Insights into penumbral formation from high-resolution photospheric and chromospheric observations

Abstract

We investigate the mechanisms driving the formation of a sunspot penumbra, focusing on a matures pore in NOAA AR 13010 using high-resolution spectropolarimetric observations from the Swedish 1-m Solar Telescope (SST). By employing non-LTE spectropolarimetric inversions of the Fe I 6302 Å and Ca II 8542 Å lines, we trace the magnetic field structure from the photosphere to the chromosphere. The study concludes that penumbra formation is driven by a combination of top-down and bottom-up processes. A preexisting, strong (>500 G) and nearly horizontal magnetic field in the chromosphere is identified as a necessary boundary condition for the onset of penumbra formation. While this horizontal field sets the stage, localized flux emergence is the mechanism that initiates the actual growth of penumbral filaments, with strong field-aligned horizontal redshifts (2–3 km/s) accompanying their development.