A Study of Equatorial Coronal Holes during the Maximum Phase of Four Solar Cycles

The 11 yr solar cycle (SC) is characterized by periodic changes in solar activity indicators such as the number of sunspots, coronal holes, and active regions (ARs), as well as the occurrence rate of solar energetic events such as filament eruptions, flares, and coronal mass ejections. In this work we performed a statistical study of the equatorial coronal holes (ECHs) and ARs during the maximum phase of the last four SCs: SC 21 (1979-1982), SC 22 (1989-1992), SC 23 (1999-2002), and SC 24 (2012-2015). We compared the number of ECHs and ARs, separations between their centroids, solar wind speed, pressure, and the number of intense geomagnetic storm (IGS) data over these four cycles. We note a strong anticorrelation between the number of ARs and ECHs. We found that the number of close ARs and ECHs, solar wind speed, and the number of IGS increases with average sunspot maximum number for even cycles and decreases with average sunspot maximum for odd cycles. Also, we find strong odd-even trends in the relation between the wind properties and the numbers of close AR and ECH. These results obtained from the annual average data suggest a possible link between ECH and AR proximity and the solar wind phenomena, though odd-even trends point to the importance of other effects (e.g., Sun-Earth magnetic alignment) as well.