DKIST Unveils Four-Lobed Stokes U Profiles in a Magnetically Complex Sunspot Penumbra

We present DKIST spectropolarimetric observations of a sunspot penumbra revealing spatially coherent four-lobed Stokes U profiles that persist after polarimetric crosstalk correction. The observed Stokes vectors are well reproduced by stratified inversions containing gradients in velocity and magnetic parameters, including line-of-sight variation of the magnetic azimuth. Forward synthesis experiments in which the magnetic azimuth is artificially held constant with optical depth substantially modify the Stokes U morphology while leaving Stokes I, Q, and V comparatively unchanged, indicating that azimuthal variation is an important contributor to the observed linear polarisation signatures within the tested one-component atmospheres. We explored degeneracies using both two-component atmospheres and one-component constant-azimuth models: the former generally converged toward pathological solutions, while the latter provided a plausible degenerate explanation for much of the observed four-lobed Stokes U morphology. A supervised machine-learning classifier applied to Stokes V profiles shows that the associated magnetic complexity is spatially confined to the penumbra-light-bridge boundary where the four-lobed Stokes U profiles and reverse-polarity fields are observed. These results identify four-lobed Stokes U profiles as a potential spectropolarimetric signature of complex line-of-sight magnetic structuring in sunspot penumbrae, providing new observational constraints on fine-scale penumbral magnetic topology.