White-Light Polarization and Large-Scale Coronal Structures

Abstract

The results of the white-light polarization measurements performed during three solar eclipses (1973, 1980, 1991) are presented. The eclipse images were processed and analysed by the same technique and method and, consequently, the distributions of the polarization and coronal intensity around the Sun were obtained in unified form for all three solar eclipses. The mutual comparisons of our results, and their comparison with the distributions found by other authors, allowed the real accuracy of the current measurements of the white-light corona polarization, which is not worse than ±5%, to be estimated. We have investigated the behaviour of the polarization in dependence on heliocentric distance in helmet streamers and coronal holes. Simultaneous interpretation of the data on polarization and intensity in white-light helmet streamers is only possible if a considerable concentration of coronal matter (plasma) towards the plane of the sky is assumed. The values obtained for the coronal hole regions can be understood within the framework of a spherically symmetrical model of the low density solar atmosphere. A tendency towards increasing polarization in coronal holes, connected with the decrease of the hole's size and with the transition from the minimum to the maximum of the solar cycle, was noticed. The problem of how the peculiarities of the large-scale coronal structures are related to the orientation of the global (dipole) solar magnetic field and to the degree of the goffer character of the coronal and interplanetary current sheet is discussed briefly.