About Sun spots
Sunspots on the 'surface' of the sun emit amounts of magnetic energy.
This causes the E- and F-layers on Earth to ionize thus making it possible to use these layers as a reflector for HF radiowaves.
The number of sunspots cycles every 11 years. There even seems to be a 100 year cycle.
Watch the weekly solar storm forecast by Dr. Tamitha Skov, Space Weather Woman