Long-lived coronal loops in solar active regions

¹ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
² Institut für Sonnenphysik (KIS), Georges-Köhler-Allee 401A, 79110 Freiburg, Germany

^⋆ Corresponding author: naganna@mps.mpg.de

Received: 4 July 2025
Accepted: 14 September 2025

Abstract

Coronal loops are plasma structures in the solar atmosphere with temperatures reaching millions of Kelvin, shaped and sustained by the magnetic field. However, their morphology and fundamental nature remain subjects of debate. By studying their cross-sectional properties and how they change along the loop and in time, we can understand their magnetic structure and heating mechanisms. In this study, we investigated the cross-sectional intensity profiles, both spatially and temporally, of two unique coronal loops, observed in the periphery of two distinct active regions by the Extreme Ultraviolet Imager (EUI/HRI_EUV) on board the Solar Orbiter spacecraft. The main results of this study are fourfold. 1. The lifetimes of these two loops (loop1 > 120 min & loop2 > 50 min) are longer than the typical timescales of radiative cooling and thermal conduction. 2. Their widths determined by the full width at half maximum of the single-Gaussian fit to the cross-axis intensity profiles are greater than 6–7 pixels of EUI/HRI_EUV, indicating that the loop cross-section is uniformly filled on well-resolvable scales. 3. These loops exhibited an almost constant width, both spatially and temporally (width for loop1 is 2.1 ± 0.4 Mm and for loop2 is 1.3 ± 0.2 Mm), indicating that they are stable non-expanding structures. 4. We present observational evidence that the one of the loops (loop2) is not braided, which strongly suggests that the non-expanding nature of this multi-stranded loop along its length cannot be attributed to the twist of the magnetic field lines. In conclusion, we find that these coronal loops are long, stable, multi-stranded, non-expanding structures with a uniform cross-section that persist in the corona for an unusually extended duration. This not only challenges our current understanding of the structure of the coronal magnetic field, it also raises critical questions about the mechanisms responsible for the remarkable stability of these loops.