Study on two Methods for Nonlinear Force-free Extrapolation Based on   Semi-Analytical Field
  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. 
Liu S., Zhang H.Q., Su J.T. Song M.T
astro-ph.SR