Abstract
He i 10830 Å synoptic maps, obtained at the Kitt Peak National Observatory during 1974–1979, show that the Sun's polar coronal holes have contracted significantly during 1977–1978. Prior to the accelerated increase of sunspot activity in mid-1977, the area of each polar cap was on the order of 8% of the Sun's total surface area (4πR 2), whereas toward the end of 1978 these areas fell below 2% of 4πR 2. Synoptic polar plots show that the vestigual holes had irregular shapes and were often well removed from the poles themselves. These results are consistent with the changes that one would expect when the polar magnetic fields are weakening just prior to sunspot maximum.
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Sheeley, N.R. The evolution of the polar coronal holes. Sol Phys 65, 229–235 (1980). https://doi.org/10.1007/BF00152791
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DOI: https://doi.org/10.1007/BF00152791