The extreme space weather events in October 1788

Solar eruptions launch interplanetary coronal mass ejections (ICMEs) and cause geomagnetic storms and equatorial extension of the auroral oval. Their rare and unique nature has made analyses of historical events extremely important to increase their data availability. In this study, we analyzed the space weather event of 1788 October, which was characterized with simultaneous auroral observations. We etended archival surveys and confirmed the auroral visibilities down to Barcelona (46.0 degrees MLAT) on October 21/22 as well as Mizuhara (27.5 degrees MLAT) and Rome (44.8 degrees MLAT) on October 22/23. The end of auroral reports overlapped with a reported declination disturbance at Mannheim, indicating a Delta D amplitude of >= 1.15 degrees. Two positive excursions of Delta D were recorded, lasting for several tens of minutes. Upward field-aligned currents could have flowed poleward of Mannheim associated with substorms. We identified the equatorial boundary of the auroral oval down to 46.5 degrees ILAT in the European sector and approximately <= 41.6 degrees ILAT in the Japanese sector. This is compared with the reported equatorial auroral boundaries during extreme storms. The long storm sequence indicates the arrival of multiple ICMEs, thereby enhancing solar activity at that time. This sequence is indeed contextualized immediately after the maximum of Solar Cycle 4. Because sunspot observations are extremely scarce around 1788, it is challenging to identify the source active region. This in turn makes these auroral records valuable for the analyses of long-term solar activity before the onset of the Dalton Minimum.

Automated summary

This study analyzed a solar activity event in October 1788 that caused auroral observations in Europe and Japan. Multiple ICMEs arrived during a long storm sequence, enhancing solar activity at that time.