The occurrence of coronal holes during the sunspot cycle

In order to learn the nature of coronal holes for a long period, magnetic disturbances represented by the geomagnetic index C9 (though proxy for coronal holes) for the period from 1884 to 2002 are examined, selecting only those that recur every 27 days during at least six solar rotation periods and separating the rising and declining phases, and also distinguishing one and two coronal holes in one solar rotation. It was found that the occurrence of coronal holes per year (N) during both the rising and the declining phases tends to be more frequent for greater numbers of the peak value of sunspots (R) in each sunspot cycle. The frequency per year is about twice as large for the declining phase as for the rising phase. The N-R relationship is 0.0080R+0.302 for the rising phase and 0.0152R+0.203 for the declining phase. (C) 2014 Elsevier Ltd. All rights reserved.