Do the Chinese Astronomical Records Dated AD 776 January 12/13 Describe an Auroral Display or a Lunar Halo? A Critical Re-examination

Abstract

The enhancement of carbon-14 in tree rings around AD 774/775 has generated wide interest in solar activity at that time. The historical auroral records have been examined critically. Of particular interest was the "white vapour" observed in China on AD 776 January 12/13. Both Usoskin et al. (2013, Astron. Astrophys. 55, L3; U13) and Stephenson (2015, Adv. Sp. Res. 55, 1537; S15) interpreted this record as an auroral display. Subsequently, Neuhäuser and Neuhäuser (2015, Astron. Nachr. 336, 225; NN15) proposed five "criteria" for the likeliness of aurorae and on this basis rejected an auroral interpretation. Instead, they interpreted it as a lunar halo, and suggested there were no auroral records as a proxy of solar activity in the interval AD 774-785. We consider if their "lunar halo hypothesis" and their auroral criteria could be of use in future researches on historical auroral candidates. We first show a counter-example for the lunar halo hypothesis from a parallel record on 1882 November 17, which was seen as a whitish colour, in the southerly direction, and near the Moon. We then consider NN15's criteria on colour, direction, and sky brightness and investigate other counter-examples from early-modern auroral observations. We also consider the extension of the white vapour in AD 776 according to the distribution of Chinese asterisms, and show that its large extension was inconsistent with the lunar halo hypothesis. Conversely, the streaks of white vapour penetrating the eight Chinese asterisms can be reproduced if we consider auroral-ray structures at altitudes between 97 km and 170 km, along geomagnetic field lines between the L-shells L=1.55 and 1.64. Our investigations show that we should consider candidate auroral records in historical documents not on the basis of the newly suggested a priori criteria by NN15 but on all the available observational evidence.

28 pages, 6 figures, accepted for publication in Solar Physics on 5 February 2019

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Cited by in corpus (4)

• Occurrence of Great Magnetic Storms on 6-8 March 1582
• Candidate Auroral Observations during the Major Solar-Terrestrial Storm in May 1680: Implication for Space Weather Events during the Maunder Minimum
• The Celestial Sign in the Anglo-Saxon Chronicle in the 770s: Insights on Contemporary Solar Activity
• A possible case of sporadic aurora observed at Rio de Janeiro