A COMPARISON OF TWO METHODS FOR SOLAR NONLINEAR FORCE-FREE FIELD EXTRAPOLATION LILIANA DUMITRU Astronomical Institute of Romanian Academy Str. Cutitul de Argint 5, 040557 Bucharest, Romania Email: lyly@aira.astro.ro Abstract. In this article we construct the 3D magnetic field of solar active region NOAA12673 using two nonlinear force-free field (NLFFF) methods. The event chosen for the study was the X9.3 class flare on September 6, 2017, which was the largest flare of solar cycle 24. We analyzed the evolution of the free-force field parameter α around the flare occurrence. We used the vector magnetogram from Helioseismic and Magnetic Imager (HMI) instruments on board of the Solar Dynamics Observatory (SDO) in the full disk format and Spaceweather HMI Active Region Patch (SHARP) format, for two hours before and after the X9.3 class flare. The comparison of the two codes, that use different algorithms and two different sets of data, shows that there are similarities and differences. Our investigation revealed that, in spite we found the same behaviour of the alpha parameter evolution during the major flare development, after that different results were obtained. Key words: Sun – solar corona – active region – solar flare – magnetic field extrapola- tion – force free-field. 1. INTRODUCTION Coronal magnetic field modeling is a very important element in the analysis of solar activity. Over time there where different methods of computation of the coronal magnetic field derived from the observed photospheric one. The most complicated and realistic are those numerical codes using the non-linear free force field (NLFFF). The NLFFF models use the vector magnetograms obtained from photosphere obser- vations and by numerical simulations the coronal magnetic fields are computed, since no direct observations of the coronal magnetic field are possible. Various authors used such numerical methods to construct magnetic coronal field, by extrapolating the photosphere magnetic fields (Wheatland et al. (2000), R´ egnier et al. (2002), Lee (2002), Wiegelmann (2004), Wheatland (2004), Valori et al. (2005), Wiegelmann et al. (2006), Wheatland (2006), Wheatland and Regnier (2009), Tadesse et al. (2009), Amari and Aly (2010), Wheatland and Leka (2011), Wiegelmann et al. (2012), Jiang and Feng (2012), Valori (2016)). NLFFF models have been tested on various types of data obtained from space missions such as magnetograms obtained by Michelson Doppler Imager (MDI) in- Romanian Astron. J. , Vol. 29, No. 2, p. 177–186, Bucharest, 2019