Thermodynamic and magnetic evolution of an eruptive C-class solar flare observed with SST/TRIPPEL-SP

¹ Institute of Theoretical Astrophysics, University of Oslo P.O. Box 1029 Blindern N-0315 Oslo, Norway
² Rosseland Centre for Solar Physics, University of Oslo P.O. Box 1029 Blindern N-0315 Oslo, Norway
³ Institute for Solar Physics, Dept. of Astronomy, Stockholm University, AlbaNova University Centre SE-10691 Stockholm, Sweden
⁴ Max Planck Institute for Solar System Research Justus-von-Liebig-Weg 3 37077 Göttingen, Germany
⁵ Thüringer Landessternwarte Sternwarte 5 07778 Tautenburg, Germany
⁶ Statkraft AS Lysaker, Norway

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Received: 20 October 2025
Accepted: 21 December 2025

Abstract

Solar flares are complex phenomena driven by the release of magnetic energy, but a large energy reservoir is not sufficient to determine their eruptive potential; the magnetic topology and plasma dynamics play a key role. We investigated the thermodynamic and magnetic properties of the solar atmosphere during the rise, peak, and decay phases of a C5.1-class flare and filament eruption in active region NOAA 12561 on 2016 July 7 to understand the origin and atmospheric response of this event. High spatial and spectral resolution spectropolarimetric observations of the chromospheric Ca II 8542 Å line and nearby photospheric lines were obtained with the TRIPPEL-SP spectropolarimeter at the Swedish 1-m Solar Telescope. Using nonlocal thermodynamic equilibrium (NLTE) inversions and non-force-free field (NFFF) magnetic extrapolations, we followed the event’s evolution from its precursor to its decay. Before the flare, our analysis reveals a complex, sheared magnetic topology with a high free energy content (∼2 × 10³⁰ erg). In this precursor phase, we detected persistent localized heating (temperature increase of ∼2000 K) with strong downflows (∼10–20 km s⁻¹) deep in the atmosphere. This heating was co-spatial with a bald-patch region, suggesting that low-altitude magnetic reconnection could destabilize the filament of the region. The flare’s rise phase was marked by the filament’s eruption, with a total speed greater than ∼70 km s⁻¹, when combining inversions and plane-of-sky motions. Following the eruption, the free energy decreased by ∼30% as post-flare loops formed, connecting the flare ribbons and channeling the released energy into the lower atmosphere. The flare ribbons exhibited significant heating to ∼8500 K and downflows up to ∼10 km s⁻¹, consistent with energy deposition along reconnected loops.