At www.nature.com/news/mysterious-radiation-burst-recorded-in-tree-rings/ … a spike in C14 levels indicates a massive cosmic event of some kind – in AD774/5. The atmospheric level of C14 must have jumped significantly – but apparently, supernovae and solar flares have been ruled out. The C14 isotope is formed when highly energetic radiation from outer space hits atoms in the atmosphere producing neutrons. These collide with nitrogen 14 which then decays to C14. This is in fact happening all the time because of background radiation, producing a continuous supply of C14 – but the spike in 774/5 is thought to be exceptional.
The article claims there were not reports of aurorae at the time, or a supernova (a new bright star). As far as a solar flare is concerned it is thought it would have had to be really big causing aurorae much further around the globe than normal flares. In the comments to the article somebody mentions an Irish saint, St Romboldus, and a possible mention of aurorae, while another commenter says that in the Anglo Saxon Chronicle of 775 there is mention of wonderful serpents seen in the sky over Sussex and a red crucifix was seen in the sky (elsewhere). Some digging around for other reports of aurorae may be fruitful.