OPTICAL POLARIZATION OBSERVATIONS OF THE SOLAR CORONA DURING THE TOTAL SOLAR ECLIPSE OF 1991 JULY 11

We report here on the results of optical polarization observations of the outer solar corona during the total solar eclipse of 1991 July 11, with specially designed telescopes. The observations were carried out near the summit of Mt. Popocatepetl in Mexico at an altitude of 5250 m to reduce atmospheric-scattered light. Polarization vector maps around the Sun were obtained at wavelengths of 532.5, 596.5, 720.0, and 801.5 nm. Strong polarization was observed in the regions where coronal streamers dominate and is thought to be due to Thomson scattering by free electrons. No excess polarization was observed at around 4-5R. from the Sun on the ecliptic plane. The concentration of dust grains which was observed several times as the circumsolar dust ring in the past was weak at this time. Our observations together with the previous ones clearly show the transient nature of this dust ring. Since its existence seems to be related to solar activity we suggest that the Lorentz force on charged grains could explain this phenomenon.