Chromospheric heating and magnetic topology above the shared penumbra of a delta-spot: Multi-line inversions and multi-height magnetic-field extrapolations

Abstract

We analysed observations of the Fe I 617.3 nm, Ca II 854.2 nm, and Ca II H lines obtained with CRISP and CHROMIS at the SST. Spatially coupled non-LTE inversions constrained the chromospheric atmosphere, while the WFA provided estimates of the chromospheric line-of-sight magnetic field. We combined these photospheric and chromospheric constraints with an HMI magnetogram as input to a multi-height field extrapolation. We characterised the reconstructed topology using the twist number, squashing factor, current density, and field-line connectivity. Results. The Ca II H magnetic signal is concentrated mainly above the strongest photospheric field concentrations, whereas Ca II 854.2 nm yields stronger and more spatially extended line-of-sight fields. The chromosphere above the shared penumbra is approximately 300 K hotter than nearby quiet regions. The selected brightening follows a chromospheric loop, with enhanced temperature and a transition from blueshift to redshift along the structure. The extrapolation recovers field strengths broadly consistent with the inversions and reveals a left-handed, flux-rope-like core following the polarity inversion line. Enhanced currents and connectivity gradients occur near parts of its boundary, where field lines connect the twisted structure to overarching loops. Conclusions. The temperature and velocity patterns and magnetic topology are consistent with reconnection between the twisted polarity-inversion-line field and the surrounding loops, depositing energy in the chromosphere and driving plasma along reconfigured field lines. These signatures do not uniquely establish reconnection, but show that combining high-resolution spectropolarimetric inversions with multi-height extrapolations can relate chromospheric energy release to the local three-dimensional magnetic structure.