Study on two Methods for Nonlinear Force-free Extrapolation Based on Semi-Analytical Field

TL;DR

This study evaluates two semi-analytical solutions of force-free fields to test and compare the effectiveness of boundary integral equation and improved AVI methods for nonlinear force-free extrapolation, demonstrating high correlation at certain heights.

Contribution

The paper introduces improvements to the AVI method and compares two semi-analytical solutions to assess the accuracy of nonlinear force-free extrapolation methods.

Findings

Correlation coefficients >90% for BIE and improved AVI below height 10.

Correlation coefficients >80% for the second semi-analytical field below same height.

Both methods provide reliable extrapolation results up to about 15% of the lower boundary extent.

Abstract

In this paper, two semi-analytical solutions of force free fields (\citeauthor{low90}, \citeyear{low90}) have been used to test two nonlinear force-free extrapolation methods. One is the boundary integral equation (BIE) method developed by \citeauthor{yan00} (\citeyear{yan00}), and another is the approximate vertical integration (AVI) method developed by \citeauthor{son06} (\citeyear{son06}). Some improvements for the AVI method have been taken to avoid the singular points in the process of calculation. It is found that the correlation coefficients between the first semi-analytical field and extrapolated field by BIE, and also that by improved AVI, are greater than $90\%$ below a height 10 of the $64 \times 64$ lower boundary. While for the second semi-analytical field, these correlation coefficients are greater than $80\%$ below the same relative height. Although the differences between the semi-analytical solutions and the extrapolated fields exist for both BIE and AVI methods, these two methods can give reliable results for the height of about 15$\%$ of the extent of the lower boundary.