Jonathon Allen quotes an entry in the Anglo-Saxon Chronicle that might account for the increased cosmic-ray flux in AD 774–775 (Nature 486, 473; 2012). Other medieval texts recall another celestial phenomenon from around the same time, which may or may not be pertinent.

In the context of Charlemagne's campaign against the Saxons, the annals of the monastery of Lorsch, Germany (Annales Laurissenses), mention an image witnessed in AD 776 as “two shields burning with red colour and moving above the church itself”. The Chronicon of Sigebert of Gembloux notes that “when the Saxons besieged the castle of Heresburch, the glory of God appeared to all, surely as two shields burning with the colour of blood and making certain motions through the air, as if at war”. The phenomenon seems to have been observed during the day, suggesting that it was very bright if indeed it was a cosmic event.

The Anglo-Saxon Chronicle also describes a heavenly red crucifix, a colour that is a traditional motif of battle-related portents. The date disparity between the shield sighting and the AD 774–775 event might be explained by an extended period of auroral activity. Also, the Anglo-Saxon Chronicle entry is linked to the Battle of Otford, thought to have occurred in AD 776.