Reconstructing the evolution of polar coronal holes in the past using sunspot number.

After the discovery of the close link between the evolution of the size of polar coronal holes and the following evolution of sunspot number for solar cycles 20 and 21, it is possible to reconstruct the magnitudes and changes of polar coronal holes in the past, using the available long time records of sunspots and assuming that the correlation parameters have remained the same. In this paper, the polar coronal hole size is reconstructed for the last six decades and the adequacy of such a reconstruction is tested against cosmic ray records on Earth, whose variation has been shown to run very closely to the evolution of polar coronal holes size. The results of this comparison suggest that, in general terms, such a reconstruction is valid and, consequently, that the time of lag between the evolution of polar coronal hole size and sunspot number seems to have been constant over the analyzed period.