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A series of three imaging K-cornameter systems have been used to monitor the structure and evolution of the lower solar corona, at altitudes of 1.3 to 1.5 solar radii from the visible limb, for a 27.2-year period beginning in 1965. Analysis of this data set demonstrates three aspects of the long term variability of the sun's outer atmosphere: (1) Some of the largest spatial scale structure demonstrates evolution on a time scale which is long compared with the rotational period of the sun. (2) Over the period of observation the polarized brightness distribution of the large-scale corona exhibited a periodic zonal variation which was not symmetric about the rotation equator of the sun. (3) Suppression of the axisymmetric zonal polarized brightness distribution reveals the existence of a persistent sectoral structure.
The second point is interpreted as being consistent with the hypothesis that an approximately periodic magnetic variation is superposed on the traditional large-scale magnetic cycle (22-year) zonal magnetic field evolution. This perturbation is qualitatively consistent with the existence of a variable zonal quadrupolar field with a period of approximately half of the 22-year magnetic cycle. The third point is consistent with the existence of a semi-permanent east-west assymetry in the large-scale magnetic field.
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