topology of Quiet Sun Magnetism in the Photosphere

“…We are unable to distinguish between gradients in atmospheric parameters along the LOS and smearing of signals in the plane of the solar surface due to the spatial point-spread function of the telescope. We concur with the advice provided by Cabrera Solana et al (2005) and Quintero Noda et al (2021) and suggest that observations of the Fe I 6302.5 Å line with the deep photospheric Fe I 15648.5 Å line should be prioritized, as observations of diagnostics that sample a range of heights in the photosphere could provide a method for distinguishing between these two otherwise plausible scenarios when simultaneously inverted. These observations would additionally benefit from the demonstrated ability of the Fe I 15648.5 Å line to observe higher linear polarization fractions (Martínez González et al 2008b;Lagg et al 2016;Campbell et al 2023).…”

Large Photospheric Doppler Shift in Solar Active Region 12673. I. Field-aligned Flows

“…This results in effectively an "averaged" B along the LOS, which does not necessarily correspond to any specific height. Previous studies also suggest that the inferred Doppler velocity and magnetic field are sensitive to different optical depths (Quintero Noda et al 2021). It is thus not expected that the inferred v can closely reproduce the observed v l and adhere to the induction equation at the same time, even in the absence of noise.…”

Exploring magnetic loops and serpentine fields in the quiet Sun with the GRIS-IFU

“…Observations with the NIR line pair at GREGOR with the GREGOR Infrared Spectrograph (GRIS; Lagg et al (2016); Martínez González et al ( 2016)) have demonstrated higher efficacy at measuring linear polarization at similar spatial resolutions. This is despite the fact that modelling with simulations by Steiner et al (2008) shows the vertical field being prominent in the deep photosphere, where the NIR line pair is sensitive (Quintero Noda et al 2021), with the horizontal component of the magnetic field becoming increasingly important in the upper photosphere, where the visible line pair is sensitive, with convective overshooting responsible for expelling horizontal fields to greater heights. Campbell et al (2021a) have used the Integral Field Unit (IFU) mode of GRIS to examine the dynamics of small-scale magnetic features, including magnetic loops.…”