11-year dynamics of coronal hole and sunspot areas


 The paper presents study the 11-year dynamics of solar activity on the basis of new observational material on coronal holes (CHs) and sunspots in the period from May 13, 2010 to May 13, 2021. We used the Heliophysics Event Knowledgebase (HEK) to obtain information on CHs areas. For 11 years of observations, we processed about 18000 CHs. Slightly more than 8000 are polar, the rest are nonpolar CHs. The statistical volume of the presented material is quite extensive and gives grounds for the study of the dynamics of different types of CHs during the cycle. Our research has shown: in the 24th solar activity cycle, the South led for polar CHs and the North led for nonpolar ones. We established a relationship between the number and area of CHs and the phase of the solar cycle. The number and daily total area of polar CHs increases at the minima of solar activity and decreases at the maximum of the cycle. This is consistent with the general concept of polar CHs as the main source of the solar dipole magnetic field. An asymmetry in both the number and areas of polar coronal holes in the northern and southern hemispheres is observed. It is shown that the areas of nonpolar CHs change quasi-synchronously with sunspot activity, which suggests a physical connection between these two phenomena.


Introduction
The solar cycle is a dynamo process in which poloidal magnetic field is twisted by differential rotation into a toroidal field and then that toroidal field is transformed by cyclonic convection back into a poloidal field of the opposite sign (Parker 1955;Dikpati and Charbonneau 1999). The original polarity is then recovered after the second solar cycle. The 11-year cycle is characterized by periodic changes in measures of solar activity such as the number of sunspots, coronal holes, and others. During a solar maximum, the sunspot number reaches a maximum, coronal holes extend to low latitudes and are diminished in polar regions, coronal cavities and prominences migrate toward the pole (Li et al. 2008;Shimojo 2013;Karna 2015). At the same time, the polar open magnetic field weakens, decreases through zero, and then emerges again with the opposite polarity.
Regions on the Sun with open magnetic fields are sources of the fast solar wind, and thus play a crucial role in Heliophysics as a main driver of geomagnetic activity. Key structures indicative of open magnetic fields are coronal holes, which are large regions of the corona that are unipolar, low density regions that appear to be dark when observed both on the solar disk and above the solar limb in EUV observations (Cranmer 2009). During solar maximum the area of the polar coronal holes diminishes while the area of equatorial coronal holes increases.
In order to statistically investigate the relationship between coronal holes and sunspots during the solar cycle, we study the cyclic variations in their areas. The study of variations in the areas of polar and nonpolar coronal holes was carried out earlier. For example, Fujiki et al. (2016) investigated the long-term trend of the distribution of coronal holes in the Sun from 1975 to 2014. Lowder et al. (2017) detailed examination of the latitude dependence of coronal hole properties over cycles 23 and 24. Our paper focuses on an analysis of the 11-year dynamics of the two types of coronal holes in the N-and S-hemispheres based on SDO epoch data observation.

Data, method and statistics
Our study used extreme ultraviolet images of the Sun in the Fe XII line (19.3 nm), obtained with the AIA/SDO. To localize coronal holes and determine their areas, we used the Heliophysics Event Knowledgebase (HEK) (Hurlburt et al. 2012), which is available at http://www.lmsal.com/hek/ hek_isolsearch.html. Information on coronal holes was extracted with the Spatial Possibilistic Clustering Algorithm (SPoCA) which was described in detail by Barra et al. (2009) and Verbeeck et al. (2014). The software performs the segmentation of the Sun images on active regions, quiet Sun, and CHs and make records into the HEK catalogs every four hours. These databases made it possible to obtain an array of daily total CHs areas for the period May 13, 2010-May 13, 2021. The area measured in square mega meters. For 11 years of observations, we processed about 18000 CHs. Slightly more than 8000 are polar, the rest are nonpolar CHs. The statistical volume of the presented material is quite extensive and gives grounds for the study of the dynamics of different types of CHs during the cycle. In (Andreeva and Malashchuk 2020) we explained our separation of CHs into polar and nonpolar. Below we briefly describe it. Statistics on the number of all types of coronal holes during the study period are presented graphicaly in Figure 1. It can be seen that the maximum, determined by the total number of CHs (lower left panel plot) is due to two maxima: first in the South, then in the North (middle and upper plots). At the same time, we show that the number of holes crossing the equator was maximal in 2012, the first maximum of the 24th cycle. We can also note that the maximum of the total number of CHs (lower left panel plot) is determined by the maximum number of all nonpolar CHs (lower middle panel plot). Their number was significantly higher in 2011-2015. At the same time, especially in 2013, the total number of polar CHs (lower graph in the right panel) was minimal. The right panel shows that the South led by the number of polar CHs during the growth phase, while the number of polar CHs in both hemispheres had maximum values during the decline phase and the 24-25 cycle minimum. The dip in the right panel is artificial, as we are not looking at data for the entire year 2021, but only before May 13.
The physical conclusion: nonpolar holes repeat the cycle. Polar holes behave in accordance with the general dipole of the solar magnetic field. Their physical nature is quite different and nonpolar CHs must most likely be associated with the sunspots activity. We drew this conclusion from the number of coronal holes. Now we turn to the analysis by the areas of coronal holes (Sch).

Relationship between the coronal holes and sunspots during 11-year
During the study, we analyzed 17717 CHs, of which less than 7% crossed the equator. We assumed that the area of CHs crossing the equator is divided into 2 parts, and each of them refers to the corresponding hemisphere, depending on its location. In our opinion, we did not make a big mistake, because there were quite few such CHs. Given this assumption, 9064 CHs were observed in the N hemisphere and 8653 CHs in the S hemisphere. As in our earlier works, here we study two groups of CHs ( Figure 2): polar (CH_pol) and nonpolar (CH_nonpol). CH_pol begin at the poles, i.e., at 90 degrees latitude, and descend, depending on their development and cycle phase, sometimes to middle and even low latitudes. CH_nonpol are mid-and low-latitude CHs that are not associated with the pole. The division of CHs into 2 groups, which we adopted in our study, is described in detail in (Andreeva and Malashchuk 2020).

Dynamics of coronal holes area
As a result of the processing of the array of CHs areas extracted from the EUV images by the SPoCA, we obtained the temporal variations of the daily total CHs area (Sch) for the entire visible surface of the solar disk and separately for both hemispheres (Figure 3). In all three plots of Figure

Dynamics of sunspots area
Data about daily sunspot areas taken from http:// solarcyclescience.com/AR_Database/daily_area.txt. Figure 7 show changes in the daily total sunspots area (gray lines) on the visible surface of the solar disk, N and S hemispheres (from top to bottom, respectively). The red curves are the sunspots area smoothed over 6 months. An asymmetry is observed in the distribution of the maximum of the sunspot areas. 2012 on the North, 2014 on the South.

Relationship between coronal holes and sunspots in hemispheres
Investigating the CHs dynamics, we analyzed the relationship between the areas of the two types of CHs and sunspots in different phases of the solar cycle. We see the hemispheric asymmetry of the polar CHs and the sunspots areas ( Figure 5 and Figure 7, middle plots; Figure 6 middle plots and Figure 7 bottom plots;). The daily total area of polar CHs increases at solar minima and decreases at the cycle maximum. This is consistent with the general concept of polar coronal holes as the main source of the solar dipole magnetic field. In both hemispheres ( Figure 5 and Figure 6, bottom plots), there is a tendency to an increase in nonpolar CHs at the rasing phase and maxima solar activity ( Figure 7 middle and bottom plots), to a decrease at the decay and minima phases. This confirms the assumption that nonpolar CHs are most likely associated with sunspots activity.

Conclusion
The dynamics of CHs during 11-year cycle was studied. Comparison of the variations in the daily total CHs areas with the sunspot areas made it possible to reveal some features of the dynamics of the polar and nonpolar CHs areas in the period May, 13 2010 -May, 13 2021. In the 24th solar activity cycle, the South led for polar CHs and the North led for nonpolar ones. There is a connection between the number and area of CHs and the phase of the solar cycle. The number and daily total area of polar CHs increases at the minima of solar activity and decreases at the maximum of the cycle. This is consistent with the general concept of polar CHs as the main source of the solar dipole magnetic field (e.g. Bilenko 2002;Sanderson et al. 2003;Wang 2009;Obridko and Shelting 2011;Fujiki et al. 2016;Lowder et al. 2017). An asymmetry in both the number and areas of polar coronal holes in the northern and southern hemispheres is observed. It is shown that the areas of nonpolar CHs change quasi-synchronously with sunspot activity, which suggests a physical connection between these two phenomena.