Records of Auroral Candidates and Sunspots in Rikkokushi, Chronicles of Ancient Japan from Early 7th Century to 887

TL;DR

This study analyzes ancient Japanese records of sunspots and auroras from the 7th to 9th centuries, comparing them with solar activity proxies to understand historical solar and auroral phenomena.

Contribution

It provides the first systematic survey of auroral and sunspot records in Rikkokushi, correlating these with lunar phases and solar irradiance data to assess historical solar activity.

Findings

Identified 1 sunspot and 13 auroral candidate records in Rikkokushi.

Detected a gap in auroral activity records from late 7th to early 9th century.

Found higher potential for aurora observation due to historical magnetic latitudes.

Abstract

In this article, we present the results of the surveys on sunspots and auroral candidates in Rikkokushi, Japanese Official Histories from the early 7th century to 887 to review the solar and auroral activities. In total, we found one sunspot record and 13 auroral candidates in Rikkokushi. We then examine the records of the sunspots and auroral candidates, compare the auroral candidates with the lunar phase to estimate the reliability of the auroral candidates, and compare the records of the sunspots and auroral candidates with the contemporary total solar irradiance reconstructed from radioisotope data. We also identify the locations of the observational sites to review possible equatorward expansion of auroral oval. These discussions suggest a major gap of auroral candidates from the late 7th to early 9th century, which includes the minimum number of candidates reconstructed from the radioisotope data, a similar tendency as the distributions of sunspot records in contemporary China, and a relatively high magnetic latitude with a higher potential for observing aurorae more frequently than at present.