Investigating the inferred spatiotemporal evolution of magnetic fields during an X1-class flare
Abstract
Several observational studies have reported sudden temporal changes in chromospheric magnetic fields inferred during solar flares using the weak-field approximation (WFA) and nonlocal thermodynamic equilibrium (NLTE) inversions. We investigate whether these variations reflect the rapid evolution of the solar atmosphere or can be influenced by opacity changes caused by flare heating. We perform spatially coupled and regularized NLTE inversions of high-resolution observations and use the inferred atmospheric models to study the formation of the Ca II 854.2 nm line. We also analyze snapshots from a 3D radiative magnetohydrodynamic flare simulation to investigate the evolution of the mapping between column mass and geometrical height. We find that opacity effects alone cannot, in many cases, explain the temporal variations in the reconstructed magnetic fields, suggesting that they reflect real solar evolution. However, in approximately 36% of the regions exhibiting rapid magnetic-field changes, opacity effects are sufficiently strong to bias their detection. Ejected or condensed cold material can also shift the line formation to a different depth, producing an apparent sudden change in the inferred magnetic field. The simulation further shows that, once the flare ribbon forms, a given column mass maps to deeper geometrical layers. These results highlight the need for caution when interpreting the temporal evolution of chromospheric magnetic fields during flares, as neither the WFA nor NLTE inversions based on 1D hydrostatic equilibrium can fully account for these effects.
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BibTeX
@misc{arramy2026investigating,
title = {{Investigating the inferred spatiotemporal evolution of magnetic fields during an X1-class flare}},
author = {Dimitri Arramy and Jaime de la Cruz Rodríguez and Adur Pastor Yabar and Jorrit Leenaarts},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.20125},
eprint = {2609.20125},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.20125},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Investigating the inferred spatiotemporal evolution of magnetic fields during an X1-class flare
AU - Dimitri Arramy
AU - Jaime de la Cruz Rodríguez
AU - Adur Pastor Yabar
AU - Jorrit Leenaarts
PY - 2026
DA - 2026/09/17
PB - arXiv
DO - 10.48550/arxiv.2609.20125
UR - https://arxiv.org/abs/2609.20125
AB - Several observational studies have reported sudden temporal changes in chromospheric magnetic fields inferred during solar flares using the weak-field approximation (WFA) and nonlocal thermodynamic equilibrium (NLTE) inversions. We investigate whether these variations reflect the rapid evolution of the solar atmosphere or can be influenced by opacity changes caused by flare heating. We perform spatially coupled and regularized NLTE inversions of high-resolution observations and use the inferred atmospheric models to study the formation of the Ca II 854.2 nm line. We also analyze snapshots from a 3D radiative magnetohydrodynamic flare simulation to investigate the evolution of the mapping between column mass and geometrical height. We find that opacity effects alone cannot, in many cases, explain the temporal variations in the reconstructed magnetic fields, suggesting that they reflect real solar evolution. However, in approximately 36% of the regions exhibiting rapid magnetic-field changes, opacity effects are sufficiently strong to bias their detection. Ejected or condensed cold material can also shift the line formation to a different depth, producing an apparent sudden change in the inferred magnetic field. The simulation further shows that, once the flare ribbon forms, a given column mass maps to deeper geometrical layers. These results highlight the need for caution when interpreting the temporal evolution of chromospheric magnetic fields during flares, as neither the WFA nor NLTE inversions based on 1D hydrostatic equilibrium can fully account for these effects.
ER -