Nonlinear force-free modeling of flare-related magnetic field changes at the photosphere and chromosphere

TL;DR

This study models the magnetic field changes during a solar flare using nonlinear force-free models and compares them with observations, revealing discrepancies especially at chromospheric layers that suggest changes in field magnitude are involved.

Contribution

It introduces a method to construct nonlinear force-free models of the coronal magnetic field during a flare and compares these with observed magnetic field changes at different solar layers.

Findings

Moderate agreement between models and observations at the photosphere.

No agreement at chromospheric layers, indicating limitations of force-free models.

Observed magnetic field changes likely involve magnitude alterations, not just directional changes.

Abstract

Rapid and stepwise changes of the magnetic field are often observed during flares but cannot be explained by models yet. Using a 45 min sequence of SDO/HMI 135 s fast-cadence vector magnetograms of the X1 flare on 2014-03-29 we construct, at each timestep, nonlinear force-free models for the coronal magnetic field. Observed flare-related changes in the line-of-sight magnetic field $B_{\rm LOS}$ at the photosphere and chromosphere are compared with changes in the magnetic fields in the models. We find a moderate agreement at the photospheric layer (the basis for the models), but no agreement at chromospheric layers. The observed changes at the photosphere and chromosphere are surprisingly different, and are unlikely to be reproduced by a force-free model. The observed changes are likely to require a change in the magnitude of the field, not just in its direction.