Hα line polarization in the major flare of 2002 July 23.: I.: Observations and data analysis

On 2002 July 23, a major 2B/X4.8-class flare was observed in Halpha lines with the Large Solar Vacuum Telescope of the Baikal Astrophysical Observatory using the spectropolarimetric method. Linear polarization of 3%-10% has been detected in Halpha lines of the flare, particularly in the Ha lines with a central reversal. They are mainly radial on the solar disk and appear at the impulsive phase of hard X-ray and gamma-ray bursts. The polarization changes its direction in a spatially limited small region (similar to4"-5") and within a short period of time (similar to10 s). Moreover, the linear polarization is limited to only some relatively small regions of the flare.