Solar activity around AD 775 from aurorae and radiocarbon

摘要

A large variation in 14 C around AD 775 has been considered to be caused byone or more solar super-flares within one year. We critically review all knownaurora reports from Europe as well as the Near, Middle, and Far East from AD731 to 825 and find 39 likely true aurorae plus four more potential aurorae and24 other reports about halos, meteors, thunderstorms etc., which werepreviously misinterpreted as aurorae or misdated; we assign probabilities forall events according to five aurora criteria. We find very likely true auroraein AD 743, 745, 762, 765, 772, 773, 793, 796, 807, and 817. There were twoaurorae in the early 770s observed near Amida (now Diyarbakir in Turkey nearthe Turkish-Syrian border), which were not only red, but also green-yellow -being at a relatively low geo-magnetic latidude, they indicate a relativelystrong solar storm. However, it cannot be argued that those aurorae(geo-magnetical latitude 43 to 50 deg, considering five differentreconstructions of the geo-magnetic pole) could be connected to one or moresolar super-flares causing the 14 C increase around AD 775: There are severalreports about low- to mid-latitude aurorae at 32 to 44 deg geo-magneticallatitude in China and Iraq; some of them were likely observed(quasi-)simultaneously in two of three areas (Europe, Byzantium/Arabia, EastAsia), one lasted several nights, and some indicate a particulary stronggeo-magnetic storm (red colour and dynamics), namely in AD 745, 762, 793, 807,and 817 - always without 14 C peaks. We use 39 likely true aurorae as well ashistoric reports about sunspots together with the radiocarbon content from treerings to reconstruct solar activity: From about AD 733 to 823, we see at leastnine Schwabe cycles ...