David M. Willis

Scientific Interpretation of Historical Auroral Records

Recent research on the scientific interpretation of historical auroral records is summarised, with special emphasis on topics that provide physical insight into possible variations in geomagnetic disturbance and solar activity during the past two millennia.

Evidence for Recurrent Auroral Activity in the Twelfth and Seventeenth Centuries

The existing evidence for recurrent auroral activity during the third decade of the twelfth century is reviewed and new evidence for recurrent auroral activity during the third decade of the seventeenth century is presented. The earliest known drawing of sunspots appears in The Chronicle of John of Worcester, which was compiled in the first half of the twelfth century. In this medieval chronicle, the Latin text describing the sunspots is accompanied by a colourful but idealised drawing, which shows the apparent sizes and positions of two sunspots on the solar disk. The date of this observation of sunspots from Worcester, England, is firmly established as 8 December 1128. About 5 days later, on the night of 13 December 1128, a red auroral display was observed from Songdo, Korea (the modern city of Kaesong). This auroral observation was recorded in the Goryeosa, the official Korean chronicle of the period. In addition, five Chinese and five Korean descriptions of auroral displays were recorded in various East Asian histories between the middle of 1127 and the middle of 1129. These ten Oriental auroral records correspond to six distinct auroral events, which provide evidence for recurrent, though possibly intermittent, auroral activity on a timescale almost exactly equal to the synodic-solar-rotation period (approximately 27 days). The existing catalogues of East Asian auroral observations, together with new auroral observations gleaned from two additional Korean histories, namely the Seungjeongwon Ilgi and the Jeungbo Munheon Bigo, are used to investigate auroral activity in the interval 1624–1627. Strong evidence is found for recurrent auroral activity between about the middle of 1625 and the middle of 1626. There is also some evidence for prolonged auroral activity at this time, sometimes extending over several consecutive nights. It is shown that these patterns of auroral activity are most unlikely to have occurred by chance. Further extensive searches of the Seungjeongwon Ilgi are required to consider the patterns of auroral activity during the long interval 1623–1894. Such searches should help to elucidate the nature of geomagnetic storms during past centuries.