MAGNETIC FIELDS AND THERMODYNAMICAL CONDITIONS AT PHOTOSPHERIC LAYERS OF X17.2/4B SOLAR FLARE OF 28 OCTOBER 2003

We present the magnetic field measurements and data of semi-empirical modeling for the exclusively powerful solar flare of 28 October 2003 of X17.2/4B class, which was in active region NOAA 0486. Observations were made with the Echelle spectrograph of the horizontal solar at telescope the Astronomical Observatory of the Taras Shevchenko National University of Kyiv. Our data relate to the peak phase of the flare and to a place of the photosphere outside sunspots. Nine spectral lines were selected for analysis, including such well known lines as FeI 5247.052, 5250.212, 6301.515, 6302.507 angstrom, etc. Three "non-split" lines FeI 5123.723, 5434.527 and 5576.097 angstrom were studied too (their effective Lande factors are, in fact, -0.013, -0.014 and -0.012, respectively). Magnetic field strengths in the flare were determined by three methods: (1) by splitting of the "center of gravity" of I +/- V profiles; (2) by the amplitude of Stokes V profiles using " weak field approximation" and (3) using the PANDORA code, which allows to create the semi-empirical model of the flare. We found that in the brightes places of the flare the effective magnetic field B-eff was 0-200 G in the middle photosphere (FeI 6302.5 and 5250.2 lines), 600-1200 G in the upper photosphere and temperature minimum zone (FeII 4923.9 and 5234.6 lines), and had S polarity. In the FeI 5434.527 line, the weak splitting of emission peaks near its core was observed; its value was 10-20 m angstrom. If this splitting is interpreted as a manifestation of the Zeeman effect, then the corresponding magnetic field is about 25-50 kG. The magnetic polarity of this "superstrong" field should be N, i.e. opposite to the polarity of other FeI lines with greater Lande factors. A semi-empirical model of the flare was build with the PANDORA code, which allows to determine the magnetic field and thermodynamical conditions on both photospheric and chromospheric levels using non-LTE approximation. It was found that the magnetic field in the middle photosphere was 1000-1200 G. The temperature in the flare increased up to 1000 K for lg tau(5) approximate to -3. Turbulent velocity v(t) is almost the same as in an undisturbed atmosphere, but in the range from lg tau(5) = 1 to lg tau(5) = -1; from lg tau(5) = -1 to lg tau(5) = -3 a sharp increase vt was found where v(t) reaches about 3.5 km/s.