Sunspots produce intense radiation which make the ionosphere reflect radio signals. The quantity of sunspots is directly related to the short-wave reflecting effectiveness of the ionosphere . Propagation of VHF Radio Signals -VHF (above 30 MHz) are less affected by solar activity although at 5O MHz very long distant propagation can occur at a sunspot maxima.
It is not surprising that the effectiveness of the ionosphere as a "mirror in the sky" follows the sunspot number in its eleven year cycle. The density of the ionosphere ( which is correlated to sunspot maxima) determines the maximum usable frequency.