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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 normalised 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 $\pm$ 0.2 yr. In order to verify and quantify accurately this periodicity and study its prevalence in time, we employed the RGO-USAF/NOAA 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 $\pm$ 0.02 yr and 3.83 $\pm$ 0.06 yr, which coexist with another three discontinuous components with more marked time-varying periods with means of 5.4 $\pm$ 0.2 yr, 9.0 $\pm$ 0.2 yr, and 12.7 $\pm$ 0.3 yr. Moreover, during 1910--1937, only two dominant signals with averaged periods of 4.10 $\pm$ 0.04 yr and 7.57 $\pm$ 0.03 yr can be clearly observed. Finally, in both signals, periods are slightly longer for plages in comparison with sunspots.