A 600-day periodicity in solar coronal holes

Abstract

CORONAL holes are large-scale structures of lower density and temperature in the solar corona. They appear as large dark features in X-ray and radio images, and as bright areas in infrared He I images of the Sun. The longest series of data is the He I (10,830 A) observations published within Hα synoptic charts1. Here we analyse outlines of coronal holes from these charts to show the variation of coronal hole area over the period 1977–89. We find that the total area of coronal holes enclosed within a latitude band of 10–50° S shows a 592-day periodic variation throughout the 13-year interval, which includes all of solar cycle 21 and the first three years of cycle 22. The corresponding northern latitude region does not show this periodic behaviour. There is some indication of an association between the coronal hole variation and other solar phenomena such as the occurrence of super-active regions and the reported 153-day periodicity.