Eleven and 87 year sunspot cycles

It is well known that the sunspots vary over an eleven-year cycle. As the sunspot number increases, the ionosphere is more ionized and radio propagation on the high shortwave bands from about 10 - 30 MHz. Also, the higher the sunspot number, the more likely is the appearance of Northern Lights.

Here's spectral analysis of the monthly sunspot numbers from 1750-2020. First I show Fourier based spectrogram analysis. The y-axis is frequency or inverse years and there are arrows marking 11 years as well as the Gleissberg cycle of about 87 years. Note how the 11 year cycle has split into two periods since the 50's. There is a weak subharmonic at 5.5 years also. Due to the 90 year analysis window length, the time scale ends 45 years before 2020, i.e. in 1975. 

The raw sunspot data is shown in the second figure. It is evident that we are right after a solar minimum now early in 2021. This is the sunspot series version 2.0. The data source is the Royal Observatory of Belgium and the input file is the monthly data from 1750 up to December 2020, file name: SN_m_tot_V2.0.txt.

The last figure shows an alternative spectral analysis with the Burg maximum entropy method implemented as a spectrogram:

It shows the same lines, but now the 22 year cycle also appears. Note how the spectral lines are narrower. 

The development of this analysis software was done as part of a project I did some years ago on Climate Data Oscillations and the Magnitude Squared Coherence at the University of Oslo.