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.