Energetics and Emission in a Simulated Solar Flare Initialised by a Non-Force Free Magnetic Field

TL;DR

This study compares two 3D resistive MHD simulations of a solar flare, showing that non-force free magnetic initial conditions lead to more energetic and realistic flare models, aligning better with observations.

Contribution

It demonstrates that non-force free magnetic extrapolations significantly influence flare energetics and observable features, improving the realism of solar flare simulations.

Findings

Non-force free model releases twice as much magnetic energy as NLFF model.

Non-force free model produces brighter, more extended EUV emission resembling observations.

Assumptions in initial magnetic field significantly affect flare energetics and signatures.

Abstract

Solar flare simulations are commonly initialised using non-linear force free field (NLFF) extrapolations derived from photospheric vector magnetograms. However, the force free assumption neglects plasma forces and may limit the available free magnetic energy. In this work, we perform a controlled comparison of two three-dimensional resistive magnetohydrodynamic simulations of the X2.1-class flare that occurred on 2011 September 06 in NOAA Active Region 11283. The simulations differ only in their initial magnetic configuration: one is based on a conventional NLFF extrapolation, while the other employs a non-force free extrapolation. Both models are evolved in an identical stratified atmosphere using the same numerical framework, enabling direct assessment of how the initial magnetic assumptions influence flare dynamics and energetics. We find that the non-force free model undergoes more extensive magnetic restructuring and releases approximately twice as much magnetic energy ($\approx4.4 \times 10^{31}$ erg) as the NLFF case ($\approx2.3 \times 10^{31}$ erg), bringing the energy budget into closer agreement with expectations for X-class flares. Synthetic extreme ultraviolet emission in the 94A channel is computed for both simulations and compared with observations from the Solar Dynamics Observatory. The non-force free model produces a brighter and more spatially extended emission structure that more closely resembles the observed flare morphology and light curve. These results demonstrate that assumptions made in constructing the pre-flare coronal magnetic field can significantly affect flare energetics and observable signatures, and suggest that non-force free extrapolations provide a promising pathway toward more realistic data-constrained flare modelling.