Historical Auroras in the 990s: Evidence for Great Magnetic Storms
  Recently, a significant carbon-14 enhancement in the year 994 in tree rings has been found, suggesting an extremely large cosmic ray flux event during a short period. The origin of this particular cosmic ray event has not been confirmed, but one possibility is that it is of solar origin. Contemporary historical records of low latitude auroras can be used as supporting evidence for intense solar activity around that time. We investigated the previously reported as well as the new records found in contemporary observations from the 990s to determine potential auroras. Records of potential red auroras in the late 992 and early 993 were found around the world, i.e. in the Korean Peninsula, Germany, and the Island of Ireland, suggesting the occurrence of an intense geomagnetic storm driven by solar activity. 
Hisashi Hayakawa, Harufumi Tamazawa, Yurina Uchiyama, Yusuke Ebihara,   Hiroko Miyahara, Shunsuke Kosaka, Kiyomi Iwahashi, H...
astro-ph.SR physics.hist-ph physics.space-ph