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

The 11-year Solar Cycle (SC) is characterized by periodic changes in solar activity indicators such as a number of sunspots, coronal holes, active regions (ARs), as well as the occurrence rate of solar energetic events such as filament eruptions, flares and coronal mass ejections (CMEs). In this work we performed a statistical study of the equatorial coronal holes (ECHs) and ARs during the maximum phase of the last four solar cycles: 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 storms (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 (which are potentially interacting), 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 suggest a possible link between ECH-AR interactions and the solar wind phenomena, though odd–even trends point to the importance of other effects (e.g., Sun-earth magnetic alignment) as well.