Polarization of Emission Lines in Astronomy.VI. Observations and Interpretations of Polarization in Green and Red Coronal Lines during 1965 and 1966 Eclipses of the Sun

Abstract

Observations of the polarization of the green and red coronal lines obtained during the 1965 and 1966 eclipses lead to the following results: Our polarization results in the green line agree with those that one would expect theoretically from the conditions at eclipse, i.e, the degree of polarization in the green line varied between 2 and 25 per cent in the range 1.0 «= R/Ro «= 1.5, and the directions of polarization (electric vector) were approximately radial. These results substantiate the careful theoretical work done by Blaha on the cross-section for electron collisional excitation of the green line and by Charvin on the polarization of the coronal green line. In order to clarify a severe contradiction between the polarization observations of others (p° < 83 per cent) and the theoretical predictions (Pt 0 per cent) of the polarization in the red coronal line, we observed the polarization of the red coronal line and of the adjacent coronal continuum during the eclipse of November 12, 1966. We find that the average degree of polarization in the red coronal line was 2.0 ± 3.5 per cent. Our results strongly support the theoretical predictions that the red coronal line cannot be polarized. Accurate measurements of polarization in the various coronal lines provide another dimension in the fixing of their identifications and in estimating the abundances of the parent ions We find, however, that the polarizing properties of observing instruments can introduce large systematic errors and that these errors must not be neglected as they have been in the past