Casting the Coronal Magnetic Field Reconstruction Tools in 3D Using the MHD Bifrost Model

Abstract

Quantifying coronal magnetic field remains a central problem in solar physics. Nowadays the coronal magnetic field is often modelled using nonlinear force-free field (NLFFF) reconstructions, whose accuracy has not yet been comprehensively assessed. Here we perform a detailed casting of the NLFFF reconstruction tools, such as π-disambiguation, photospheric field preprocessing, and volume reconstruction methods using a 3D snapshot of the publicly available fullfledged radiative MHD model. Specifically, from the MHD model we know the magnetic field vector in the entire 3D domain, which enables us to perform "voxel-by-voxel" comparison of the restored and the true magnetic field in the 3D model volume. Our tests show that the available π-disambiguation methods often fail at the quiet sun areas dominated by small-scale magnetic elements, while they work well at the AR photosphere and (even better) chromosphere. The preprocessing of the photospheric magnetic field, although does produce a more force-free boundary condition, also results in some effective 'elevation' of the magnetic field components. This 'elevation' height is different for the longitudinal and transverse components, which results in a systematic error in absolute heights in the reconstructed magnetic data cube. The extrapolations performed starting from actual AR photospheric magnetogram are free from this systematic error, while have other metrics comparable with those for extrapolations from the preprocessed magnetograms. This finding favors the use of extrapolations from the original photospheric magnetogram without preprocessing. Our tests further suggest that extrapolations from a force-free chromospheric boundary produce measurably better results, than those from the photospheric boundary.

Record-breaking Coronal Magnetic Field in Solar Active Region 12673

“…These magnetograms were employed as the bottom boundary conditions to perform NLFFF reconstruction of the coronal magnetic field. Specifically, we employed two different versions of the optimization method Fleishman et al (2017) found that existing methods of the photospheric magnetic boundary condition preprocessing, proposed to remove the forced component of the magnetic field, in fact result in a corrupted height scale, and do not improve quality of the reconstruction. Therefore, we did not apply any preprocessing, but used the bottom boundary condition as is.…”

Rapid Variability in the SOL2011-08-04 Flare: Implications for Electron Acceleration

“…It supplies the user with a possibility of automated production of 3D magneto-plasma data cube built for a user-defined field of view and spatial resolution. The 3D magnetic model is created by nonlinear force-free field reconstruction (Fleishman et al 2017) initiated with the corresponding photospheric vector magnetogram automatically downloaded from the HMI/SDO site. We employed the GX Simulator for 3D modeling of the flaring loops and computation of the associated electromagnetic emission.…”

Nonlinear Force-free Modeling of Flare-related Magnetic Field Changes at the Photosphere and Chromosphere

“…The highresolution chromospheric movie of the flare shows a very complex small-scale (subarcsecond) structure with changing loops, which do not seem to be reproduced by this global lower-resolution NLFFF model and this could be a major contributor to the observed discrepancies. Tests of a nonlinear force-free reconstruction on boundary data from a radiative magneto-hydrodynamic simulation show that the NLFFF model performs poorly in representing the field structure in the chromosphere (Fleishman et al 2017). The low atmosphere is not force-free, and in particular the gas pressure and gravity force are dynamically important.…”

“…In summary, for the purpose of reproducing the observed field changes at the photosphere and chromosphere, the force-free field model appears to be inadequate, because a NLFFF reconstruction based on photospheric boundary conditions does not include the physics of the chromosphere (Fleishman et al 2017). A possible next step is a magneto-hydrostatic model (e.g.…”