If you want to really understand the interaction of solar activity and HF propagation read this very well written article by Paul Harden, NA5N. The 8-page PDF is a paper presented at the 2005 Dayton Hamfest FDIM Symposium and is entitled, "Solar Acitivty & HF Propagation". Paul provides excellent diagrams, examples and explanations in a style that's very accessible. He also offers valuable insights for QRP operators. This is a must read paper.