The cyclic behaviour in the N-S asymmetry of sunspots and solar plages for the period 1910 to 1937 using data from Ebro catalogues

The heliophysics catalogues published by the Ebro Observatory during 1910-1937 have been converted into a digital format in order to provide the data for computational processing. This has allowed us to study in detail the North-South (N-S) asymmetry of solar activity in that period, focusing on two different structures located at two different layers of the solar atmosphere: Sunspots (Photosphere) and solar plages (Chromosphere). The examination of the absolute and normalized N-S asymmetry indices in terms of their monthly sum of occurrences and areas has made possible to find out a cyclic behaviour in the solar activity, in which the preferred hemisphere changes systematically with a global period of 7.9 ± 0.2 yr. In order to verify and quantify accurately this periodicity and study its prevalence in time, we employed the Royal Greenwich Observatory-United States Air Force/National Oceanic and Atmospheric Administration sunspot data series during 1874-2016. Then, we examined each absolute asymmetry index time series through different techniques as the power-spectrum analysis, the Complete Ensemble Empirical Mode Decomposition With Adaptive Noise algorithm or the Morlet wavelet transform. The combined results reveal a cyclic behaviour at different time-scales, consisting in two quite stable periodicities of 1.47 ± 0.02 yr and 3.83 ± 0.06 yr, which co-exist with another three discontinuous components with more marked time-varying periods with means of 5.4 ± 0.2 yr, 9.0 ± 0.2 yr, and 12.7 ± 0.3 yr. Moreover, during 1910-1937, only two dominant signals with averaged periods of 4.10 ± 0.04 yr and 7.57 ± 0.03 yr can be clearly observed. Finally, in both signals, periods are slightly longer for plages in comparison with sunspots.