It is tempting to speculate that the ancient text of the Anglo-Saxon Chronicle might offer a clue to the cause of the mysterious, dramatic cosmic-ray event in AD 774 (240–242; 2012). et al. Nature 486,
A chronicle entry for the same year (see go.nature.com/wwkw5j) hints at the presence of a supernova largely hidden behind a dust cloud, which would scatter and absorb all light bar a trickle of red. The resulting supernova remnant would be invisible.
The entry notes: “This year also appeared in the heavens a red crucifix, after sunset; the Mercians and the men of Kent fought at Otford; and wonderful serpents were seen in the land of the South-Saxons.”