The solar flare of AD774/5 (sometimes AD776) is the subject Mike Baillie has earmarked for his 2013 AGM talk – but I'm sure it will include lots of other interesting add-ons. A new paper in Geophysical Research Letters might be worth having a look at – go to http://wattsupwiththat.com/2013/03/12/solar-proton-event-seen-in-tree-ri… … has worked out it was one order of magnitude more powerful than a solar flare in 1989 and such events are not as rare as people might imagine.
See the article at http://onlinelibrary.wiley.com/doi/10.1002/grl.50222/abstract/