Detection of impact linear polarization of the line observed in the flare of 23 July 2002 with the large solar vacuum telescope

We have studied Stokes parameters in the 2B/X4.8 proton flare observed with the Large Solar Vacuum Telescope (LSVT) on 23 July 2002. In most sections along the dispersion, we have found no evidence for linear polarization in the Ш line. Short-term (tens of seconds), small-scale (2-4") regions with nonzero Stokes parameters Q/I and U/I (2-6 %) are only observed at the flare initiation in 78 out of 664 sections. Orientation of linear polarization is radial and is usually apparent in self-absorption nodes in the core of the Ηα line. Polarization directions on the first two spectrograms (obtained 4 minutes after the X-ray flare initiation) are both radial and tangential, with the maximum Р-13-14 %. The observed linear polarization is interpreted as an impact polarization caused by bombardment of the chromosphere by electron beams during the flare.