Astron. Astrophys. 319, 664-668 (1997)

3. Analysis of the observational data

In our case, when the polarizer was successively adjusted to four positions differing by , the calculation of the green corona intensity and of its degree of polarization was performed according to the following formulas (Saito & Yamashita 1962):

Here and are the intensities of the polarized components of radiation, is the intensity at physically the same point of the image with the polarizer in the k position. The -values are defined according to the signs of and in (3).

Obviously, a certain amount of white-light coronal radiation is contained in the green-line images, recorded during the eclipse, and namely the radiation which has passed through the narrow-band filter applied in our experiment. That is why, the degree of polarization, derived immediately by using Eq. (1), is deteriorated to some extent by the polarization of the white-light corona. This influence increases with distance from the solar limb and is particularly evident in the region of the north polar coronal hole. Very low radiation is detected on the green-line images in the region of this hole, but, at the same time, the degree of polarization calculated according to (1) is 30-35%. The comparison of these values with the degree of polarization observed for the white-light corona reveals that both the quantities are practically identical in the north polar region. This fact allows us to assume that over the north polar hole almost all the radiation passing through the interference narrow-band filter comes from the white-light corona and the contribution of the green line itself is negligible in this region. This evaluation and assumption alone enable us to estimate, in general, the effect of the white-light radiation on the measured green-line corona degree of polarization.

In this procedure, we used the measured polarization and the intensity of the white-light corona, as derived from the images taken with an exposure of 1/125 s. First of all, it was necessary to reduce the intensities calculated both for the green-line corona and the white-light corona to a common base of relative units. By comparing the intensities of the white-light corona with the green-line intensities observed in the region of the north polar coronal hole, we have found that logarithms of the white-light corona intensities are for 0.25 higher than the corresponding values for the green-line corona. In other words, to reduce all the intensities to a common relative scale, it was necessary to add the value 0.25 to all the uncorrected values of the green-line intensities observed. In this case the ratio W of the purely green-line intensity to the intensity of the white-light radiation is 0.8 over the north pole. Note, that the ratio W should be understood as a ratio of the green-line radiation to the white-light corona radiation passing through the narrow-band filter (i.e., to the white-light corona radiation integrated over the whole passband of the filter). On an average, the values of the same ratio are 2.5 - 3.0 in the majority of coronal regions around the sun, and they even increase up to 8.0 - 9.0 in the brightest regions of the green-line corona (Fig. 1). It should perhaps be mentioned that the visible diameters of the sun and moon differed considerably during the July 11, 1991 long-lasting eclipse. That is why, many of the bright coronal regions situated lower than 1.08 at the east limb and 1.15 at the west limb were covered by the moon. (The difference between the east and west limbs is due to the green-line images having been taken during the first half of the eclipse totality.)

Fig. 1. Ratio W of radiations in the green-line and in the white-light corona passing through the narrow-band filter. Isolines are drawn with step = 0.15 and the numbers 1, 2, 3 correspond to logW = 0.15, 0.45 and 0.75, respectively

Since the angles of the direction of polarization of the white-light corona and that of the green-line corona do not generally have to be identical, it would be, in fact, necessary to subtract vectorially the contribution of the white-light corona from the observed polarization of the uncorrected green-line corona. Nevertheless, in our case, we used a different procedure. Since all the intensities measured on the four green-line images and those on the four corresponding white-light images were reduced to a common scale of relative intensities, we were able to subtract immediately a contribution of the white-light radiation separately for each position of the polarizer. The differences = - (where indices l and c relate to the line and continuum, respectively, and i denotes the number of the polarizer position) were calculated for each pixel of all the matrices individually. The calculations according to Eqs. (1) - (3) were then repeated - now, however, for the corrected radiation of the green-line corona. This procedure of the linear subtraction of intensities is adequate to the vectorial subtraction of the polarization values. Performed calculations revealed that elimination of the white-light contribution reduces the obtained polarization for about 5%, this effect being more pronounced in the equatorial regions than in the regions of the high-latitude streamers.

The maximum error in p, caused by the photometric processing of the coronal images, does not exceed 3-5%.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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