The best concise propagation explanation I have ever seen is by Paul Harden, NA5Nhttp://www.qrparci.org/mambo/pdf/FDIM81.pdf
I attended this talk at the QRP ARCI sponsored "Four Days in May" event at Dayton in 2005 and
anytime I want a propagation refresher this is where I go.
I'm not looking for an explanation here, just some links or resources where I can learn about solar activity and propagation. I've searched the web but I have been able to find anything the was concise and well organized.