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

¹ Astronomical Institute of the University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
² Institute for Solar Physics, Dept. of Astronomy, Stockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden
³ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, N-0315 Oslo, Norway
⁴ Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029 Blindern, N-0315 Oslo, Norway

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 17 July 2025
Accepted: 31 October 2025

Abstract

Context. Penumbra formation occurs within hours and remains not fully understood. Two competing theories suggest either a top-down mechanism, whereby the chromospheric magnetic field becomes more horizontal, or a bottom-up process involving flux emergence near the sunspot umbra.

Aims. We investigate penumbra formation and test both hypotheses by inferring and analyzing the full magnetic field from the photosphere to the chromosphere above a forming sunspot.

Methods. We applied Milne-Eddington, weak field approximation, and the multiatom non-local thermodynamic equilibrium inversion code STiC to obtain the model atmosphere that reproduces the Stokes profiles of Fe I 6302 Å and Ca II 8542 Å observed at the Swedish Solar Telescope, complementing the analysis with images taken in Hβ.

Results. A penumbra formed only in the southern sector of a sunspot in AR 13010 on May 15, 2022. We find that a preexisting, strong (> 500 G) and nearly horizontal magnetic field in the chromosphere is a key prerequisite. Penumbral filaments grew southward where localized flux emergence was observed, accompanied by strong redshifts (2–3 km s⁻¹) throughout the atmospheric layers. These redshifts likely represent field-aligned horizontal flows, possibly combining the onset of the Evershed flow and a siphon-like flow driven by field-aligned pressure gradients.

Conclusions. Our findings indicate that penumbra formation results from both top-down and bottom-up processes. While strong horizontal chromospheric magnetic fields are a necessary boundary condition, flux emergence initiates filament growth. The fallen flux tube model is ruled out, based on clear bottom-up signatures, though it remains unclear why the chromospheric and photospheric fields become more horizontal concurrently. Extended spectropolarimetric data are needed to get a deeper insight.