How to optimize nonlinear force-free coronal magnetic field extrapolations from SDO/HMI vector magnetograms?

TL;DR

This paper presents an improved nonlinear force-free extrapolation method for modeling the solar coronal magnetic field using high-resolution SDO/HMI magnetograms, accounting for measurement errors and optimizing boundary conditions.

Contribution

The authors developed a new extrapolation code that incorporates measurement errors and preprocessing, enhancing the accuracy of coronal magnetic field modeling from SDO/HMI data.

Findings

The new method effectively balances force-free constraints with measurement deviations.

Preprocessing improves compatibility of photospheric data with force-free conditions.

The approach yields more accurate magnetic energy estimates and field line projections.

Abstract

The SDO/HMI instruments provide photospheric vector magnetograms with a high spatial and temporal resolution. Our intention is to model the coronal magnetic field above active regions with the help of a nonlinear force-free extrapolation code. Our code is based on an optimization principle and has been tested extensively with semi-analytic and numeric equilibria and been applied before to vector magnetograms from Hinode and ground based observations. Recently we implemented a new version which takes measurement errors in photospheric vector magnetograms into account. Photospheric field measurements are often due to measurement errors and finite nonmagnetic forces inconsistent as a boundary for a force-free field in the corona. In order to deal with these uncertainties, we developed two improvements: 1.) Preprocessing of the surface measurements in order to make them compatible with a force-free field 2.) The new code keeps a balance between the force-free constraint and deviation from the photospheric field measurements. Both methods contain free parameters, which have to be optimized for use with data from SDO/HMI. Within this work we describe the corresponding analysis method and evaluate the force-free equilibria by means of how well force-freeness and solenoidal conditions are fulfilled, the angle between magnetic field and electric current and by comparing projections of magnetic field lines with coronal images from SDO/AIA. We also compute the available free magnetic energy and discuss the potential influence of control parameters.