The Evolution over Time and North–South Asymmetry of Sunspots and Solar Plages for the Period 1910 to 1937 Using Data from Ebro Catalogues

In this work, we analysed both the evolution over time and the north–south (N-S) asymmetry of certain solar features with regards to their morphological type and area. We examined and compared simultaneously 4212 photospheric sunspot groups and 5781 chromospheric solar plage groups using data from the Ebro Observatory catalogues for the period 1910–1937. We found that the most frequently recorded groups of sunspots and solar plages are the smallest ones, and the total number of occurrences of these groups is, on average, about 23% higher for solar plages than for sunspots. Concerning the N-S asymmetry, when the northern hemisphere becomes predominant, it dominates the other hemisphere for a longer time than the opposite case and shows a higher asymmetry. In addition, we found that the normalized N-S asymmetry index, δ, of solar activity in terms of the occurrence of sunspots and solar plage groups follows approximately the same behaviour regardless of the morphological type, but presents a slight dependence on their area, since sunspot and solar plage groups show higher values of the δ index, the larger their areas are. Furthermore, we noticed three highly significant extremes in the N-S asymmetry, located around the minima of the 14th, 15th and 16th solar cycles (1912 [S]; 1924 [N]; 1933 [N]). However, the general trend is not equal in both structures, since in contrast to sunspots, solar plage groups present lower values of asymmetry throughout practically all the period under consideration. These facts could be interpreted in terms of magnetic cancellation processes and/or merging of close-by faculae.