The Celestial Sign in the Anglo-Saxon Chronicle in the 770s: Insights on Contemporary Solar Activity

The anomalous concentration of radiocarbon in 774/775 attracted intense discussion on its origin, including the possible extreme solar event(s) exceeding any events in observational history. Anticipating such extreme solar events, auroral records were also surveyed in historical documents and those including the red celestial sign after sunset in the Anglo-Saxon Chronicle (ASC) were subjected to consideration. Usoskin et al. (Astron. Astrophys. 55, L3, 2013: U13) interpreted this record as an aurora and suggested enhanced solar activity around 774/775. Conversely, Neuhauser and Neuhauser (Astron. Nachr. 336, 225, 2015a: N15a; Astron. Nachr. 336, 913, 2015b: N15b) interpreted after sunset as during sunset or twilight; they considered this sign as a halo display and suggested a solar minimum around 774. However, so far these records have not been discussed in comparison with eyewitness auroral records during the known extreme space weather events, although they were discussed in relationship with potential extreme events in 774/775. Therefore, we reconstruct the observational details based on the original records in the ASC and philological references, compare them with eyewitness auroral observations during known extreme space weather events, and consider contemporary solar activity. We clarify the observation was indeed after sunset, reject the solar-halo hypothesis, define the observational time span between 25 March 775 and 25 December 777, and note that the parallel drawing of lunar halo display in 806 in the ASC shown in N15b was not based on the original observation in England. We show examples of eyewitness auroral observations during twilight in known space weather events, and this celestial sign does not contradict the observational evidence. Accordingly, we consider this event to have happened after the onset of the event in 774/775, but to have shown relatively enhanced solar activity, with regard to other historical auroral records in the mid-770s, as also confirmed by the Be-10 data from ice cores.