A recent study accepted for publication in Astronomy & Astrophysics on 15 December 2025 (https://arxiv.org/abs/2512.11160) provides new insights into the physical nature of sunspots by showing that their outer boundary is defined by a specific magnetic field strength rather than by brightness alone.

Sunspots are dark, magnetically intense regions on the solar surface where strong magnetic fields suppress convective heat transport. Although their internal structure has been studied in detail, the origin of the outer boundaries of sunspots has long remained unclear. In this study, the researchers analysed spectropolarimetric observations of a long-lived sunspot in active region NOAA 11591/11612 over two successive passages across the solar disk.

During the sunspot’s stable phase, the authors found that the commonly used intensity contour marking the sunspot boundary closely coincides with a contour of constant magnetic field strength of about 625 Gauss. This value corresponds to the equipartition field, where the energy densities of the magnetic fields and convective motions are equal. The results demonstrate that the outer boundary of stable sunspots is a physically meaningful magnetic threshold separating magnetically dominated plasma from normal solar granulation.

When the sunspot decays, this correspondence breaks down, and bright granulation in a super-equipartition regime can be observed between the contours. This behaviour provides new constraints on how sunspots lose stability and eventually dissolve.

This study is particularly timely as the Sun is currently in an active phase of its 11-year cycle, characterised by increased numbers of sunspots and enhanced auroral activity on Earth. Therefore, understanding the magnetic structure of sunspots is essential for advancing both fundamental solar physics and space weather research.

The two supplemental videos show the sunspot evolution during the two disc passages, with intensity and magnetic (purple) contours outlining the spot boundary (video No.1, No.2).
 

This research was conducted within the framework of the Czech–German collaborative project Unveiling the Principles of Solar Magneto-convection, funded by the Deutsche Forschungsgemeinschaft (DFG) under project BE 5771, and the Czech Science Foundation under project 23-07633K. 

Our colleague Dr. Ivan Milic had a talk at the Physics Faculty at the University of Freiburg on Monday, October 27th at 16:00. This talk was a part of his habilitation application at the Physics Facutly.

Ivan has been teaching master courses at the Physics Faculty since 2023. This talk presented some content from these courses, combined with some example applications in high resolution spectropolarimetry.

Poster for the talk can be found here (see also the accompanying figure): https://www.mathphys.uni-freiburg.de/physik/data/seminare/abstract/PhysKolloquium_Milic.pdf

A. Vicente Arevalo receiving his award with the poster at the Bad Honnef conference.

It was wonderful to see our researchers participating in the past solar physics conference in Bad Honnef, a great gathering for the broader solar physics community to share and discuss the latest scientific developments. During the event, our colleague A. Vicente Arevalo was honored with the Best Poster Award for his work, presented in collaboration with T. del Pino Aleman, J. Stepan, and M.J. Gonzalez Martinez. Their poster, titled "3D inversion of a solar prominence" , details an innovative 3D meshfree inversion framework designed to be significantly faster and more memory-efficient than traditional grid-based methods. We extend our heartfelt congratulations to the team for this excellent recognition, which highlights the exciting research being shared by our institute.
 

Poster file