Sunspot Activity and Coronal Holes during the Declining Phase of Cycle 24

Abstract

The dynamics of coronal holes (CHs) during the declining phase of solar cycle 24 is studied based on observational data from the Atmospheric Imaging Assembly (AIA), the 19.3 nm channel, of the Solar Dynamic Observatory (SDO). Some features of changes in the areas of polar and mid- and low-latitude CHs in the period April 1, 2014–December 31, 2019, are revealed by a comparison of the variations in the daily total CH area with the index of the sunspot number. The division of all CHs observed in the considered period into polar and nonpolar shows that polar-CH areas dominate and increase while nonpolar-CH areas constantly decrease toward the cycle minimum. Polar-CH areas account for ~80% of the total area of all CHs considered over the studied period. The total polar-CH area increases toward cycle minimum, which is consistent with the general concept of polar CHs as the main source of the Sun’s dipole magnetic field. However, the second subset, which contains selected isolated CHs, shows an interesting behavior in the declining phase of solar activity that was not previously observed. Their areas slowly decrease from the maximum to minimum of the solar cycle. This trend is qualitatively consistent with a decrease in sunspot activity toward the minimum, which suggests a physical relationship between the two phenomena.