Comments on solar linear force-free field and application of FFT analysis

We begin with some comments on the methods of computation of the magnetic fields of solar active regions in terms of force-free field, and point out some mistakes in the solutions. Then we propose a method of quasi-linear force-free field, and probe the application of fast Fourier transform (FFT) in its computation. The magnetic field of the active region NOAA 9165 on 2000-09-15 is taken as an example, and three different methods are used in the determination of the configuration of magnetic lines. The results are then compared with the fine Hα filtergrams of Ganyu Station of Purple Mountain Observatory.