First Results of Mg I (3p 1P1-4d 1D2) Line Linear Impact Polarization during the Solar Flare on 2001 June 15

The results of the analysis of the first spectropolarimetric observations of the 3p (1P(1)-4d (1D(2) Mg I line at 5528.4 AA made during a solar flare are presented in this paper. The line is found to be polarized with a polarization degree at the line center that reaches up to 3% and a direction of polarization nearly parallel to the local transverse magnetic field. After eliminating scattering, the Zeeman effect, and the intensity gradient as possible origins of the observed polarization, this polarization is interpreted as due either to a low-energy proton beam or to the return current associated with electron beams.