Behavior of the solar coronal holes around the maximum activity of the cycle 24

Abstract

The behavior of coronal holes (CHs) as open magnetic field lines and a substrate for the high-speed exit of solar winds is an essential aspect of solar activity. This paper studies the statistical properties of CHs around the maximum activity of cycle 24 (13 May 2010 to 31 October 2017) observed by the Atmospheric Imaging Assembly at 193 Å images. We showed that the size of CHs follows the power-law distributions indicating the scale-free behavior of CHs. The butterfly diagram of CHs demonstrates that the overall number of CHs shows a similar distribution in the two hemispheres. Large CHs with sizes ⩾2.57×105 arcsec2 (top 3% of observed CHs) more commonly appear at latitudes above 30∘ closer to the poles. We found that the largest CHs with sizes ⩾7.71×105 arcsec2 (top 0.03% of observed CHs) occur in the northern hemisphere. The appearance and disappearance of large CHs exhibit the asymmetrical behavior between the two hemispheres. Also, it is observed that CHs tend to have a latitudinal drift towards the poles in each hemisphere. A significant correlation between the number of sunspots and the eccentricity of equatorial CHs appeared near the solar disk center revealed that most of the CHs have elongated shapes at the maximum solar activity. In contrast, they have less abnormality during solar minimum. Applying detrended fluctuation and rescaled range analyses to the time series of CHs, we realized that the CHs have a long-term correlation and form a self-organized system.