Decrease of hot regions in the polar corona of the Sun during 1957–2002

Abstract

The Kislovodsk series of the green (FeXIV 530.3 nm; KI$_{5303}$) and red (FeX 637.4 nm; KI$_{6374}$) corona has been used to calculate the mean monthly intensities at high latitude (45$^\circ$–90$^\circ$) zones for 1957–2002. The ratio KI$_{6374}$/KI$_{5303}$ was observed during the minimum sunspot activity. It was shown that this ratio increased by more than a factor 2 during the last 45 years at the high latitudes (45$^\circ$–90$^\circ$). This may be interpreted that the fraction of cool regions in the polar corona has more than doubled over these years. We suggest that this increase in the amount of cool regions is related to the increase in the area of the polar zones occupied by magnetic field of a single polarity at the solar minimum and possibly to a corresponding increase in the area occupied by polar coronal holes, while the magnetic field strength itself has not or barely increased.