A Great Space Weather Event in February 1730

Aims. Historical records provide evidence of extreme magnetic storms with equatorward auroral extensions before the epoch of systematic magnetic observations. One significant magnetic storm occurred on February 15, 1730. We scale this magnetic storm with auroral extension and contextualise it based on contemporary solar activity. Methods. We examined historical records in East Asia and computed the magnetic latitude (MLAT) of observational sites to scale magnetic storms. We also compared them with auroral records in Southern Europe. We examined contemporary sunspot observations to reconstruct detailed solar activity between 1729 and 1731. Results. We show 29 auroral records in East Asian historical documents and 37 sunspot observations. Conclusions. These records show that the auroral displays were visible at least down to 25.8{\deg} MLAT throughout East Asia. In comparison with contemporary European records, we show that the boundary of the auroral display closest to the equator surpassed 45.1{\deg} MLAT and possibly came down to 31.5{\deg} MLAT in its maximum phase, with considerable brightness. Contemporary sunspot records show an active phase in the first half of 1730 during the declining phase of the solar cycle. This magnetic storm was at least as intense as the magnetic storm in 1989, but less intense than the Carrington event.Comment: 30 pages, 5 figures, and 2 tables, accepted for publication in Astronomy & Astrophysics on 25 April 2018. The figures and transcriptions/translations of historical documents are partially omitted in this manuscript due to the condition of reproduction. They are available in the publisher versio

SEKKI phenomena on September 17,1770

第6回極域科学シンポジウム[OS] 宙空圏11月16日（月）　国立極地研究所１階交流アトリウ

Possible cause of extremely bright aurora witnessed in East Asia on 17 September 1770

Extremely bright aurora was witnessed in East Asia on 17 September 1770, according to historical documents. The aurora was described as “as bright as a night with full moon” at magnetic latitude of 25°. The aurora was dominated by red color extending from near the horizon up beyond the polar star (corresponding to elevation angle of ~35°). We performed a two‐stream electron transport code to calculate the volume emission rates at 557.7 nm (OI) and 630.0 nm (OI). Two types of distribution of precipitating electrons were assumed. The first one is based on the unusually intense electron flux measured by the DMSP satellite in the March 1989 storm. The distribution consists of hot (peaking at 3 keV) and cold (peaking at 71 eV) components. The second one is the same as the first one, but the hot component is removed. We call this high‐intensity low‐energy electrons (HILEEs). The first spectrum results in an auroral display with a bright, lower green border. The second one results in red‐dominated aurora extending up to the elevation angle of 35° when the equatorward boundary of the electron precipitation is located at 32° invariant latitude. The poleward boundary of the precipitation would be 42° invariant latitude or greater to explain the auroral display extending from near the horizon. The origin of the HILEEs is probably the plasma sheet or the plasmasphere that is transported earthward to L ~ 1.39 due to enhanced magnetospheric convection. Local heating or acceleration is also plausible