Precise determination of the orientation of the plane of polarization in the solar corona

It has been shown by Molodensky (1973), that precise measurements of the position of the plane of polarization in the corona may allow us to observe overthermal electrons in the solar corona. For such measurements during the eclipse of 10 July 1972, a method based on the photographic recordings of the corona by means of a cineset and with an automatically rotating polaroid has been developed. A technique has also been developed for determining the position of the plane of polarization by means of isophotes obtained with polarization filters. This technique uses the photometric data for determining phase shifts between the apparent intensity variation curve and a similar curve expressing the rotation phase of the polaroid. The results of the measurements for h/r _⊙ =0.5 to 0.9 allow us to conclude that:

1. The plane of polarization (E-vector position) coincides very exactly with the tangential direction in the region of N-W limb. The maximum deviations of this plane amounts to 1–1.5°, and the mean-square deviations in this region amount to ～0,3° at h/R _⊙ ≈1. This coronal region was the least active one and there were no spots there.
2. The corona near the E limb consisted of two ‘fans’ divided by a thin beam. In that region some deviations of the plane of polarizarion from the tangential direction were revealed. Those deviations were of the order of 3°. During the time of the eclipse there were some groups of spots behind the E limb (but close to this limb). The observed deviations were apparently connected with those groups.
3. Calculations have been made of the turn of the plane of polarization caused by an inhomogeneity in the radiation field from the photosphere and due to the presence of spots. The effect qualitatively coincided with that shown by the measurements.